|
Spray Foam Attic Ceilings - Reduce Enery bill by 30%?
Last Post 24 Aug 2010 11:47 AM by Dana1. 39 Replies. |
|
| Author |
Messages |
 |
sumlee
New Member
Posts:7
 |
| 11 Mar 2010 12:13 PM |
|
Does anyone have any research on if having attic ceilings with spray foamed or a radiant barrier helps reduce your energy bill by 15-30% in warm climates such as central Florida. By reducing the attic temperature from 150F to normal outside temperature. I am interested in to know if anyone who lives in a warm state like FL,TX,AZ etc...has had their attic ceilings spray foamed (hurricane adhesive) or use a radiant barrier and has seen a significant reduction in their energy bill? As the claim is since all the AC air ducts are in the Attic and the attic temperature is at 150 to 160F it causes the AC unit to work harder and adding this insulation barrier will reduce the attic temperature to the ambient temperature outside and thus your AC unit is on less. Now I just move to FL from NY where I used my AC 1 month out of the year instead of year around in Florida. Any Help on this matter would be great. I have gotten quotes for Hard Foam at approx. $1.50 to $1.80 per sq ft since its not summer season yet. In summer it jumps to $2.50. So for 2200 sq.ft. living space its $6500 |
|
|
|
|
|
|
Matt G
New Member
Posts:93
|
| 11 Mar 2010 03:46 PM |
|
You are asking about two entirely separate things... Insulation and Radiant barriers. I don't live in that hot a climate, but there are a few things to read and think about:
The University of Fla has done a bunch of research on radiant barriers in the attic. If you Google it you will find it easily. BTW - I don't think they make any claims of 30% savings for radiant barriers.
Re the Foam attic insulation, I as assuming you presently have a vented attic space? If so, I'm guessing the idea is that you will be turning the area into a sealed attic. Here is the thing though: If you have any moisture problems - even if unknown at this time - like bath or kitchen fans vented up there, no moisture barrier under the house (slab?), a damp crawl space, etc, etc, since the attic will no longer be vented, the moisture will have no way to escape and the house will essentially become a incubator. Or, maybe your AC unit will be short cycling and not removing enough moisture from the air. Mold grows great in an incubator! Not saying foam isn't a good idea - it can work great - I'm just saying that you need to proceed with caution. Foam, in combination with other circumstances could be a very bad thing. I'd do it though if I thought everything else was good to go... (and I had the $$$$)
BTW - my ESWAG is that 30% savings is a little high but it all depends on the existing conditions in your home which no one here knows.
|
|
|
|
|
Dana1
Senior Member
Posts:6991
|
| 11 Mar 2010 04:14 PM |
|
The more foam insulation you have, the less effective (and less cost-effective) radiant barriers become. Radiant barriers work GREAT at getting the most out of standard-density fiberglass at R-values of R20 or less. At R30 it's barely measurable.
Whether or not your cooling bill drops by 15-30% depends a whole lot on factors other than just your attic, but also includes how much existing insulation you have. But if you have a lot of solar gain from glazing, that will continue to dominated the cooling load no matter what you do to the attic. In most instances heat transfer through the attic isn't more than half the cooling load.
Sealing the attic with foam puts it inside the pressure and vapor boundary of your house- it's humidity will track the humidity levels inside the house. With a vented attic it tracks with the outdoor humidity which is often quite high. Sealing the attic is the right thing to do, in most cooling dominated climate cases. (The things Matt talks about- bath & kitchen fans vented to the attic instead of the outdoors are in fact code violations in the first place, and should be rectified whether the attic is sealed or not.) If you use at least 2" of closed cell foam you will have a class-II vapor retarder on the outside pressure boundary structure, which is ideal in cooling dominated climates. That would also add another R13 or so to the total insulation, which may be close to doubling real R-value of the typical R19 batt installation. Leave any attic-floor insulation in place. The semi-conditioned attic space will run warmer that way, but it'll be at lower humidity since it's now inside the pressure boundary where the AC is drying things out, and conditioned space will see a reduced cooling load. |
|
|
|
|
jonr
Senior Member
Posts:5341
|
| 11 Mar 2010 09:30 PM |
|
Take a look at http://www.buildingscience.com/documents/reports/rr-9801-vented-and-sealed-attics-in-hot-climates
Cooling costs can go up with a sealed, roof insulated attic, even if there are ducts up there. A white roof might be a better investment.
|
|
|
|
|
sumlee
New Member
Posts:7
|
| 12 Mar 2010 06:55 AM |
|
Thanks for all the feedback as I appreciate the information. The house I have is 1 story and an open attic system so there are vents on the roof and sides. My impression was with using InsulStar Plus(hard Foam/hurricane adhesive) on the attic ceiling/underneath roof would not seal the attic as the hard foam would just act as an insulator to the roof temperature. So if its 90F outside and the roof is at 150F the inside of the attic will not only be the same as the actual outside temperature 90F instead of being the same as the roof. |
|
|
|
|
dpilati
New Member
Posts:10
|
| 12 Mar 2010 08:02 AM |
|
That seems like a lot of money to not seal the attic. I'm curious what the conclusion is. I live in NC and went with the radiant barrier (we are actually a heating dominant climate). The foam was a lot of money. We have a very tight ceiling with foam in spots and tight ducts. Foam would probably take the 150 down to 120 if you are not sealed.
In my house, the windows are the dominant heat gain but a FL house might be designed differently. Since the heat gain helps in the winter, there is a different dynamic in window planning. |
|
|
|
|
Dana1
Senior Member
Posts:6991
|
| 12 Mar 2010 10:00 AM |
|
Posted By jonr on 11 Mar 2010 09:30 PM
Take a look at http://www.buildingscience.com/documents/reports/rr-9801-vented-and-sealed-attics-in-hot-climates
Cooling costs can go up with a sealed, roof insulated attic, even if there are ducts up there. A white roof might be a better investment.
Curiously, that document comes to the opposite conclusion (as does numerous and more recent field data, not mere models as in the above.) See p.11. "White roof" doesn't necessarily mean "cool roof" either. It's the combination of solar reflectance and infra-red emissivity that makes it work. High reflectance and low emissivity doesn't result in reduced cooling load. |
|
|
|
|
Dana1
Senior Member
Posts:6991
|
| 12 Mar 2010 10:17 AM |
|
Posted By sumlee on 12 Mar 2010 06:55 AM
Thanks for all the feedback as I appreciate the information. The house I have is 1 story and an open attic system so there are vents on the roof and sides. My impression was with using InsulStar Plus(hard Foam/hurricane adhesive) on the attic ceiling/underneath roof would not seal the attic as the hard foam would just act as an insulator to the roof temperature. So if its 90F outside and the roof is at 150F the inside of the attic will not only be the same as the actual outside temperature 90F instead of being the same as the roof.
The temperature of the attic depends on many things- don't assume it'll track outdoor temps with any precision. All closed cell foams are effective adhesives (similar chemistry to Gorilla Glue). But to have any insulation effectiveness it needs thickness. It generally runs ~R6 per inch of thickness. It's emissivity is high, so if you have a skim coat of 1/2" it may glue it up real nice, but it'll only add R3 between the roof an interior, and it'll still radiate heat to the interior. If you go to 2" you get R12-R13 which will reduce the temperature of the interior surface significantly resulting in far lower radiated heat flux. From a hurricane protection point of view, sealing up the vents reduces the upward pressure experienced by the roof in high winds considerably. A survey of the literature on sealed vs. vented attic issues can be found here. If you don't have the time to wade through it all, just ponder the conclusions, starting on p44. |
|
|
|
|
sumlee
New Member
Posts:7
|
| 12 Mar 2010 10:21 AM |
|
I am trying to get clarification from the foam company on whether it will be sealed or not? As i cant remember what foam estimator said. So given that all my AC ducts are in the attic the attic temperature has no real effect on work load the AC unit is doing? So cooler attic temperature doesnt mean a lesser work load for my AC unit? |
|
|
|
|
sumlee
New Member
Posts:7
|
| 12 Mar 2010 10:23 AM |
|
Information from another person I know:
I did a tremendous amount of research when we built our home (end total sq ft. 4100) regarding spray foam vs. blown insulation w/ radiant barrier. You will read a lot of debate over the use of spray foam in Florida. We very seriously considered using it, however we opted to go with blown insulation (R35) with radiant barrier. There are a few things to consider with spray foam. When I did my research, it seemed that much of the information about spray foam with conducted within their own industry. Most test homes were located in the northen part of the United States. These homes do not see the long term sun exposure that a Florida home will see. There is a debate over how spray foam effects the roofing system. Many believe that because there is no gap between the roofing structure and the insulation, this causes the roofing to "bake". Even though your attic may be cooler, the singles have to absorb the heat that would normally pass through the roof. Supposedly, this may cause a shortened roof life. In our case, we built with ICFs. Our home was extremely air tight without the spray foam. The use of the spray foam would have created a "too airtight" situation and forced us to include a system that would bring air into the house to feed the air condition system. It seemed like there was too much of a debate at this point over the benifits and costs for us to proceed with spray foam. If the research becomes more solid, then we would consider using it in the future. (We really were dead set on using it in the beginning.) As far as the radiant barrier goes, we did cover the entire roof system as well as all gable walls facing east and west. Our electric bill runs in the $350-$400 range in the summer for 4100 sq. ft. Our electric bill this past month was $236. I am sure that the ICF is the major contributor to this, but I seriously believe that the radiant barrier (second floor is frame construction) has made a big difference. |
|
|
|
|
sumlee
New Member
Posts:7
|
| 12 Mar 2010 10:33 AM |
|
From another person I know:
I don't have any documented research regarding E-bill savings but in speaking with my trusted A/C contractor in N. Florida, the radiant barrier film attached to the bottom of the roof trusses is the way to go. It creates an air gap between the roof surface & attic space that allows the hot air created by hot roof surface to vent directly through the soffit and ridge vents without ever entering your attic. He says the attic temp is close to if not the same as outside air temp. Simple laws of thermodynamics suggests your A/C will work more efficiently without fighting heated attic ducts .... thus less enery used ... thus lower E-bill.
I WOULD NOT recommend spray foam. One, it causes the roof temp to increase dramatically because there's nowhere for the heat to dissipate to except back toward the roof surface. If you have shingles thats a real bad thing. You need an air gap. Two, if you have any type of water leak, large or small, you'll never know until its too late .... the entire roof would be rotted .... possibly underlayment AND the trusses ... BIG $$$ ... & it would be a perfect environment for mold growth. Spray foam is really meant for keeping out cold not heat. The hurricane adhesive claim is a marketing ploy. I've been through Andrew (164MPH - 1 mile from my house) & if your roof is constructed properly ... the roof sheathing will not come off.
As for cost, the house I'm building is 5228 SF CBS house and my insulation cost for the entire project ... combo blown-in R30/batt R-19 for attic ceilings, sound walls, fire caulking, vents etc INCLUDING radiant barrier applied to attic trusses as I described is only $5040. So the hard foam is not only a bad idea but a RIP OFF in my opinion. Get some quotes for the radiant barrier.
|
|
|
|
|
sumlee
New Member
Posts:7
|
| 12 Mar 2010 10:47 AM |
|
This was also useful information
http://www.dca.state.fl.us/FBC/commission/FBC_0210/Green_Roofs/FSEC-CR-1496-05.pdf |
|
|
|
|
Dana1
Senior Member
Posts:6991
|
| 12 Mar 2010 11:02 AM |
|
Posted By sumlee on 12 Mar 2010 10:21 AM
I am trying to get clarification from the foam company on whether it will be sealed or not? As i cant remember what foam estimator said. So given that all my AC ducts are in the attic the attic temperature has no real effect on work load the AC unit is doing? So cooler attic temperature doesnt mean a lesser work load for my AC unit?
A cooler attic temperature DOES reduce your cooling load, as long as it's not cooler simply because it's being actively cooled by the cooling system. But it takes some real R at the roof deck to lower the attic temperature much. If you're splitting the insulation, some on the roof deck, some at the attic floor the attic may be warmer than if it's all at the roof deck, but the reduced load of the fully conditioned space due to the greater insulation value more than compensates for the losses from the ducts & mechanicals being in a higher temp environment than the conditioned space. There's no moisture control rationale for a ventilated attic in a cooling dominated climate- NONE! When the attic is inside the pressure boundary of the building it's humidity tracks the interior humidity, so when it heats up in mid-day it's relative humidity is lower than the conditioned space. When it's ventilated to the outdoors high humidity outdoor air enters during the day only to condense moisture on the cooler joist-tops (cooled by the AC in the rooms below). Only in heating dominated climates do you need to worry about sealed-attic condensation, and then primarily when splitting the R-value between roof deck & attic floor such that the attic runs substantially cooler than the conditioned space below. There's very little energy rationale for a passively ventilated attic in a cooling dominated climate, and even active ventilation results in at most single-digit percentage reductions in cooling load- spending the money on more insulation instead of a fan is money better spent. There is increased hurricane damage potential with ventilated attics. How much insulation to you currently have in the attic? Are the ducts sealed? (With mastic or aluminum tape.) Are the ducts insulated? How thick is the foam they're spraying? In my neighborhood closed cell foam costs ~$1.20 per board foot (1 square foot at 1" thick) installed. If you're getting at least an inch, that's R6. If you have R19 batts between joists 16" o.c. that's roughly R15-16 when the roof is hot, so you'll have reduced your heat gain by bit more 1/4. If you have only R11 batts between joist 16" o.c. that's roughly R10, and and additional R6 means you've reduced the heat gain by a bit more than 1/3. Sealed vs. ventilated won't make large difference in the heat gain, but it's huge in terms of moisture control and hurricane protection. If hurricane protection was part of what you're paying for, seal it. Sealed attics are almost always the right thing to do in cooling dominated climates. Sealed or not, it's almost always cost effective to go to R25-R30 with lower-cost fiber insulation if you're starting out with R11-R19. Radiant barriers can be cost effective if you only have R11-R19, but at R30, not so much. The folks at Oak Ridge Nat'l Labs have worked out present value tables for various upgrades based on anticipated utility savings for several US cities. Basically, paying up to the amount in the box in the table per square foot will pay for itself using fairly conservative assumptions about energy pricing and interest rates, etc. Hurricane protection isn't in the equation, but you can use the R-value of your sprayed-on glue as part of the total R before-upgrade to decide whether you should blow some cellulose topping on the existing insulation. |
|
|
|
|
Dana1
Senior Member
Posts:6991
|
| 12 Mar 2010 12:25 PM |
|
With due respect for your friends, insulating the roof deck only "dramatically" increase peak shingle temps on low-slope roofs (2:12 or lower.). The bulk of shingle cooling is always convection & radiation to the exterior. The roof deck & felt add about R0.75-R1 of insulation between it and any paltry cooling an attic provides.
And "keeping out cold, not heat", is only a matter of which side of the thermal barrier you're on, eh? Foam performs well up to temps of 200F- far outperforming the cheap low density fiberglass batts found all over the cooling-dominated parts of the US per R-rating.
From a roof leak & energy point of view going with a higher R-value of 4-8" open cell foam is preferred, but it doesn't provide the structural rigidity you're seeking. But if you use it to also seal the venting it does still provide significant hurricane resistance. When a roof leaks above open cell foam it drips through pretty much where the leak is, and it'll dry toward the air-conditioned interior since it's fairly vapor permeable. A 1" lift of closed cell foam is still semi-permeable and the roof deck can dry, but it's water-proof, and finding the leaks can be difficult. At 2" or more closed cell foam is semi-IMpermeable, slowing inward-drying considerably. You need at least a half-inch of closed cell foam to get the glue-as-structure effect, and an inch is better. A hybrid approach is to use closed cell foam over the rafters to glue it to the roof deck, and open cell (or nothing) on the roof deck itself, putting the bulk of the R-value in far-cheaper cellulose at the attic floor, insulating any ducts or mechanicals in the attic. |
|
|
|
|
sumlee
New Member
Posts:7
|
| 12 Mar 2010 12:39 PM |
|
Hi Dana1,
I am just looking for what is a most effective way in reducing my energy bill and everyone keeps saying its cause my attic ceiling isnt insulated (the attic floor is with R30 8 inches originally in 2002 but now its at 4inches so that like R13) and thus the attic temp is 50/60F hotter than the outside temp and causing my AC to be on more. All these Foam Vendors keep saying the issue is at the attic ceiling adding new insulation to the attic floor will do nothing since that high attic temps are heating up my AC ducts that are all in the attic. |
|
|
|
|
NFC
New Member
Posts:59
|
| 12 Mar 2010 01:14 PM |
|
You could always just spray foam the ducts in the attic to seal and superinsulate them. That will save you money. Unless you use the attic for livable space, spraying the entire envelope does not make much financial sense to me. You could probably seal the ducts and radiant barrier the roof for less than spraying the attic. |
|
|
|
|
Dana1
Senior Member
Posts:6991
|
| 12 Mar 2010 03:07 PM |
|
Posted By sumlee on 12 Mar 2010 12:39 PM
Hi Dana1,
I am just looking for what is a most effective way in reducing my energy bill and everyone keeps saying its cause my attic ceiling isnt insulated (the attic floor is with R30 8 inches originally in 2002 but now its at 4inches so that like R13) and thus the attic temp is 50/60F hotter than the outside temp and causing my AC to be on more. All these Foam Vendors keep saying the issue is at the attic ceiling adding new insulation to the attic floor will do nothing since that high attic temps are heating up my AC ducts that are all in the attic.
Blow 6" of cellulose on top of what you've got, seal and insulate the ducts to at least R6. If your roofing color isn't fairly light/white install aluminized fabric type radiant barrier at the roof deck (with at least 2" of space between the RB & roof deck. If the ducts are low enough it's often cheap to just bury them in blown cellulose. To keep the cellulose from blowing around when there's high winds creating air current in one soffit and out the other, misting the top of the cellulose to a damp (not soupy) papier-mache' consistency causes it to crust over. Alternatively, wet-spray cellulose instead of dry-blown keeps it's shape & position due to it's water-activated adhesives. If dry-blown, insist on "borate-only" cellulose, since sulfates have metal corrosion issues if ever wetted, some minor wetting is likely with open soffits in a gale-force storm. Insulating the ducts to R6 is at least as effective as insulating the (much larger area) with an inch of closed-cell foam. Using cc foam to seal & insulate the ducts in one step is often possible. The foam guys are overselling. Restoring the attic floor R-value to R30 or above will be FAR more effective than an inch of closed cell foam at the roof deck, with or without a radiant barrier. But 5-6" of open cell foam at the roof deck + sealing the attic would be better, in which case the need to insulate the ducts isn't as dire. With R30+ on the floor, a radiant barrier at the roof deck has a very marginal effect on the heat flux through the attic floor, but RB will reduce the uptake of the less-well insulated ductwork. But if you can bury the ductwork in 6" of cellulose even that factor fades. For a set of well-vetted recommendations for FL attics, skim your way through the relevant portions of these documents. Blown cellulose is preferable to batts or blown fiberglass here for a couple of reasons. The higher density makes it less likely to be moved by air than blown fiberglass, but any blown insulation gets more perfect coverage, fills in all the gaps. It also behaves as a hygric buffer, distributing any condensation or lighter bulk moisture incursions, wicking it away from structural materials. But if it's ever saturated with a major roof leak the wet sections need to be scooped out & replaced, since it'll take forever to dry. In general it makes structured MORE moisture tolerant, and less prone to mold issues. Also it loses less R-value with high temperature differences than fiberglass does, and is opaque to infra-red, whereas heat radiating into the upper couple of inches of fiberglass under a hot roof deck pretty much wipes out the insulating value of the upper few inches, with the hottest layer of the insulation occurring an inch or so from the top. |
|
|
|
|
dpilati
New Member
Posts:10
|
| 12 Mar 2010 03:49 PM |
|
Ok - I've heard that covering the ducts with blown fiberglass is not a good idea. Now cellulose might be different. I thought the issue had to do with condensation on the outside of the ducts and mold issues....
If you aren't supposed to cover with fiberglass, what should you do to increase the duct insulation. Someone mentioned foam but that is not an easy DIY option. Any reasonable method to increase the R on your ducts as a DIY? I was going to use rigid foam board on the air handler and plenums. I suppose you could wrap with R-13 batts but it wouldn't be pretty. I also start to wonder about vapor barrier as that subject always confuses me. My ducts now have a black plastic vapor barrier and if I wrapped with batts with the paper vapor barrier, would that be a problem? |
|
|
|
|
Dana1
Senior Member
Posts:6991
|
| 12 Mar 2010 04:38 PM |
|
Posted By dpilati on 12 Mar 2010 03:49 PM
Ok - I've heard that covering the ducts with blown fiberglass is not a good idea. Now cellulose might be different. I thought the issue had to do with condensation on the outside of the ducts and mold issues....
If you aren't supposed to cover with fiberglass, what should you do to increase the duct insulation. Someone mentioned foam but that is not an easy DIY option. Any reasonable method to increase the R on your ducts as a DIY? I was going to use rigid foam board on the air handler and plenums. I suppose you could wrap with R-13 batts but it wouldn't be pretty. I also start to wonder about vapor barrier as that subject always confuses me. My ducts now have a black plastic vapor barrier and if I wrapped with batts with the paper vapor barrier, would that be a problem?
Cellulose passes far less attic-air through to come into contact with the colder ducts, so there's less condensation potential, and it wicks away any condensation that does occur. I haven't studied this in detail, but I know that it's done, and haven't heard of problems with it. It's not necessisarily the preferred method though, since it can blow with air currents or sag with repeated condensation cycles. R13 batts have similar issues to blown FG- it's difficult or impossible to make it air tight. Kraft facers on batts are semi-impermeable (partially permeable), and
not much of an issue in an AC duct-wrap situation. Air-tightness is
far more important than vapor permeabilty here but moisture WOULD be
able to dry eventually through a kraft facer. They're less permeable
than 1" of XPS rigid board though. Rigid foam board with a combination of mastic/foam/FSK tape seals can probably get you there with square ducts, but probably not with round. DIY closed cell foam kits (eg TigerFoam, FomoFoam, FrothPak, etc) are reasonable for small projects like this, and seal perfectly. (This WOULD be the preferred method.) Give it at least an inch, but more than 2" is probably overkill. Even if the bottoms of the ducts aren't accessible, it'll be at least a 3/4 fix, since the radiated heat gain is primarily from above, and of greater magnitude than the air-conducted heat gain. The surface area of the ducts is a small fraction of that of the attic floor and less of a heat-gain issue. But it's still a significant secondary factor due to the fact that the ducts are much cooler,than the conditioned space, for a higher delta-T. The primary issue here is the thinness of the floor insulation, and blown or wet-sprayed cellulose is a decent option. Box over any recessed lighting fixtures and seal them with caulk or 1-part foam before installing the insulation. |
|
|
|
|
tradermom
New Member
Posts:2
|
| 12 Mar 2010 05:49 PM |
|
Dana:
Need some help in resolving a condensation problem we created by installing 2 1/2" Rigid Polyiso Board insulation. We thought we would green the small 1959 beach side home we were renovating here in Mid Florida. Since the exposed wood ceilings are also the wood deck of the roof, there is no attic to add insulation to. We did some research and found a supplier of Polyiso board who assured us we could use it in this application. We installed the boards directly to the underside of the wood deck (the inside ceilings)and between the beams with adhesive and had no problems for months. Since we are not living there we keep the thermostat very low and several weeks ago after we had finished installing new tongue and groove over the polyiso board and between the beams (we used an air gun to install and shot nails into the sides of the beams, we never hit the deck) we noticed water coming between the beams and polyiso board and also at the top of the ridge of the ceilings throughout the house. We spoke with a roofing contractor and he is recommending that we install 4 sloped vents every 8 feet along the ridge since he believes that the air is being trapped and needs venting (he says these are used in mobile homes that alos have no pitch). However, the roof has a very shallow pitch and we are not certain this will resolve the problem. We did not tape or seal the polyiso boards to the beams and we are thinking that this might be a contributing factor. We do not want to create a long term problem and need some help in how to solve this. As a last resort we are considering installing a new roof so we can create a 1-2 inch air gap above the Polyiso board and existing deck. This wold be very expensive and we opuld prefer not to do this. Any feedback would be greatly appreciated.
Thanks,
Tradermom
|
|
|
|
|
Dana1
Senior Member
Posts:6991
|
| 12 Mar 2010 06:44 PM |
|
It's rare to have condensation issue during the heating season in FL. For it to be a condensation issue during the heating season it would mean warm humid air from the interior is getting past your seal job on the iso and condensing on the cooler exterior side of the insulation. With the house being minimally heated, that's highly unlikely to be the source of moisture unless the interior is at a super-moldy near 100% relative humidity level even though you didn't (but should have) air-sealed the edges of the cut-to-fit iso at the beams.
For it to be condensation, warmer moister air has to find a surface colder than it's dew point. The dew point of 68F 35% RH air is about 40F. The dew point of 60F 60% RH air is about 45F. Even when you're not there you need to keep it under 60% RH inside to limit mold growth.
But without a ventilation gap between the roof deck and foil faced iso any moisture that finds it's way to the roof deck has a hard time getting out. High inward vapor drive from the sun hitting a dew or rained-on roof is more likely to be the culprit on a flat-ish roof in FL. If the exterior of the roof is dry the roof deck will be drying out whenever it's ~40F or above, but if it stays wet for long periods, it can't, and the vapor drive is all inward.
I'm not sure what exactly the vents would be venting, if there's currently no air gap between the iso and the roof deck, no matter what the slope. If it was interior air condensing (but I don't think it is), vents to a paper-thin an intermittent gap might pull enough interior air in to make the problem WORSE, not better. If it's from inward vapor (which it probably is) it's unlikely to fully resolve the issue (it may in spots, but not everywhere.)
I fear you may have to bite the bullet on this. Vented roofing deck pre-laminated to iso is available from Hunter, Atlas, et al, and might have been the "right" thing to do from the get-go. Stuff like that may still be the right solution, since 2.5" of iso is only R15, less the thermal short-circuit of the beams. A 2" Hunter Cool Vent (tm) or similar application above the existing roof deck would give you a thermal break over the beams, and plenty of ventilation air for the inward vapor drive of sun on wet roof. But it's not a cheap fix, for sure. |
|
|
|
|
jonr
Senior Member
Posts:5341
|
| 12 Mar 2010 10:30 PM |
|
> Curiously, that document comes to the opposite conclusion (as does numerous and more recent field data, not mere >models as in the above.) See p.11.
You're misreading something. See page 11, row 8, column 3 - an increase in cooling cost of 1.1%. |
|
|
|
|
adkjacUpstateNY
Basic Member
Posts:167
|
| 14 Mar 2010 08:41 PM |
|
Tradermom... I think you made a lobster trap with the foam board and should tear it out. Then if you want an insulated roof.... do the roofdeck commercial style since it is near flat. |
|
|
|
|
Matt G
New Member
Posts:93
|
| 15 Mar 2010 06:03 AM |
|
Back to the initial poster's question, if it were an attic with rafters (as opposed to trusses) and if there is good access - ie not a low pitch roof, I wonder how it would work to sheath the underside of the rafters with say - 1/4" OSB, and then spray foam to the underside of that? You would still have your roof ventilation, but the attic would be either a conditioned or semi-conditioned space that was not vented to the exterior. Thoughts? |
|
|
|
|
Matt G
New Member
Posts:93
|
| 15 Mar 2010 06:25 AM |
|
Posted By Dana1 on 12 Mar 2010 06:44 PM
.
.
>
But without a ventilation gap between the roof deck and foil faced iso any moisture that finds it's way to the roof deck has a hard time getting out. High inward vapor drive from the sun hitting a dew or rained-on roof is more likely to be the culprit on a flat-ish roof in FL. If the exterior of the roof is dry the roof deck will be drying out whenever it's ~40F or above, but if it stays wet for long periods, it can't, and the vapor drive is all inward. <
.
.
.
Inward vapor drive penetrating roofing materials? That is the first time I have heard of that... I guess it is as good an explanation as any. I gotta wonder what role the foil facing on the rigid insulation plays. Did she say it had foil facing? I wonder which way it faces - up or down? The whole thing sounds rather odd though. Good example of something going really wrong when someone didn't do their homework prior to changing the characteristics of a thermal envelope. I know hindsight is 20-20, but the thing about putting up the foam but not sealing the edges to me indicates a lack of due diligence. If the edges had of been sealed it may have just masked the problem though and the whole roof assembly would rot off. Sounds like it is on a slow burn now... |
|
|
|
|
tradermom
New Member
Posts:2
|
| 15 Mar 2010 08:55 AM |
|
Dana1:
We took down the finished wood tongue and groove and caulked all the spaces around a few of the iso boards this past weekend. This past week was very rainy and than went to very warm humid. We waited for a hot sunny day and cool temp. inside to remove a small section of the caulking/foam and found that where we sealed the iso boards to the beams the wood deck above was dry; in the other areas where we had not remained wet. We did purchase the material from a local distributor who I called several times to discuss the project first. Even after the problem started and we were was asking for explanations we were never told to seal the boards. In addition we had several licensed contractors come out and they too never told us to seal. In hindsight though, it seems so obvious. Either way we will be monitoring the situation closely to see if we need to do more than this in the future.I also have contacted the Poyiso manufacturer to further discuss the situation. Lesson learned, I will always spend more time doing my research first. Thank you all for your concern and feedback. |
|
|
|
|
Dana1
Senior Member
Posts:6991
|
| 15 Mar 2010 09:58 AM |
|
Posted By Matt G on 15 Mar 2010 06:25 AM
Posted By Dana1 on 12 Mar 2010 06:44 PM
.
.
>
But without a ventilation gap between the roof deck and foil faced iso any moisture that finds it's way to the roof deck has a hard time getting out. High inward vapor drive from the sun hitting a dew or rained-on roof is more likely to be the culprit on a flat-ish roof in FL. If the exterior of the roof is dry the roof deck will be drying out whenever it's ~40F or above, but if it stays wet for long periods, it can't, and the vapor drive is all inward. <
.
.
.
Inward vapor drive penetrating roofing materials? That is the first time I have heard of that... I guess it is as good an explanation as any. I gotta wonder what role the foil facing on the rigid insulation plays. Did she say it had foil facing? I wonder which way it faces - up or down? The whole thing sounds rather odd though. Good example of something going really wrong when someone didn't do their homework prior to changing the characteristics of a thermal envelope. I know hindsight is 20-20, but the thing about putting up the foam but not sealing the edges to me indicates a lack of due diligence. If the edges had of been sealed it may have just masked the problem though and the whole roof assembly would rot off. Sounds like it is on a slow burn now...
EPDM membranes are highly vapor retardent, but roofing felt is highly permeable- even the heaviest construction felts barely make class-III vapor retardency levels. Without being able to inspect, (or even measure temps/humidities) this diagnosis is highly speculative, but it's surely not condensation of interior air against the cool roof deck (as is common in cold/very-cold climates) unless the temperature of the roof deck stays below the dew point of the interior air for extended periods (days, not hours) which would be a rare condition in FL. The foil facers on the iso limit the ability of the roof deck to dry toward the interior. In high vapor-drive cycling of the roof this it potentially what allows water to accumulate in bulk and drip. Most of the drying of roof decks still has to be toward the exterior though. As the roof deck heats up in the sun it's vapor pressure increases considerably With a highly retardent foil vapor retarder on the interior the micro-gap of air between the insulation and foil takes on that moisture while it's hot, but at night when it cools the wood doesn't easily resorb the water, and it condenses. Spray polyurethane in thicknesses less than 2" thick is semi-permeable, and would still allow some inward drying, but without the air gap to take on water to later condense liquid water never forms & runs. Placing unfaced iso with a ventilated nailer deck above the structural roof: A: decreases the peak temperature of the structural roof deck during sunny periods, lowering it's inward vapor drive ...and... B: Allows the nailer-deck to dry to both sides during periods of high vapor drive, keeping the moisture from finding it's way down to the structural roof ...and... C: retains the ability of the structural roof to dry toward the exterior, into the ventilation gap below the nailer deck Simply adding a ventilated secondary roof above the original may be enough to fix the problem, but putting some vapor-permeable insulation like fiber-faced iso or EPS between the structural roof and ventilation gap is more assured, since it lowers the inward drive toward that mis-placed interior side vapor retarder (the foil facers on the interior iso). |
|
|
|
|
adkjacUpstateNY
Basic Member
Posts:167
|
| 15 Mar 2010 11:55 AM |
|
Lobster traps... that's so often what we are making since the 70's oil embargo mess.... and now with DIYers and most contractors (even building scientists) trying their best... we are close at all times to making lobster traps. watch Holmes on Homes... he and many of us are ripping apart nearly new construction... that is built 90% or more by the books... the most bestest modern Joe Lstiburek advice... (http://www.buildingscience.com/index_html) who is the Zen Master of all things moisture/insulation related,
and what pray tell are we finding?
massive water problemos... huge volumes of trapped water, mold, wet insulation soaking wet wood so full of water it sinks if dropped into water.
I see a problem that others here see... that we must build assemblies that work if they are not perfectly assembled. Either during assembly, or as time goes on and parts of the assembly natural age and become less perfect. One example of this is rainscreening... such a great way to build and side a wall.
So much of what we do... does one thing but also does another less good thing... unintended consequences are biting us bigtime in the home building business these last 40 years...
Hopefully thru informative exchanges of real experiences and knowledge on boards like this... we will get thru this phase of American home building and make it to a day where we proudly build sustainable long lasting low to zero energy use homes.
aj
the lobstertrapslayer |
|
|
|
|
engineer
Veteran Member
Posts:2749
|
| 15 Mar 2010 09:55 PM |
|
I have 3" of closed cell spray foam applied to underside of 6:12 roof, sealing 3rd floor attic, 2" under 2nd floor wing roofs as well as insulating and air sealing drive-under 3 bay garage from rest of house. Every inch of ductwork is within the conditioned envelope and plays an important role in why my 3400 SF house near Jacksonville, FL heats and cools with 2.5 tons of HVAC at an average HVAC cost of <$40 per month. That result is despite 40+ windows and 8 exterior doors as well as 3 kids constantly in and out of all 8 doors.
I ran the design through two industry standard Manual J compliant load calc software packages and both predicted the 2.5 ton load, heating and cooling, to within 5%.
My builder, now partner has been using closed cell foam in this manner for 15 years. We don't build a house without it. Despite dewpoints often in the 70s continually for 4+ months we are able to hold interior RH to 50% or less without added dehumidification. |
|
Curt Kinder <br><br>
The truth is incontrovertible. Malice may attack it, ignorance may deride it, but in the end, there it is - Winston Churchill <br><br><a href="http://www.greenersolutionsair.com">www.greenersolutionsair.com</a>
|
|
|
adkjacUpstateNY
Basic Member
Posts:167
|
| 15 Mar 2010 11:47 PM |
|
Curt... great post for me to learn a bit about Florida insulating... I definitely like spray foam over piecing board foam all day long.
Tricky part up this way is the cost of closed cell, and the wetness of building materials as we build being trapped if sprayed with closed cell and the frame is still soaking wet.
aj |
|
|
|
|
sasanoa
New Member
Posts:6
|
| 16 Mar 2010 07:21 AM |
|
I'm sure some of you understand this but it bears repeating.
Home Performance is Climate Specific.
Building construction needs to vary with local climate conditions. Ask yourself which way do you want the temperature to move? What is your preferred Relative Humidity, in relation to the outside of the building shell? Each of these has a great effect upon how you should layer your building components.
|
|
|
|
|
adkjacUpstateNY
Basic Member
Posts:167
|
| 16 Mar 2010 11:25 AM |
|
sasanoa... yes indeed about climate specific... ..
I am however very wary of layers that create trapped moisture.
and layers that create capillary water migration into layers that then trap it it
call me the lobsterman... but I feel I have seen this type of water problem lots... and really try my best to avoid construction plans that create this.
aj the lobstertrapman |
|
|
|
|
Dana1
Senior Member
Posts:6991
|
| 16 Mar 2010 12:06 PM |
|
Posted By engineer on 15 Mar 2010 09:55 PM
I have 3" of closed cell spray foam applied to underside of 6:12 roof, sealing 3rd floor attic, 2" under 2nd floor wing roofs as well as insulating and air sealing drive-under 3 bay garage from rest of house. Every inch of ductwork is within the conditioned envelope and plays an important role in why my 3400 SF house near Jacksonville, FL heats and cools with 2.5 tons of HVAC at an average HVAC cost of <$40 per month. That result is despite 40+ windows and 8 exterior doors as well as 3 kids constantly in and out of all 8 doors.
I ran the design through two industry standard Manual J compliant load calc software packages and both predicted the 2.5 ton load, heating and cooling, to within 5%.
My builder, now partner has been using closed cell foam in this manner for 15 years. We don't build a house without it. Despite dewpoints often in the 70s continually for 4+ months we are able to hold interior RH to 50% or less without added dehumidification.
Predicating it on the "... added dehumidification...", is a bit specious, since dehumidification is happening any time the AC is running. Simply properly sizing the AC for the climate pretty much takes care of the dehumidification. When it fails to, it's usually a sizing error. That said, 2-3" of cc foam under the roof deck does manage inward vapor drive from roofs well, and air sealing keeps humidity from air infiltration under control. CC foam does a great job of both air sealing and vapor control, but that's not to say it can't be done by other means. Sounds like you have tight construction methodology well under control (head and shoulders above the average, I'd gather.) I suspect doing the roof decks with 5" of half-pound foam would yield a similar result though. Despite it's higher vapor permeability, half pound foam is a good air barrier, and infiltration is the first 90% of the battle. (I'm sure there are good models out there for calculating the increase in latent load you'd see.) Of course there are structural reasons for going with cc foam in hurricane zones too. It's a premium product with many great features when properly applied. Misapplication of vapor retardent materials is rampant though, especially by those applying standard methodologies developed for one climate inappropriately to a quite different climate. Closed cell foam is one of those materials, but gross errors aren't as common as with foil facers or poly sheeting. |
|
|
|
|
gregpruitt55
New Member
Posts:1
|
| 20 Aug 2010 08:32 AM |
|
I live in a hot climate in Texas. I see a lot of responses to the question. I think in the 6 years i have been spraying retorofits in attics, I have some experience. We are currently spraying 5 to 10 per week. I can unabasedly say that on the low average, nearly everyone sees a 30 percent reduction in their overall utility bills. I have seen a few that have been at 25%. I have had some much higher; even one at 66%. It works extremely well if installed properly. You do have some contractors doing a pour job as in anything, Sometimes the lowest bid is not the best. Pople are profusely thanking us. On two story homes, often the upstairs is so hot befor we come, that the customer has ceased trying to use it during the summer. When we get through, the upstairs is as comfortable as the downstairs. I have never seen an upstairs heat problem solved by new A/C. But we have solved the problem on every job we have been on. We have also solved problems with moisuture dripping of of ducts in hot humid weather. I am also a builder, and get called in often to review problems in homes, or in other foam aplicators work. Foam is fantastic! Not to consider foam in a new home is nearly a crime. In a normal new house it will pay for itself in 1-2 years and save the customer depending on the size of the house from $30,000.00 to 70,000.00 in a twenty year period. How could you not use it. In an existing home the payback is 3-5 years. We seal the whole attic, and cover the rafters with about 1.5". I speak from experience, and if this did not do what it says, I would not reccommend it. It simply works! Radiant barrier is a joke compared to foam. Radiant brrier will reduce attic temperaturesby 15 degrees. YOu still have a hot attic, at about 125 during the hot part of the day. That is better thatn 140-150, but still very hot..I have been in many foam roofs, an the temperature be 77 degrees and sometimes less. It usually is between 3-10 degrees of the inside fo the house. In a hot climate, I will challenge any expert to prove this wrong! In new houses, we have many 4200 sq foot homes with $150.00- $200.00 electric bills in the summer. i could go on and on, but you get the picture. The secret is doing it right the first time. We caulk every joing in the exterior wall. Most contractors do not. On average, any foam job will out perform any fiberglass job, but a foam job done well will outpreform the average foam job by abut 20-35%. |
|
|
|
|
Dana1
Senior Member
Posts:6991
|
| 20 Aug 2010 11:58 AM |
|
Greg- out of curiosity, what R-value are you typically foaming your Texan roofs, and are you doing primarily closed cell or mostly open cell?
Covering the rafters with the foam as well as the roof deck is good practice (and not the typical foam job in my limited experience.) I'm sure that's a HUGE benefit under a low-pitched roof in TX! |
|
|
|
|
Alton
Veteran Member
Posts:2157
|
| 20 Aug 2010 02:08 PM |
|
Dana1,
Is it ok to cover the bottom part of the rafter or truss with closed cell foam? Is there any need to leave the rafter exposed so it will let moisture escape or is that not a problem once the attic is sealed? In my home the floor has been foamed and I am now trying to decide the best way to insulate the attic. Since this is an existing home, I am considering removing the fiberglass batts and just spraying the attic floor (drywall ceiling) and the duct work with about 3" of closed cell foam. If it is better to seal the attic completely, then do we leave the fiberglass batts in place and also spray the bottom part of the rafters? Looking forward to your thoughts on this. |
|
Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
|
|
Dana1
Senior Member
Posts:6991
|
| 20 Aug 2010 03:27 PM |
|
Posted By Alton on 20 Aug 2010 02:08 PM
Dana1,
Is it ok to cover the bottom part of the rafter or truss with closed cell foam? Is there any need to leave the rafter exposed so it will let moisture escape or is that not a problem once the attic is sealed? In my home the floor has been foamed and I am now trying to decide the best way to insulate the attic. Since this is an existing home, I am considering removing the fiberglass batts and just spraying the attic floor (drywall ceiling) and the duct work with about 3" of closed cell foam. If it is better to seal the attic completely, then do we leave the fiberglass batts in place and also spray the bottom part of the rafters? Looking forward to your thoughts on this.
In most of Alabama you never need to worry about interior air condensing in the attic in winter with a sealed-attic configuration- even the average daily outdoor temps will be above the dew point of the interior air. Applying up to 2" of closed cell foam on the rafters would still allow them to dry toward the interior should they take on water from summertime vapor drives (or minor leak events.) Short answer- yes, it's OK to cover the rafters/trusses. Sealing the attic venting and putting 2-3" of cc foam on the roof deck, 1-2" on any exposed rafters and leaving the batts in place would give you a much higher R value overall. The ducts would then be in a warm, (but not hot) attic, but inside the pressure-boundary of the building. Inside the pressure boundary the dew point of the attic air would be the same as in the conditioned space (even thought it's temperature would be higher) so the risk of condensation on the AC ducts would be low- about the same as on ducts inside the fully conditioned space. The efficiency loss by the ducts being in a somewhat warmer environment will be more than cancelled by: A: The higher overall R value of the roof and B: The lower air infiltration drive of having the ducts completely inside the pressure boundary of the house. Mastic-sealing then insulating the ducts to at least R6 (even with cheap fiberglass or blown/heaped cellulose) would cut even those conducted duct losses by a major fraction. Board-feet are board-feet, and cost about the same whether applied to roof & rafters or to attic floor & ducts. Even if that R19 is only performing at R10 on the hottest days and even if you have to drop the thickness at the roof deck to 2" to come up with the same amount of foam, sealing the attic will usually be a bigger performance boost than tossing the batts and laying down R20 in foam between the joists (especially if you're foaming over the rafters to provide a thermal-break.) In fact, the batts will perform better after you drop the attic temp with
roof-deck insulation, since with a lower delta-T across them they'll be closer to their labeled rating. The loss of R that occurs with fiberglass batts is a function of temperature difference, and you'll have cut that by half or more. It's probably at least R10 under the hot roof deck now, but should be better than R15 with 2"+ of cc foam on the roof deck. For the same amount of CASH you can improve the overall R going with open-cell foam, which has almost no downside in places as warm as Alabama (but the closer to the gulf you are, the better.) In colder climes wintertime vapor diffusion from conditioned space can load moisture into the roof deck with open cell, where it is much-impeded with 2" of closed cell. See: http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems If yours are 2x6" rafters, a full cavity fill of half pound foam has about the same or slightly higher R as 3" of closed cell, but at only ~2/3 the installed price, and the amount of "extra" needed to fully cover the rafters is also less additional square-footage. The low density stuff won't add structural rigidity the way closed cell would, but that wasn't part of the package you were considering. Something to think about if you end up with sticker-shock on a closed cell roof-deck quote. Last, probably least, any cooling dominated climate, going with a " cool roof" materials when it's time to re-roof is cost effective (practically price-neutral in many cases), and can cut peak-loads & and attic temperatures substantially (usually better than radiant barriers, depending on what the prior roofing was). The key performance aspects to keep in mind are the product's aged solar reflectivity and emissivity numbers (higher is always better). Combined, in a standardized set of parameters they make up a (somewhat messy to calculate) solar reflectance index (SRI). Anything over 75 for an SRI is good, anything over 85 is GREAT. The Lawrence Berkeley National Labs has a handy downloadable spreadsheet calculator in which you can punch in the aged reflectance & emissivity numbers listed for the product on the coolroofs.org rated products directory, to compare one against another. (Not all white is created equal, and some significantly off-whites/tans/grays can beat the SRI of many white finishes.) It's not a replacement for real insulation, but it's well worth considering when re-roofing (or on new roofs, of course.) |
|
|
|
|
Alton
Veteran Member
Posts:2157
|
| 20 Aug 2010 05:22 PM |
|
Thanks Dana1. Very clear and understandable.
|
|
Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
|
|
tigerfan6
New Member
Posts:47
|
| 23 Aug 2010 07:41 PM |
|
Two, if you have any type of water leak, large or small, you'll never know until its too late .... the entire roof would be rotted .. If this is the case, doesn't it also apply to a SIP roof, and to a lesser extent, nail base on a conventional deck? |
|
|
|
|
Dana1
Senior Member
Posts:6991
|
| 24 Aug 2010 11:47 AM |
|
Posted By tigerfan6 on 23 Aug 2010 07:41 PM
Two, if you have any type of water leak, large or small, you'll never know until its too late .... the entire roof would be rotted ..
If this is the case, doesn't it also apply to a SIP roof, and to a lesser extent, nail base on a conventional deck?
To the extent that it's true (I'd strongly disagree about the "entire roof" aspect), it would be the case whenever the deck is insulated from the inside with a water proof product. I practical terms the damage would usually be fairly localized & limited in scope (with the rare exception to prove the rule.) Vented nail-base over a structural deck (on purlins or furring) is more resilient to leakage events. |
|
|
|
|
Active Forums 4.1
|