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Spray Foam Attic Ceilings - Reduce Enery bill by 30%?

Last Post 24 Aug 2010 11:47 AM by Dana1. 39 Replies.

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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 User is Offline

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 User is Offline

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 User is Offline

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 User is Offline

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 User is Offline

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.