which saves ... ALL of the leakage driven air-infiltration losses and ALL of the stack effect infiltration losses

NOTHING gets rid of ALL of these losses.

If, in the winter, you have high indoor relative humidity, it will move into a cooler semi conditioned attic and you will have excessive humidity (ie, mold or water) there. So either also seal the house->attic barrier well and add a dehumidifier (it may ice up) or put vent holes in the house->attic barrier (which means the attic will be fully conditioned).

HI Jonr:

About outdoor winter humidity within nearly having 3-states,  15-35 mi just below lake Erie, causing outdoor AHtP to be mounted 16 to 24" off the ground, from moisture-snow belt stuff:
- indoors can be 25%-35% rH in single digi's while outdoors gets close to the 'Indian' white-death mist near 40%rH, in county-wide areas. Ventilated, vented attics do not seem to collect much moisture if at all  in winter, NOT looking closely, just aware of outside humidity that is HIGHER than the home.

? Could you mean a  higher-in-home layer of more humid air is transferring rH to attic rH?

Posted By jonr on 10 May 2012 08:15 AM

which saves ... ALL of the leakage driven air-infiltration losses and ALL of the stack effect infiltration losses

NOTHING gets rid of ALL of these losses.

If, in the winter, you have high indoor relative humidity, it will move into a cooler semi conditioned attic and you will have excessive humidity (ie, mold) there. So either also seal the house->attic barrier well and add a dehumidifier (it will probably ice up) or put vent holes in the house->attic barrier (which means the attic will be fully conditioned).

OK 95-99% of the duct-leakage driven infiltration and 95-99% of the stack effect infiltration, fine.

The dew point of the attic air will seasonally track that of the conditioned space air unless that attic floor is VERY air & vapor-tight.  Even with kraft-facers on the top side of batt insulation at the attic floor this would be the case.  It's just not cold enough in FL for wintertime moisture to become a mold issue in the attic unless the conditioned space air is allowed to rise to an unhealthy level.  If you hold the line at 60%RH @ 70F indoors in winter, the dew point of that air is ~55F, which is below the mean January OUTDOOR temp in Gainesville runs ~57F.  If the mean temp in the attic is splitting the difference between indoor & outdoor temps between it'll stay dry enough in the attic that mold is not an issue.  At an average temp 66F that 55F dew point air is still under 70% RH, the threshold for a serious mold issue.

Ventilating the house with outdoor air at any reasonable rate with the HRV in January would put the average dew point in the 48-50F range, which would put the attic at less than 60% RH @ 66F.

You can verify it with this tool if it's too painful to read a psychrometric chart.

Don't build your house to just avoid problems on an AVERAGE winter day. 72F @ 60% means dripping wet (100% RH) in a semi conditioned attic when it drops to 42F outside (which happens). You don't want this - so pick one of the solutions.

And yet, that dripping wet scenario literally never happens in real life.

Even if at peak lows gets below the dew point in the attic, without a steady source of moisture it wouldn't be actually dripping- it would only sheen over a bit, condensing only the amount of water it takes to balance the vapor-pressure. It has to be substantially below the dew point for an extended period to get to the point where it drips. If it suddenly drops from the winter-average weather down to say 10-15F for the daily high, you may condense visible liquids that may actually run or drip, but that's still not a mold hazard unless it stays that way for weeks.

If you felt you needed insurance, 3" of cellulose overblow would sufficiently buffer any peaks, but I doubt it would be necessary. When it's cold the conducted heat losses of the ducts will warm the attic above the mid-point between interior & exterior temps, making condensing conditions even briefer & shallower.

When it's colder out, it's drier out. Unless you're actively adding humidity to the ventilation air in winter to KEEP it at 60% RH (a bad idea in any building except maybe a tropical zoo exhibit) it won't be anywhere near that high.

You guys have been great; I have learned so much and have not expected so much interaction. As a person that does a lot of internet research before I do something I find these interactions very helpful. So you are not only helping me in this current issue but helping those who will come after me in their research.

Back to Dana's comments. What you say makes sense but goes against common approach here in Florida, which is to vent the attic as much as possible to keep it cool. I just went up to the attic with my temperature gun. General air was 115 +-, the boards of roof in the mid 130s, those face the sun right now. It is about 87 outside now.

The outside temp even in the hottest time is mid 90s, so why wouldn't a good venting with the outside air do the same thing as keeping it in an acceptable range? Seems that would be a cheaper solution.

Doug

Posted By DougLeppard on 10 May 2012 03:47 PM
You guys have been great; I have learned so much and have not expected so much interaction. As a person that does a lot of internet research before I do something I find these interactions very helpful. So you are not only helping me in this current issue but helping those who will come after me in their research.

Back to Dana's comments. What you say makes sense but goes against common approach here in Florida, which is to vent the attic as much as possible to keep it cool. I just went up to the attic with my temperature gun. General air was 115 +-, the boards of roof in the mid 130s, those face the sun right now. It is about 87 outside now.

The outside temp even in the hottest time is mid 90s, so why wouldn't a good venting with the outside air do the same thing as keeping it in an acceptable range? Seems that would be a cheaper solution.

Doug

Venting it with 90 degree air won't drop the attic to 90 degrees, and won't stop the radiated heat transfer from the 130F roof deck from penetrating an inch or two into the (presumed fiberglass) insulation, causing it to be 5F or more hotter than the attic air temp (!), and radiating directly to the ducts. It also won't block stack-effect from drawing cool & dry conditioned space air into the attic.

If you power-vent the attic depressurizing it to lower the attic temp and cool the roof deck it generally increases total power use due to induced infiltration losses adding more load to the AC than the reduced attic temp was taking lowering. In FL this is primarily from the increased latent load: It takes roughly 2x the air conditioning energy to lower the dew point 20F than it does to lower the air temp 20F, so when sucking in 95F air when the dew point is 75F,  about 2/3 of the power for conditioning that to a comfortable & healthy 75F with a 55F dew point is the latent load.

If you insulate the roof deck but the attic was still 115F due to the lack of venting, the radiated heat transfer to the fiberglass (and ducts) is much lower and the stack effect infiltration is pretty much stopped.  The ceiling temperature in the conditioned space would be lower, despite the same air temp in the attic, due to the reduction in radiated heat transfer into the fiberglass.  You could get similar or better radiated heat transfer reductions with radiant barrier, but that doesn't address the infiltration issue, and with the high summertime dew points of central FL, infiltration is a big chunk of the air conditioning picture.

I highly recommend reading this document prepared by the Florida Solar Energy Center on the subject.  The common approach was based on bad assumptions, not science, and was even enshrined in code for many years, but the folks who actually measure stuff have shown that attic venting has few benefits in a FL climate, and many drawbacks. (eg: Night time condensation inside of attics where the radiation-cooled roof deck is below the outdoor dew point is common, and the moisture content of the wood in those attics is usually higher than in conditioned/semi-conditioned sealed attics.)  Average shingle temps rise in the unvented insulated roof deck case,  but the cost of a ~10% reduction in shingle life is a drop in the bucket compared to the energy savings over the 22 rather than 25 year shingle.  Shingle color makes a larger difference in shingle life and peak temperature than vented/unvented attics.  A sealed attic has lower uplift forces under high winds, making it more hurricane proof, etc. It's all in there- read away!

Dana I read all 68 pages of that report. Man that was a long report, but good, I also recommend reading it.

BUT how much does this cost to retrofit a house that already has blown insulation and soffits? We have about 1500 sq ft under roof (2 story 3000 sq ft house).

Doug

Open cell foam at is about 35-40 cents per board-foot (=square foot at an inch of depth). A shot of 3" is about R10, and is enough to get a good air seal. If you have 9" of blown fiber between joists on the attic floor (which is nominally R30 center-cavity, but performs at about R20 with thermal bridging of the joist or R15 at peak roof deck temps if it's low density fiberglass), that's probably good enough. If it's more like 5" on the attic floor go with 6" on the roof deck, or a 3" at the roof deck and a 3-5" cellulose overblow on the attic floor goods.

The 6' of open cell foam at the roof deck is a better solution overall, since that brings the ducts more inside the thermal envelope, and the cost of the foam drops with the volume installed, so the cost delta may not be all that big, and you'd be dealing with one insulation contractor, not two, both needing to make the smaller job worth their time.

So, for 1500' at 3" you'd be looking at something a bit over $1500, for 6" it'll be three grand. The per-board-foot price drops as the volumes increase, and it's also a competitive business- don't be afraid to push back on price- most can still make money at 30 cents a board-foot on a 10,000 board-foot job, and until the building market recovers it's a buyer's market. There may be subsidies available through the utilities, state/other to take the sting out of it too.

If you become a calculating-estimating energy nerd like me you'd at least do a Manual-J type estimate on how much that reduces the load, and work that against the average cost of geo per ton. Near me geo runs about $9K/ton (sometimes more, rarely less). If I can peel a ton off the size of the geo for under 5 grand in foam, it's money in the bank, and even at 10 grand/ton it's rational even in the intermediate term. But if going with a ductless air source it would have to be about three grand/ton of load reduction to make mid-term economic sense. It takes a sharp pencil and a crystal ball on what future electricity and interest costs are to draw the line sharply.

The payback in comfort of a high-R low-gain house isn't measured in utility dollar-savings, but it's there too. When in doubt, improving the building a bit more than what might be a no-brainer cash investment isn't a terrible way to go, because the payback on comfort is fairly immediate and long lasting. But it takes a bit of analysis to figure where that building-envelope improvement make the best economic & comfort sense overall. eg: A house with a lot of high-solar gain west facing windows may not be all that comfortable at any roof-R, and even with an R-50 roof the peak loads might not come down very much. Knowing and treating your ACTUAL site-factor, orientation, and construction detail gains/losses is better than following some code or other prescribed set of R and U values, which is where modeling tools (or even Manual-J), can provide a lot of insights.

It's great when someone else does all the work around here...especially for a Florida member!

I echo pretty much everything Dana has written. We get a lot of pushback from the foam companies about needing to remove existing attic floor insulation in the course of foaming the roof deck. The reasoning might make sense in Minnesota but a Florida attic will never get cold enough to condense water. 60% RH at 70*F is considered high RH in summer, never mind winter. Even with foam, an attic will always gain some solar heat on a sunny day, even a "winter" sunny day in Florida.

All our new construction and most of our moderate to deep energy retrofits incorporate foam. It deals effectively with Florida's worst HVAC bad habit - putting the coolest air (AC supply air) in the hottest part of the house. Typical attic has hundreds of feet of R4-R6 ductwork. Duct air leaks are lost to the home, increasing infiltration, and attic heat penetrates ductwork.

A foamed attic moves the ductwork into conditioned space, and allows significant downsizing of HVAC. We took a 7 ton house to 5 (arguably should have gone 4), another 7 ton house to 4, two 5s to 3s, and a 3 down to 2.

I do take exception to one of Dana's points...Foam may be a commodity, but the skill and dedication of the installer is NOT. For foam to work it isn't just a matter of coating the roof deck to the average depth stated in the contract. It is crucial that the foam achieve a very tight air seal, and that is hard work, especially with lower pitch hip roofs. Failure to achieve good air sealing dramatically raises attic RH. This occurs because the sensible load is removed, lowering attic dry bulb temperature, but leaving several hundred CFM50 in missed air leaks allows infiltration to continue. Approximately 9 months per year here, outdoor air absolute humidity is higher than indoors

In some cases it is necessary to pull down the soffits and foam from outside in order to properly air seal. That costs time and money, but a failed foam job does more harm than good.

I've made pretty good beer money coming in behind one of the bad actors here with my own favorite foam crew. A blower door and theatrical smoke reveals problems fast.

Unless you're actively adding humidity to the ventilation air

Unfortunately, this is exactly what people, pets, plants, cooking, showers, etc do. The indoor humidity level (which is the issue here) depends on all of these things and how much leakage and HRV ventilation you have. 60% RH is very possible.

You may have misinterpreted "actively adding humidity" Any budding building scientist or HVAC pro able to fog a mirror (pun intended) knows that human activity adds moisture

"Actively adding humidity" probably means operating a humidifier...

In that case, the original statement is simply wrong.

60% RH at 70*F is considered high RH in summer, never mind winter.

Not that the above is particularly relevant to indoor RH, but tonight's forecast for Orlando: 71° F, Humidity:79%

I know first-hand that engineer tells it true when he writes: "Foam may be a commodity, but the skill and dedication of the installer is NOT."

That's not to say that competition in the foam biz isn't at it's toughest level in over a decade with the current state of the construction biz (at least in my neighborhood, but reports from FL indicate that it may be even more competitive there than here), and that installed pricing is more negotiable now than ever. Using contractors who specialize in air sealing would be a first-level filter of who to even request a quote from.

Posted By geome on 09 May 2012 03:46 PM

Posted By knotET on 09 May 2012 03:10 PM
now how do we help the threadholder at the top?

Cease replying with incomprehensible rants would be a great start.

(You read my mind)

Filters often make me uncomfortable- i.e. people who specialize.
"Specialized" companies are often band wagon variety which means new to an industry to take advantage of government subsidies.
A 50 year old insulation contracting firm who branched into airsealing may well be the better pick.
With only 2 exceptions I'm aware of "geo only" HVAC guys in my AO are new companies with little track record.

I'll buy that. The insulation industry in general doesn't have the greatest record, so filtering by longevity and reputation would be another important factor.

I'm only aware of 6 or so foam contractors in my million+ population AO. That includes the guy I clean up after, another who indelibly stained a customer's driveway with one of the foam components and then changed his phone number, one more who flat out insists that the existing ceiling insulation must go (nonsense, as Dana well explained), another whose prices were way high last I checked.

So that leaves me two active, with a third in waiting, a new entrant relocated from Charlotte who is making all the right noises. His work is scheduled for test sometime in June. Of the active two, one is distant, sometimes pricey, but does well with challenging projects. The other, cheaper, tends to be sloppy but fixes most of the problems I find, but his lower cost is eaten up by my increased quality control time.

I wish it were as wide open and cost competitive as Dana wote. A guy who does audits for me part time occasionally threatens to buy a foam rig and have at it...we'll see.

I'm surprised there are so few in your area- there are at least a dozen within a 45 minute drive of my house!

A friend of mine (not a contractor) who always seems to have 3 projects going has recently taken to going back to his more-favored contractors with, "Y'know, I've given you some business in the past 3 years and I know you do good work, but this other outfit has quoted me $X.XX for that job. I'd give it to you if you can match it." and it seems to be working. Five years ago it was hard to even get people to show up to quote a job unless it was over 5000 board-feet, and even then you might have problems scheduling it. I got bumped on the schedule twice on one project, eventually getting it done 2 weeks after the originally scheduled date. Now it seems they show up with the truck the next day on a verbal OK over the phone.

Markets WILL vary...

I strive to avoid beating down one contractor with another guy's lower price. If the other guy's lower price indeed represents better value, then I'll go with it, and the original guy will have a chance to offer better value on the next project.

Likewise when I propose a system and am confronted with price pressure I do NOT arbitrarily lower my price. Instead I detail my bid with the customer and ask him to select what components or options he is prepared to forego and then reprice without those services / options / features. If a competitor beats me, then the customer was better suited for them or I need to reconsider my costs / processes that contributed to the failed bid. Distance figures in - a competitor 10 minutes from a client 50 miles from my base has an advantage. Demeanor figures in - if I really hit it off with a potential client, I'll shave my margins in anticipation of easy execution and heartfelt referrals. Conversely, if a client seems likely to be hard to please, I'll cover that probablility, increasing the chance someone else gets to die taking that hill.

I want long term relationships with my subs so that they work with me in times of need...sometimes we feast, sometimes we fast. Likewise I want my clients to make an informed choice from the alternatives I place before them. Sometimes my subs bring me clients, sometimes I direct my clients to subs.

I haven't been at this particularly long, but long enough to have figured out that the backbone of the business is referrals, and the only way to garner those is to treat everyone fairly - give a little, get a little; make a living, not a killing.