Foam Insulation in the attic, but not in exterior walls?

16 years ago

Alright folks...first, the facts. Near Dallas, TX, I am building a 1 1/2 story 3400 square foot home with 2600 on the main floor.

Now, here is the great debate. Just read this article on foam insulation vs cellulose vs fiberglass.

I particularly liked this phrase:

"At advanced Insulation we are energy pragmatists and not energy purists. We realize no one product makes the home and that budgets often drive the decision making process. This is why we advocate the use of foam on the underside of roof sheathing and not necessarily on 2x4 or 2x6 exterior walls."

I'm definitely an energy pragmatist and not a purist. I just want something that will cut down the bills in the Hot Texas Heat and not have me go bankrupt doing it. We are definitely foaming the attic. But our budget originally had the whole house with foam insulation.

But would it be worth it, energy wise, to switch out the foam in exterior walls for fiberglass and use the money the saved for better energy efficient windows? Or how about upgrading to 2x6's instead of 2x4's?

Decisions, Decisions, Decisions....

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Comments (5)

sfhc21
Original Author
16 years ago
I guess I should link to the article too. You can check it out here:

http://www.advancedinsulationinc.com/sprayfoam/default.htm
sierraeast
16 years ago
You might also consider radiant barrier.

Here is a link that might be useful: radiant barrier
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Hi, I had never heard of Air Krete before -- interesting product. They say the R per inch is 3.9, which is a touch better than dense pack cellulose at 3.7 -- probably not enough to make much difference. A well done cellulose job will dense pack the cellulose in the wall cavity such that it fills very well and never settles. It can also be done poorly and leave voids and settle, so I'd try to learn as much as you can about how good the cellulose guy is. One question I would have about the Air Krete is whether they ever have any problem with the wall drying out. It sounds like they put quite a bit of moisture in the wall. Another think to think about is vapor barriers. If you have no vapor barrier in the wall now, I would think about using a vapor barrier paint on the walls, and also sealing up all the holes (e.g. outlets). The cellulose and (I would guess) the Air Krete are permeable to water vapor, and should handle some water vapor migration out the walls, but its good to be on the safe side. Maybe you could let us know what the cellulose bid is? Years ago a friend and I were thinking of starting an insulation business. We went out on a job with a couple guys who wanted to sell their business. This was the Ureaformaldehyde foam, which has since be discredited. Anyway, these guys intentionally sprayed in the wrong mix of the the resin and catylist because the catylist was cheaper -- so they did a bad job just to save themselves a few bucks. My point is, get a good contractor :) Gary
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It seems to me that you should answer two questions. Do you want conditioned space in your attic? How much money are you willing to spend? Our story: In our 1/2 story remodel we have conditioned living space. It had always been living space (at least for 70 or more years of our home's 97 years). During the depression there had been vermiculite in the space above the ceiling. In the 1950's parts of it got fiberglass bats in the space above the ceiling. (It had always been finished like an after thought, pressboard walls, and even cardboard over 1xs during the depression!) When we gutted the poorly finished space, we decided to have cathedral or loft type ceilings. Our roof does not have any vented soffits, nor a ridge vent either, so we had to have spray foam insulation in order to do the cathedral ceilings and have some kind of insulation. It did cost a small fortune (in our estimation), but it is highly effective. Between that and foam backed roof sheathing we have a cold deck roof. Unless the snow load is wet/heavy, we don't have to shovel it; the entire roof melts evenly with virtually zero icicles and no ice dams. The 1/2 story space needs very little heat, there is more sound proofing to the outside than before, and this will sound odd, but the foam literally glues your house together. Kind of hard to describe. Our space has been inspected by structural engineers, don't worry! But before the foam was sprayed you'd arrive at the top of the stairs and walking in certain spots created "creaking". After the foam was installed, no more creaking. We are very satisfied with spray foam. YMMV Others who will post here clearly prefer blown in cellulose. Since I have no experience with that type of insulation, I hope they will post about it. Hope you get the information you need and end up with a space that meets your needs.
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bill1, are you concerned about water retention/moisture absorption with the cellulose? As you might have read on a recent thread of mine, I'm considering retrofitting the empty wall cavities with insulation and topping off insulation in the attic. Our auditor had recommended dense-pack cellulose, and many others agree, including the authors of this very informative paper about passive climate control on historical buildings (www.wag-aic.org/1999/WAG_99_baker.pdf). But after talking things over with my father, who restores historical homes in northern maine, I'm avoiding the cellulose, as he says he spends his days tearing down walls that have rotted because of the water retained by cellulose. I have two estimates scheduled this week with local insulation companies, and the one that has the best reputation (at least as far as I can tell) has this on its website: "If Fiberglass insulation becomes wet, usually it will not lose any R-Value if it able to completely dry out. Cellulose insulation usually needs to be replaced once it becomes wet." Anyway, just curious what your thought process is these days. Best of luck!
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Michigan occupies building climate zones 5 - 7. Zone 5 is the warmest zone that can still comply with the R20 cavity-only requirement. It looks like Ontario has adopted similar IRC mandatory exterior insulative sheathing. We use the performance compliance path on every home, one of the many advantages to including a third party energy rater. Most raters are not keen on getting too far away from code prescriptions. For some of the prescriptions, like blower door minimums, it's cost-effective to exceed code. Most high performance builders are exceeding code with exterior insulation as code encourages risky wall strategies, hence the recommended minimum table I first posted about. Even leaky, all natural homes can have poor indoor air quality. Outdoor air ventilation is now required by current IRC codes and a smart idea for any inhabited enclosed space. It's a life-safety requirement and long overdue measure for modern homes.
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dallasbill
16 years ago
sierraeast... He is doing a sealed attic, with foam in the roof underside, thus negating use of an RB -- no air space.

I have read many places about the use of spray foam in walls not providing the bang for the buck. If I were building stick, I'd seriously look at 2x6 with 1 inch foam on the outside to negate any thermal bridging. I'd also be anal about caulking everything. But, having built ICF here in Dallas now, I'd never do stick again... ;-)

We built ICF and due to design we have two stick walls open to unconditioned space. A half-wall in the gable attic above the master -- it is the top half of the 28 ft vaulted middle living area on the other side. We spray foamed this so that it joins the foam on the attic floor. The open-cell foam completely covers over the studs, as well as the A/C vents that exit those walls.

The other is a 15 foot long, 9 ft high wall, above the garage, which forms the outside wall of the media room. It's a double 2x4 wall, that is foamed on the outer 2x4 studs only -- the rest is dead air back to the sheetrock on the inner 2x4 studs. It was done so the windows would be deep-set and matched to the others on the 11in ICF wall. When I shoot that stick wall (winter or summer) with an infrared gun, it is the same temp as the foamed (no attic) ceiling above and the ICF wall.

I think that you are asking the right questions. Thermal bridging is your biggest issue after air infiltration. Just make sure you get a proper manual J and manual D for your HVAC to match all that prep work up front!
sierraeast
16 years ago
Sorry, i didn't catch the part, " we are definetely foaming the attic". I thought their decision options were still open so i gave out radiant barrier as another alternative.
bungeeii
16 years ago
There have been recent threads here that you could read from.

I suggest you view these videos and then share them with your builder;
http://www.codecollegenetwork.com/la/attic.html

My home is a combination of cellulose, fiberglass and foam. Each has their strengths.

Foam Insulation in the attic, but not in exterior walls?

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sfhc21
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sierraeast

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Foam Insulation in the attic, but not in exterior walls?

Comments (5)

sfhc21Original Author

sierraeast

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dallasbill

sierraeast

bungeeii

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sfhc21
Original Author