Vented attic or unvented attic with spray foam?
green654
| Posted in General Questions on
Two concerns with seemingly opposing solutions: mold growth and chemical sensitivities. Seems like spray foam in attic to create a conditioned air tight space is the way to go to avoid mold in attic assuming the spray foam is done correctly. However, I’m concerned about the chemical off gassing and all the unknowns with spray foam insulation. That makes me lean towards a vented attic but then the risk of mold developing and a greater. Hard to find a balanced solution. Any suggestions?
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Replies
Green654,
If you have chemical sensitivities, it would be safer to avoid using spray foam. But spray foam is sometimes the best option for insulating complex assemblies (like 1.5 story capes) or cathedral ceilings with limited rafter depth.
Whatever you do, do a good job air sealing the interface between the attic and conditioned space. That's the best way to mitigate your exposure to chemicals and allergens.
Mold can grow on spray foam too. Mold feeds on the dust in that case, not the spray foam itself.
Do a good job of air sealing your attic, use continous soffit vents in addition to a good ridge vent (not the "shingle over a strip of foam" style), and you'll be a good shape with a vented attic, for a lot less money. The key is proper venting of the attic air space, and good air sealing of the attic floor. Use loose fill (blown) cellulose to insulate the attic floor.
Spray foam alone will not prevent mold growth.
Bill
Whether a vented attic might see mold is highly climate dependent. I've never seen the slightest sign of any in mine (not very dry SE Michigan).
The way I see it the spray foam industry has given their product for free or almost free to all the DIY TV shows. It is a huge marketing victory they are selling selling the least green product for the most money while glossing over that it is the most likely product to fail. They seem to have falsely convinced most people that any home without their product is second rate.
I will say there are times and places where spray foam is the best/only real option but in a new build covering large areas with spray foam is a big red flag in my eyes. I see spray foam as a lazy answer use to avoid doing the work required make a proper design. Almost any new construction problem has a much lower cost solution without spray foam.
I see a vented attic as the safe low cost solution for an R60 attic. The only reason to select an unvented attic is if someone has made some poor choices (half story construction, HVAC in the attic and sloped ceiling) that have driven them to take the risky and expensive option of a conditioned attic.
The way I see it there are 3 types of attics vented, conditioned and moldy. If you can’t being yourself to call your attic conditioned it seem likely you will be unwilling the heat and cool the attic if you don’t there will be times when the air in the attic be below its dew point and it will rain in the attic if it happens enough things will get ugly.
Walta
>"while glossing over that it is the most likely product to fail. "
That's probably a bit harsh. I wouldn't say spray foam is any more likely to fail than any other product, but it IS probably the most expensive way to insulate. I completely agree that spray foam isn't really needed in most applications, and often doesn't even really gain you anything. Do a good job of air sealing and use more conventional types of insulation, put the monetary savings towards things that will accomplish more (like thicker exterior rigid foam).
I only really recommend spray foam for certain niche applications where it's really the best or only option, mostly that means unvented cathedral ceilings and cut stone foundation walls. Sometimes rim joists too. Spray foam is a waste in walls in nearly all applications, the same goes for most attics. The only time I ever specced spray foam for a wall was for a small wall in one room of my own house, and the only reason I did it there was because I had to use spray foam in the roof (unvented cathedral ceiling), and it was only $100 more for the crew to do the one wall at the same time while they were there. The insulating performance of the spray foam in that wall is no different than any of the walls I insulated with R15 mineral wool batts.
Bill
Bill I will give you harsh. Could we agree if I said “most likely insulation product to fail on the market today”?
OK I forgot about asbestos. Spray foam is definitely preferable to asbestos. Now you went and made me say something nice about spray foam. LOL
Walta
I've seen many, many spray foam installations that are fine. I think there are a few bad installs that have gotten a LOT of attention, which gives the entire industry a bad name. I just don't think it's accurate to say spray foam is most likely to fail, I haven't seen anything to really show that. I think if you want to point at an insulation product most likely to be suboptimally installed, that's probably going to be a low-density fiberglass batt.
There are many reasons to not use spray foam in "regular" applications (walls especially, but also most attics), I just don't think the likelihood of failure of the product is one of those reasons.
Bill
There is no reason to think a properly ventilated attic will develop mold problems, or that any mold or materials that off-gassed in the attic would affect the house below. The primary reasons to go with an un-vented assembly are so that the space can be incorporated into the living area, or used for mechanical equipment.
This is also a decision what needs to be made at the design stage, so that an appropriate roof structure can be chosen for whichever option you go with.
I very slightly disagree. In Summer, with reverse stack effect, a little attic odor is likely to enter the interior. Maybe enough to bother someone with chemical sensitivities.
On the other hand, anyone with such sensitivities should run slightly positive building pressure in Summer - which would prevent this.
For us, we went with spray foam +rockwool against the inside of our roof decking because we have a staircase that leads to our floored attic, so we did not see a good way to air seal between second floor and attic.
There is offgassing even now, several years later, but we do not find that the smell comes down into the second floor (it did at first, slightly, but I think only for a week, maybe?) The offgassing happens on some sunny, warmish days... but it's only noticeable if you're up in the attic.
FWIW - Foaming an attic requires creating an auxiliary opening (~4 sqft) to properly ventilate during the SPF process. See the industry guidance to get a sense of the volume of air required to manage off0gassing during the immediate curing phase. Then the temporary hole is sealed with rigid foam board. This is almost never done, unfortunately. Also, visualize the ease of applying a smooth layer on continuous roof deck, versus trying to get a uniform layer at the various acute angles of a roof. Then see articles that describe extremes in humidity and you get a sense of challenges. It's hard to do and few get it right.