Is slight condensation in a conditioned unvented attic with closed-cell foam ever OK?
I recently moved into a new home that has a conditioned attic in Cleveland, Oh. Over the last few weeks it’s been brutally cold for this area. Temps have consistently stayed in the single digits for weeks now. All of this is unusually cold for this area. With that said, these unusually cold days have worried me, and one main insulation concern continued to bug me. Are the wall and attic cavities collecting condensation?
To give you a little background, the attic is unvented and not lived in. Scissor trusses are throughout. The rafter, are made up of 2×12’s and are filled with 3″ of closed cell foam with the remainder of the cavity being filled with cellulose. Cheese cloth and Tyvek holds the cellulose in place.
So, in order to remedy my condensation concerns, I cut a small slit in the Tyvek, stuck my hand up through the cellulose, and check for moisture on the underside of the closed cell foam. Two things immediately became apparent. One, about 40% of the rafter bays had some condensation. It wasn’t a ton, but it was enough feel with your hands. It seems like it should dry out when the weather slightly warms. Two, in some of the bays, there was an air gap between the cellulose and the bottom of the closed cell foam. By doing these checks an array of questions popped into my head, so for simplicity’s sake, those questions are below. Any technical specs that may be relevant is below as well. Thank you for any help in advance!
- I know there is an overall attic R-Value required by code. Is there also a code requirement for the minimum thickness of closed cell foam under the attic sheathing? If so what is that for climate zone 5?
- Is there are chart that lists the minimum closed cell thickness requirements per climate zone?
- When it’s below the normal winter temperature for a particular climate zone, is condensation within a wall or attic cavity normal and expected as long as it can dry out inwards?
- Is a small amount of condensation in attic rafter bays ever ok? Especially on the coldest day for a particular climate zone?
- Does the cellulose need to be touching the bottom of the closed cell foam? Should there ever be an air gap between the foam and the cellulose? It seems that, over time and with the cellulose settling, an air gap will inevitably form.
- If the slight attic condensation is NOT ok, what is the most cost effective fix? Unfortunately, I don’t have 10’s of thousands of dollars to fix this problem if it is indeed one at this point.
- Home is in Cleveland, OH which is climate zone 5
- The attic is not a livable area. There’s scissor trusses throughout the attic.
- The attic is unvented
- The attic rafters are 2×12″‘s
- There is around 3″ of closed cell insulation directly under the roof sheathing
- The rest of the rafter cavity is filled with cellulose.
Temperature / RH
Current Outside Temp : 5-15 deg Fahrenheit
Current Inside Attic Temp / RH : 72 deg Fahrenheit with 25% RH.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Perhaps Dana will chime in with a thorough response. In the meantime, consider reading this article on the flash and batt method: https://www.greenbuildingadvisor.com/articles/dept/musings/flash-and-batt-insulation
Note that you need R-20 of closed cell foam installed against the roof sheathing. It possible the foam is thin in areas, but your new built house may have a lot of excess moisture left over from the construction process. You might want to buy an inexpensive humistat to gather some data.
The code requires that builders using this type of assembly -- one that combines foam insulation with fluffy insulation -- in Climate Zone 5 to install a minimum of R-20 foam, and aim for the entire assembly to have at least 41% of the assembly R-value in the form of foam insulation.
The principles behind this requirement are explained in these two articles:
Combining Exterior Rigid Foam With Fluffy Insulation
(While the second of these two articles focuses on rigid foam rather than closed-cell spray foam, the principle is the same for either type of foam.)
Your roof assembly is close to these code requirements, although your closed-cell spray foam installer assumes an R-value of R-6.7 per inch for the spray foam, which is optimistic. (R-6 is more realistic.) You probably have about 8.25 inches of cellulose with an R-value of R-30.5, putting your foam-to-whole-assembly ratio at about 37% instead of 41%. But you're close, as long as the spray foam installer really did install 3 inches of spray foam, rather than a layer that is closer to 2 inches than 3 inches.
If this were my house, I would probably assume that the assembly will dry out when warm weather returns. But I would keep my eye on things.
Thank you for your detailed answer.
After further investigation of the roof insulation, the closed cell foam under the roof sheathing is only 2" thick. Obviously, this is against code and needs to be addressed. So as any homeowner would do, I called the installer and told him of the issue. He thinks the air gap between the closed cell foam and blown in cellulose is the reason for the condensation. There's an air gap because the cellulose is currently being held in by Tyvek and Cheese Cloth. And that Tyvek sags a bit underneath the attic rafters. It's not much, but there is a some sagging.
The solution to this sag, suggest by the installer, is simple. He suggests adding 1 x 4 furring strips to the underside of the rafters and add more cellulose to each rafter cavity. Doing so would push the cellulose back up make contact with the closed cell foam. This may indeed work. However, I don't like this solution as it still violates code. It doesn't address the lack of closed cell foam. It should be at least 3" thick for Climate Zone 5.
What are your thoughts on the above solution? If it were up to me I'd like to completely remove the cellulose and fill each rafter bay entirely of closed cell foam ( 8" - 10" total ). I'm ok with the high price of doing so. I don't want to worry about this issue for the rest of my life. The only issue is with potential roof leaks. Is having that much closed cell insulation and issue when it comes to leaks? If it is, what would you suggest to solve my dilemma?
Your roof assembly has two code violations. The first violation is that the spray foam layer is too thin. The second violation is that the fluffy insulation is not in contact with the cured spray foam as required by code.
The fact that you can feel condensation on the interior side of the spray foam layer indicates that these problems are real.
If I were you, I would insist that the contractor correct these code violations at the contractor's expense. The cellulose will need to be removed; additional spray foam will need to be installed; and a better method for retaining the cellulose will need to be developed so that the cellulose can be installed using the dense-pack method to prevent slumping.
I have a few more questions. Any answers are greatly appreciated. Thanks you so much.
The insulation contractor will be tearing out the cellulose and addressing the lack of closed cell foam. Right now the closed cell foam is only 2" thick under the attic sheathing. As you know, that's a code violation in Climate Zone 5. It should be at the very least 3" thick.
Instead of adding an inch or two of closed cell foam and repacking the 2" x 12" rafters with new cellulose, I'm thinking of having the contractor just fill ever rafter bay entirely with 10" closed cell foam. A couple of questions come up if we go this route.
1. Regardless of cost, are there any unforeseen consequences of filling rafter bays with 10" thick closed cell foam?
2. There is already 2" of closed cell foam under the attic sheathing. If the contractor adds another 8" of closed cell foam is there anything to worry about? I worry that by spraying on top of the old foam layer may create some air-pockets between layers. Thus, causing future condensation problems. Is this a valid concern?
Q. "Regardless of cost, are there any unforeseen consequences of filling rafter bays with 10 inch thick closed cell foam?"
A. No, as long as your spray foam contractor knows that the closed-cell spray foam needs to be installed in multiple lifts, with time between each application to allow the heat from the foam's exothermal reaction to dissipate.
Q. "There is already 2 inches of closed cell foam under the attic sheathing. If the contractor adds another 8 inches of closed cell foam is there anything to worry about? I worry that by spraying on top of the old foam layer may create some air-pockets between layers. Thus, causing future condensation problems. Is this a valid concern?"
A. No, as long as your spray foam contractor know what he or she is doing.
At 3" most closed cell foam is between ~0.5 perm, which is sufficient for protecting the roof deck. A mere 2" is enough, even for zone 6A at a total of R49 if it's not a light colored "cool roof". See the Table 3 summary in this document, the Minneapolis row, 2" ccSPF + spray fiberglass column:
And that's with fiberglass, which doesn't buffer and distribute the moisture accumulation the way cellulose does.
In a 11.25" cavity with 3" of ccSPF and 8.25" of cellulose you have at least R18 of foam (it could be as high as R21), and a most R32 of cellulose (R30 is more likely). At the worst case it's being a ratio of only 36%, but with the cellulose buffering the moisture it's not worth sweating it, and CERTAINLY not worth replacing the cellulose with more closed cell foam. You might have R21 out of a total R51 if it's an R7/inch foam, and R3.7/inch cellulose, which would meet the prescriptive ratio. Cleveland is also not the coldest edge of zone 5A either, so the ratio doesn't need to meet the full 41% to be reasonably safe even with non-buffering fiber.
The primary problem is the high vapor permeance of the Tyvek. Code demands a least a class-III vapor retarder on the interior at the prescriptive ratio, and Tyvek is north of 25 perms. Putting half-inch wallboard over the Tyvek and painting it with half-perm "vapor barrier latex", would meet code, since the latex would be a class-II vapor retarder. Alternatively, 2mil nylon (Certainteed MemBrain) between the Tyvek and unpainted wallboard would get you there with variable vapor permeance, a class-II vapor retarder when dry, but class-III vapor permeance (or higher) when it needs to dry.