Closed- and Open-Cell Spray Foam in Walls and Ceiling
Build_ny
| Posted in GBA Pro Help on
I am building a house in climate zone 6a. I have gotten a few spray foam quotes and have a question about one of them.
I need to get r25 in the walls (2×6). They suggested 1.5″ of closed cell, followed by 4″ of open.
Also, the ceiling (2×10) needs r49, for which they suggested 3″ closed and 7″ open.
I guess I’m worried about creating a dew point in the wall cavity, could either of these lead to that?
Thanks
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There are two issues here. One is meeting the energy code. Because spray foam is the most environmentally damaging insulation you can use, it's a good idea to use the least amount that will meet code. (A better idea is to choose materials that aren't environmentally damaging.) When first installed, closed cell foam is about R-7/in and open cell is about R-3.6/in, so your wall will meet the energy code. The roof will be about R-46, unless they are inflating the value of one or both materials.
More important is long-term resistance to moisture accumulation, for which you should use the aged values, about R-5.5/in for the closed cell and R-3.6/in for the open cell. Using those values, your walls have about 36% of the total R-value in the impermeable (closed-cell) layer, which is prescriptively adequate for climate zone 6. In the roof you will have about 45% in the impermeable layer, which is not enough for climate zone 6; you need at least 50-51% in the closed cell layer.
It can take many years or decades for moisture-related problems to become obvious, so your installer may think they are recommending a safe assembly when really they aren't. Or if you're lucky, there is a good chance it will be ok as spec'd, but I never drop below code-minimum ratios unless someone with a lot of experience and letters after their name have done a hygro-thermal model that shows the assembly will be safe.
Thanks for the detailed response. It's much appreciated.
The 2x10 rafters are furred up 3/4" to make them a true 10". I was told that his open cell is r4 per inch, once over 5".
So, just to make sure I am understanding correctly, I should have a minimum of 50% of my r value in the impermeable layer (for the ceiling)? If I use the aged r value I would need 4.5" minimum?
One side of the ceiling is framed out of 2x12, also furred up 3/4", is it a bad idea to just go all open on that side (this was an option on a different quote, idk how it's not only r48)?
Thanks again, I really appreciate the help.
I see large amounts of spray foam in new construction plans as a big red flag for poor planing and an easy fix from someone with little regard for your budget. How far along in construction are you? Is it too late to move some of the insulation to the exterior of the roof or walls?
Have you read the GBA article 5 cathedral ceiling that work?
Walta
Many installers use overly optimistic R values for spray foam, as do a lot of the manufacturers. The R value when the material is brand new is better (higher) than when the material has aged a few years, and what you want is the aged R value, the so-called "LTTR" (Long Term Thermal Resistance) R value. For closed cell spray foam, I assume about R6/inch. For open cell spray foam, I assume about R3.5/inch. I see Michael has slightly different assumed R values, but as you can tell, we both assume a fair bit less than what you were quoted.
The other issue you have, which is much less with the "top it off with open cell" installation method they are recommending, is that closed cell spray foam almost never goes in exactly at the target installation depth -- you almost always have thiner and thicker spots, and it does NOT average out the way many think it does. That's another issue that comes up.
When you're done, you would end up with around R21 or so in that 2x6 wall assembly using the installation method they are recommending. You can get R23 with mineral wool batts, which is what I'd use, along with an interior side smart vapor retarder for moisture control. I am not a fan of spray foam in walls.
For the ceiling, why are you considering spray foam at all? Loose fill (blown) insulation is much, much cheaper, and performs just as well, as long as you've air sealed properly ahead of time, which isn't all that difficult to do -- especially with a new build. That's only really possible if this is a vented attic though. Is your ceiling assembly a cathedral ceiling? With a cathedral ceiling, spray foam is sometimes the only option, since it is the only fully-adhered insulating material that can be applied directly to the underside of the roof sheathing without problems. I completely agree with Michael about the required ratios, but I like to be more conservative and go "one step up". If code requires 40%, go with 50% to be safe. Basically err on the side of caution, and apply more closed cell in realtion to open cell in terms of total R value. Apply however much closed cell you need to hit that R value, plus some extra for some margin of error (those thick and thin spots I mentioned earlier), then fill the remainder of the space with open cell which is typically trimmed flush with the lower edges of the joists/rafters/trusses. If you need to eek out a bit more R value for the assembly, you can put a layer of rigid foam under the joists/rafters/trusses behind the ceiling drywall. A 1/2" layer of polyiso will get you a bit shy of R3 this way, for example.
My recommendation here would be to put mineral wool batts in the walls, or use high density fiberglass batts if you want to save some money. Use blown cellulose in the attic if it's a vented attic, use spray foam if it's a cathedral ceiling. Use the savings from not using as much (or any) spray foam to put up some exterior rigid foam to beef up your walls R value, which will be a MUCH better performing wall compared to spray foam between the studs. Keep in mind that you have to hit the minimum R values for that exterior rigid foam for your climate zone too, you can't just put up enough to hit some target total wall R value the way you could with the underside of the ceiling in the way I described earlier.
Bill
Bill,
We are in total agreement that closed cell SPF should be avoided but I strongly disagree with your suggestion of mineral wool. For reference, HFO-based closed-cell SPF is roughly the same global warming potential as mineral wool. Open-cell SPF usually uses water as the carrier and is actually less than half the GWP of mineral wool. It seems silly to suggest a product just as equally bad or much worse (in the comparison to OC). I prefer good old-fashioned fiberglass myself, as you also suggested. Cellulose is a good option as well.
Reference: https://www.efficiencyvermont.com/Media/Default/docs/white-papers/The_High_GHG_Price_Tag_on_Residential_Building_Materials.pdf (page 9, table 1)
Thanks,
Matthew
I don't use that kind of information to determine the suitability of a product for a particular application. I am only looking at the product in terms of it's viability in the application at hand in most cases, unless there are no other differences between materials. Even with your own data (going by some of their graphic representations), mineral wool is close to ccSPF in only one category, and much better in others.
It is always important to consider the entire system here, and never be overly focused on any one aspect of that system. If you want to select materials based on how they are manufactured, that is different from selecting them based on how they will perform over time in the structure (I select primarily based on the second attribute). There are nearly always tradeoffs between different materials, so you have to pick the best balance of properties/features/processes, and not focus on any one thing to the exclusion of others.
If you want to go with fiberglass, I would suggest considering high density fiberglass, which is becoming more available in recent years, since it is closer to mineral wool in terms of the ease of properly installing the product.
Bill
Spray foam is something you design out of new build. Besides the cost, it is also something where the install error (rare but not uncommon) could mean having to tear down the whole place to remediate.
If you have designed yourself into a corner, spray foam might be the only way out.
One suggestion for your walls is to strap them out with 2x2s and install 2x8 batts. This will get you a higher assembly R value without any spray foam.
Your ceiling is tougher especially if it is all cathedral and venting was not part of the design. In that case, spray foam tends to be the only solution.
You can still avoid some foam by doing a flash and batt, so for an r49 ceiling that would be ~4" of closed cell SPF and R24 (5.5" mineral wool) batt. This can fit into either 10" or 12" rafter space just make sure the batts are pushed tight against the foam in case of the 12" rafters.
P.S. All open cell foam in rafters is bad idea in zone 6, very risky assembly.
Walta, Bill, and Akos,
Thanks for your replies. I have built myself into a corner. The siding and roof are on and the ceilings are all cathedral, so I am stuck with the foam there.
Can anyone link me to the code minimum ratios that Michael was referring to? I found an article on here titled "the ratio rule for hybrid roof insulation " but I'm a little confused. Is it the ratio column in that table?
Any info on why all open is a bad option would also be much appreciated, I assume you would recommend against high density open cell as well ( a third quote recommended I fur down the 2x10 side and do whole house in high density open cell)?
Build_ny,
The article Walta mentioned is worth a look. It may offer you more options. The ratios for each assembly are listed there.
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Perfect, ty
For open cell only in cold climate, take a look at table 3 here:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf
Martin covers the topic here: https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
If you're meeting or exceeding the recommended ratios, there's no reason open cell foam won't work just as well as batts or cellulose; it's all-open cell roofs that are very risky. High density open cell is vapor-permeable so although building codes allow it because it's airtight, it's still risky. When it's warm inside and cold outside, interior moisture REALLY wants to get outside.
There are other reasons to avoid foam, such as it doesn't always cure properly and it glues whatever it touches together. When there are renewable options available it's unfortunate to use a finite, polluting resource like petroleum, no matter how endless the supply may seem. It can do some things that other materials just can't do, and for those situations I use it without guilt. But in most cases there is an equally good, equally affordable alternative.
I have to ask have you install dozens of recessed lights in your cathedral ceiling?
Hint this is another very poor choice.
Walta
Only 4, luckily they are all on the 2x12 side so hopefully they don't weaken my system too much