new build spray foam insulation
Last Post 09 May 2023 01:01 PM by pusherwr. 10 Replies.
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tjguy57User is Offline
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16 Jan 2019 12:45 PM
Hello, Looking for feedback on spray foam insulation in a new build. This is a one story house, nudura crawlspace, 2x6, zone 5, 3000 elevation, zip wall and roof sheathing. Contractor typically sprays open cell all around the envelope. Appreciate your thoughts.
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16 Jan 2019 06:37 PM
If the siding is back ventilated it's fine to use open cell foam in walls in zone 5 without interior vapor retarders. If the siding is going to be tight to the ZIP use "vapor barrier latex" primer on the interior finish gypum.

If the roof ZIP is a vented cathedralized ceiling with the requisite 1" soffit-to-ridge venting open cell is fine without interior vapor barriers. Using half-inch fiberboard (asphalted or bare MDF) under the vent channel as a substrate for applyin the open cell foam is preferred, since fiberboard is both moisture tolerant and water vapor permeable. Applying the open cell directly to the underside of the roof deck is somewhat more risky, but can be protected from interior moisture drives by vapor barrier latex on the ceiling gypsum.

In open attics open cell foam is an expensive and less-green approach than blown cellulose (preferred) or blown fiberglass. There are much cheaper ways to air seal an attic floor than R49 of open cell foam (which would have to be installed in three lifts with a cooling/curing period between, which is both a quality and fire-hazard-while-curing issue.)

Open cell on foundation walls is nearly always a bad idea, prone to becoming waterlogged/saturated, or allowing mold growth on the finish wall.
tjguy57User is Offline
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16 Jan 2019 07:21 PM
Dana,

Thanks for your response. So we are using vinyl siding and the attic will be unvented. The spray foam will be sprayed on the underside of the roof and gables. Shingles are the roof material.
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16 Jan 2019 09:14 PM
With vinyl siding in zone 5 you won't need anything more vapor retardent than standard interior latex on wallboard on the interior side of the walls.

With the unvented roof it would be prudent to install either 2 mil nylon (Certainteed MemBrain) as a "smart" vapor retarder between the ceiling gypsum and foam/rafters, or paint the ceiling gypsum with "vapor barrier latex", reducing it's vapor permeance to about 0.5 perms. That prevents the foam and roof deck from loading up excessive moisture over a winter, but still allows adequate seasonal drying.

Do spell out that you don't want them to install more than the manufacturer's recommended thickness per pass. (usually 125-150 mm, or 5.5-6"). It's a lot easier to head-off a bad open cell job (or a fire) by insisting they do it by the book than it is to deal with it after the fact. It's amazing how many installers think it's OK to install R30 or even R38 in single pass. They usually manage to avoid the fire, but not the shrinkage voids in the foam.

Other installers will insist that "R20 is all you need", when in fact it would be a code violation. What those words really mean is "That's all that makes short-term financial sense for the homeowner, and it's the most we can install safely in a single pass, maintaining our gross margins by not having to wait for the stuff to cool off between lifts."
tjguy57User is Offline
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16 Jan 2019 09:33 PM
Dana,

Would it make sense to install closed cell in the rafters and open cell in the walls. What you describe at the end of your last post is close to what I am hearing. Thanks again
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16 Jan 2019 10:19 PM
In zone 5 it takes R20 of closed cell for dew point control on R29 of fiber, going by the IRC's prescriptives in chapters 7 & 11. If you go higher total R, the ratio foam-R/total has to remain ~40% or more.

Assuming 2x12 rafters, 3" of HFO blown closed cell foam (a much greener blowing agent) runs about R21, and leaves 8.25" of space below. Half-pound open cell in that 8.25" would have to be installed in two lifts, but would deliver about R30.5, for a total R of 51.5, and a ratio of R21/R50.5= 41.5 which is right on the money.

HFO blown foam can be installed in fairly thick lifts (over 5" for some of them). With HFC blown closed cell foam you'd need to install it in lifts of 2". It would take 4" of HFC blown open cell foam (~R24) to provide sufficient dew point control at code-min, and would leave 7.25" of space for other insulation. Rather than 2-lifts or a (cheater-thick single lift) of open cell foam, R30 rock wool batts would be much cheaper /faster/ greener and are designed for 2x8 framing, which is 7.25".

Three inches of HFO blown foam costs about as much as four of HFC blown foam, but doesn't use HFC245fa as a blowing agent, which is an extremely powerful greenhouse gas that is soon to be banned in this application under the Kigali amendment to the Montreal Protocol (the ozone-layer protection agreement.) But open cell foam is usually quite a bit more expensive than rock wool.

Of course 4" of HFO blown foam (R27-R28) in a 2x10 rafter would leave 5.25", enough for an R23 rock wool batt that would perform at R22 at the slightly reduced thickness for a total of R49-R50, and a huge dew point margin.

The other benefit of rock wool it's fire resistance. Foam is flammable, and needs a timed thermal barrier against ignition. As little as 1.5" of rock wool is a sufficient thermal barrier to not need the half-inch gypsum to serve that function.

While you can hit code min with 7" of HFO blown foam or 8" of HFC blown foam, the much shorter thermal bridging through the rafters makes it under perform compared to thicker, lower R/inch materials at any given R-value. In walls the framing fraction is high and that hit is really significant, in roofs somewhat less so, but it's still real.
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27 Mar 2020 10:03 AM
I recommend it. Good for almost any kind of property. I have installed an insulation with the help of this spray foam installer. It was really convenient not only in managing temperature but also in reducing noise and covering up wall cracks. Most importantly, it is eco-friendly and easy to maintain, making it a nice choice for those who want to do something for the environment.
My name is Xaview Caroll. I work as an architect in a prestigious company. If there’s one thing I treasure the most, it’s my family.
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07 Mar 2023 04:49 AM
Hello. I am acting as GC for my new construction home build and I was wondering if anyone here has had experience with spray foam insulation for an entire house? I really think it will make a difference over time but I am having trouble coping with the costs vs benefits. Has anyone sprayed their own home, or had it done by a professional? In your opinion, was it worth it? Is it noticeable? Its sooooo expensive!! Oh - and an important bit of info: I plan on spraying the insulation by myself. I can't justify hiring a contractor, so if it is to be, it is up to me.
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09 Mar 2023 09:05 PM
Foam insulation is not a DIY product. I started my career in insulation. I've seen newbies try it with disastrous results.
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17 Apr 2023 08:32 PM
I'm not a fan personally, go with Rockwool if you can. My dislikes are it can/will leak air if you're using it as an air barrier. I view it as a giant thermal mass, which can be solved of course, but I'd rather use fluffy stuff or blow-in for instance.
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09 May 2023 01:01 PM
Would it make sense to install closed cell in the rafters and open cell in the walls. What you describe at the end of your last post is close to what I am hearing. Thanks again
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