Closed cell spray foam vs fiberglass batts for exterior walls

jhouse2023

last year

We are building with a builder in southwest PA (climate zone 5) building a ~2700 sq ft, 2 story home. The default exterior wall insulation is R15 fiberglass batts between 2x4 studs with 1" R5 foam board on the exterior. They said they can replace the fiberglass batts with 2" of closed cell spray foam (R14) for ~$11k.

I know the spray foam on paper has a lower R value, but I am guess the air-tightness would make up for it. Is it worth upgrading to the spray foam, or should I stick with the default option? What problems have people had with spray foam? I heard it is an absolute pain if you want to add/change anything electrical later, so we should probably figure that all out now rather than later, but is there anything else that would make you hesitant?

R14 Closed Cell Spray Foam

R15 Fiberglass Batts

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Comments (6)

Keen B
last year
We removed batts from attic of our current "old" home and replaced with foam. AC bill dropped considerably and humidity levels changed enough we could feel it. This helped us decide on using it throughout our new home construction. Even with higher ceilings and a larger interior (and a hotter region) the A/C bill is lower there than in our current home with batts in exterior walls.
3onthetree
last year
You have two basic methods to meet R20 for walls: insulate between studs with a single R value; or insulate between studs with a lower R-value but make up for it with an additional continuous layer of added R-value.

Your builder uses a standard of R15+R5 (R13+R5 is Climate 5 code) for the latter method compliance, and with that allows use of cheaper 2x4 framing. Most builders choose the former method and frame with 2x6, which meets an R20 with fiberglass batts.

If you were to go to the expense of spray foam, 3" thick (your builder is stating closed cell at 7.0/inch R-value) would meet R20 code just between the stud spaces, so you would not need the extra R5 exterior insulation or any of it's extra detailing and work.

But it doesn't sound like the builder offers a different option than the continuous 1"/R5 exterior insulation, so IMO paying $11K more for 2" spray foam substitute instead of fiberglass batts is just giving him a bigger boat.
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Open cell spray foam can effectively attenuate sound as part of a structure with additional features. See this piece from Spray Foam Magazine with research on spray foam in walls. Illustration below is from the article. Filling an entire conventionally built void with foam will increase the sound transmission. Not to mention that most sound transmission in a building is through flanking pathways, such as HVAC and non-sound attentuated components. Comprehensive research on sound attenuation assemblies published by the National Research Council of Canada, available here (PDF), notes that: "Sound-absorbing materials are only effective when used in assemblies with the faces correctly isolated as described above. Using sound-absorbing materials in a rigidly connected assembly is a waste of money. When the two layers forming the wall cavity do not have rigid connections between them, adding sound-absorbing material to the cavity increases the STC by about 10 points." (emphasis added) This post was edited by worthy on Sun, Sep 22, 13 at 9:54
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"madpebs87 "plus then you might actually have to seal some of the holes in the ceilings." Wouldn't you do this anyway? You foam the roof deck/rafters, do nothing to the ceiling for insulation, why would you not seal all the plumbing/lighting etc holes that go up into attic? Great thread (bookmarked) giving me lots of info when i build in 3 years in coastal ga." been out of town so just getting to answer this. when the attic is vented, you absolutely want to seal all openings between living & attic space. when the attic is unvented & foam sealed, then you want air exchange between living & attic space. you can get this exchange from not sealing the openings that trades make & never seal, or seal it up tight and dedicate an exchange air pathway. problem is...seldom does anyone seal these openings, so adding a dedicated pathway is redundant. people think they built tighter than they actually did in my experience. but it is all just a guess until you blower door test to see what leakage actually exists. build tight, ventilate right, and TEST to find out what performance actually is. OP, I have to defer to folks in climates with basements, as we have too high of a water table in La. for basements. I have no hands on experience with basements, 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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Looking at ccspf of R7.5, plus fg batts between the studs to meet Ontario Building Code (OBC) min. of at least R24. Here's the architect's drawing for XPS on the exterior. (Instead of the polyethylene pillow on the rim joists, I will use ccspf. As well, if I use ccspf for the walls, the Blueskin can't be used as the heat of the spray destroys the adhesion, says the spray contractor.) Ratio of exterior to interior insulation. I've built several homes in our Climate Zone 5 with 1" XPS on the exterior, unfaced fg batts on the interior. I lived in one for six years and kept up with that buyer for a few years. No visible problems. Here's Dr. Joe: "I grew up with this wall in Ontario - 2x6 wall with R-5 foam sheathing (Photograph 2) coupled with unfaced fiberglass batt cavity insulation covered with an interior 6 mil poly air/vapor barrier (Photograph 3). This wall is currently being built all over Minnesota and Wisconsin. It is still being built in Ontario. And it works." * ********* spray foam under the slab I hadn't heard of that before the spf contractor made that point too. Thanks for the confirmation! I will decide when closer based on price. ********** a liquid applied air barrier in lieu of the spray foam As you say, no insulation value to that. What I typically see here on high-end infills is simply housewrap over sheathing. relocating to a more southern climate Actually, before the upper end housing market collapsed here, we were all set to move south--to New Hampshire. We're not fans of heat. Or alligators!
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David Cary
last year
last modified: last year
In general, spray foam in the wall does not return well. Attics are a different story. They are not at all the same. That being said, generally spray foam is too costly to be a good value.

When you have rigid foam to the outside of framing, that will typically be taped and provide a pretty good air barrier. This air barrier reduces the value of the spray foam.

So spray foaming a bad ceiling barrier can be very helpful but spray foaming a well sealed wall can be not particularly helpful.

I spent a pretty good amount of time doing research (Zone 4 - Raleigh) and I did 2x6 with blown fiberglass and 1/2 inch foam to the outside. California corners/insulated headers etc. Spent about $10k over conventional (we are allowed to do 2x4 with batts here) and got almost all of that back from the utility company as a rebate.

2x4 framing with 1 inch of foam was one of the options. At the time, my builder prefered 2x6 - and sound abatement was important for us. For your climate 1 inch of foam with batts in 2x4 is a little light to me but that doesn't make $11k for 2 inchs of wall cavity foam something to consider.

I would rather pay $5k for 2 inches of foam to the outside but I bet that isn't an option. I would rather pay $1k to upgrade the attic insulation. I would pay to make sure ducts are in conditioned space. Lots of things are worth it but not this.

$11k after rebates would buy a lot of solar panels....
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Jeffrey R. Grenz, General Contractor
last year
Agree w @David Cary

Priorities

1. Foam the roof deck (called a radiant barrier)

2.insulate the attic w glass

3. leave the walls as the exterior foam performs

remember that with all that your windows are still under R4 so pay attention to glass and thermally broken frames.

Just completed one with these specs and they barely use their HVAC system. Solar wouldn't pay off for their home.
3onthetree
last year
^ a foamed roof plane over an open attic renders the attic an unvented conditioned space, so insulation on the attic floor would be the same as insulating between a 1st and 2nd floor - no insulation value.
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Charles Ross Homes
last year
Design of a wall assembly needs to consider both the thermal performance and drying potential.

I agree that a continuous layer of rigid foam insulation on the exterior of the sheathing enhances performance of a wall assembly designed for climate zone 5. Properly installed and with seams taped, it's a great air barrier, a great secondary drainage plan, and it will limit the potential for condensation in the wall cavity. That said, continuous foam insulation on the exterior will limit drying of the wall assembly to the interior side only. Sandwiching the sheathing between a continuous layer of rigid foam and 2" of closed cell foam will further inhibit drying to the interior side which could cause problems. If you're going to insulate with foam, I'd keep it on one side of the sheathing. If you're able to get the code-required R-value with exterior foam and cellulose insulation in the cavity, that would be my recommendation.

Closed cell spray foam vs fiberglass batts for exterior walls

Comments (6)

Keen B

3onthetree

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David Cary

Jeffrey R. Grenz, General Contractor

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