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Insulating stud bays: Spray closed-cell foam directly on 1x6 diagonal sheathing or use asphalt felt with air gap?

Hi All,

We're renovating a historic 1935 brick home. Once the interior paster was removed from the walls, we found 1x6 diagonal sheathing (in great shape). While the stud bays are open, we'd like to add insulation to the house.

The house is framed with 2x4 (1.75x3.75 actual) studs.

My original plan was to line the stud bays with asphalt felt, leaving 1/2 to 3/4 air gap to allow the stud bay to dry out, and seal it up with 1" of closed cell foam, and fill the rest of the bay out with open cell foam.

I'm now wondering if I could spray the closed cell foam directly on the interior side of the 1x6 diagonal sheathing without creating a moisture issue. Skipping the asphalt felt would speed up the process, and allow a bit of extra open cell foam to be applied to the bay.

The fear would be that foam application directly to the sheathing would create a moisture issue, and the sheathing rot would go unnoticed behind brick and layers of foam.

Asked by getco
Posted Jul 12, 2018 11:21 PM ET
Edited Jul 13, 2018 6:43 AM ET


5 Answers

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For a full answer to your question, it would be good to know (a) your geographical location or climate zone, and (b) whether or not there is some type of WRB (for an older home, that would usually be asphalt felt) between the sheathing and the bricks, and (c) whether or not there is an air gap between the WRB (if one exists) and the bricks.

You should be able to use a large-diameter bit to carefully drill an inspection hole in the sheathing to gain more information on your wall construction.

Answered by Martin Holladay
Posted Jul 13, 2018 5:08 AM ET


Hi Martin,

Thanks for taking the time to answer.

A) We're in zone 4A (And closer to 3A than 5A).

B) There is a 4" break in the sheathing where the interior portion of a concrete window sill meets the studs. Inspecting here, there is no WRB.

C) There is a 1" gap between the sheathing and the brick, but no WRB.

- g

Answered by getco
Posted Jul 13, 2018 10:01 AM ET
Edited Jul 13, 2018 10:02 AM ET.


In Zone 4, I think it will be fine if you install the closed-cell spray foam directly to the interior side of the wall sheathing. The bricks and mortar are vapor-permeable, and the air space will help with outward drying. This wall assembly should be able to dry to the exterior.

Of course, bad flashing details at windows, or excessive globs of mortar droppings that block the air space, can always result in moisture damage. These factors are hard to predict. So a little common sense (along with an inspection of the exterior and its susceptibility to water entry, and an assessment of the roof overhangs) always helps.

Answered by Martin Holladay
Posted Jul 13, 2018 10:18 AM ET
Edited Jul 13, 2018 10:20 AM ET.


Thanks Martin!

There's something about the irreversibility of closed cell foam that has me in a 'measure many times, cut once' state of mind.

- g

Answered by getco
Posted Jul 13, 2018 10:40 AM ET


Why use any closed cell foam at all?

In zone 4A it's usally better (and certainly cheaper) to use open cell foam for the full cavity fill. The extra step of a flash-inch of closed cell really doesn't buy you anything, except an additional R0.1 or so in whole-wall performance. (Adding strips of 1/4" XPS to the interior side stud edges and going with a full 4" of open cell foam would be a much bigger improvement in performance, at much lower cost. See: http://www.greenbuildingadvisor.com/articles/dept/musings/installing-clo... )

The all open cell foam solution is a lot greener too. The inch of closed cell foam uses more polymer than a full 3.75" cavity fill of open cell foam, and the blowing agent is water, not HFC245fa (an extremely powerful greenhouse gas.) While HFO1234ze blowing agent closed cell foam is available, it's 25-40% more expensive more expensive per inch than the HFC blown goods in my area.

With only open cell foam in the cavities an no closed cell, the drying capacity toward the interior is about 3-4x higher than it would be with an inch of closed cell, assuming latex paint as the only interior side vapor retarder.

The total COST of a full 3.75" cavity fill of half-pound open cell foam would be less than the cost of 1" of HFO blown closed cell foam in my area, and barely more than the cost of 1" of HFC blown closed cell foam.

In zone 4A the conditions where using only open cell foam becomes a problem in that stackup is if a highly vapor retardent interior finish (eg foil or vinyl wallpaper) were used, in which case the exterior moisture drives of the brick could potentially increase the moisture content of the wallboard to troubling levels during the humid dog-days of summer if you air-condition to a low temp. So use latex paint instead.

Answered by Dana Dorsett
Posted Jul 13, 2018 6:08 PM ET
Edited Jul 13, 2018 6:09 PM ET.

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