Foam board around the soffit?

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Old 03-10-12, 07:05 AM
Yukon Youngun's Avatar
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Foam board around the soffit?

It looks like I am going to finally put up the drywall on the outside wall of my walkout basement and am considering adding 1" foam board first to minimize loss through the studs and boost the performance a little. House is framed with 9' walls and in what will be a t.v. room the builder installed soffits for the can lights all around the perimeter.

Should I add foam to the top of that wall inside the soffit as well? Seal it too? Would it make any difference to replace the glass batts at the rim joist with foam?


 
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Old 03-13-12, 09:01 PM
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Installing the foam board interior will keep the stud cavity colder giving more potential for mold/condensation by lowering the "dew point" of the wall. What is the exterior cladding? Is the cladding on a rainscreen?How Risky Is Cold OSB Wall Sheathing? | GreenBuildingAdvisor.com

Adding f.b. to the exterior reduces your vapor retarder/barrier requirements for your Zone. Which Zone are you in- #4, or #5?Find nearest city: Chapter 11 - Energy Efficiency

Chapter 11 - Energy Efficiency

The exterior frame wall requires fire-blocking at the soffit framing of vertical/horizontal joint: Chapter 3 - Building Planning

The faced f.g. should not be side stapled as pictured for convective loops: Insulating with Fiberglass Batts - Oikos Green Building Library

The rim f.g. should be removed/installed after air-sealing with foam board/canned foam: Info-408: Critical Seal (Spray Foam at Rim Joist) — Building Science Information

Gary
 
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Old 03-24-12, 05:00 AM
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Thanks for your comments.

I recently have installed new siding on this side of the house -- so 2x6 walls, OSB, Tyvek and vinyl siding outside. That project is in another forum here, and was focused on air and water sealing. Tyvek is pretty thoroughly taped all around.

I am near the northern edge of zone 4.

Still trying to understand what all the firestop requirement means. I assume it applies to finished wall where "hidden" spaces exist, as opposed to the open, unfinished walls I have now. It seems to indicate that I basically want to put drywall on the wall and ceiling surfaces within the soffit just as if the soffit was not there.

Yes, I was planning to move the facing tabs to the stud edges as I go. There are numerous places where the builder shoved the f.g. into gaps and around electrical boxes where I intend to fill the spaces properly.

I was hoping the job outside would be sufficient to satisfy the sealing requirements for the rim joist in this area. (Do I need to consider outside air penetration and inside convection separately?) But since I definitely need to address the adjacent walls parallel to the joists, it would not be too much extra effort to treat these spaces as well. I already have 2" foam board that I have not used yet from my attic project that I would put in these spaces and seal in with spray foam.
 
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Old 03-24-12, 05:46 AM
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Regarding firestops...

You do not have to drywall the interior of the soffit, but you do need solid wood blocking (2x4's laid flat) between the wall studs where the bottom of the soffit meets the wall. (and any gaps behind that wood blocking and the masonry wall would have to be air sealed with fire rated spray foam.) A fire inspector just doesn't want fire to have the ability to burn within the lower section of wall, then jump up inside the soffit and into the floor. The solid blocking is your fire stop in that it would have to completely burn through before fire could jump into the soffit. Limiting air movement also limits fire movement.

It's sometimes hard to see what a fire inspector requires but they have a good eye and are trained what to look for.
 
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Old 03-24-12, 03:04 PM
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I just thought it might be easier to put longer pieces of drywall up (if I could get them inside in the first place) than to cut individual 2 x 6 blocks.
 
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