Cantilevered bumpout

Users who are viewing this thread

Jonathan Mcintyre

New Member
Messages
1
Reaction score
0
Points
1
Location
Farmington, Michigan
So today I had 2 inches of closed cell spray foam applied to my rim joists. There are a couple of cantilevered bumpouts that proved to be kinda tough to get to, so most of those are done, but I think there must be a place in a couple of spots where the foam didn't get in completely as I can feel drafts and my Flir camera shows temp differences.

Now, each of the bumpouts have 2 joist bays that have a duct running in them (the register is on the kitchen floor above), so they are really impossible to reach in, and tough to get sprayfoam in there. There are 2 bays that are straight back, and 2 that are angled.

So, I wanted to figure out how best to finish this up.

http://imagebin.ca/v/2SM35xWHMpRs

Here's a pic of what I am looking at. The brown is the spray foam, the pink is fiberglass, and the turquiose box is where I am thinking of cutting out a piece of XPS rigid foam and then foaming that closed.

BUT, then I thought... wouldn't it be a bad idea to close that off if I am still feeling a small draft from that particular bay? In terms of condensation, etc?

I thought perhaps that I could go to the exterior and caulk/recaulk, and then add a 1/2" piece of XPS to the bottom of the bumpout, and seal that. Thoughts?



http://www.hgtv.com/remodel/mechanical-systems/sealing-floors-above-unconditioned-spaces

This expresses my original concern, so I am trying to figure out how best to air seal this....

"If a cantilever isn't properly sealed, it will leak air, causing the floor to feel cold. Condensation is the worst result of air leaks in cantilevers. Moist air in the home can pass into the floor cavity and condense on the coldest surface it finds—the back side of the sheathing or band joist—causing mold to grow there. Cantilevered floors don't receive direct sunlight, so they don't dry quickly, and condensation can cause the floor to rot.

Air-seal any place where the band joist meets the floor to prevent outside air from entering the cavity. Air-seal all gaps and penetrations in the cantilever. Air-seal the connection between the cantilever and the interior floor system to reduce the infiltration paths contributing to heat loss. After installing insulation, install continuous sheathing, such as insulating foam sheathing, on the underside of the cantilever. Then air-seal the sheathing material to the bottom side of the cantilever with caulk."
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
Air seal the band joist as well as possible with can foam. Use OSB (which is a "smart" vapor retarder) instead of XPS (which may be too low permeance to allow much drying toward the interior) as the air-dam at the overhang, and FILL the cavity with blown cellulose or high-density fiberglass with a compression-fit, no gaps.

There may be refinements to this recommondation based on which zone are you in on this map, and the depth of the joists:

iecc-climate-zone-map-energy-code-warm-moist-line-800.jpg
 
Top
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks