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Old 01-14-2015, 07:28 PM   #1
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Cantilever Overhang Insulation


I have read many of the threads on here regarding insulating this area of the home. I am just getting started and wanted some expert advice.

I have ~18 inch overhang on the back of the house over the foundation. Prior to doing anything there was always a cold draft and a draft upstairs under the cabinets in the kitchen that lie above it.

The previous insulation was just fiberglass faced insulation batts that were stuffed haphazard, see the photos. Once these were removed there was a direct communication to the outside through 3-5 mm gaps between the 2x4s. on the "floor" of the cantilevered portion.

So in the space, I placed R 7.5 or 1.5 inch that i glued down to the "floor" of the cantilevered space. I also used great stuff gap and crack sealer to fill the gaps between the 2x4s from the outside (I am sort of limited as it is ~10-20 degrees here during the day.

There is a noticeable decrease in the amount of air through this area but not perfect.

My questions:
1) Is rigid foam an adequate "air barrier" or do I need to consider adding some sort of house wrap from the outside?

2) What R value batt would you use above the rigid foam board I've installed?

3) after placing fiberglass batts, would you still seal off the cantilevered portion with another vertical piece of rigid foam?

Thanks in advance.

Cantilever Overhang Insulation-img_0193.jpg

Cantilever Overhang Insulation-img_0195.jpg

Cantilever Overhang Insulation-img_0196.jpg
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Old 01-15-2015, 04:17 PM   #2
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Rigid foam is plenty air barrier when sealed.

You need to seal and foam up the exterior rim board and get that all sealed up. After that, leave the batt insulation out so that warm air can circulate there and keep the underside warm.

Ideally, you would insulated from the exterior and thermally uncouple the framing but getting all the gaps sealed up is the critical part.
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Old 01-15-2015, 04:29 PM   #3
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Unfortunately I cannot seal the outside in the winter too well, I'll take a picture of the underside when I can.

So if it is all sealed from outside and around the rigid foam, that should be enough air barrier without the home wrap and I dont need to seal the opening.

What R value would you put above that rigid foam? I have about 6" of space. I purchased unfaced R 30 which is technically attic insulation per the packaging
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Old 01-15-2015, 09:31 PM   #4
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The problem is the thermal bridging from the joists, they are still as cold as ambient temps with R-1.25 per inch.... the FB should have been installed under the joists to decouple from ground, (and on rim joist vertically as said), then cavity fill with a gap above it--- http://www.buildingscience.com/docum...n-crawlspaces/

to warm your toes; http://www.buildingscience.com/docum...al-performance

Seal it off vertically with FB over the concrete wall/mud sill; no air to degrade the fiberglass cavity batt (air permeable), then cover with plywood for ignition barrier.

Gary
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Old 01-16-2015, 09:17 PM   #5
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I have a 24" catalever area where my fireplace sits. Last wintwr the whole area and wall below the fireplace was ice cold from the cold air and even though the whole area 4 joists wide was stuffed with fiberglass it didn't do much. I hired a spray foam company to do all my rim house and they filled that whole cavity with foam as well. So this winter the wall is warm, no air leaks and problem is solved. If you do t want to go this expensive route I would say calk and seal all the joints, great stuff the larger gaps, use rigid foam to box the face and foam those gaps and stuff the rest with fiberglass.
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Old 01-17-2015, 10:28 AM   #6
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Obviously a gas insert type fireplace?

Did then clearance the foam from the flue?
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Old 01-17-2015, 11:18 AM   #7
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Lots of space. It's one of those decorative ones so it's up higher in the wall and no where near the foam in rim joist
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Old 01-18-2015, 10:35 PM   #8
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Put foam board (and seal) on that vertical board in the center of picture 1. That's your rim joist and is should have been insulated when you did the bottom soffit.
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