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Old 02-17-2010, 09:28 AM   #1
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Insulating Attic Conversion

I purchased a cape cod style home in Raleigh, NC a few years ago that had half of the attic converted to a bonus room. The roughly 400sf room is conditioned but it is not comfortable in the winter and summer months. In the winter, the room is often 5 degrees cooler than the downstairs (one zone -thermostat downstairs).

I've sketched a section of the attic room envelope. The previous homeowner added a 3" blue rigid foam between the 2x6 rafters and then added a 2" rigid foam running continuously under the rafters. The 1/2" drywall is attached through the 2" rigid into the bottom of the rafters with long screws i assume. The knee wall has a faced R-13 batt and the 2x10 floor joists have an R-30 batt.

I went to the U.S. Department of Energy's website and they recommend sealing the backside of the wall with rigid foam and installing a draft blocker in the floor joists (I've attached a link/photo).

The thing is, won't sealing a rigid foam which acts as a vapor retarded to the backside of the knee wall trap moisture and cause problems? Also, the way the room was built, I have 1/2" plywood running under the knee wall and stopping roughly 2' from the outside perimeter wall. Rather than cutting out this plywood to install a draft blocker, can i just continue the plywood to the perimeter and seal the joints?




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Old 02-10-2011, 09:47 AM   #2
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What kind of venting do you have in place already?


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Old 02-10-2011, 04:28 PM   #3
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I'd add some foam board (which will need covering with drywall if attics are communicating) or better yet, house wrap to attic side of knee wall.;
The plywood will work to stop the convective loops in the R-30 (low density) insulation;

It won't trap moisture because of it's permeability rating; Tyvek; 58 ? perms, 2" foam board; 1.5 perms. Try to wrap the knee wall top plate in foam to air seal it from heat loss through the drywall joints there.

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Last edited by Gary in WA; 02-10-2011 at 04:30 PM. Reason: added link
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Old 02-11-2011, 12:42 PM   #4
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A radiant barrier might help as well, by reflecting sun heat back to the outside during the summer and heat back into the attic during winter.

Look them up. There are several types, including "spray-on" coatings for hard to reach places.
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