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dustycreek 09-14-2012 09:43 AM

Insulating an attic framed with 2x6
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Hey, I have a question about how to best insulate my attic. It's a 1.5 story wartime house, the upper floor comprised of two bedrooms. Noticing it can get really hot in there, while working at least in the warmer months - stiflingly so at times. I'd like to keep it cooler.

I've attached a diagram to illustrate my thoughts.

It's framed with 2x6 rafters and ties, then has 2x4 knee walls. Currently, the space behind the knee walls is insulated with cellulose; the knee walls, rafters and collar ties have old rockwool insulation, then more cellulose blown in within the space below the peak. I'm planning on ripping all this out and re-insulating; admittedly, this will only be for one side of the house, as the board is still up in the other bedroom.

Looking at the diagram, my thought was maybe to go with either 1" or 1-1/2" blue ridgid foam first, leaving around a 1" air baffle between the foam and exterior sheathing to allow air to pass between the peak and the space behind the knee walls. Below the foam, I would place batts of 3-1/2" fibreglass insulation, followed directly by drywall.

How does this sound? You can see in the diagram, there are passive vents installed on either end of the house for the peak space and both sides of the house for the space behind the knee walls. Do I even need baffles? Would vapor barrier be a good idea?

Cost/space ARE a factor. The rooms are small; I can furr out the rafters if needed but prefer not to.

Edit - house is located just outside of Toronto, summers can be hot and winters can be cold.

All suggestions/information appreciated, thanks!

dustycreek 09-14-2012 11:59 AM

I'll add, alternately I'm now considering instead going with 1" (or whatever is common) foam baffles and then as much fibreglass as I can fit in.. guessing R20 is probably too much unless I peel some off.

Windows on Wash 09-15-2012 04:56 AM

It is pretty well thought out, however, I would plan on covering the rafters with rigid foam prior to drywalling. They will still be super cold in this application and in that climate, you will have some condensation issues.

Rip down some 1.5" foam board and apply those strips to the underside of the roof deck to create continuous vent channels. Fill the rest of the rafter bays with rigid foam cut to fit the width (do not use fiberglass). Seal all the seams with sealant/foam for air seal. Cover the rafters with a rigid foam to eliminate the thermal bridging and cold rafters. Cover with drywall.

This should give you the best assembly and still maintain the venting.

You will not need a vapor retarder if you use the right foam board and how much you cover the rafters with will determine your final R-Value.

I would not put less than an R-38 in that assembly.

Air seal and kneewall floor. Blow in additional insulation. Install joist blockers. Foam seal the kneewall and back with rigid board as well.

Air seal the top attic floor. Blow in R-50.


Gary in WA 09-15-2012 10:15 PM

Well said! ADA the drywall to seal the knee wall cavities on room side (esp. around electrical boxes) and caulk the wall/floor joint (required in U.S. Codes now), weather-strip the attic access opening;


dustycreek 09-16-2012 10:15 AM

Thanks for the replies guys, and the detailed write up Windows On Wash. I'm going to cost everything out and decide what to do.

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