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2 x 6 cathedral ceiling insulation plan

I have a 1 1/2 story cabin that I am insulating. The second story was constructed using engineered attic trusses. The top chords are 2 x 6 and the knee walls are 2 x 4. Metal roof with soffit vents and ridge vents. My plan is to fur the sloped top chord with a 2 x 4 on edge. This would provide me with 9" of space. I will put allow 2" of ventilation space and install 1.5 " XPS rigid foam board as my baffles, followed by 5.5" of roxul comfort batts, 6 mil poly vapour barrier and drywall. The 2 x 4 kneewall I will build out also with 2 x 4, giving me 7 inches of space to fill. I will place 1 inch rigid foam as an air barrier for the back of knee wall, followed by 5.5 inch roxul, and vapour barrier, drywall. I will air seal everything with caulking and great stuff spray foam. Does this sound like a good method. Should I still be concerned with thermal bridging? Would an additional layer of rigid foam before the drywall be advised?

Asked by Dawn Roberts
Posted Aug 30, 2014 8:07 PM ET


2 Answers

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Dawn add your location data. add pictures. No poly should be used most of the time. Instead of more space for batt insulation you should just put rigid insulation on the living space side of your build up. use standard vent chutes from a building supply. High R fiberglass is fine, I don't see any good reasons to spend more for Roxul, spend more for interior rigid foam. Do not use rigid foam for vent chutes, rigid foam is not being used correctly if placed in framing instead of on framing.

i build just like I posted and I like the build.

Next you will get a dozen alternatives from others and who knows if they build, good luck.

Answered by aj builder, Upstate NY Zone 6a
Posted Aug 30, 2014 8:34 PM ET


For a complete discussion of your options, see How to Build an Insulated Cathedral Ceiling.

I don't recommend that you install any insulation in your kneewall. It usually makes more sense to continue the insulation installed in the sloped roof assembly all the way down to the floor. To learn why, see Two ways to insulate attic kneewalls.

If you don't take my advice, and you end up putting insulation in your kneewall, made sure that you include carefully air-sealed blocking between the joists under the kneewall bottom plate and between the rafters above the kneewall top plate.

Answered by Martin Holladay
Posted Aug 31, 2014 5:09 AM ET
Edited Aug 31, 2014 5:10 AM ET.

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