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Small Cabin Forum / Cabin Construction / Best Insulation Option for 2x6 Rafters
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# Posted: 20 Nov 2022 02:36

I’ve got 2x6 rafters for a single pitch roof of 2/12. spaced 16” o.c. and am unsure how to optimize insulation R-value while avoiding any potential mild issues.

I have 12” overhangs and will be adding a soffit all the way around and am installing a standing seam metal roof.

I’m hoping to retain as much heat as possible for efficiency.

Initially my plan was to install 2x6 Rockwool but that would sit up against the roof sheeting as well as the finished ceiling. I’ve done a little reading here and it sounds like that’s not advised due to blocking airflow so was thinking of using Rockwool 2x4 insulation in order to leave a 2” gap between the insulation and roof sheeting and then adding a radiant barrier to the bottom side of the rafters before installing drywall or wood ceiling.

Will that be safe as far as creating airflow and preventing mold?

If the 2x4 material is the way to go, is there anything else I can do to improve heat retention?

# Posted: 20 Nov 2022 08:00

Plywood decking above the rafters to seal it up, 1x2 purlins over that, metal roofing over that.

# Posted: 20 Nov 2022 08:34

What you described was how I insulated and vented min. I used R13 fiberglass with a paper vapor barrier. The vapor barrier made it fast and easy to staple in place. It left an air space for the vented soffits and ridge vent to work. In the ridge vent, I installed a bug screen that was a black, continuous roll of material not unlike a scotch guard pad. I think it all works well.


# Posted: 20 Nov 2022 11:04

I would put foamboard in the bays and can foam around them. You can even just put the foam board on the bottom of the rafters if you want. Your never going to get a good R value with 2x6 so really really left trying to get as much air sealing as possible. Foam takes care of the air sealing and adds as much ar value as you can per inch.

How big is the roof? You could do a DIY spray foam kit or even add furring strips under the rafters.

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