What is the best way to insulate 2×6 walls without exterior insulation?
I currently undergoing some remodeling and I have new construction second floor walls with 2×6 studs 16″ OC. I am in climate zone 5. We are in the proccess of having Hardie board installed and exterior insulation is not an option right now. I am also hesitant to build and second interior wall as discussed in other answers. I know exterior is the best option but that ship has sailed. What can I do to maximize my insulation from the interior with both value and performance. Do I flash and batt to stop air flow? Do I use rigid and batt? Is my best option just rockwool batt? Is there anything I an do to help mitigate the thermal bridging in the 2×6 studs. I get so many mixed answers and contradicting answers I’m not sure where to begin. Or if someone can point me in the right direction of some articles that would be great as well.
I also have a second question about how I should insulate my attic which has HVAC/insulated mechanical room. I was planning on insulating with blown in but others have suggested I use closed which would clearly be costly.
Thanks,
Justin
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Replies
Justin,
Nothing is "best."
For the highest R-value per inch (but a significant negative environmental impact compared to other insulation options), install closed-cell spray foam.
For more R-value at the expense of some loss of interior space, add continuous interior rigid foam, Bonfiglioli strips, or "Mooney wall" horizontal strapping.
Bonfiglioli strips consisting of 1" polyiso and 1x strapping would bring the total cavity depth to 7.25", which is the proper depth for R30 rock wool batts. The R6-ish edge strips would cut the framing losses by half.
Some tips on Bonfiglioli strips:
https://www.finehomebuilding.com/membership/pdf/9750/021250059.pdf
Tips on cleanly cutting foil faced polyiso into 1.5" wide strips (or any width, really):
https://www.finehomebuilding.com/2009/04/15/theres-a-better-way-cutting-rigid-insulation-2
Detail the exterior sheathing as the primary air barrier. Since the siding is already up it's too late to tape all the seams, but a full perimeter caulking of the framing to the sheathing with polyurethane caulk is pretty fast with a powered caulking gun. Any horizontal seams in the sheathing can be taped. All electrical & plumbing penetrations of the framing or sheathing get can-foamed, including lateral runs through studs.
The end result will be measurably higher performance than a typical R20 cavity + R5 continuous insulation on the exterior approach. In zone 5 as long a there is an air gap between the Hardie and the housewrap you don't need an interior side vapor barrier. If there isn't an air gap, install 2-mil nylon (Certainteed MemBrain) or Intello Plus, detailed as an air barrier under the wallboard.