Spray foam insulation in attic?
I want to insulate a walk-up attic in a 100+ year old home in climate zone 6. I need to add insulation because there is only about a foot of blown in cellulose under the floor boards in the attic. I don’t want to blow in more cellulose because I am thinking about finishing the attic for more living space. I am thinking about having closed-cell spray foam insulation applied against the roof sheathing. The rafters are only 2x4s and I am thinking about firring them out two to three inches to accommodate 5-6 inches of foam. I do not want to firr them out any more than 3 inches because I will lose too much head room. I am also concerned about thermal bridging through the wood rafters. I’ve thought about adding 1.5″ to 2″ thick rigid foam strips to the bottoms of the rafters as a barrier instead of wood. Does anyone have advice for the best way to insulate? I would appreciate it.
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T.S.,
There is no "best" way. If you are concerned about losing head room, then you should probably install closed-cell rigid spray foam, which has the highest R-value per inch.
If you install 2x4s at 90 degrees to the rafters, you can gain depth and address some of the thermal bridging issues. However, you might want to talk to an engineer first, since your 2x4s are probably undersized for the load. You may need to sister new 2x6s or 2x8s to the existing rafters for structural reasons before coming up with an insulation plan.
Once you've addressed any structural issues, this article will explain everything you need to know about your insulation options: How to Build an Insulated Cathedral Ceiling
And given that you are asking on a green building site, you might be interested to know that spray foam has an outsized climate impact because the gas used to blow the bubbles in it has more the 1000X worse climate impact than CO2.