Exterior insulation with interior spray foam
I’m rebuilding an 1800’s farmhouse and insulating as well as possible. The interior has 3” closed cell spray foam (I know a guy) and 3” rockwool in the form of safe n’ sound batts. However, since this wasn’t new construction, I wasn’t able to avoid thermal bridging without framing secondary walls.
Now it’s time to do siding, so I figured it’s my chance to have a layer of continuous insulation. I’m worried about anything that would air seal both sides of these old walls given the closed cell foam on the inside. Should I be? Finding info on super insulating historic buildings is difficult, and not sure which direction to go.
thank you.
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See here to alleviate concerns about multiple air barriers. And if you use unfaced EPS, Winter vapor won't be trapped. I'd use DrainWrap underneath the EPS.
Awesome, thank you. Is EPS at any reasonable thickness still sufficiently vapor permeable? And preferable to XPS?
It’s also come to my attention that rigid mineral wool exists and is extremely permeable. However, I see very little mention of it, so there must be issues I’m not aware of...
Unfaced EPS is more permeable than XPS and is better for the environment. Exactly how much drying any wall/ceiling needs depends and involves opinion. You could use this calculator:
https://www.appliedbuildingtech.com/fsc/calculator