Continuous insulation on existing homes
I reside and work in Colorado in Climate Zone 7. Polyethylene sheathing is typically used as a class I vapor retarder under the drywall in existing homes. When able to add exterior insulation we opt for EPS foam due to it being most vapor open, typically 1-1/2” or 2” thick. We take care to air seal to the best of our ability, but drying potential of the walls is always concern. Reasoning here is that if liquid water is controlled and air leaks are minimized there should be no concern with moisture issues, but is considered risky practice. I was unable to find recommendations or best practices when it comes to exterior insulation on existing homes with vapor barrier installed. Lets asume that removing drywall is not an option. Any advice or suggestions would be appreciated.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Check Building Science Corp. IIRC they have some articles on how it can be done.
Brstina,
This article addresses your questions: "Rethinking the Rules on Minimum Foam Thickness."
Martin, thank you for the link. Your article answers some of my questions. When you talk about walls with too thin exterior insulation you mentioned necessity to reduce air leaks. This absolutely makes sense to me. If you are able to quantify level of air leaks where would that land? 3-5 ACH50 or more than that? Thank you for your replay.
In the high-dry Colorado mountains it may be worth using rigid rock wool rather than EPS, for better fire resistance. It has the added benefit of being VERY vapor open (at any thickness.) It's very pricey stuff by comparison, but is definitely worth considering in wildfire zones. (Metal roofing and UNvented attics also enhance exterior fire resistance.)