GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Insulation Retrofit with No Sheathing

TomFid | Posted in General Questions on

Our house was originally built by DIY solar enthusiasts, then later added on to by a clueless architect and builder. We’re looking at replacing a bunch of windows and siding, so we’d like to upgrade insulation at the same time. There are a bunch of complications, so I’m interested in general ideas about how to proceed. We’re in USDA Zone 4, or zone 6B via the map here, at about 5300 feet, in Montana where it’s overall very dry. Wildfire hazard is a significant consideration.

The old part of the house has 2×4 walls with R-11 cavity fiberglass and 1″ polyiso foam sheathing. There’s no plywood sheathing; instead there’s diagonal bracing. There are polyethylene vapor barriers on the inside under the drywall, and on the outside on top of the polyiso. The siding is 6″ horizontal pine shiplap on 1.5″ stringers to space it off the foam. We’ve had issues with pest intrusion (mice) – they can burrow through the foam at the bottom edge. I think the outside poly is somewhat degraded, and the polyiso isn’t glued to the framing, so there’s probably significant air intrusion. With a thermal camera, I can see the studs from the inside, so there’s also some bridging. The roof is 2×10 rafters with a cathedral ceiling, so access is difficult.

The new part of the house has 2×6 walls with OSB sheathing. The outside is tyvek wrap, not well detailed. On top of that are cedar shingles. The inside was flashed with an inch or so of closed-cell foam before filling cavities with R19 fiberglass. I think there are some issues with the junction between the new and old parts, based on what looks like cold streamers with the thermal camera. The roof is a scissors-truss arched ceiling with blown fiberglass.

So … what would you do with this mess? For the old part, I’m thinking that it would make sense to pull off the siding and stringers, add an inch or two of foam, and re-side with something fireproof. I’d rather not lose space on the adjacent deck, so metal seems like a good option. Presumably it would make sense to remove the poly sheet and tape the seams of the polyiso, or next new layer. But how can we deal with pest intrusion at the bottom edge, and improve air sealing between the framing and foam?

For the new part, I’m thinking that adding enough foam to eliminate condensation worries on the sheathing might be hard. Perhaps instead it would be better to put that $ into the windows, and just replace the siding while upgrading air sealing? We’re currently thinking about pretty high performance Litezones for some fixed glass, and Fenstur for operable windows. For the roof, I’d do a little more air sealing and baffling to keep the eave vents open, then blow in more insulation to R60 or so.

I’m mindful of the tradeoffs between upgrading the envelope, and just adding efficient heat or solar. Given my so-so experience with efficient appliances, my bias is to put the money into the envelope. But what am I missing here? Any thoughts much appreciated.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |