Insulating an old farm house
We bought a 111 year old farm house in northern Michigan 12 years ago. When we moved in, it did not have any insulation. Over the years, we’ve blown in cellulose into the wall cavities and roof and replaced the windows. A builder-friend also recommended placing 1 inch polyisocyanurate foam with foil on the inside wall just behind the drywall. We have followed this recommendation as we have renovated room by room. This year we are going to tackle the outside and replace the rotting/damaged original siding. I would like to add 1-4 inches of staggered foam on the outside as well but can I? From the reading I have done, I am fearful that there will be moisture buildup in the walls because it will not be able to dry to the inside. Local builders have told me everything from “don’t do it” to “it’ll be just fine” to “apply 1 inch of XPS foam and tape the seams”. Suggestions?
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Ryan,
I think that this is a judgment call, and that your decision should be based in part on the condition of the wooden components that you inspect when the siding is removed.
If everything is bone dry, I think that you can safely install several layers of rigid foam on the exterior side of your wall sheathing. Make sure that the R-value of the exterior rigid foam is enough to keep the wall sheathing above the dew point during the winter. That's going to take some figuring, because you'll need to add up the R-value of the cellulose and the interior foam, and aim for enough exterior foam to end up with the right percentage of exterior foam for your climate zone. For the concepts behind the calculations, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
If your wall has rot or signs of moisture problems, it's time to rethink the idea of exterior rigid foam until you figure out what happened.
Of course, it's always safe to install exterior mineral wool, because mineral wool is vapor-permeable. For more on this approach, see Wrapping an Older House with Rock Wool Insulation.
Using unfaced EPS instead of XPS would give you a higher exterior R before it's drying rate to the exterior was severely impinged. At 2"/R10 most XPS would be running between 0.5-0.7 perms which is well into Class-II vapor retardency. At 3"/ R12.6 1.5lb density "Type-II" EPS would be just under 1-perm, and 1lb Type-I EPS (about R11.8 @ 3") would run about 1.5 perms, more than 2x the drying rate of R10 XPS.
At 1" XPS is typically ~1.2perms, give or take, but only R5, not much dew point control against an R15-R20 interior-side R.
Are you in the U.P. (mostly US climate zone 7), or on the main part of MI, zone 6? In zone 6 a 3" layer of ~ R12 EPS is sufficient dew point control for up to R23 or so on the interor side of the sheathing. (Say 4" of cellulose + 1.5" of interior side polyiso.)
Thank you for your expertise Martin and Dana. We are in zone 6 just bordering 7. I'll pass the information on to our builder.