Those fiberglass batts will be replaced with R-15 unfaced mineral wool batts, which I intend to use throughout the house. ![]() Anecdotal, for certain, but no issues in this case. I could have simply left the fiberglass batts installed, but I was curious to inspect for damage and see what the internet has been yammering about in terms killing old houses by installing insulation within stud cavities. There was absolutely no moisture damage behind those batts, the studs and sheathing bone dry. Judging by the branding on the fiberglass batts, the insulation was installed roughly 40 years ago. Apparently one of previous owners did a bit of remodeling. In these areas, drywall had been installed over the batts. It should be noted that a small number of the exterior walls were previously insulated with fiberglass batts. Now I’m curious what each of you would tell me. The reconciliation between my gut and brain tells me to use a vapor-retarding primer on the drywall. My brain tells me to use a semi-permeable membrane such as Certain-Teed’s Menbrain product. My gut says to keep it simple, don’t try to overly seal a building which is not designed to be sealed tight. Despite the likelihood of vapor permeating through the drywall, into the cavity, and condensing on the backside of the sheathing, I am still hesitant to add a control layer which discourages drying to the interior. What I’m mostly battling with now is the decision to use a vapor barrier on the interior, just behind the new drywall. However, due to the inevitability of imperfection, and the simple fact the home does now have an exterior weather barrier/house wrap, I’ve elected to fill the stud cavities with moisture resistant mineral wool batts. I am only minorly concerned with potential issues related to water entry from the exterior not being able to dry out once insulation has been added to the cavities. Where there were any gaps/cracks (found exclusively around windows), I have sealed the areas with spray foam. There are almost no signs of water coming from the exterior in. Now that the interior of the walls have been striped, the studs and backside of the sheathing are exposed, allowing me to inspect for any signs of water intrusion. The plaster will be replaced with drywall. The interior plaster and lath has been removed due to significant cracking throughout the house. ![]() ![]() The cladding is original cedar clapboard. I am about do what many in the building science world consider a big no-no: insulating the exterior walls of a previous uninsulated, 100 year old home. I’d like to get your opinion on something.
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