Condensation on Vapor Retarder Over Mineral Wool Insulation
noticed something odd today. Might not be odd, but I thought it was. I have a few bays of my exterior wall (above grade) that have mineral wool insulation in 2×6 wood framing. This differs from the rest of the bays that have dense-packed cellulose – the reason for this was that these few bays used to be where a window was located – but I filled it in since we are making some changes to that part of the basement. Didn’t want to mess around with cellulose for literally like 16 square feet of fill.
Anyhow, I noticed that these bays have some visible moisture on the back side of the VR (Certainteed Membrain) today where the neighboring cellulose bays do not. What is this all about? This was a recent change (filling in the window) so i am very curious if this is expected or if I should make a change prior to closing it up?
My wall assembly – vinly siding, Tyvek wrap, 7/16 OSB, cellulose / mineral wool, VR-Membrain.
Was a very mild day, about 70F, very low humidity in central WI.
Edit – that day about 7PM, the visible moisture disappeared. The next day was warmer and more humid outside (we ran a/c) and it appeared again during the day. Later that night, it disappeared again. Is this cycle something i should expect forever? Is it dangerous? Why mineral wool bays exhibit this behavior but not cellulose?
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