Insulating an Atypcial Wall
Hi Folks, I have a very goofy situation I’m trying to resolve. Someday maybe I can build new and build right, but for now I have to solve problems inherited from others. Here’s a long explanation of the situation, but the short question is how much should I worry about condensation in a wall that can’t dry very easily, and should I scrimp on insulation in it because of that concern?
I have an exterior wall of about 40 square feet in a first floor bathroom, behind a fiberglass tub and shower area. There is a one foot drop or soffit in the ceiling over this area that houses an exhaust fan that vents out the wall. The wall is currently stripped down to the studs below the soffit, it will be covered with GoBoard.
The climate zone is 6 and the average January temperature is 20F. The house is 19th century, so there are true 2×4 studs, although they are somewhat planed down to achieve a plumb wall. There is 1″ wood sheathing for about 60% of this space, and 3/4″ fiberboard of some sort in the remainder — my guess is that when they installed the tub, they actually cut a big hole in the wall to get it into the room, which has only a 24″ door. They also cut through all the studs and replaced them in a non-standard way, so there are extra ones, irregularly spaced, sometimes sistered, etc. The fiberboard is outside of the original sheathing, so the depth of the wall cavity there is about 5″.
The wall had been insulated with R11 kraft-faced fiberglass, except below the rim of the tub, where there was blown-in cellulose and no wallboard, it just spilled out behind the tub. That area under the tub can be reached through an access panel in the adjacent room, and also had a couple square feet open to the basement due to large uncovered cutouts for the plumbing. On the exterior side of the sheathing, there is felt paper, one inch of 40-ish year old XPS, another layer of 30lb felt paper, and vinyl siding.
Despite the various red flags that this may raise, I did not find any water damage or mold in the wall. I’d like to keep it that way — the fiberboard especially is a concern to keep dry. The 1/2″ GoBoard has an R value of 2.3 and is rated at less than 1 perm, so I’m planning to use unfaced fiberglass in the wall. In the soffit, I can cover the studs with 1″ foil-faced polyiso, sealed around the edges — in case the exhaust fan leaks moisture into the soffit. I managed to wiggle 3/4″ EPS board in behind and below the tub ledge to form a wall against the studs — I choose EPS here because I thought that at least some portion of the wall needs to be able to dry inwardly.
So then we come to the problem of the fiberglass. To fit a depth that varies from 3.75 to 5 inches, it appears that using R19 or R21 is necessary. I learned that compressed fiberglass has a higher R value than if you just trim it even with the studs. I’d get about R13-14 out of the R19, or R15-16 out of the R21 when compressed.
However, reading the Building Science article, “BSD-163: Controlling Cold-Weather Condensation Using Insulation”, I am wondering if less R value here is better. When I do the calculation in that article for determining the interior temperature of the sheathing (inside temp 70, outside 20, 35% RH), with both R19 and R21 compressed FG, I see temps below the dew point. If I want to avoid that, I would need to limit my fiberglass insulation to approximately R6! I don’t know how to do that, but if I trim the R19 to the actual wall thickness, I could get it down to around R11-R12 (and total wall value of R19).
If you’ve gotten this far, thank you! Now let me know what I am doing and thinking wrong and how to improve or at least not worsen this goofy wall!
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