Humidity management and wall/roof/floor insulation details
Hello and thanks in advance for your advice. This is an extension of an earlier thread on perm ratings.
The build site on the Big Island of Hawaii is at 4000′ elevation in a “nano-climate” cloud forest. See the attached JPG for stats on the typical day – high of 73-75, low of 55-57, with humidity ranging from 65% to 100%. Record lows can get down to 40 degrees F. Thermally, this is a very benign environment with little change across the seasons. No big R values needed here… let’s do big windows instead!
There are few houses in this specific area, and none that have really been impartially evaluated in any way. Meaning, just because the neighbor built a certain way – doesn’t mean that is an existence proof and I should copy it. Most of the construction in this region happens at a much lower/dryer/hotter elevation, so there is not a lot of local expertise to draw on for my build.
We will build with 2×6 framing, and pressure-treated plywood sheathing. It will be off-grid, so we need to be conscious of the ongoing cost of dehumidification. Proper air sealing is, of course, paramount.
My questions all revolve around sealing up the house against the humidity and managing any condensation risk. I’m going to probably break this subject into separate questions to focus on just one variable at a time.
With all the backstory behind us then, the first question is whether condensation is a significant risk or not.
Assuming we don’t do closed cell foam in$$ulation, the first condensing surface will be on the inside of the plywood sheathing. The condensation charts suggest that if the interior of my sheathing gets to 40 degrees, then to prevent condensation (with an interior temperature of 70 degrees), I’ll need to dehumidify to around 35% – on these exceptionally cold and relatively rare days, at least. For a typical day around 55 degrees, I’d only need to get to around 60% RH.
From my experience with occasional VRBO stays in the region, I am confident that I can affordably get to 60%. I’m not as sure about 35%. What if I don’t make so much PV power on those days, and only get to 50% RH?? I can always fire up the generator and burn gasoline/propane, but I’d prefer not.
It would greatly simplify construction and minimize costs if I could avoid any exterior foam sheet insulation, outside of the sheathing and behind the siding. And this insulation challenge is multiplied when you consider the underside of the pier/post area and the roof.
Wouldn’t it be nice if I could just “paint” the entire exterior envelope – the outside of plywood sheathing, top of roof sheathing, and underside of any T&G plywood flooring with a vapor barrier like Prosoco’s R-Guard VB. It’s all highly visible, and we could focus all our efforts on a high quality air/vapor seal. Anything inboard of this layer would dry to the inside.
I tend to think that this is the best solution, but I guess the big question is the decomposition risk of
“occasional” condensation on the inside of the sheathing, should I fail to hit my goal of 35% RH and end up at 50% for a couple days a year. The cellulose insulation would act as a bit of a buffer, absorbing to some degree any liquid water.
Am I insane for thinking I can pull this off?
Sincere thanks, Mark
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