Vapor Permeability of Roof Membrane
Modern Shopdominium. 32′ x 80′. All 7/8s” 24 gauge corrugated steel run vertically on walls and roof. No rain screen because the corrugated will self drain and vent. Shop on first floor treated as a completely separate building for insulation and heating purposes. Truss created attic room, 17′ wide x 8.5′ high x 80′ long. 8/12 pitch. No vertical walls on exterior of attic room. 3/4″ plywood sheathing on trusses. Attic room completely conditioned. No vents. Montana Zone 6B.
Now the explanation, which I assume will result in me being shamed and ostracized!
I get it. Really, I do. I’ve read all of buildingscience’s and other’s articles on the subject I could find. I understand sorption and dew points and all of that. I understand about insulation ratios and venting. I know what – in an ideal situation – I’m supposed to do. Insulate the roof above with board insulation and/or insulate under the sheathing to hit the ratio to avoid sorption. Etc., etc., etc. But I simply cannot – or will not – do it.
First, outside insulation is probably not a good idea with the corrugated.
Second, I can’t afford the ideal solution.
Third, much of the discussions is a bit like debating the number of angels that can dance on the head of a pin. Lots of great theory, but a bit short on reality. Blower door tests? Really? You guys must have made a heck of a lot more money than I did in my 7 decades!
I’m not going to stack layers of board insulation on top of the sheathing. Why? Because the corrugated metal producers warn me that on an 8/12 pitch roof, the length of the screws necessary to reach through 11 inches of rigid insulation and into the plywood and trusses will allow too much “sliding” of the heavy metal, and trying to put some sort of a damn or ledger at the end of the run of the metal to prevent the sliding of a 20′ length of steel is just asking for trouble. So use shingles? I don’t want to. I don’t like the look. There. I said it. I like the look of a Scottish or New Zealand black barn. Yeah. I know. Montana’s climate is different than either of those countries.
Moreover, the cost of top mounted insulation in that quantity is nothing less than enormous. It’s approaching silly, really. Coughing up the cash in today’s ruined economic climate to reach r48 in the roof of a cathedral-like space is nuts.
So just vent the space and blow in insulation? Nope. Don’t want to. I like open ceilings and the very modern look of exposed trusses.
Insulating the underneath of the sheathing from the inside isn’t any more feasible. I have 2” x 8” top chords in the truss. To increase the depth of that chord in order to house the necessary amount of insulation to insure that the dew point isn’t reached at the plywood (the first cold plane) will cost far too much money. “Silly fool”, you say. “You’ll pay one way or another.” Maybe.
Yes, even keeping the interior at a max of 68 degrees, and the humidity at 30%, the moisture may (ok, probably will) condense on and in the plywood. But the rockwool insulation won’t absorb or adsorb the moisture. So I will have sort of damp plywood for several months. But not soaking wet. I’ll run the HVAC etc. systems. Or I’ll just sprinkle of bunch of those “Do not eat!” desiccants I find in beef jerky around. I like beef jerky.
The climate numbers support my assumption. Even though I am in Zone 6B here in Montana, the 28 degree median or average or degree days or whatever it’s called in the three critical winter months won’t result in a soaked structure.
No vapor barrier against the underneath of the insulated truss bays, and wall finish comprising spaced MDF 2′ x 4′ panels on 1.5″ hold-offs on the ceiling (with 3/8s” spacing between the panels) will allow for the structure to dry to the inside. Yes, I know – I’ll probably only end up with an r 30-something. OK. No codes and no inspections where I live. And cord after cord of firewood on the property. More than I can ever use. I do appreciate the irony of writing these sentences on green building site. Yet another reason to hold me in disdain.
What’s that? At least spray closed cell foam for an inch or two under the sheathing before you batt the rest of the 8”. Nope. Too expensive around here. And even if I did, I could never achieve a high enough r value in the interior of ceiling alone to prevent the warm interior air from condensing against the cold plywood. And I just am not going to couple internal insulation with rigid foam on the deck.
Deride me all you want. The building is now costing me 45% more than I planned last Fall. If it rots and falls apart, I’ll never admit to you guys.
So please save the lectures and the condemnations and answer the following: it is better to install an impermeable membrane on the roof plywood (e.g., Ice and Water Shield), which would prevent the structure from drying to the outside, or I should install a permeable membrane on the plywood, which would keep out liquid water and (with the corrugations acting as vent channels) but would allow the structure to dry to the outside AND the inside. And what product should I use? (I want a self-adhesive membrane to cover the entire roof.)
I realize that discussions of these topics have become battles between religions. And many of you may think that I am condemned to hell (or at least purgatory) for even considering this approach. But I’m 70, and it’s too late to seek penance anyway.
So, help me out here! Thanks.
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