wall assembly for wet climate garage walls - smart vapor retarder maybe?

I am halfway through building my 3 car garage, 24x34 just finishing the foundation (city made me build it like a house lol)

So I am planning the HVAC and insulation and wall assembly. I am in zone 4 marine in the PNW on the ocean. Not usually below freezing and not too hot in summer, but very rainy from Nov to March. I want to be able to use the garage with heat or cooling intermittently. Most of the time no heating no cooling, but perhaps 4 hours a week I'd like turn on a small mini split to make it nice to work in. I'm willing to put in 3.5 inches of rockwool on walls and ceiling, and 2 inch of foam under the slab.

Because it will be largely left unheated in winter I fear the moisture may build up on one side or the other. The garage doors will probably not do much to control moist air from getting in.

My wall assembly will be mostly: stucco-rainscreen-Tyvek-OSB-rockwool-vapor retarder inside. I would put drywall on eventually but maybe years later.

ChatGTP suggests that just using poly on the inside could be problematic in such an intermittently heated and cooled space. Might be much better to use a 'smart vapor retarder' like Intello Plus or Membrain, to facilitate extra drying control and lower the risk of condensation.

The upside of Intello Plus is it has fibres in it making it pretty strong to sit in place without drywall for a couple years, while the Membrain looks kind of weak like poly.

Also since they city makes me build this to the standard of a house (2 foot deep foundation, $3k in #5 rebar just in the stem walls and footings, super well drained), heavy duty and tight, it might help to use premium materials like Rockwool and Intello in case I or someone else wants to turn it into a residential cottage or business office of some sort in 10 or 20 years' time. Of course everyone just uses regular poly for their vapor batterier behind the drywall but that is for houses which will be constantly heated and cooled.

What do you think? go for the expensive Intello or Membrain? Or just use poly and drywall with paint and hope there is no condensation to worry about?

currently about to finish the backfill and pour the slab

The vapor barrier is dependant on climate. I'm not sure where this is. 2 ft frost line feels like Virginia or similar North to South? (I've always lived in areas with 3 1/2' to 4' deep frost so no question on warm side vapor barrier. For you, certainly under slab vapor barrier. If synthetic stucco, I don't think the wall could never dry outward. If cement and lime stucco, it can dry out so I would put an interior vapor barrier in.

location: pacific northwests zone 4 marine - ~100km north of Seattle. Wet winters above freezing, dry mild summers.

for the stucco I will install a rains creen product so the stucco is not a problem. Plus 2 foot overhangs for the roof is in the plans.

Read again I will use tyvek on the outside of the OSB under the rainscreen and tar paper and stucco. I am concerned only about the interior vapor controls. Normally houses use poly here with no issues, but with the space not being conditioned most of the time I expect the moisture to get inside the garage fairly easily and linger. So maybe a more hi tech vapor retarder and mineral wool insulation would guarantee adequate drying?

Wouldn't use visqueen. Call tyvec and other manufacturers you listed their the professionals. New products coming out all the time.

update in april: the foundation is finally at long last done lol. 50k spent

walls going up now and trusses come in 2 weeks. drop trusses for 2x6 outlookers

I have decided to just put two layers of tar paper as is traditional for under stucco and not bother with a rain screen. But on the back wall (invisible from my property) I will likely just put hardiboard on furring strips over the continuous tar paper.

LVL just arrived. And my back just went haywire with morning pain ugh. I feel fine in the afternoon so I guess I work half days on this job lol.

Now for the roof I'd like to save $500 and use 5/8 OSB as it costs just over half the price of 1/2 plywood here. BUT it weighs 20lbs more per sheet than the 1/2" ply in the specs. With my back acting up maybe I just go with plywood and enjoy the extra water tolerability long term of it as a side benefit.

nearly 6 inches thick on the right side so that could handle a Bendpak lift maybe. 4600psi and 3" of insulation foam underneath I got for very cheap. Around $3000 in rebar in this whole job now!



type 2 3" foam. almost enough for the whole slab for $250usd from a builder nearby who had it surplus.

I am hoping with insulated doors, some 3" rockwool inside and I could get by with a 110V 10,000btu Senville mini split I have on hand left over from another project. I winter down south so would only need some shoulder season heat when it is maybe 8C (40F) outside, and cooling when it gets up to 25C (75F) outside, which is kind of our range of temperatures here.

hey what do you guys think of the idea of not finishing with batts of insulation nor putting in any drywall, no roof venting but just spraying foaming the inside of the sheathing walls and roof, then conditioning the whole space rafters and all? I know it is not not as fire safe as drywall and rockwool though, esp with a lot of exposed lumber. Closed cell spray foam.

I guess all the electric had better be in conduit then.

bcsaltchucker said:

BUT it weighs 20lbs more per sheet than the 1/2" ply in the specs. With my back acting up maybe I just go with plywood and enjoy the extra water tolerability long term of it as a side benefit.

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Once the framing is done have your supplier delivery with a lift truck, have him lift all the roof sheathing to the roof framing.... why kill yourself getting it up there, my preference would be 3/4" ply. 1/2" ***** for roofing, it is the go to for cheap contractors.

great idea for flying sheathing on with the crane! I am having them fly the trusses onto the top plates too.

I am planning for a standing seam roof so obviously will be on furring strips. So I don't quite see the need to go to 3/4 ply and I would not be having fun moving 3/4 around on my own lol. BTW it is my own garage. The metal goes on and then nothing will be touching the roof for 50 years I imagine, long after I am dead. Engineer specified 1/2" ply in the schedule and noticeably did not give any OSB as an option.