When my garage(32X40 with 16 foot walls) was framed in the contractor reinforced the doors end with plywood over the insulation, but did not put in a vapor barrier. I'm in the process or insulating, etc. the rest of it, in preparation to get the slab-heat hooked up and running.

On the door end, should I:
A) vapor barrier over the plywood before dry-walling over it
B) waterproof-tape the seams of the plywood then drywall
C) forget it and drywall
D) drywall and seal the drywall with a vapor barrier paint. And yes- no overlapping seams.

C

D.
You don't want the water vapor condensing on a vapor barrier within the wall. In a cold climate the vapor will be driven from warm to cold. There is no water vapor in frozen air.
Use a heat source that doesn't add additional moisture to the inside air.

You've got vapor barrier on the outside why would you want it on the inside also?

Phil

Where you put the vapor barrier is dependent on your climate. Hot/tropical would be on the outside. Cold/dry would be on the inside.
The Tyvek, which is widely used for wrapping the outside of buildings is not a vapor barrier, It is a moisture barrier. Water vapor moves through Tyvek, water doesn't.

How about we look at the Code? 2012 IRC Section R702.7 requires vapor retarders on the interior side of framed walls in specific Climate Zones. Class II vapor retarders include Kraft-faced Fiberglas batts. Class III includes latex or enamel paints that are only permitted in certain vented cladding or insulated sheathing systems.

Did your contractor perchance install Kraft-faced batts?

Bottom line is that one doesn't want warm moist interior air from occupied spaces to migrate through the wall assemblies and condense on cold exterior surfaces. If your structure has a slab-on-grade floor and the design professional/contractor didn't do a good job on the slab vapor barrier, you can get a surprising amount of water vapor migrating though the slab and into the tempered interior space as well....

As mentioned, you absolutely want vapor barrier between the sheetrock and the insulation.

in AK it would seem that in your case the moisture would be coming from your slab heating and you want to prevent that from going through your inside walls and hitting the cold outside wall and condensating on the inside of the sheeting. therefore i would vote for D on your finished walls and to put battened insulation facing IN on all the unfinished walls and drywalling over that. I.E, barrier facing the heat source as close as possible. i have ripped out a lot of interior walls of houses that were insulated like that and never found any signs of moisture in the walls.

I was deficient in providing information.

The in-slab (to-be) heated floor is laid on several inches of rigid closed cell foam, and I believe a 10 mil (maybe only 6) sheet-plastic vapor barrier under that, so ground moisture migration should be minimal.

It was laid 5 years ago, the door end of the building in question was insulated with unfaced R-21 batts at that time, overlaid with the plywood as stated. This is the only concern I have, as to where to place a vapor barrier. It WILL get a vapor barrier, on reflection - I was posting previously at the end of an 18 hour day- not quite all there, as it were. Or as much as I am ever all there....

It has been stud-wall only since, wrapped with tyvex under T- plywood siding, so moisture accumulation wouldn't have been a problem such with free passage.

I am now in the process (hopefully finishing today!) of insulating the other 3 sides of the building with un-faced R21 batts between the 2X6, 24" OC studs, and lesser OCs in some places. This will get a vapor-barrier of 6 mil sheet plastic between insulation and the sheet rock. (I was going to put plywood backing under the drywall, but I'm already over budget! The reserve is past history!)

The into- hillside, 8' concrete retaining wall opposite the doors-end was moisture sealed by the concrete contractor, and outside faced with 2 inch closed cell styrofoam, which I again covered with sheet plastic. Just for luck, I installed a French drain at footing level before back-filling that end with good sand/gravel pit run. That took me a couple weeks last summer, what with water/mechanical compacting every foot or so, and building concrete stack-walls off the end of the retaining wall, back into the hillside both sides at the same time. It ain't gong nowhere, and again ground moisture infiltration should be taken care of. Even (IF) the planned, heated floor greenhouse eventually gets put in on that end of the building (lawn level outside, with entry door into into same-level loft area of the garage).

The interior of the concrete retaining wall I furred in with 2X4, 2' OC or less and will get 3 1/2 inch unfaced batt insulation/ 6 mill sheet plastic vapor barrier as well. Above that on the frame only part, will go the R21 batts with sheet plastic VB as on the side-walls.

OK - back to the door end. Question is, will sheet plastic vapor barrier between the plywood and the sheet- rock cause rot problems in the plywood or studs? I shouldn't think so, and would be my preferred solution, but I can live with the VB paint over the drywall if necessary. Or maybe both?

This, by the way, is not in Arctic Kotzebue, but on permafrost-free/ milder climate Kenai Peninsula.

DO NOT double vapor barrier! If it were mine, and I've done a lot of these (In lower 48), I would want the vapor barrier to the inside of the insulation. I would pull down the plywood, put up the vapor barrier sealing any penetrations or holes in the barrier with Tyvec Tape. Then put up the plywood and drywall. No VB paint.

Kraft. It's not just for sandwiches.....

Why would you want both the plywood and the drywall on the wet side of the barrier? Great conditions to grow mold.
Why would you use Tyvek tape, which is not a vapor block, on a vapor barrier? Tyvek is specifically designed to allow moisture vapor to pass through.

You didn't mention...E

Is removing the plywood and installing a proper vapour barrier not an option? If it took all day to remove and install, its worth it.

Yeah, I thought of that. Lot of work! And probably wasted plywood that would need replacing. If contractors would use screws instead of nail guns with galvanized nails (or do it right to start with) it would make life a lot easier.

The vapor barrier will of course go on the inside.

I'm just wondering with the plywood between it and the insulation, if the plywood would soak up water from the outside ambient air and eventually rot. I don't see why it would, anymore than the studs do. I'd prefer the plywood to be inside the vapor barrier also, but that probably isn't going to happen unless I get adamant negative responses, not just from 24hr, but several local building sources.

I have a few days to think about it. The salmon are calling my name ("dip", "dip" - I'm assuming they are talking about the net.... ). Likely go get 10 or 12 on tomorrow morning's tide (the week-enders will be gone, and the after-worker folks not arrived), then process both the fresh and the 20 lbs or so left from last year (will brine, smoke, and can those)

Think of it like this....."conditioned" and "un-conditioned". In a perfect world, your insulation separates the two. Limiting condensation caused by sweating. The vapor barrier always goes on the conditioned side of the insulation. Think about "craft-faced" Owens Corning fiberglass insulation. The paper or "Craft-face" goes toward the room.
Any punctures or holes in the VB is a potential for moisture problems. I was taught to be anal when sealing any and all holes/penetrations in the VB. I mentioned Tyvek tape because it's common. There's a bunch more. The tape is plastic, water can't penetrate. Hope I helped.

las
If its that big of deal then use your 6mil poly on the plywood

tuck tape all joints and be certain there aren't any holes

make sure you acoustical seal the bottom edge of the poly

seal all edges if you have to, then drywall, it will be fine.

Being it's your shop and not living space, there should be far less vapor produced inside the shop than a home. I'd be more concerned with ventilation for grinding dust, welding fumes, etc.

VB with poly (sealing holes, seams, and edges) and go on with life.

Help a slow learner who is lurking his way through this thread for future use on my own barn:

What is "tuck tape"? (or tuck taping, I don't know if it's a material or a method.)

What is "acoustical seal"?

TIA !!

Tuck tape....its red and is commonly used by insulators, it sticks like a bugger works good.

Acoustical seal.... is similiar to silicone, comes in a tube and needs a caulking gun, it never seems to dry hard and is a real pita if you get it on you, its great if it gos where you want it to and it will provide an exellent seal, which is what you want.

Finally some answers I like! Affirmation, anyway, of what I wanted to do.

I'm better now......

Only 3 fish in 3 hours today. Forgot the commercials were raping the resource today.

And the waders have a leak, ball-level, left side. I'm looking for it. The leak, that is, but for awhile there in that glacial water............