I have a detached garage with vinyl siding over plywood, no house wrap or insulation. I want to insulate the inside and install drywall. Do I need to just install poly on the inside (warm side) or do I have to pull off the vinyl siding and install house wrap between the siding and plywood?  Thanks Dan in Ontario

Your cold climate wound dictate the use of an interior vapor barrier, so install the poly just under the sheetrock.

Re the exterior, the purpose of housewrap is to act as a secondary weather barrier - the vinyl siding being the primary. Please be aware that vinyl siding leaks - it is designed to leak. So the housewrap, or other building paper is designed to protect the sheathing while still allowing some drying to the exterior. It may be that your situation now is that there is some leakage - the sheathing is getting wet - but it has an opportunity to dry both to the outside and largely to the inside since the sheathing is exposed there. If you install the interior vapor barrier you no longer have drying of the wall assemble to the interior - only to the exterior. Further, depending on what kind of insulation you use it could act as a reservoir to store moisture slowing the drying process even more. This could result in accelerating the rot of the sheathing and possibly a moisture problem - mold/mildew.

You didn't really say if you plan to heat the structure. If so, and you decide to pull the siding, you may want to cover the building with a rigid foam and tape the seams - rather than using housewrap. This will serve the dual purpose of helping to protect sheathing from exterior moisture and providing some additional insulation and a thermal break to decouple thermal transfer through the wall. If you were to do this I'd recommend using the rigid foam that has the highest perm value you can find so as to allow the building the ability to breath. or again, dry to the exterior.

If you were to remove the siding and install housewrap only, you would likely be able to reuse the vinyl siding provided you were careful during removal. It helps to do it when it is warm as plastic is brittle when cold. Removal of the doors and windows would likely be necessary so as to see that they are flashed properly; the housewrap should really go under these and additional flashing is required - I'm assuming there is none now. If you build up the thickness of the wall with rigid foam you would probably need to R&R the doors and windows so as to get the sheathing under those, and again, get it flashed properly. The vinyl siding would not be able to be reused as the structure is now (slightly) larger. The good news is that a lot of the time that goes into a vinyl siding job is the trim around the soffits and fascia - this part would likely not have to be re-done.

You thought this was going to be easy.... but you had to ask.... :-)

Why are you inclined to add insulation and drywall? Are you planning to heat the space, if only now and then for projects? Is the garage for parking a car or something else?

The plywood provides some measure of air barrier, but assuming the seams were not sealed then the garage likely leaks a lot of air through it. Garage doors typically don't seal well either at the edges and panel joints. If you pull a wet or snow covered vehicle into the garage, you will introduce moisture. You could get by inexpensively by using faced fiberglass batts, a low-end insulation, with the facing edges stapled to the framing. That ought to provide sufficient vapor retarder to avoid condensation issues under the plywood.

If you are planning on heating the space and using it for other purposes, you'll have to explain before we can advise accordingly.

When I lived in SE Idaho at 6,000 feet elevation where we would get 4 feet of snow on the ground and -20 in the winter, I woul back my car out of the garage (not at -20) and hook the hose to the hot water faucet and wash it outside then bring it in once it was clean. The hot water would eveporate before it could freeze. My garage was in a daylight basement so it wasn't extremely cold.

PS: I hate sheetrock in a garage, but that seems to be the trend now days. Sheet rock is cheap and the ladies like it