Rockwool and vapor barrier question

[Zachary Hoyt]

I have put R23 rockwool in my new shop's walls, and R30 in the roof. I was going to put up 6 mil plastic on the inside to be a vapor barrier, but a wise neighbor came by today and said he thought I shouldn't. The more I read about this online the more mixed up I get, so I am hoping someone here can point me in the right direction.

I am in a cold climate, about 60 miles from the Canadian border but at a higher elevation, so things that can grow there can't grow here. The workshop will be heated by a wood stove in the winter, and has rough cut 2x6 walls and 2x10 rafters. There are foam rafter vents, soffit vents and a vented ridge on the metal roof. The floor is insulated with 2" of XPS foam on top of the boards, with OSB subfloor on top of that. I am planning to line the walls with 7/16" OSB, and have not yet figured out what if anything to put on the inside of the rafters. The walls have a woven housewrap on the outside of the studs and rough cut board and batten over that. I'll be very grateful for any advice.

[Jim Becker]

Here in the north, it's normal to use a vapor barrier to the inside side of the wall.

[Zachary Hoyt]

Thanks for the help. I'll go ahead as planned, then. I'll be looking forward to seeing the pictures when the crew comes to build your new shop, it should be very exciting.

[Jim Becker]

Thanks for the help. I'll go ahead as planned, then. I'll be looking forward to seeing the pictures when the crew comes to build your new shop, it should be very exciting.

I can't wait! I'm hoping to find a way to do a time lapse without spending any more money, but at worst, there will be many still images taken.

[Steve Rozmiarek]

Here in the north, it's normal to use a vapor barrier to the inside side of the wall.

Plus 1. Vapor barriers are confusing, their use depends on what you are trying to do. In your case it's to stop air infiltration through the insulation. Wool or fiberglass insulation isn't great at stopping drafts, so the vapor barrier is there to do so. This improves the performance of the insulated envelope. The only practical place to put it is between the conditioned space and the insulation. Kraft paper backed fibreglass is designed for the same reason.

There are circumstances where vapor barriers can trap humidity in wall and roof components. Avoiding that is where it gets confusing. Usually those situations are found in a building that was designed to "dry itself" with air movement. Not typical in buildings designed to be insulated. For example, an old barn with lap siding over studs with no sheeting or wrap will have to be treated differently if it gets upgraded to an insulated space.

One caveat, your wood stove will require air coming into the building to burn and send up the chimney. Plan for that so that you aren't forcing it to migrate through the walls or roof. If you don't it could create channels of differential moisture inside of the building components that can build condensation. Better to open the door periodically or crack a window a bit.

Oh, don't leave the vapor barrier exposed anywhere. It's delicate and practically guaranteed to get holes. It's also required to be covered for fire code.

[Paul F Franklin]

Building science and the understanding of how air and moisture move into and through wall assemblies has evolved a lot over the last two decades, and practices that were (and often still are) common are no longer considered best practices. IMO, one of the best and most reliable sources of current info on the topic is the Green Building Advisor site. IIWM, I'd start there.

[Malcolm McLeod]

Building science and the understanding of how air and moisture move into and through wall assemblies has evolved a lot over the last two decades, and practices that were (and often still are) common are no longer considered best practices. IMO, one of the best and most reliable sources of current info on the topic is the Green Building Advisor site. IIWM, I'd start there.

I have not visited this 'GBA' site, but there is some interesting info available via a search for 'hot roof design'. Of note is how much air leakage occurs thru a typical vapor barrier after dry wall is hung. Can lights, electrical boxes, and any screws thru the ceiling/wall board effectively wreck a vapor barrier.

[Bill Dufour]

They no longer seem to sell fiber insulation with foil backing. theory is foil has to have an airspace or it will not reflect the heat.
Bill D

[Jim Becker]

They no longer seem to sell fiber insulation with foil backing. theory is foil has to have an airspace or it will not reflect the heat.
Bill D

I used foil faced fiberglass in the ceiling of my old shop. I did have to order it and used it primarily to reflect heat back into the shop in the colder months. It was subsequently covered by acoustic tile. One of the unfortunate things is that many folks thought they could leave foil faced insulation exposed which is not true and since a lot of it was being installed by homeowners.

[Bill Dufour]

I have not seen any real studies but many respectable sites say the foil is useless if it is touching the drywall or, in Jims case, the ceiling tile.
Bill D

[Jim Becker]

Bill, there are a lot of radiant products currently used in the building trades that "touch" other building materials, such as sheathing with a foil radiant barrier on the inside surface.

[Kris Cook]

One caveat, your wood stove will require air coming into the building to burn and send up the chimney. Plan for that so that you aren't forcing it to migrate through the walls or roof. If you don't it could create channels of differential moisture inside of the building components that can build condensation. Better to open the door periodically or crack a window a bit.

I am heating with a wood stove in my new shop. I did get my mini split hooked up finally but will rely on the wood stove when it gets really cold. I added an intake kit to my stove which fortunately had a 4" port. I used an aluminum blast gate to control the air flow from outside.

[Jim Becker]

Kris, that makes sense for your geography because you generally have a heavy heating season with temps that are very often below the range for even the best minisplits (which are heat pumps at heart) to be able to pull heat out of the cold air. The split should cover you well for spring/fall and summer for sure.

[Tom Bender]

Hi Zachary
Congrats on the new shop.

Yes a vapor barrier is a good thing, it can keep the insulation from getting saturated and frozen.

OSB walls and ceiling might be a gloomy space. An option might be foil backed drywall which provides the vapor barrier. You would probably need to add 4' high paneling to toughen the walls and you would need to be more careful how you hang things on the walls. I have very little hung and it makes the space bigger.

[Zachary Hoyt]

Thank you all for your advice. Yes, OSB floor, walls and ceiling will be a bit gloomy, but I have 12 LED light bulbs out there so I can see pretty well. Right now it's a mix of Rockwool, plastic and OSB depending where you look, so all OSB should be about the same. I may paint the OSB at some point, it seems like it would be a good idea but from what I understand it takes a lot of paint to soak into the pores. I am guessing that I wouldn't do any painting till next spring or summer, and by then I'll have had time to see if it feels necessary. I have a lot of things to hang on the walls, so OSB will be handier for that, and it's less prone to damage from being bumped into with things. Right now I'm just using the shop to prepare trim boards for the house, so it's a lot of planing, table saw work etc, but the workflow will be much different once I start building instruments again. For a while I'll be doing both, which will be messy, but that's life.