How should I insulate my workshop

Question

I have a 20x14' workshop in my backyard. When I had it build, I had the installer place rigid insulation form under the concrete slab but everything else was not insulated. Since then, I've installed insulation batts in the walls and a vapour barrier and then drywall on top of that. It is till cold inside since cold air from the outside can enter in through the ridge vent at the top or the 8 soffit vents that line the side. This year my plan is to insulate the ceiling as shown in the middle picture below. There will be an air gap between the roof and the insulation, then another vapour barrier and a radiant barrier on top of everything.

I'm hoping with some space heaters the inside will now remain warm when the outside is cold. I am worried what will happen in the summer though. I don't want the inside to turn into an oven so I'm trying to think what to do to prevent that from happening. I could put an AC unit in one of the windows for the summer months to cool the inside ... but is there another way? I don't want to punch any holes in the walls (through the insulation) because I'm worried birds or critters are going to try to climb into the workshop during the winter months.

I was wondering if I could put some fans or vents in the ceiling insulation to allow hot air to escape out through the soffit vents or the ridge vent.... would that work?

Any suggestions would be greatly appreciated!

Answer 1

Solar load (which heats the building to 95 on a 68 degree day) is a real problem. Insulation will make things better, not worse.

The #1 thing you can do is paint the roof white. Yes, I know that's hard and weird, but it really does work.

Soffit and roof vents, obviously, make it a lot harder to heat a shed. Maybe the architect was concerned with condensation inside your shed, and want to make sure moist warm air from the daytime is cycled out by nightfall.

If you're going to hang a ceiling and have a void between ceiling and roof, it helps get rid of solar heat if you move a lot of air through there. That will carry away the heat that comes through the roof before it reaches the ceiling. Of course this would be unhelpful if you are trying to heat below the ceiling.

Answer 2

Insulating it works both ways - it will be cooler than the outside in hot weather, due to less heat gain from the exterior, if you keep the windows closed when it is hotter outside. When it is cooler outside, open the windows. Hopefully you have, or will add, screens in the window openings.

It will not be 95 inside when it's 68 outside unless you leave the space heaters running, or it's been over 100 during the day, and you did not open the windows when it cooled down in the evening.

Answer 3

The first thing to do is isolate the part of the roof that's ventilated from the interior. There's no point in insulating a space that has huge holes in it which allow exterior air inside.

Your plan seems like a good one, especially the part about the radiant barrier facing the airspace. However, you have it in the wrong place: radiant barriers should face down, not up. The reason for this is if a radiant barrier faces up, eventually dust will settle on it and completely destroy its effectiveness. As a result, you should install or paint a radiant barrier on the underside of your roof decking, facing down into the airspace.

Beyond that, any advice has to be climate-specific. Your choice of temperatures, units, and spelling of "vapour" lead me to believe you're in the UK or Canada--both predominately cold and wet climates with mild humid summers. The reason why this is important to know is because you added a plastic vapor barrier into the wall under the drywall. This is a bit of an error, not necessary in the best case, and harmful in the worst case, because in a wet climate like yours (I am guessing), if you air condition the interior, the poly vapor barrier may become cold enough to be a condensing plane for any moisture that is within the wall itself. These conditions (wet summer, air conditioning, poly vapor barrier) gets you half way to "mold-and-rot-machine" conditions that will exist if additionally the wall sheathing is OSB, if there is stucco or brick veneer applied right over a single layer of tar paper, and if there are no roof overhangs. If this is your wall stack-up, it is imperative that you change one of those conditions, preferably two or more.

The correct choice in this climate would have been a "smart vapor retarder" or nothing at all coupled with rigid foam or mineral wool exterior insulation outboard of the wall sheathing.