Basement ceiling insulation

[blz2dwl]

Long time listener, first time caller.

My woodshop is in my basement; a partial basement under half of the house, about 400sqft. When we moved in 2 years ago, the ceiling had batt insulation between the joists and a clear vapor barrier under it. Unfortunately, it was sagging in most places and had plenty of evidence of mouse activity. The ceiling is also only 7'. I'm just 5'7", so I can live with it. But I had to tear the insulation and vapor barrier out, as it was disgusting. The open rafters make it feel much more spacious. And since it's a heated basement, I assumed the insulation was not needed.

Fast forward to winter, and the part of the house over the partial basement is noticeably colder. It appears the insulation is needed after all. But what kind to use to maximize my situation?

I have seen batt insulation in a white poly bag, which would brighten the space and contain the fiberglass, which would be preferable. It might also keep the rodents out, but I am not sure. Other options?

 

[CombatNinja]

If the basement shop is heated to the same approximate temp as the house above, there is no reason the floor it should be colder. Do you keep it heated 24/7 like the house or just turning it on when you want to work down there?

Something is not making sense and it sounds like you might have massive air leakage around the rim joist.

 

[Crazyjake8493]

Something is not making sense and it sounds like you might have massive air leakage around the rim joist.

That'd be my guess. Assuming the basement is heated along with the floor above, there really shouldn't be any basement ceiling insulation. I'd air seal the rim joist with spray foam or silicone, insulate the rim joist bays with some 2" thick PolyIso foam, air seal around any basement windows, and if it's still cold then consider finishing and insulating the basement walls.

First-story floors over a crawlspace should be insulated, but not over a basement. That's just redundant.

 

[nadogail]

Spray foam for the win.

 

[ace10]

Rockwool

 

[The Cobbler]

what temp is your workshop vs the rest of the basement & house?

 

[raco232]

2nd the Rockwool

 

[blz2dwl]

Great points. I probably have not been keeping it as warm as the upstairs. And I don't think it is sealed all that well. When I took the batt insulation out, it left the ends of the bays at the rim joist empty. I stuffed some insulation in the ends, but it's by no means "sealed".

So let's get more specific: can I seal the rim joist and insulate the sill plate area myself? With what, spray foam? From a can or is there a larger DIY kit of some kind?

Thanks, all!

 

[loganb]

Rockwool for the ceiling joists, a bigger spray foam kit for the rim joist area.

 

[blz2dwl]

That'd be my guess. Assuming the basement is heated along with the floor above, there really shouldn't be any basement ceiling insulation. I'd air seal the rim joist with spray foam or silicone, insulate the rim joist bays with some 2" thick PolyIso foam, air seal around any basement windows, and if it's still cold then consider finishing and insulating the basement walls.

First-story floors over a crawlspace should be insulated, but not over a basement. That's just redundant.

Thanks! So seal the joints at the sill plate/rim joist with silicone caulk or the minimal-expanding spray foam and then cut 2" foam board to fit each bay, stood vertically?

 

[patterg2003]

If you are trying to eliminate the cold in the space and it is heated then stopping the cold at the walls and rim joist would more likely be the better solution. Bare concrete walls have next to zero R value. There really is no need for insulation in the ceiling unless the basement space is only heated intermittently. As previously suggested seal where the floor meets the basement and consider installing vapour barrier & insulation to the values recommended for the region where the house is located. Insulating the ceiling will not improve eliminating the cold in the basement and only would serve to make the floor warmer to walk on & reduce noise from the shop activities. If the temperature in the basement can be maintained with improved wall insulation then you could paint the joists & overhead wood white to improve lighting and make the space feel taller. Depending on the codes for your area it may be possible to stud the walls and use spray foam without a vapour barrier. It would give a good insulation value and would make for a perfect air seal. The studs will give a place to attach the drywall to cover the foam insulation that will likely be required by code for fire safety. The advantage of a good level of insulation in the walls will be that it will require a lot less heat input to maintain the temperatures and will give a payback in the energy savings.