Insulation for basement shop

[Michael Ginsberg]

My soon to be shop which is now bone-dry due to a trench-sump pump system, I think needs to be insulated before I finish with the sheetrock. I was planning on putting a plastic vapor barier on the walls before the studs. I have a choice between pink or purple foam (which needs to be cut to 16 inches), or conventional fiberglass. The basement does not get very cold (or hot). The shop space is 30' x 30', Which insulation would you suggest, and why? Perhaps skip it altogether. Also, the shop will be ducted with heat and AC when needed and will have a dehumidifier.
Thanks for the suggestions.

[Thomas Bank]

I wouldn't use fiberglass in a basement - too much potential for mold to grow in it even with the vapor barrier. I'd stick with rigid foam and another layer of plastic over the rigid foam and studs before the sheetrock. That will keep any humidity in the basement from getting into the wall as well.

And you want a closed cell foam to further limit any chance of moisture in the wall. Not quite sure what the purple foam is - around here I see pink or blue. But ask for closed cell and you should be ok.

[Jim Andrew]

Closed cell styrofoam doesn't absorb moisture, so it is the vapor barrier. You didn't mention what kind of walls your basement is built of, but too bad you have to use studs, you will use up some space. Your foam will glue directly to concrete, if you use the proper glue, careful of construction adhesive, as it disolves foam.

[Michael Ginsberg]

Thanks guys. I want to use sheetrock to give it a painted finished look. The walls are poured concrete. The purple foam is thicker than the other colors. I am also wondering what R-value I need. Do I rip the foam on the table saw to width, or make several passes with a knife? What adhesive do you suggest?

[Jim Matthews]

This is my idea for cutting rigid foam board...

I stole it from FHB, so now it's MY idea. This really works.
You need a curved taping knife, the thickness of the steel and curved profile make this work so well.

[Gary Pennington]

We're in Maine, near the coast so a little north of you. Part of our basement is my shop, in the winter it's usually 50 degrees for most of the winter and very dry. I had to turn on the humidifier a few weeks after turning off the dehumidifier from the summer. This fall we had the rim joists insulated with spray foam by an insulation contractor. It's made a phenomenal difference in shop comfort, the temp has hovered around 60- 62 all winter and there's been minimal need for humidification. I try to keep the humidity in the 30-40% range year round. Past years the humidifier has run all winter on high speed, this winter I ran it on low and it had no problem keeping up.
We have a perimeter roughly 180 feet long, the cost was $860. I had to give them 3 feet clear around the entire perimeter and cover everything. I also removed the existing fiberglass but they disposed of it for me. I believe that was the best $860 I've spent in quite some time.

[Craig Michael]

1 1/2" foam would be fine. Get the ones that have the grooved edges so they seal well. Don't use plastic between that and the studs, the foam is a moisture barrier and having a second isn't a good idea. Use pressure treated for the bottom sill of your walls and use a foam sill sealer under it to keep it off the concrete. Before you put up anything, check the outside walls and seal up any air gaps with cans of foam. Also make sure you don't close up any silcock shutoffs, etc. And check the insulation in the rim joists and make sure that is sound, fix it if not.

What will really make the space feel much warming is using a product like dri core on the floor. It works well.

[Thomas Bank]

Closed cell styrofoam doesn't absorb moisture, so it is the vapor barrier.

The issue with adding vapor barriers on either side of the foam is that you typically do not get a full seal along the studs and then get moisture travel around the insulation. Vapor barriers on either side of the foam is possibly overkill. But I'd certainly want it on the concrete wall side and another layer under the sheetrock is cheap insurance.

For cutting foam, score it with a utility knife and snap it just like you do with sheetrock. Trying to cut it with a tablesaw is going to result in particles of it everywhere - particularly because it will cling to everything by static electricity!

[Jim Falsetti]

Building Science Digest has an excellent writeup on insulating basement walls here. As others have noted, fiberglass is not recommended. Also, it doesn't appear a separate plastic vapor barrier is recommended.

If you contact the manufacturer of the foam boards they will most likely recommend a suitable glue product for their material.

Jim

[Will Winder]

I used rigid foam up against the foundation, framed over that, then special ordered some "Stone Wool" insulation at the big box store instead of fiberglass.

[Dan Hintz]

I DryLok the walls/floor, then put rigid foam over the top.

[Jim Andrew]

Your building center should know what adhesive works, but be sure to read the instructions, because some sales people just don't have a clue.

[Craig Michael]

I used rigid foam up against the foundation, framed over that, then special ordered some "Stone Wool" insulation at the big box store instead of fiberglass.

Why would you insulate between the studs? The rigid serves several purposes, it's a vapor barrier, it's very good insulation with a high R value, air seal, etc. If you do 1 1/2" or 2" rigid, that is plenty of insulation and since the basement is below grade it's even better. Putting anything between the studs is a waste and will make no difference at all.

[Will Winder]

Why would you insulate between the studs? The rigid serves several purposes, it's a vapor barrier, it's very good insulation with a high R value, air seal, etc. If you do 1 1/2" or 2" rigid, that is plenty of insulation and since the basement is below grade it's even better. Putting anything between the studs is a waste and will make no difference at all.

I don't claim to be an expert, but from my research it seemed like no two people do basement insulation the same way. If it wasn't a shop I would have used thick foam and no framing. For my shop I liked the idea of doubling up two types of insulation, with thin XPS excess moisture can slowly get out and shouldn't bother the stone wool.

I only left a 1" gap between the wall and the framing and decided to put some rigid foam in the gap. Then I read about the fire concerns regarding XPS since I was using OSB instead of drywall so I added the fire resistant stone wool. Maybe next time I'll drop the XPS and just use the stone wool (cheaper, easier, and lets me build the wall thinner).

The building science document here seems to suggest the way I went as an option:

Walls with 3.5 inches of extruded polystyrene (XPS) and no
vapor barrier performed the best in this analysis. However,
walls with 0.75 inches of extruded polystyrene and 3.5 inches of
fiberglass batt insulation in the cavity would perform well as
long as interior humidity was controlled below 50 percent
during the summer. Increasing the extruded polystyrene to 1.0
or 1.5 inches would improve performance even with higher
interior relative humidity during the summer months.

[Doug McClentic]

Vapor barriersshould only be put on the warm side of insulation. This is because as moisturemoves through the wall from warm tocold it condenses and will damage insulation. It is true that CLOSED cell foaminsulation will act as a vapor barrier but you have to have an air tight seal aroundthe foam to make that happen. The ideas of attaching it to the concrete walland then framing inside of it are good. This will create a complete thermal break between the source of the cold and your shop. You would still have to seal around allthe edges etc. to make to completethe vapor barrier. Open cell foam will not be able to be used as a vaporbarrier.

In a basement situation I would not use typical drywall / gypsum board. It is amold magnet and will be very tough to keep the humidity low enough to preventmold growth especially in a closed area like the walls against concrete. Iwould use mold resistant drywall instead. Make sure to pressure treated wood atall contact points with concrete and insulate a couple feet around the perimeterof the floor at the rim joists.
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