How to install foam insulation board on a concrete wall

I’ll be installing 2 inch Thermax to the inside of my poured concrete foundation. I don’t yet own a hammer drill, but I’m looking for advise on the best way to attach this material to the wall. I’ve read about glue only, plastic fasteners, metal fasteners, and strapping. Which are the easiest to install and will last the life of the house?

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As I've covered extensively in my waterproofing section, basements are cool, damp places. You can't just throw any ole insulation up. You need to be aware of your options, and have a game plan going into your finishing project.

Your choice really comes down to blanket insulation (aka fiberglass) or XPS.  Most homes use fiberglass, but rigid foam insulation, although more costly offers a few extra features. Let's take a look.

Blanket Insulation: This is very common mainly because it is very cost effective. Usually made of fiberglass and it typically has some type of facing attached: paper, foil, or vinyl. This was the option of choice in Jason's basement and I wouldn't discourage anyone from going this route.  

[NOTE from Jason: The builder for my house had pre-installed my fiberglass insulation so the choice was pretty easy for me. Living in the moderately warm climate of Virginia and not having any obvious moisture concerns and a limited budget I opted to just use what was there.  Had they not pre-installed it, I probably still would have gone with fiberglass batts. ]

XPS - a.k.a Extruded Polystyrene - a.k.a Rigid Foam Board: This is what I installed, and the topic of this article. Boards come of varying thickness up to 2in.

The board is more expensive than blanket insulation. Average cost for a 4'x8'x2'' sheet was $30/sheet. My typically sized basement took about 35 sheets. That adds up to a cool Gee! Obviously a good chunk of change. By comparison, drywall for that same basement was $400.

Other: There are other forms of insulation like loose-fill which is usually blown in or the sprayed in foam type. The trick there is to make sure you have the proper density to ensure adequate thermal capacity. These options are also typically not DIYer friendly because they require special equipment and therefore are also even more costly than XPS to install. 

XPS Benefits:

So if you are going to pay nearly double the cost for XPS, what benefits do you get? I argue peace of mind. I've contended that at some point, every basement is susceptible to water. Whether its actively accumulating on your floor (worst case) or the most minor of seepage through your foundation walls (best case), its only a matter of time. Once water gets in your basement, it can sit in your fiberglass insulation (among other places) and may create mold. 

XPS is extremely water resistant and subsequently mold resistant. Unlike the fiberglass insulation, it will not hold the water needed for mold to develop and spread. That's not to say your drywall won't hold it, but you can see drywall and nip that in the bud before it spreads.

Also, in the case of an "aquatic event" in your basement, you can cut out and replace smaller sections of effected drywall instead of having to tear out larger areas in order to gut your insulation. How easily can you see your insulation? Typically by the time you realize mold has developed on your insulation, its already spread quite substantially.

Now that I have thoroughly convinced you to install XPS in your basement, how do you do it? It's quite easy actually...

Installing rigid foam insulation

Allow me to break down the major components.

Prep Work - I've talked about basement waterproofing paint before. I recommend you do this before XPS installation for added peace of mind. I had no problems applying the XPS to my painted walls. Aside from any other wall treatments you want to do (I highly recommend reviewing my interior waterproofing article) the prep work basically involves making sure you have a clean dry surface. Take a shop vac and wet towel to your walls to get rid of all those cob webs!

Tools/Materials Needed:

Box Cutter - Used to score the XPS

Drywall Saw - Not 100% required but I found that with 2'' thick XPS, the box cutter did not sufficiently score the XPS to be able to snap pieces off. It did OK when scoring along the 4' lengths, but not to well along the 8'. If you don't have one, this Stanley is under $10 and gets great reviews. You'll need it when installing or repairing drywall anyway

Caulk Gun - Another frequently used tool on non-basement finishing projects. You'll need this to apply the adhesive

Foam Board Adhesive - You can get this at your local HD, s

XPS - Duh! I had to make multiple trips to HD to get the 30+ sheets home. Not because of the weight, but the size is awkward to transport. I had to strap them to the roof of our Pilot. Driving with 10 sheets at a time looked silly enough - couldn't imagine doing it with 30. Plus I spread the purchase over 2 credit card billing cycles to help ease the financial burden. 

Application Tips -

1. Don't be stingy with the adhesive. Apply liberally - specifically in the corners and along edges of a piece.

2. Use something to hold the boards in place while the adhesive cures. I had all my framing lumber already in the basement so I just used that. You don't need a tremendous amount of pressure - just something to keep the board in place and firmly up against the wall.

Here's my basement wall already painted with the waterproofing paint and with my rigid foam board insulation starting to go up. Note: the pink panther guy is there, so you know it's good.

3. Like drywall, install full pieces covering windows and later once the adhesive is fully cured, come back and cut out your openings with the drywall saw.

4. Try to keep the seams tight. Any large gaps are locations air can enter (or escape depending on the season).

5. Finish the job with some tyvek tape across the joints. Larger gaps (where unavoidable) can be filled with expandable foam. 

Simple right? This job is certainly low on the DIYer difficulty scale. Pretty low on time commitment too. Certainly a bit higher on cost, but if your budget allows it, its provides an excellent insulating option and peace of mind. If you have any questions, please leave them in the comments below!

- Adam