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Insulation Is Boring, Except For One Thing

I’ve been struggling with putting together this post on XPS insulation.  I think for two reasons.  One, XPS is not the insulation I would recommend as the go-to insulation for an underslab application, especially if you live in termite or carpenter ant country.  Two, how do I write about underslab insulation and keep it interesting?  It’s a tall order, because, frankly, insulation is boring.  Then I started thinking about why I get excited about it.  It’s because of what it can do as part of the bigger picture.  It’s key to saving energy and staying comfortable.  Thinking about those two things, I find insulation, in all it’s varieties strangely interesting.

Selecting the right insulation is just a matter of being informed and I hope I can help by sharing a little of what I know.

So, rounding the corner on the third and last type of underslab insulation let’s talk about XPS.  Without getting into the details of how XPS is made (see wikipedia for that), XPS is an acronym for eXtruded PolyStyrene closed cell foam insulation.  Not to be confused with Expanded PolyStyrene insulation or EPS.  It’s hard to keep track of which is which because they both start with “ex” and end with PolyStyrene.

XPS is probably the most commonly specified underslab insulation, primarily because it has good waterproof qualities.

Like foamglas and EPS, it too has it’s pros and cons.

Pros and Cons (in no particular order):

XPS is waterproof.  

If insulation becomes wet, it loses some of it’s insulating ability.  The tests show that XPS absorbs virtually no water.  That’s a good thing in an underslab application.

XPS is not the least expensive insulation.

XPS is less expensive than foamglass, and more expensive than EPS.

XPS comes in colors.

XPS is easy to identify because it is blue or pink and sometimes green. The color is just a thing that manufacturers add to differentiate between brands.

XPS is a good insulator.

XPS is a good insulator, with an average R-value per inch of 5.

XPS is designed for use under slabs. 

XPS is designed for use under slabs. That means the manufacturers will warranty it.

XPS provides a good place for insects to nest. 

XPS provides a good place for insects to nest. If you are in termite or carpenter ant country you need to think twice about using XPS. The termites don’t eat the insulation, but they bore holes in it, reducing the effective R-value.

XPS is strong.  

It is available in densities that can support loads like concrete.

XPS is flammable.

The building code requires that foam be protected with a non combustible barrier to keep it from burning.  In an underslab application the concrete provides a good barrier between you and the possibility of fire.  It is more of a concern in walls.

XPS has added fire retardents.

Blame the building code for this.  The manufacturers have to add it.  The popular one to add is Hexabromocyclododecane and no one has anything good to say about it.

XPS shrinks

XPS insulation has shown some evidence that it shrinks over time.  Little gaps opening between boards reduces the effective R-value.

XPS is not a good friend to the planet.

XPS is known for having a very high (huge is more like it) Global Warming Potential because of the gasses used for blowing agents.  This is the big reason I would look for other alternatives before specifying XPS.  The good news is there are alternatives now that have much lower GWP’s.  The bad news is that XPS manufacturers haven’t adopted them yet.  Hopefully, that changes soon as more people start to ask about it.


Insulating well can be a sincere expression of love. It not only saves you and your loved ones money, does its bit to save the planet, but maybe most importantly insulation keeps those you love warm and comfortable.