Menu Close

EPS Foam For Under Slab Insulation: Pros And Cons

When it comes to insulation under the slab, foam is the most commonly used material. In my last post I talked about foamglass as an alternative to foam. Today, it is about the go-to material for most under slab applications: foam. There are two foams commonly used for sub slab insulation, Expanded PolyStyrene (EPS) and Extruded PolyStyrene (XPS). EPS today, XPS for a later date.

EPS is a closed cell foam and recognizable by it’s white color. Yes, it’s the same stuff those takeout containers and coffee cups are made of.

The process of making EPS requires energy, but doesn’t require CFC’s or HCFC’s which are greenhouse gases that contribute to global warming. Overall, EPS is a fairly friendly environmental choice. The energy saved by using it far outweighs the energy used to produce it.

EPS has an R-value of about 4 per inch, it is widely available and is the least expensive foam option. It comes in various densities and thicknesses and is advertised for use under slabs. Like all foams it is lightweight and easy to handle. It does tend to abrade easily along the edges, so care must be taken when installing it.

While foam is the default choice for underslab insulation it’s not all roses. Let’s just say that there are some issues.

Issue #1 – Moisture Absorption:

In an under slab application, it is possible for foam to absorb moisture. If foam absorbs moisture, it loses R-value. So, a foam that is R-4 when dry might only be R-2 when wet.

There is testing data for moisture absorption, but there is some question about how reliable it is because it is a short term test, not a long term test as would be found in the real world. Suffice it to say that it is likely EPS will absorb moisture if it is there to be absorbed, but the jury is out on exactly how much. There is some argument that EPS has an ability to drain moisture because of its cell structure, but I haven’t seen a test that quantifies that.

Denser foam tends to have better moisture absorption ratings.  Interestingly, some manufacturers add facers which leads to significantly better moisture absorption ratings than straight up EPS and they have the testing information to back it up.  In any case, foam should be installed so that any potential moisture is managed.  Think base course that provides a capillary break or even sub slab drainage for extreme situations.

Issue #2 – Flame Retardants

EPS is flammable. Because of this, the law requires that manufacturers add flame retardents. The commonly used retardent Hexabromocyclododecane HBCD has “potential reproductive, developmental, and neurological effects”,  so says the EPA.  As a halogenated flame retardant it is on the RED list.  A UN group of chemical experts has recommended that it be eliminated from the global marketplace to protect human and environmental health.  Basically, nobody has anything good to say about it.

Change is in the works and the EPA is looking at rules to eliminate or limit the use of HBCD. It’s not happening tomorrow though, so in the meantime beware.

Issue #3: Insects

They don’t eat it, but they like to nest in it and tunnel through it. They, being termites and carpenter ants. Inviting them to stay in your sub slab insulation is just an invitation to come into the house if they can find a way in.

Some EPS manufacturers add borate to their foam. This helps. Flashing and other insect deterring strategies will also help.

The building code is explicit about foam and termites.  If a building is in an area with a probability of very heavy termite infestation you can’t put foam under a slab, unless your local building official says OK to an “approved method”.  I’m not sure what the “approved method” is, but would guess that it might be the addition of an insect resistant chemical to the foam.  It’s worth talking to the local building official about that one.

2012 International Residential Building Code

R318.4 Foam plastic protection. 
In areas where the probability of termite infestation is “very heavy” as indicated in Figure R301.2(6), extruded and expanded polystyrene, polyisocyanurate and other foam plastics shall not be installed on the exterior face or under interior or exterior foundation walls or slab foundations located below grade. The clearance between foam plastics installed above grade and exposed earth shall be at least 6 inches (152 mm).


1. Buildings where the structural members of walls, floors, ceilings and roofs are entirely of noncombustible materials or pressure-preservative-treated wood.
2. When in addition to the requirements of Section R318.1, an approvedmethod of protecting the foam plastic and structure from subterranean termite damage is used.
3. On the interior side of basement walls.

Issue #4: Shrinkage

EPS Foam Shrinks. This is a problem because the joints between foam can potentially open up over time. That little gap then becomes a path for heat to travel, diminishing the R-value.

Expanding joint sealant can be installed to combat this issue, but it is important to make sure the sealant is approved by the manufacturer because certain chemicals can melt EPS, and that would be a bummer mistake. Some manufacturers design a tongue and groove system to help address this issue. Installing the foam in layers and staggering the joints will also help, but this should be reviewed with the structural engineer.

EPS is used all the time for underslab applications and the benefits are numerous.

As with anything, it makes sense to understand what the issues are and how to address them so that they don’t become problems. There may be some change in the near future with the flame retardent HBCD and if that goes away, I think the other issues can be managed so that EPS foam is a viable option for under slab insulation in a high performance modern home.