Short answer: EPS and XPS foam board are both used under concrete slabs. Your plans or engineer set the foam type, its minimum compressive strength and the R-value. Garages, shops and other heavier slabs call for firmer board. Match what the plans name, then pick the thickness that reaches the R-value.
Why slabs get foam
A slab sitting on cold ground loses heat downward and out through its edges. Foam board under the slab and along the edges slows that loss, keeps floors warmer and is common under heated and radiant floors. Utah enforces the 2021 IECC with state amendments for homes. Utah County and Salt Lake County are in climate zone 5B, Washington County (St. George) is 3B, and some mountain counties are 6B. Your building department will tell you what your slab needs.
What "compressive strength" means
Foam under a slab carries the weight of the concrete and everything on it. Compressive strength tells you how much load a board takes before it squashes a set amount. ASTM C578, the standard for polystyrene insulation board, sorts EPS and XPS into Types and sets a minimum compressive strength for each Type. It is measured in pounds per square inch (psi) in a short-term lab test at 10% deformation or yield.
Plans and specs usually call out a Type or a minimum psi. If yours do, use board that meets it. The lab figure is a minimum for the Type, not a safe long-term working load. Your engineer sizes the foam for the real slab load, so don't pick a Type from the tables alone.
EPS Types: minimum compressive strength
Per the ASTM C578 standard, minimum values.
| ASTM C578 Type | Density, min. (lb/ft³) | Compressive strength, min. (psi) |
|---|---|---|
| Type XI | 0.70 | 5 |
| Type I | 0.90 | 10 |
| Type VIII | 1.15 | 13 |
| Type II | 1.35 | 15 |
| Type IX | 1.80 | 25 |
| Type XIV | 2.40 | 40 |
| Type XV | 3.00 | 60 |
Source: ASTM C578 EPS performance requirements table, EPS Industry Alliance.
XPS Types: minimum compressive strength
Per the ASTM C578 standard, minimum values.
| ASTM C578 Type | Density, min. (lb/ft³) | Compressive strength, min. (psi) |
|---|---|---|
| Type XII | 1.20 | 15 |
| Type X | 1.30 | 15 |
| Type IV | 1.45 | 25 |
| Type VI | 1.80 | 40 |
| Type VII | 2.20 | 60 |
| Type V | 3.00 | 100 |
Source: XPS material standards table, Extruded Polystyrene Foam Association.
Important: these numbers are the published minimums in the public ASTM C578 standard (ASTM C578). They are not Rapid Insulation product claims, ratings or guarantees. Check the label or submittal for the specific board on your job, and follow your plans and engineer.
EPS or XPS under concrete?
Both are rigid polystyrene foam and both have Types from light to very firm. EPS comes in more densities and we cut it to any thickness, so you don't have to stack sheets to hit an R-value. XPS comes in stock thicknesses with a smooth skin. For a side-by-side, read EPS vs XPS under a slab.
R-value and thickness
R-value adds up with thickness: total R is about R per inch times inches. Divide the R-value your plans call for by R per inch and round up.
Typical R-value
| Density | R per inch |
|---|---|
| 1.0 lb EPS | R-3.9 |
| 1.5 lb EPS | R-4.2 |
| 2.0 lb EPS | R-4.4 |
Typical values at 75°F from the Rapid Insulation EPS data sheet (REV004). Ask us for the full sheet.
Example: if your plans call for R-10 under the slab, 10 ÷ 4.4 is about 2.3, so 2-1/2 inches of 2 lb EPS gets you there. More in What is the R-value of 4 inch EPS?
By job
- House slabs and basement floors: follow the plans for Type and R-value; the edges matter as much as the middle.
- Garages and shops: vehicle and equipment loads call for firmer board. Use the Type or psi your plans name.
- Radiant floors: foam under the tubing keeps heat moving up into the slab, not down into the ground.
- Slab edges: vertical board at the perimeter, cut to the slab depth.
- Frost-protected shallow foundations: foam wings and edge board are designed per your plans; build them exactly as drawn.
Install tips
- Lay foam flat on a compacted, level base. Voids under the board are weak spots.
- Put down the vapor retarder your plans call for.
- Butt joints tight, stagger them and tape the seams.
- Protect edges and walk boards during the pour so the foam isn't crushed or shifted.
Quick answers
Can I use EPS under a concrete slab?
Yes. EPS and XPS are both used under slabs. Your plans or engineer set the type or minimum compressive strength; match or exceed it.
What compressive strength does under-slab foam need?
It depends on the slab and its loads, so it comes from your plans or engineer. The ASTM C578 standard lists a minimum compressive strength for each foam Type; plans usually name the Type or the minimum psi.
Is XPS better than EPS under concrete?
Not automatically. Both are polystyrene and both have Types from light to very firm. EPS comes in more densities and can be cut to any thickness; XPS comes in stock thicknesses with a smooth skin.
How thick should foam under a slab be in Utah?
Use the R-value your plans or building department require, divide by the foam's R per inch and round up. Utah County and Salt Lake County are in climate zone 5B and enforce the 2021 IECC with state amendments for homes.
Do I still need a vapor retarder with foam under a slab?
Usually yes. Lay the vapor retarder your plans call for. Foam board is not a substitute unless your plans say so.
Get it cut
Send us your slab layout or plans and we'll cut EPS to the thickness you need. Cut and fabricated in American Fork, Utah. See foam board insulation, EPS foam sheets, XPS foam board or buy standard sheets online.
Sources: ASTM C578; EPS Industry Alliance, ASTM C578 EPS table; XPSA, XPS material standards; Rapid Insulation EPS data sheet REV004 (Typical R-value); U.S. DOE Building Energy Codes Program, Utah; IECC climate zone map, PNNL.