Why Insulate Under a Slab
Once the concrete is poured, what’s under it is there for the life of the building. Without insulation, much of the heat in a slab goes down into the ground instead of into the room, and with radiant floor heat you end up paying to heat the soil.
You only do it once, so do it right.
What to Look For
- Made for below grade. Rigid foam that keeps its R-value in wet, buried conditions.
- The right strength. Enough compressive strength (PSI) for the loads on the slab.
- A vapor barrier. Lexar has a 6-mil vapor barrier built in, so there’s no separate plastic to roll out.
- Termite treatment. Lexar is treated with Preventol®. Termiticide is built in and protects your home from termites.
EPS vs. XPS Below Grade
XPS (pink and blue foam board) gets much of its insulating value from blowing agents trapped in the foam, and those gases escape over the years. EPS insulates with trapped air, so there’s nothing to escape. In a 15-year below-grade study, EPS kept 94% of its R-value and XPS kept 52%. XPS also held nearly four times as much water.
Source: EPS Industry Alliance, Technical Bulletin 103 (2008). EPS vs. XPS after 15 years below grade, St. Paul, MN.
Other Options Under a Slab
Polyiso is made for roofs and walls, not for the ground. Like XPS, it relies on blowing gases that escape over time, and its R-value drops as it gets cold, right when you need it under a slab.
Spray foam on gravel under a slab costs significantly more, takes a specialty crew, and comes out uneven in thickness. Rigid EPS gives you a known R-value across the whole pad.
R-Value Under a Slab
Lexar EPS is about R-4.2 per inch at 75°F and R-4.8 to R-5.0 per inch at 25°F. 2 inches is the most common thickness.
| Mean temp | 15 PSI | 25 PSI | 40 PSI | 60 PSI |
|---|---|---|---|---|
| 75°F | 4.20 | 4.20 | 4.20 | 4.40 |
| 40°F | 4.60 | 4.70 | 4.70 | 4.70 |
| 25°F | 4.80 | 4.90 | 5.00 | 5.00 |
R-value per inch of thickness, from the Lexar spec sheet. 2" is the most common thickness; other thicknesses are available.
What PSI Do You Need?
Houses, garages, and most slabs over rebar or mesh.
Shops with tractors or heavy equipment, commercial garages, and engineered jobs.
Industrial loads and cold storage.
On big or heavy-equipment jobs, check with your engineer. Not sure? Ask us and we’ll help you pick.
How Lexar Goes Down
Lexar is fanfold: 4x24 runs that cover 96 square feet each, with a printed 6-inch grid.
- Step 1
Prep your grade like you normally would.
- Step 2
Unfold the runs to cover the pad.
- Step 3
Tape the seams.
- Step 4
Staple your PEX on the printed grid and pour.
Under-Slab Insulation FAQ
What is the best insulation to put under a concrete slab?
Rigid foam made for below-grade use, and one that keeps its R-value in wet, buried conditions. In a 15-year below-grade study by the EPS Industry Alliance, EPS kept 94% of its R-value while XPS (pink and blue foam board) kept 52%. Lexar is EPS made for under the slab: 4x24 fanfold runs with a 6-mil vapor barrier and a 6-inch PEX grid built in.
Why EPS instead of XPS (pink and blue foam board)?
XPS gets much of its insulating value from blowing agents trapped in the foam, and those gases escape over the years. EPS insulates with trapped air, so there's nothing to escape. In a 15-year below-grade study, EPS kept 94% of its R-value and XPS kept 52%. XPS also held nearly four times as much water.
What PSI do I need?
Houses, garages, and most slabs over rebar or mesh use 15 PSI. Shops with tractors or heavy equipment, commercial garages, and engineered jobs step up to 25 or 40 PSI. 60 PSI is for industrial loads and cold storage. On big or heavy-equipment jobs, check with your engineer. Not sure? Ask us and we'll help you pick.
Is it a vapor barrier?
Yes. Lexar has a 6-mil vapor barrier built in: 3-mil poly film laminated to each face. Tape the seams and there's no separate layer of plastic to roll out underneath.
Is it treated for termites?
Yes. Lexar is treated with Preventol®. Termiticide is built in and protects your home from termites.
What R-value is Lexar?
About R-4.2 per inch at 75°F (R-4.4 for 60 PSI), rising to R-4.6 to R-5.0 per inch at colder temperatures. 2 inches is the most common thickness. Lexar comes in 15, 25, 40 and 60 PSI.
Where can I buy Lexar?
Through our network of building suppliers across the country. Request a quote and the Lexar team will get you a price and connect you with the right dealer, or call one of the dealers on the map yourself.
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