Why Radiant Floors Need Insulation Underneath
Without insulation under the slab, much of the heat from the tubing goes down into the ground instead of up into the room, so you pay to heat the soil. Insulation that holds its R-value keeps that heat working for you year after year.
Built for PEX
Printed 6-inch grid
Staple straight in
6-mil vapor barrier
96 sq ft per run
“My two sons and I were able to install it quickly and easily, and the 6-inch grid lines made laying out and installing the PEX extremely simple.”
Heat That Stays in the Floor
XPS (pink and blue foam board) gets much of its insulating value from blowing agents that escape over the years. EPS insulates with trapped air. In a 15-year below-grade study, EPS kept 94% of its R-value and XPS kept 52%.
Source: EPS Industry Alliance, Technical Bulletin 103 (2008). EPS vs. XPS after 15 years below grade, St. Paul, MN.
R-Value
| 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.
Four Steps to a Radiant Slab
- 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.
Radiant Floor Insulation FAQ
Do I need insulation under a heated (radiant) floor?
Yes. Without insulation under the slab, much of the heat from the tubing goes down into the ground instead of up into the room, so you pay to heat the soil. Lexar's printed 6-inch grid also lays out the PEX, which staples straight into the board.
What PEX spacing does the Lexar grid fit?
The 6-inch grid printed on Lexar lines up 12-inch residential and 18-inch commercial PEX spacing, with no chalk lines, and the PEX staples straight into the board.
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.
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.
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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Find a dealer near you, or send us your project and we’ll get you a price within 24 hours.
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