How Fanfold Works
Lexar fanfold ships folded into a 4x4 cube and opens right over your grade to a 4x24 run: 96 square feet in one piece, three times a 4x8 sheet. The folds every 4 feet help it lie flat.
96 sq ft per run
A third of the end joints
25 runs vs. 75 sheets
About 46% faster
What Fanfold Insulation Is Used For
Lexar fanfold is made for below grade, where it has to hold its R-value in wet, buried conditions:
- Under concrete slabs
- Radiant (in-floor heat) slabs
- Foundation perimeters
- Frost protection
- Walks and drives
- Cold storage
See the under-slab insulation guide for how to choose insulation for a slab.
Fanfold Insulation R-Value
R-value depends on the foam and the thickness. Lexar is EPS, which insulates with trapped air, so it has no blowing agent to lose over time. Per inch, it’s about R-4.2 at 75°F and gets better as it gets colder. At the most common thickness, 2 inches, that’s about R-8.4 at 75°F.
| 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.
Is Fanfold Insulation a Vapor Barrier?
Lexar is. It 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. A 6-inch grid is printed on the face to lay out PEX, rebar or mesh and to give you a straight line to cut on.
EPS vs. XPS Fanfold
Fanfold comes in different foams. 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. 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.
Fanfold Insulation FAQ
What is fanfold insulation?
Fanfold insulation is rigid foam made as one long run that folds back and forth on itself, like an accordion, so it ships as a compact bundle and unfolds into one big piece on the job. Lexar fanfold opens to a 4x24 run that covers 96 square feet, three times a 4x8 sheet.
What is fanfold insulation used for?
Lexar fanfold is made for below grade: under concrete slabs, under radiant (in-floor heat) slabs, foundation perimeters, frost protection, walks and drives, and cold storage.
What is the R-value of fanfold insulation?
It depends on the foam and the thickness. Lexar EPS fanfold is about R-4.2 per inch at 75°F (R-4.4 for 60 PSI) and R-4.8 to R-5.0 per inch at 25°F, from the Lexar spec sheet. 2 inches is the most common thickness, which works out to about R-8.4 at 75°F.
Is fanfold insulation a vapor barrier?
Lexar is. It has a 6-mil vapor barrier built in: 3-mil poly film laminated to each face. Tape the seams and there is no separate layer of plastic to roll out underneath.
Why fanfold instead of 4x8 sheets?
Speed and fewer seams. Each 4x24 run covers 96 square feet, 3x a 4x8 sheet, so you handle a third as many pieces and tape a third as many end joints. It ships folded into a 4x4 cube and opens right over your grade, and the folds every 4 feet help it lie flat.
How much does one sheet of Lexar cover?
Each Lexar fanfold run is 4 feet by 24 feet, or 96 square feet: three times a 4x8 sheet. A 40x60 slab takes 25 Lexar sheets compared with 75 sheets of 4x8, with a third of the end joints to tape.
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.
Where to buy fanfold insulation
Lexar is sold through building suppliers. Find a dealer near you, or send us your project and we’ll get you a price within 24 hours.
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