Rigid Foam Insulation Calculator
Calculate 4×8 panel count, total R-value, and material cost for EPS Type I & II, XPS, and Polyiso rigid foam insulation.
Rigid Foam / Radiant Panels
Under-slab and wall rigid insulation panels
Inputs
Standard panel: 4' × 8' = 32 sq ft
4'×8' panel
Results
How to Use the Rigid Foam Calculator
Enter the total area to insulate, select insulation type, and enter the thickness. The calculator returns total panel count (4×8 ft sheets), total R-value achieved, and optional material cost. Adjust the waste percentage for cutting and irregular areas.
How It Calculates
R-value = Thickness (in) × R per inch (by type)
EPS-I: 3.6/in · EPS-II: 3.8/in · XPS: 5.0/in · Polyiso: 6.5/in
What to Know Before You Calculate
- Product R-values vary: R-values above are typical nominal values. Actual labeled R-value on the product label governs — always verify before specification.
- Compressive strength matters under slabs: Slabs require rigid foam with at least 25 psi compressive strength (XPS or EPS Type II minimum).
- Two layers vs. one: For high R-values, two thinner layers with staggered joints provides better thermal bridging control than a single thick layer.
- Facing and vapour control: Some Polyiso comes foil-faced, which also acts as a vapour barrier. Coordinate insulation and vapour barrier specifications to avoid doubling up.
Common Mistakes
- Not accounting for cuts. Irregular shapes and penetrations can waste 10–20% of material. Always add a waste factor.
- Using Polyiso in cold conditions without derating. Polyiso R-value decreases significantly below freezing. For sub-slab or exterior below-grade, use XPS or EPS.
- Forgetting the vapour barrier. Rigid foam on top of gravel subbase still requires a separate vapour barrier under the slab unless code-approved foam-as-barrier details are used.
- Using the wrong compressive strength. Standard EPS Type I (15 psi) is not adequate under most slabs. Always specify the correct grade.
Frequently Asked Questions
What is the R-value per inch for EPS, XPS, and Polyiso?
Approximate R-values per inch: EPS Type I ≈ R-3.6, EPS Type II ≈ R-3.8, XPS ≈ R-5.0, Polyiso ≈ R-6.5 (at temperature, may decrease in cold climates). Polyiso's labeled R-value can drop significantly below freezing, so XPS is often preferred for below-grade applications.
How many 4×8 rigid foam panels do I need for 1,000 sq ft?
Each 4×8 ft panel covers 32 sq ft. For 1,000 sq ft: 1,000 / 32 = 31.25, round up to 32 panels before waste. Add 10% for cutting: 32 × 1.1 = 36 panels total.
Should I use EPS or XPS under a concrete slab?
XPS (extruded polystyrene) is the preferred choice under slabs due to its higher compressive strength (25 psi minimum) and moisture resistance. EPS Type II is also code-acceptable in many jurisdictions but has lower compressive strength. Check your local building code for minimum requirements.
How thick should rigid foam insulation be under a slab?
Building codes in cold climates (Climate Zones 4–8) typically require R-10 minimum under slabs. With XPS at R-5/inch, that is 2 inches. Energy codes in colder zones may require R-15 or R-20, requiring 3–4 inches of XPS or Polyiso.
Related Calculators
Disclaimer: R-values are nominal and may vary by product and installation temperature. Always verify insulation specifications meet your local energy code and verify compressive strength requirements with the structural design before installation.
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