Which foam board is best for basement walls?

XPS or foil-faced polyiso, taped at the seams. EPS also works and is more vapor open, which some builders prefer below grade.

Does foam board need to be covered?

Yes. Interior foam requires a code-approved thermal barrier — usually 1/2 inch drywall — for fire safety.

Commonly, yes: rigid board on flat areas and canned or two-part foam to seal the edges and awkward corners.

Comparisons

Rigid Foam vs Spray Foam

Both are foam plastic insulation with high R per inch. One comes in sheets you cut, the other arrives as a liquid that expands in place. The choice usually comes down to geometry and budget.

Updated October 2026 · about 7 min read

R-value per inch

EPS: R-3.6 to R-4.2. XPS: R-5 (drifts toward R-4.5 with age). Polyiso: R-5.6 to R-6.5, but loses performance in very cold temperatures. Closed-cell spray foam: R-6 to R-7 and stable.

Cost

Rigid board is $0.45–$1.30 per sq ft per inch as material, and a competent DIYer can install it. Closed-cell spray runs $1.00–$1.75 per board foot installed. For big flat surfaces, board is far cheaper.

Gear for this job

Foam Board Insulation Panels 2 InchBrowse current prices and top-rated options on Amazon

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Air sealing

Spray foam seals itself. Board only seals if you tape every seam and foam every edge — do that and taped board is an excellent air barrier, skip it and it is nearly useless as one.

Where each wins

Board: basement walls, exterior continuous insulation, slab edges, garage doors, flat roofs, anywhere you can cut a rectangle. Spray: rim joists, irregular bays, cathedral ceilings, crawl spaces, and retrofits full of pipes and wires.

Insulating a camper van? The van insulation calculator works out board and foam for the walls, roof and floor.

Run the numbers

Rigid Foam Board Calculator Spray Foam Board Feet Calculator Basement Insulation Calculator

Frequently asked questions

Which foam board is best for basement walls?: XPS or foil-faced polyiso, taped at the seams. EPS also works and is more vapor open, which some builders prefer below grade.
Does foam board need to be covered?: Yes. Interior foam requires a code-approved thermal barrier — usually 1/2 inch drywall — for fire safety.
Can I combine them?: Commonly, yes: rigid board on flat areas and canned or two-part foam to seal the edges and awkward corners.

Insulation gear worth owning

The tools and materials that show up on almost every insulation job.

Rigid Foam Board InsulationBrowse current prices and top-rated options on Amazon Foam Board Insulation Panels 2 InchBrowse current prices and top-rated options on Amazon Spray Foam Insulation KitBrowse current prices and top-rated options on Amazon House Wrap Insulation TapeBrowse current prices and top-rated options on Amazon

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