Glossary · Insulation & windows

Rigid foam (EPS, XPS, polyiso)

Also called: rigid foam board · foam board · foam board insulation · rigid insulation · rigid board insulation · EPS

Stiff insulation boards made of foam plastic, used on foundations, under slabs and as a continuous layer over walls and roofs.

What it means

Expanded polystyrene (EPS) is the white bead board; extruded polystyrene (XPS) is denser and usually colored; polyisocyanurate (polyiso) usually comes with foil facings and gives the most insulation per inch. Polyiso's insulating value can drop in cold temperatures, so in cold climates it works best as an inner layer unless the product is rated for the cold. Taped or sealed boards can also act as an air barrier and, depending on type and thickness, slow vapor.

Why it matters in an audit

Rigid foam is how many basements get insulated and how continuous insulation is added during re-siding, both common audit recommendations.

Typical numbers

Reference values with their sources. Your auditor’s report gives the figure for your own house.

What	Value	Source
R per inch (NRCan)	EPS Type I R-3.8; EPS Type II R-4–4.4; XPS R-5; polyurethane board R-5.5–6.8 (design R-6)	natural-resources.canada.ca ↗
Range across rigid foams (DOE)	About R-3.2 to R-6.5 per inch, depending on type	bsesc.energy.gov ↗

Sources

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Related terms

Continuous insulation (ci) A layer of insulation running unbroken over the framing, usually rigid boards on the outside of walls, to stop heat escaping through studs.
Thermal bridging Heat taking a shortcut through materials that insulate poorly, like wood studs, steel or concrete, running from the warm side to the cold side.
R-value How well a material resists heat flow; higher is better, and insulation is rated with it, such as R-20 for walls or R-50 for attics.
Rim joist (header) The band of framing on top of the foundation wall around the edge of the floor; it's prone to air leaks and often poorly insulated.
Spray foam (open-cell and closed-cell) Liquid foam sprayed in place that expands to insulate and air seal in one step; closed-cell is denser and higher R, open-cell lighter.

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