Does polyiso R-value change in cold weather?

Yes, polyiso R-value drops to approximately R-5 per inch at very cold temperatures. This is important for very cold climate zones.

Can I use polyiso below grade?

No. Polyiso absorbs moisture and loses R-value below grade. Use XPS or EPS for foundation insulation.

What is the foil facing for?

The foil face serves as a radiant barrier (reflects heat), vapor retarder, and provides a surface for taping air-tight seams.

Polyiso Rigid Foam Board - building material

Insulation

Rigid Board

$0.70-$1.20 per sq ft per inch of thickness

Polyiso Rigid Foam Board: Maximum R-Value Rigid Insulation (2026)

Prepared by Lumberama's Building Materials Team

|Last updated July 25, 2026|Sources & methodology

Polyisocyanurate (polyiso) rigid foam board provides the highest R-value per inch of any rigid insulation at R-6 to R-6.5. It typically features foil facing that acts as a radiant barrier and vapor retarder. Polyiso is the standard for commercial roofing insulation and is increasingly used in residential continuous insulation applications.

Key Features

R-6 to R-6.5 per inch; Foil-faced radiant barrier; Highest R-value rigid board; Integrated vapor retarder; Fire-resistant compared to other foams

Specifications

R-value: R-6 to R-6.5 per inch (at 75F). Density: 2-3 lbs/cu ft. Sizes: 4x8 ft in 1/2 to 4 inch. Foil faced or glass fiber faced. Flame spread: 25 or less.

Applications

Continuous exterior insulation; Commercial roofing; Cathedral ceiling insulation; Duct insulation; Above-deck roof insulation

Installation Overview

Cut with utility knife. Install as continuous insulation over exterior sheathing with cap nails or screws with washers. Tape foil seams for air and vapor barrier. Install cladding over foam with furring strips or long fasteners.

Pros & Cons

Pros

✓ Highest rigid foam R-value; Foil face acts as vapor retarder; Better fire resistance than EPS/XPS; Excellent for continuous insulation; Lightweight

Cons

✗ R-value drops in cold temperatures; More expensive than EPS/XPS; Not suitable for below-grade; Foil facing complicates some assemblies; Edges can be fragile

Top Brands

Johns Manville

Dow

Kingspan

GAF

Code Requirements

Must meet ASTM C1289. Thermal barrier per IRC R316. Not recommended below grade due to moisture absorption. Continuous insulation values per IRC Table N1102.1.2.

Buying Tips

Polyiso offers the most R-value per dollar for exterior continuous insulation. In very cold climates (Zone 7-8), consider that cold-temperature R-value drop may affect performance. Use foil-faced for vapor retarder properties.