Wall and Roof Insulation
Summary
Wall and roof insulation reduces heat flow through the building envelope, as cavity insulation between framing, continuous insulation over it, or above-deck roof insulation. Compare products by R-value per inch and by where they sit in the assembly, and check moisture, fire, and air-barrier behavior along with the thermal number.
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Terminology
- R-value: resistance to heat flow; higher is better
- Cavity insulation: insulation between studs or joists
- Continuous insulation (ci): insulation over the framing that is not interrupted by studs
- Above-deck insulation: rigid board under the roof covering
- Closed-cell and open-cell foam: spray foam types with different R-value and permeance
- Polyiso: polyisocyanurate rigid board
- EPS and XPS: expanded and extruded polystyrene boards
Standards
R-value per inch (typical ranges)
| Material | R per inch |
|---|---|
| Fiberglass batt | 2.5 to 4 |
| Blown fiberglass | 2.2 to 2.8 |
| Mineral wool | 3 to 4 |
| Cellulose, blown | 3.2 to 3.8 |
| EPS | 3.8 to 4.4 |
| XPS | about 5 |
| Polyiso | about 6 to 6.5 |
| Closed-cell spray foam | about 6 to 7 |
| Open-cell spray foam | about 3.5 to 3.7 |
- Values vary with density, facing, and installation; use manufacturer data for a project
Board standards
- ASTM C578: EPS and XPS foam board
- ASTM C1289: polyiso board
- ASTM C728: perlite board
- ASTM C552 and C1902: cellular glass
- ASTM C726: mineral fiber roof board
- ASTM C208: wood fiberboard
- NFPA 276 and UL 1256: fire tests of above-deck roof insulation assemblies
Materials
- Fiberglass and mineral wool: batts and blown cavity fill
- Cellulose: blown cavity fill
- Foam plastics (EPS, XPS, polyiso, spray foam): continuous and above-deck insulation
- Cellular glass and perlite: rigid boards for roofs
Detailing
- Choose cavity, continuous, or combined insulation from the wall type and energy requirements
- Place continuous insulation outboard to reduce thermal bridging at framing
- Keep insulation continuous at corners, parapets, and roof-wall joints
- Pair insulation with the air barrier and vapor control layers for the climate
- Cover foam plastic as the code requires, and check roof assembly fire testing
- Fasten cladding through foam with attachments rated for the load
Uses
Specific uses
- Stud cavities in wood and metal walls
- Continuous insulation behind cladding
- Roof boards under membrane and metal roofing
Pros and cons
Pros
- Cuts heating and cooling loads
- Rigid boards add thermal performance over framing
- Many products for different assemblies
Cons
- Foams need fire and thermal barriers
- Moisture and air leaks reduce performance
- R-value per inch varies widely by product
Manufacturing and cost
Construction
- Batts are friction-fit between framing; blown products fill cavities; boards are fastened or adhered; spray foam is applied in place
Main manufacturers and suppliers
Sources
- Pacific Northwest National Laboratory (BASC): typical R-values and vapor retarder classifications, https://basc.pnnl.gov/information/typical-r-values-and-vapor-retarder-classifications-common-insulation-materials
- U.S. Department of Energy: guide to home insulation, https://www.energy.gov/sites/prod/files/guide_to_home_insulation.pdf
- BuildingGreen: insulation recommendations, https://www.buildinggreen.com/sites/default/files/BG_Insulation_Recommendations.pdf
- GreenBuildingAdvisor: comparing the properties of insulation materials, https://greenbuildingadvisor.com/article/comparing-the-properties-of-insulation-materials
- UpCodes: 2024 IBC Chapter 15 roof assemblies (Department of Defense adoption), https://up.codes/viewer/department-of-defense/ibc-2024/chapter/15/roof-assemblies-and-rooftop-structures