Rigid insulation boards are used on basement walls, under slabs, as continuous insulation over wall sheathing, on cathedral ceilings and on flat roofs. Each material has its own R-value per inch, so the thickness you need to reach a target depends on the board you choose.
R-values per inch
| Board | Typical R per inch | Notes |
|---|---|---|
| EPS (expanded polystyrene, white beadboard) | 3.6 to 4.2 | cheapest; slightly vapor permeable |
| XPS (extruded polystyrene, pink or blue) | 5 | good moisture resistance; R-value drops slowly over the years |
| Polyiso (foil-faced) | 6 to 6.5 | highest R; loses some performance in very cold weather |
| Mineral wool board | 4 to 4.3 | non-combustible, drains water, good for sound |
How the calculation works
- Thickness needed = target R-value ÷ R per inch
- The calculator rounds up to the next half inch, the usual step for boards
- Boards = area × (1 + waste) ÷ board area, rounded up
Example
To reach R-10 on 500 sq ft of basement wall with XPS (R-5 per inch), you need 2 in of board. EPS would need 2.63 in, so a 3 in board (R-11.4), and polyiso 1.67 in, so 2 in (R-12). With 10% waste, 4 × 8 ft boards cover 32 sq ft each, so you buy 18 boards.
How much R-value do you need?
The International Energy Conservation Code (IECC) sets minimums by climate zone. In recent editions, basement walls in most of the northern half of the country call for R-10 to R-15 of continuous insulation, and slab edges R-10. Above-grade walls in cold zones often combine cavity insulation with R-5 to R-10 of continuous exterior board. Check the code adopted where you build.
Installation tips
- Seal the joints with foil tape (polyiso) or the tape the manufacturer recommends, and use canned foam at the edges: air leaks around boards ruin much of the R-value.
- Two layers with staggered joints perform better than one thick layer.
- Foam plastics inside a living space must usually be covered by a thermal barrier such as 1/2 in drywall, because they burn.
- Below grade, choose XPS or EPS rated for ground contact and protect the exposed part above ground from sunlight and damage.
- Thick exterior foam changes the depth of windows and doors and needs longer fasteners for siding.