Rigid foam board: EPS vs XPS vs polyiso (R-value & sheets)
Rigid foam is the continuous layer that stops thermal bridging — and the three types (EPS, XPS, polyiso) trade off R-per-inch, moisture behavior and cold-weather performance. Count sheets, pick the right board.
Two formulas run the job
R = thickness (in) × R-per-inch and sheets = ceil(area ÷ 32) for a standard 4×8 board (32 ft²). Example: 2" XPS (R-5.0/in) over 640 ft²: R = 2 × 5.0 = R-10; sheets = ceil(640 ÷ 32) = 20 sheets. Run it in the rigid-foam R & sheet calculator or the rigid-foam board calculator.
The three foams at a glance
| Foam | R/inch | R at 2" | Character |
|---|---|---|---|
| EPS (expanded) | ~3.6–4.2 | ~R-7.8 | Cheapest, drains/dries, lowest R/inch, no ozone blowing agent |
| XPS (extruded) | ~4.5–5.0 | ~R-9.5 | Mid R/inch, water-resistant, common pink/blue board |
| Polyiso | ~5.6–6.5 | ~R-12 | Highest R/inch, usually faced; R drops in the cold |
Values are labeled typicals — confirm the rated R printed on the board. Full grid in the rigid-board R by thickness table.
The polyiso cold-weather catch
Polyiso posts the best R/inch on the label, but its R falls as temperature drops — exactly when you want it most on a cold exterior wall. For continuous exterior insulation in a cold zone, many builders derate polyiso (or use XPS/EPS, or layer polyiso inboard of a lower-R board). Don't design a cold-climate exterior to the room-temperature polyiso number.
Where each foam wins
- EPS: below-grade and under-slab where it may get wet (it drains and re-dries), and anywhere budget rules and you have room for thickness. Lowest embodied ozone impact.
- XPS: the do-everything board — foundation walls, rim joists, exterior continuous insulation. Water-resistant and stiff. Higher global-warming blowing agent (improving with newer formulations).
- Polyiso: above-grade walls and roofs where you want max R in min thickness and it stays warm; usually foil-faced (a built-in radiant/vapor layer).
Detailing that actually matters
- Tape and seal the seams — continuous foam is only continuous if the joints are sealed; otherwise you've added an air path.
- Ignition/thermal barrier: foam exposed to living space usually needs a code-required cover (e.g. 1/2" drywall). Set by code, not by you.
- Fastener length: continuous exterior foam pushes your cladding fasteners out — plan long screws into the framing.
- Vapor drive: foil-faced polyiso is a vapor retarder; know which side of the assembly it belongs on for your climate.
What to measure first
- Area to cover (walls: net of openings).
- Target continuous R → thickness = R ÷ R/inch for your chosen foam.
- Sheet size (almost always 4×8 = 32 ft²) and how the layout falls (odd bays waste board).
- Your climate — it decides whether polyiso's label R is honest.
Worked example: continuous exterior foam
Say you're adding continuous foam outboard of the sheathing on a 24×8 wall (192 ft² gross, ~165 net after openings) to hit a code “R-13 cavity + R-5 ci”. In XPS (R-5.0/in) that's just 1 inch; sheets = ceil(165 ÷ 32) = 6 sheets. In EPS (R-4.0/in) you'd need 1.25", and in polyiso (R-6.0/in label) only ~0.85" — but in a cold zone you'd derate the polyiso and probably spec 1" anyway. Notice the continuous layer does double duty: it delivers the R-5 and covers the studs, so the whole-wall number jumps more than the +5 suggests. Count sheets in the rigid-foam board calculator.
Layout waste and stacking to a target R
Sheets = ceil(area ÷ 32) is the floor, not the order. A 4×8 board lays out clean on a wall in full courses, but hips, gables and window returns generate offcuts — add ~10% on a cut-up elevation. To reach a high continuous R you stack boards and stagger the seams so no joint runs through both layers (two 1.5" polyiso = R-12 with a broken thermal path); tape or foam every seam, because continuous foam only works if it's actually continuous. Two thinner staggered layers almost always outperform one thick layer with aligned, un-taped joints.
Fasteners, furring and the cladding detail
Exterior foam pushes your siding out, so the details change. Plan long fasteners that reach at least ~1–1.25" into the studs through the foam, or install 1×4 furring strips screwed to the studs over the foam to carry the cladding and create a drainage/vent gap. Windows may need extension bucks. None of this is hard, but it has to be planned before the foam goes up — measure your total exterior build-out (foam thickness + furring) and buy the fasteners and trim to match, or you'll stall the siding crew.
Quick numbers to leave with
- R = thickness × R/inch; sheets = ceil(area ÷ 32) for 4×8 boards.
- R/inch: EPS ~3.6–4.2, XPS ~4.5–5.0, polyiso ~5.6–6.5.
- At 2": EPS ~R-7.8, XPS ~R-9.5, polyiso ~R-12.
- Derate polyiso in the cold — its R/inch sags below ~40°F.
- Tape/seal every seam, stagger stacked layers, and plan long fasteners or furring for cladding.
Pick the board for the location and climate, compute R and sheet count, seal the seams — that's what turns rigid foam into a real thermal break.