Polyiso R-Value Calculator
Convert polyiso insulation thickness to R-value and back, with LTTR design values, cold-weather drift, layer build-ups, and board feet for a roof area.
Pick a direction — the other value is computed.
The assembly R-value you need to hit.
LTTR is the aged value codes require. Use the derated value for cold climates.
Provide to estimate board feet.
Polyiso result
To reach R-30 at R-5.7/in.
Two staggered layers
- Target R-value
- R-30
- R-per-inch used
- R-5.7/in
LTTR is the aged design value codes require. Fresh polyiso tests higher but drifts down over time.
Use 2-layer staggered joints over 3 inches to minimize thermal bridging at board gaps.
Verify the required R-value against the IECC edition adopted by your jurisdiction.
How this calculator works
Polyiso (polyisocyanurate) is the most common above-deck roof insulation in North America. This calculator converts between polyiso thickness and R-value in both directions, suggests a practical layer build-up, and estimates board feet for a roof area — using the same R-per-inch value the design and code path expects.
Thickness and R-value, both ways
The math is linear: R-value equals thickness times R-per-inch. Enter a target R-value to get the required thickness, or enter a thickness to get the resulting R-value. The conversion carries over when you flip modes, so you can quickly check a design from either direction.
LTTR is the number that counts
Polyiso is sold with two R-values: a fresh published value (~R-6.0/in) and the aged LTTR design value (~R-5.7/in). LTTR — Long-Term Thermal Resistance — is the time-weighted figure codes and specs require, because the foam's blowing agent off-gasses over the first years of service and the R-value settles down. Always design to LTTR, not the fresh number.
The cold-weather catch
Polyiso has one important quirk: its R-value drops as it gets cold, the reverse of most insulation. Below about 40°F the effective value can fall toward R-5.0/inch or lower as the blowing agent condenses inside the foam cells. In cold climates (IECC zone 5 and up) this matters — designers either derate the R-per-inch value or hybridize the assembly with a top layer of XPS or EPS, which hold up better in the cold. The calculator includes a cold-derated R-5.0/in option for exactly this reason.
Layer build-up and board feet
For thicknesses over about 3 inches, use two staggered layers with offset joints rather than one thick board — this breaks up the thermal bridges at board edges. The calculator suggests a two-layer split automatically. Add a roof area and it returns board feet (area × thickness) for ordering.
Frequently asked questions
- What is the R-value of polyiso per inch?
- The aged design value (LTTR) is about R-5.7 per inch — the number codes and specs use. Fresh off the line, polyiso tests higher, around R-6.0+ per inch, but that drifts down over time. For cold-climate design many engineers derate to R-5.0 per inch. This calculator lets you pick which value to apply.
- What is LTTR and why is it lower than the published R-value?
- LTTR stands for Long-Term Thermal Resistance — the aged, time-weighted R-value measured per CAN/ULC-S770. Polyiso is foamed with a blowing agent that slowly off-gasses and is replaced by air, which conducts heat slightly better. Fresh polyiso therefore tests high (~R-6.0) and settles toward its LTTR (~R-5.7/inch). LTTR is the honest, code-recognized design number.
- Why does polyiso lose R-value in cold weather?
- This is polyiso's signature quirk: its R-value drops as temperature drops, the opposite of most insulation. As the foam gets cold, the blowing agent inside the cells begins to condense, reducing thermal resistance. Below roughly 40°F the effective value can fall to R-5.0/inch or lower. In cold climates designers either derate the value or hybridize the assembly with a top layer of XPS or EPS, which perform better cold.
- Should I use one thick layer or two thinner layers?
- For anything over about 3 inches, use two staggered layers with offset joints rather than one thick board. Staggering breaks up the continuous gaps at board edges, which are thermal bridges, and improves the real-world performance of the assembly. The calculator suggests a practical two-layer build-up automatically above 3 inches.
- What R-value do I need for my climate zone?
- The IECC sets minimum R-values for insulation entirely above the roof deck by climate zone — roughly R-25 in zones 1–3, R-30 in zones 4–6, and R-35 in zones 7–8 for commercial work (IECC 2021, Table C402.1.3). Toggle the climate-zone reference in the calculator to see the table, and always verify against the edition your jurisdiction has adopted.
- How does polyiso compare to XPS and EPS?
- Polyiso has the highest R per inch of the three at typical temperatures (~R-5.7 LTTR vs ~R-5.0 for XPS and ~R-3.6–4.2 for EPS), so it is thinner for the same R. But it loses R-value in the cold while XPS and EPS hold or even gain slightly. XPS resists moisture best; EPS is the most cost-effective and drains/dries well. Cold-climate roofs often layer polyiso with an XPS or EPS cover board to balance these.