Polyiso vs XPS Insulation
Polyiso vs XPS Insulation
Polyiso often offers higher nominal R per inch in roof and wall boards, while XPS is often considered for moisture-exposed rigid-board details. Temperature and facing matter.
R per inch
Polyiso is often around R-5.6 to R-6.5 per inch; XPS often around R-4.5 to R-5.0.
Roof vs below grade
Polyiso is common in roof boards; XPS is more often discussed for foundation or basement exposure.
Product variation
Facers, temperature ratings, aging method, and approvals can change the right choice.
How to use this page
The insulation comparison page is best used to narrow options, not to make an automatic design decision. Start with a realistic thickness, compare lambda, R-value, and installation limits, then check shortlisted materials against current data sheets.
The result must be read together with the application. A material that works well on an external wall is not automatically the best choice below a screed, on a flat roof, or in timber construction. Moisture, load, fire behaviour, and fixing method can outweigh a small lambda difference.
When two materials look thermally similar, look at system availability, tolerances, and whether the insulation layer can be made continuous. A slightly weaker material installed cleanly can perform better than a theoretically stronger option with gaps or thermal bridges.
After comparing materials, move to the U-value calculator and test the complete assembly. The comparison table shows material differences, but the full build-up reveals the effect of render, cladding, air spaces, and surface resistances.
If two results are close, do not choose from one number alone. Check moisture behaviour, fire performance, available board formats, system requirements, and whether the layer can be installed continuously without open joints.
For renovation work, also check substrate condition, condensation risk, and details around windows, ring beams, and balconies. A strong lambda value will not fix a layer interrupted at the most important junctions.
The final step should be comparison with the current manufacturer data sheet. The database helps narrow the direction, but the documentation for the exact product matters most for ordering and construction.
In practice, make a short decision table: thermal result, installation difficulty, moisture behaviour, fire behaviour, acoustics, and system availability. That reduces the risk of choosing a material only because one number looks best.
If the comparison is used in a supplier discussion, record the data-check date and assumed thickness. It becomes easier to separate a price or product change from a real change in thermal performance.
A useful comparison includes at least three variants: current state, economical option, and option with margin. That shows whether extra thickness still delivers a meaningful improvement.
If the comparison is tied to legal requirements, do not rely on an internet average. Use the local threshold, the correct calculation method, and data for the product that will actually be installed.
Calculator example
If the target is R-30, a polyiso planning value of R-6 per inch gives about 5 inches, while XPS at R-5 gives about 6 inches. That is only a thickness estimate.
- For roofs, check fastening pattern, cover board, facer compatibility, and system approval.
- For basement or below-grade details, check moisture exposure, drainage, and compressive requirements.
- Cold-temperature performance can change the effective choice, so use current product documents.
- Use Polyiso vs XPS Insulation as a screening page: set the target R-value, compare typical per-inch values, then check a declared product.
- If two options look close thermally, prioritize fit, moisture control, fire covering, access, and the full assembly before choosing.
- Keep manufacturer data, local code review, and installer documentation with the calculation so later changes can be checked.
The comparison narrows the choice, but the result still depends on declared lambda, thickness, moisture, and installation quality. Before making a decision, check the calculation method, definitions, and current material table. methodology, glossary, lambda table.