Thermal conductivity library

λ in W/(m·K). The source column links the document each value was read from, with the date it was retrieved — 398 of the 468 rows carry one, as a datasheet (the maker's own declaration), a certificate (BBA, NSAI or similar), a listing (a retailer), a calculator or a paper. 384 values come from a declaration or a certificate — those are the ones to put in a submission. secondary source marks the 40 cited to a retailer listing or another calculator rather than the maker: usable for design, worth confirming before you submit. unconfirmed marks the 33 whose only source is a paywalled value table (CIBSE Guide A, BS EN ISO 10456, BS 5250). They are what the industry uses, but we cannot confirm them against a document you could check for free, so check them against your own copy. placeholder marks 9 conservative stand-ins of our own, to be replaced with a declared value. no longer made marks the 13 products no longer made or sold: kept so an existing building can still be modelled with what was installed, but not for a new specification.

Download it

The whole library as CSV or JSON, with the source, retrieval date and confirmation state on every row. See our open data for the licence.

How these values were gathered

Every named product's λ is the manufacturer's declared value, read from their own datasheet or declaration of performance at the URL on that row, on the date shown. Nothing is interpolated and nothing is averaged across a range. Where a product declares a thickness-banded λ — the same board performing differently at 25 mm and at 100 mm — the band is noted rather than flattened to one number.

Generic materials are a different problem. BR 443 (2019) §3 publishes conventions for the common ones, and those are marked as confirmed. The rest are figures the industry has used for years whose only published source is behind a paywall, and rather than present those as authoritative we flag them. That flag is the point of this page: a λ with no provenance is how a U-value calculation quietly becomes wrong.

Put a value to work

U-Monkey builds the whole construction from these values — layer by layer, with bridging fractions, and an interstitial condensation check on the result. Free in your browser; the Building Control PDF is the only paid part. See also the U-value to R-value converter and what a U-value actually is.

Frequently asked questions