Cormar Primo Grande (1.4 TOG) with Duralay Durafit 500 (0.83 TOG) gives a combined 1.23 in-situ TOG. Whether that passes the 2.5 TOG underfloor heating limit and what to do.

Updated 05 September 2026.

Combined TOG result

Cormar Primo Grande and Duralay Durafit 500: combined TOG for underfloor heating

The combined TOG for Cormar Primo Grande over Duralay Durafit 500, worked against the UHMA underfloor heating rule.

1.52.5
1.23TOG in situ
Pass · limit 2.5
Published TOG stack (carpet + underlay)
carpet 1.4
underlay 0.83
limit

Combined against the UHMA 2.5 TOG underfloor-heating limit. Under 1.5 also suits a low-temperature heat pump.

Carpet TOGUnderlay TOGPassMarginalFail
Combined in-situ TOG is 1.23, under the 2.5 TOG limit, so Cormar Primo Grande with Duralay Durafit 500 is suitable for a standard underfloor heating system. It is also under 1.5 TOG, so it suits a low-temperature heat pump run.
The combined-TOG calculation
CarpetCormar Primo Grande (1.4 TOG)
UnderlayDuralay Durafit 500 (0.83 TOG)
Published sum2.23 TOG
UHMA in-situ correctionminus 1 TOG
Combined in-situ TOG1.23 TOG
Standard UFH limit2.5 TOG
Heat pump targetunder 1.5 TOG

Cormar Primo Grande is rated 1.4 TOG and Duralay Durafit 500 is rated 0.83 TOG. Laid together the combination is what the underfloor heating has to warm through.

The published TOG figures add to 2.23. The UHMA rule subtracts 1 TOG because tests showed real in-situ resistance runs about a TOG below the paper sum, giving 1.23 TOG. That clears both the standard limit and the tighter heat-pump target.

Related checks
Cormar Primo Grande TOG value1.4 TOGDuralay Durafit 500 TOG rating0.83 TOGCormar Pimlico with Duralay Durafit 5000.83 TOG, passCormar Primo Textures Ribbed with Duralay Durafit 5000.83 TOG, passCormar Primo Textures Weave with Duralay Durafit 5000.73 TOG, passCormar Apollo Plus with Duralay Durafit 5000.93 TOG, pass

Questions and answers

Cormar Primo Grande and Duralay Durafit 500: combined TOG for underfloor heating?

Combined in-situ TOG is 1.23, under the 2.5 TOG limit, so Cormar Primo Grande with Duralay Durafit 500 is suitable for a standard underfloor heating system. It is also under 1.5 TOG, so it suits a low-temperature heat pump run.