I have a large new build with in-floor radiant heat (20KW Thermolec electric mini Boiler - 2500 sq ft garage) I am going to use for a home gym.  Question: Will 4 x 6 rubber mats 3/8" thick or so, or interlocking gymtype mat flooring allow heat to transfer through them, or will a rubber mat flooring decrease performance of the in-floor heat system.  Thank You

Might there be a larger issue here. Dropping weights is not unusual in a gym setting so how about potential damage to the tubes?

Rubber should be fine at that thickness, as long as the MFG indicates the product is ok over radiant (no offgassing smells, etc).

Any foamed rubbery material will have an R-value, which will decrease the heat transfer at the same water temperature. But 10mm goods are probably no more than ~R2, and raisng the water temp ~10C would likely be sufficient to provide the same heat flux as the un-clad floor.

Rubber is usually a low R-value material. We recommend foam and slab rubber products as carpet pads regularly. You're unlikely to exceed an R-1 for a 3/8" rubber product of any kind.

Thanks Rob (I would have had to look it up, but I'm not surprised.)

Thanks for the replys.  Not worried about dropping weights; weights are rubber bumper plates dropped on a built up wood platform.

The radiant floor designer will input the floor coverings and may increase the design water temperature to account for the rubber, but as Rob suggests it will not be a deal breaker.

Drop those weights buddy, if the slab won't stand up to it, the PEX tubing will be the least of your problems.