Do you need radiant heat in a slab on grade floor?


We are looking to build a slab on grade, do we need radiant heat?

Scott Self Published: Sept. 21, 2020, 10:34 p.m.Last updated: Sept. 27, 2020, 5:04 p.m.

We are looking to build a slab on grade addition and were wondering wether it was necessary to install in floor radiant heat. We received a quote from Legalett for their geo slab it seemed more than we were hoping, is hydronic in floor more cost effective? Also, if we heated the space with a mini split heat pump and insulated beneath the slab would the floor be warm?

Responses (2)

George Lamb
George Lamb Oct. 1, 2020, 10:11 a.m. Reply

Hi Scott, I'll try to answer your questions, then discuss my thoughts a bit.

  1. While it's not necessary to install in-slab radiant heat, it's much easier to do it now than retrofitting.
  2. Hydronic in-slab will use smaller pipes, but the pipes aren't the cost-driver (compared to labour and other equipment in the system, such as pumps/fans, the heater/furnace/boiler, any valves, etc.).
  3. If the slab is un-heated, it will always be cooler than the air in the room, regardless of the amount of insulation.

The Legalett seems to have two differences compared to 'traditional' hydronic heat - the use of air as the fluid, and that the heater and pump/fan are embedded in the floor.

Using air will give more even distribution because the pipes are larger, and because the heat is transferred more slowly into the concrete, but is less 'efficient' in that it's harder to pump air (because it's compressible), and you need to pump much more of it (water having 3500 times better thermal capacity, on a volume basis). The larger pipes will reduce the strength of the slab (all else being equal).

That the system is embedded in the floor is a nice bonus (for internal space, less so for maintenance), but I see no reason why the same thing couldn't be done with a hydronic system - it would be a tankless water heater + pump.

For future-proofing reasons, I would encourage you to embed the pipe (either for air or water) now, even if it's not attached to a heating system. There's a great video on this site showing how it could be done

Cheers, George