Replies

1. 
   charlie_sullivan | Mar 14, 2021 10:30am | #1

   You don't need concrete of any kind to do a radiant floor. You can if you want, but the performance is actually better without that thermal mass slowing down the responsiveness.

2. Expert Member
   Michael Maines | Mar 14, 2021 10:40am | #2

   Wisco, do you have existing slabs and you're proposing to go over them with an additional 2"? If so, is there insulation below the existing slabs?

   I have used gypcrete a couple of times and as far as I know they have held up, but that's not much of a sample size.

   For thin concrete the main difference is to use smaller aggregate, max 3/8". Some might say to add additional Portland cement which will increase strength but also increases the risk of shrinkage cracks, so I would keep it on the low side, 2500 to 3000 psi. Micro-fibers will help reduce cracking but be sure to get the finest ones and plan on "polishing" (lightly grinding with a diamond wheel) the floor, or else you will get fibers sticking up. If you don't want the extra expense or care about the look from polishing, skip the fibers. You can use a bonding agent, basically Elmer's glue, to help adhere the new floor to the existing floor.

   The main downside of "radiant" floors (they heat by conduction and convection as well so I call the system in-floor heat) is the slow response time, which can lead to overheating or under-heating. There are various controls available to help anticipate temperature needs. Many people have another source of heat for quick response.

   The other downside of in-floor heat is that in a well-insulated and air-sealed house, the floors don't actually feel warm. They only feel warm if you have a poorly-built building envelope. But in a basement or garage, that may not be the case, or be important.

1. 
   wiscoguy | Mar 14, 2021 11:26am | #3

   Thanks for the response. My main reason for using radiant on the upper floor with concrete is for one I will already have a boiler for the garage and basement seems kinda crazy to not utilize what I already have for heat on the main floor. Even the Mitsubishi rep said I would more than likely need supplemental heat upstairs he’s from Wisconsin and even though Mitsubishi’s have come along way there’s 2-3 months here where they aren’t as efficient and there’s actually a better savings with another source of heat plus I don’t heat or cool the garage so trying use that boiler as a supplemental heat source for the cold days the Mitsubishi won’t keep up.

   The other thing I’ve learned through research is warmboard wants 13000 just for the subfloor. It’s a great product but not at that price for me.

   I can pour concrete myself I’ve done stamped walls etc in my life I try to avoid it but at a 10k savings it’s worth it for me.

   I’m not sure how much of this matter but I’m not a huge fan of vinyl floors and wood with kids and a dog seems likely to get damaged and tile can feel cold without some heat source underneath it.

   I hadn’t considered that having a good envelope will lead me to having a very conditioned and warm space that’s definitely something to consider appreciate the thoughts just trying to utilize the best given what I already have and try to maintain the most energy efficient heating at different times of year. So below a certain temp the floor heat kicks in and the Mitsubishi’s go off. Then once it gets back above 15 go back to the Mitsubishi this was my thought anyway.

3. 
   DCContrarian | Mar 14, 2021 08:18pm | #4

   Poor man's warmboard: Cut 7" strips of 5/8" plywood and fasten them on 8" centers (ie with a 1" gap between pieces. Put a piece of this into the gap:
   https://www.pexsuperstore.com/product/100-4-ft-snap-on-aluminum-heat-transfer-plates-for-1-2-pex-omega/

   Run pipe in loops.

   What goes above that depends upon how the floor is finished.