Installing a radiant heat system in slab

TimTrujillo

I’m a homeowner building my house and am getting ready to pour the slab in the basement. I’m not going to finish the basement for at least 2-3 years and I was going to put tubing for radiant heat for future use. I’m trying to do this the most cost effective way and was going to put 3/8” tubing on top of the rebar mat. Everything I’ve looked up has said you must have 2” if insulation below the slab but I just can’t afford it (basement is 1260 sq ft) and I wouldn’t be able to pour my slab before freezing temperatures set in due to have to dig out the footings to place the insulation down. My question is should I even waste my time installing the tubing without putting insulation below the slab itself? Or would it be better (obviously less cost effective) to put gypcrete on top of the slab when I’m ready to finish the basement?

TAG

You want the thermal separation ... otherwise you heat the ground. The level of inefficiency is all about climate and if any moisture is around.

I go to a local lumber yard where they did not insulate the slab and the snow melts around the foundation in the winter -- they have unlimited wood for the outdoor boiler and don't care.

How much height do you have and what's the goal with the finished floor? I have done wood floors over a slab with VB -- PT on the flat w/ 1.5 " foam board between and subfloor/ PEX above. It ends up being more expensive vs doing it correct from the start .. especially if you want tile or keep the concrete floor look

TimTrujillo

The finished floor height will be 7’8” (8’ wall with 4” slab) and as far as the finished flooring it will most likely be LVP or wood depending on what we can afford when we can finish it.

STEVEusaPA

If you can't afford the insulation, you can't afford to do a proper radiant heat installation. Don't do it without the insulation.

GroundUp

Without insulation, it doesn't pay to install the tubing. With that said, the insulation should be there regardless of heat source or the floor will always be cold even if the air is 90 degrees down there. If you can't swing $800 for foam, maybe wait to pour the concrete.

TimTrujillo

Thanks for the replies and confirming what I thought would be answer. As far as the insulation being a must under the slab can I get away with 1” foam? The cost isn’t the most prohibitive thing it’s the digging and removing the dirt from the basement that will be required to get the footing exposed for 2” foam. It is pretty much ready for 1” foam if I can go that route

hcpatel78

Insulation is must. I Just wanted to share my experience with my basemnet walls. My basemnet used to drop temperature of 55 or less in wintertime . I have 1/2 basement walls underground and 1/2 above ground. I am in northeast New Jersey. After insulating walls with R25 (R10 rigid foam+R15 Roxul ) my basemnet never drops below 65 when outside is around 30 degree. My heat loss at 70(delta T) for 0 degree outside temperature is 35K BTU/H. Before it was 350K BTU/H. So total 10 times less heat loss.....These helps not only my basement but floor above the basemnet too.... heat loss reduced due to this insulation. I am throwing these numbers to give you an idea how insulation helps to reduce Heat loss?....and eventually you will have comfort and save heating cost.
Hiren

GroundUp

1" is definitely better than none, but that extra inch of excavation for 2" is only 3 yards of dirt. You don't need to expose the footing though, just break the bond between the slab and everything else whether it's dirt or the walls. If you can afford the 2" reduction in ceiling height, there's no reason to excavate anything. Assuming you already have your basement walls laid up, just lay foam vertically inside the basement walls then lay the bottom sheets and butt them up to the vertical foam.

hcpatel78

TimTrujillo said:

Thanks for the replies and confirming what I thought would be answer. As far as the insulation being a must under the slab can I get away with 1” foam? The cost isn’t the most prohibitive thing it’s the digging and removing the dirt from the basement that will be required to get the footing exposed for 2” foam. It is pretty much ready for 1” foam if I can go that route

1" will break when you walk on it during pour. I would prefer 2" XPS. That will give rigidity and performance both.

TimTrujillo

GroundUp said:

1" is definitely better than none, but that extra inch of excavation for 2" is only 3 yards of dirt. You don't need to expose the footing though, just break the bond between the slab and everything else whether it's dirt or the walls. If you can afford the 2" reduction in ceiling height, there's no reason to excavate anything. Assuming you already have your basement walls laid up, just lay foam vertically inside the basement walls then lay the bottom sheets and butt them up to the vertical foam.

I have looked into it a bit more and found a product called Reflectix and wouldn’t lose the 2” head room. Is this product any good or should I just bite the bullet and lose the head room?

Ironman

Reflectix? That's nothing but bubble wrap. Don't waste your time or money on it.

The best investment you could make would be to use CreteHeat board: it's the insulation, vapor barrier and hold down system all in one.

TimTrujillo

Ironman said:

Reflectix? That's nothing but bubble wrap. Don't waste your time or money on it. The best investment you could make would be to use CreteHeat board: it's the insulation, vapor barrier and hold down system all in one.

 Thanks for the advice I definitely can’t waste money or time so I’ll look into CreteHeat as another option as well.

Paul Pollets

2" foam is required under the heated slab by most building codes. A layer of 3mil plastic is also placed under the foam. 1/2" pex is also installed on 12" centers. Why use 3/8?
To use less foam compromises the efficiency of the slab and as said above, tends to break when walked on.