About twenty years ago I poured a 24 X 20, 4” thick monolithic slab over three feet of rock. I was filling in the previous owner’s attempt to convert a carport into a sunken living room. Sometime later I framed it in and it has served as an unheated mudroom and general storage. Now I’d like to convert it into living space with a radiant heated floor. I have seen electric heating grids that are designed to be imbedded in the thinset when applying a tile or stone finish floor. An ideal application for me, except that my slab is uninsulated.
Anybody out there have any experience with radiant heat in an uninsulated slab? I want to avoid the floor thickness resulting from laying down a couple inches of rigid foam, sleepers, subfloor, etc. How about a foil radiation barrier under the thinset? The manufacturers are no help. They respond to my queries with their stock application recommendations.
By the way, I realize that “heat rises”, but that only applies to convection. Conduction and radiation are immune to gravity.
Replies
how thick can you go? that will determine what you do. btw, any hot water heat in the current house?
I can go about an inch and a half, including the finish floor. Maybe two inches. Beyond that, the step up gets intrusive.
There is no hot water heat. I could use the domestic water heater, but I don't want to add the complexity of pumps, mixing valves, etc. Basically, I subscribe to the no-moving-parts philosophy. Anyway, hydronics is out of the question.
Dear Nut-
Just saw your post from April- now almost July. I had a similar issue with the concrete floor installed and limited thickness capacity. I put down a layer of felt and then a layer of TEKFOIL, FOIL-BUBBLE-BUBBLE-FOIL, 1" X 3" sleepers, my hydronic PEX tubing on top of the foil and around the sleepers. then a 1/2" subfloor and finally a glued-down parquet floor. Total thickness a hair over 2". Hope this helps. The TEKFOIL gives somewhere near a R10-12 factor(:-)
Windy Wood
From the Helderberg Mountains
I had an electric mat floor heating system (WarmlyYours product) installed in a newly constructed bathroom that was built on a concrete slab. No special insulation was used. ..just the electric -coil mat placed in thinset, and finished with ceramic tile. System worked great as secondary heat. It would NOT have cut it as primary heat in serious winter weather. The manufacturer recommended putting down a barrier, but the R-value would not have been raised appareciably, so we didn't do it.
You're right to take manufacturers "results" with a grain of salt. I think the radiant products work best in northern climes as supplemental heat sources. Also, I sure hope you live somewhere where electric rates are more modest than they are here in the Big Apple!
Whether or not with electric or hydronic, this is a big radiant nut to crack.
With that thick of a slab, and the rock below, you have this tremendous heat sink that is going to suck all of your heat downward.
Now I am not saying you can't do this, just a lot of energy is going to be wasted downward.
Any type of system will warm the floor, but as for heating this space on a design day, that is questionable until a heat load calc is performed. We not only need to deal with the downward heat loss, but the heat loss of all walls, windows, doors.....
What you might want to consider in conjuction with the floor is the option of radiant walls, ceilings....
Hope this helps
Dave H.
You have confirmed my fears, exactly. Radiant ceiling heat is a distant second best. If I can't get any good suggestions for super-thin, super-efficient insulation under the thinset, I just might jackhammer the slab and start again.
>but as for heating this space on a design day, that is questionable until a heat load calc is performed.
And that's the real key. The slab beneath would be acting kinda sorta like a high mass system, and a hi mass system with lo mass (or weakly insulated) walls is the hardest kind of system to control.