How to insulate a slab from overlying tile and radiant?
Hello
I want to install a hydronic radiant heated tile floor on my existing patio that is currently an unfinished concrete slab, smoothed out and leveled but left rough. My radiant contractor says we need to isolate the radiant slab from the existing slab that will serve as a heat sink otherwise. To isolate the radiant from the concrete sub floor slab, he suggests 2″ rigid XPS (also insulating vertically at the edges), then lay the pex on the XPS, drop welded wire mesh on top of the pex tubing, and pour another 1.5″ concrete slab, Id use a small aggregate concrete mix with fiber and possibly polymer modified additives to add crack resistance. Once this slab cures, I can apply tile with modified thinset.
Does this sound feasible? My mason does pavers and is a good friend of mine for years (NOT A TILE GUY), so I trust him. He feels putting tile top on top of XPS is crazy and it would be a question of time for areas to compress and tile would crack.
Bulding Science Corp has literature on the issue but I see the slab is usually in the garage or basement slab and thus is a min of 4″ of slab with rebar thus it creates its own flex-resistant plane.
I do not own a copy of the TCNA handbook but have seen architects reference it in different articles in regards to the Hydronic Radiant Heat Pex Tubing needing a minimum of 3/4″ of mortar from the top of the tubing. Thus by using 1/2″ oxygen barrier pex with 5/8″ outer diameter, the 1.5″ slab is good but does NOT address the flexibility and longevity of this slab design.
Any constructive input?
Thanks
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Replies
I would not use a hydronic system for a small space. I really like the Schluter Ditra Heat system. It’s an awesomely easy process. There are other electric systems available that are well suited for your application. If you still want to use hydronic, Warmboard-S is a really good system and its super easy to use as well. There is no need to install concrete slabs any more, in my opinion. Make your job as simple as possible!
There are plenty of info on the web on all these systems, plus YouTube videos to educate yourself and your radiant contractor. Good luck.
I find it interesting that Warmboard's recommended insulation is R1.2 of Homasote Comfort Base. I'd use more R value.
Thanks for the input. I prefer the thermal mass a cementious slab provides. Interesting product. I don't think a layer of aluminum atop plywood can compete with a 1.5" cement slab for maintaining thermal mass. One of the modifications I am considering from the original plan is decrease the 2" XPS to 1.5" or even 1" (R 7.5, R5) and increase the slab to 2" or 2.5". Add rebar to the welded wire mesh to increase its rigidity and include an uncoupling membrane before tile installation. A Ditra or Red Guard perhaps.
Thanks though.
I'm curious - other than slowing response time and making temperature control less accurate, what do you expect the thermal mass will do?
Its not a small space, 2 attached patios, about 1400 sq ft.
I’d say it best to start with the assumption there thermal mass has pros and cons, like everything else. Sometimes it helps, other times it doesn't. I recall on this board over 5 years ago Martin telling me he does not like it and gave me a list of all its cons. Personally, I like thermal mass in a home, I have a wood burning stove that is ensconced in a heavy stone and mortar wall enclave, I leave the stove on for a few days and even if it dies out in the middle of the night in February with 5 degree F outside I wake up to a warm room, a warm wall. I like that, that's my preference.
We know thermal mass helps balance temperatures in spring and fall when days are warm but nights are still cool. That will either increase comfort or reduce the need for heating and cooling. Sure, you can’t turn it off with a switch, that’s why we have windows. Radiant heating of the surfaces in your home (walls, windows, floors) are below that of your body temperature, you will conduct and radiate heat towards them and feel cooler. If they are warmer than your body temperature, they will conduct and radiate heat towards you and you will feel warmer. Thus its now being used for cooling and heating. This is one way you can take advantage of thermal mass in a home to increase your comfort. Having heat radiating from warm surfaces in the winter is a beautiful thing, it warms the couch, the kitchen cabinets, the chair, the island, etc.. Living in Zone 5, yes please!
So what can you tell me about my design to make this happen successfully besides that Warmboard is interesting?
Sal,
To answer your original question: I agree with your mason. If you add a layer of foam on top of the existing slab, the new one needs to be deeper than an 1 1/2". Probably 3" to 4".
Thank you Malcolm.
As I digest the concept and gather input from other reputable sources, I agree with you. 1.5" is not enough. Much like those insulated slabs from buildingscience.com, it has to have enough structural integrity to serve as its own autonomous plane, resistant to cracking. Welded wire mesh, rebar, polymer additive, fiber in the mix will help but ultimately a thicker slab is needed.
4" being ideal, 1.5" being too thin. Could I get away with 2.5" to 3"? 4" slab + 1"XPS + tile and thinset kills the rise of the step. hmmm..