Radiant heating over slab on grade

PhilipXP

Hi,

we plan on installing a hydraulic radiant heating system on our main floor, which is a slab on grade. We can't be sure if there is a vapor barrier or insulation under the slab. From the different available systems we prefer one that works with an overpour (so no warmboard) and ideally has insulation and a vapor barrier included. My main concern is moisture buildup between the slab and the radiant heat system / the radiant heat system's vapor barrier, which I understand can lead to mold. I have found different information on how to deal with that:

• A vapor barrier over the slab is fine
• A dimple mat is better
• A drain board is even better

We are located in Colorado which generally has a very dry climate. There is no indication anywhere in the house that moisture buildup ever was a problem (e.g. in the basement).

Does someone have any experience with these types of situations? Any input is appreciated.

Larry Weingarten

Hello, Certainly others will chime in, but I'd want to know how much of a problem water coming up through the slab really is. One easy test is to tape a roughly foot square piece of plastic sheet down to the slab and leave it for a week. After that, is there any moisture on the plastic, or has the concrete darkened? If not, water isn't really a concern.

Also, do you have perimeter drainage around the slab?

Yours, Larry

DCContrarian

Moisture doesn't hurt concrete in the least. In fact it makes it stronger. Mold growing in the concrete on the other side of a vapor barrier isn't a threat, the soil on the other side of the concrete is chock full of mold. You want to keep it on the other side of the vapor barrier from you, that's the whole point of a vapor barrier.

A dimple mat or drain board is a terrible, terrible idea. The idea that you somehow need to ventilate the space under a concrete floor is something promoted by the marketers of drainboard, it's scaremongering and ignorant of the science. They promote the idea of a "drying space" under the floor. If there is moisture under the floor you want it staying there, you don't want it drying into your living space.

All of the above assumes we're talking about moisture coming out of the soil and into the concrete through capillary action, also known as "rising damp." If there is liquid water intrusion, it needs to be dealt with by giving the water a place to go and disposing of it. A drainboard is a bandaid patch for liquid water, it conceals it and doesn't solve the underlying problem.

DCContrarian

https://forum.heatinghelp.com/discussion/197601/radiant-heating-over-slab-on-grade

I want to push back on no Warmboard. Why?

In general, poured concrete makes a terrible residential floor. It's hard and uncomfortable. The latest thinking is that you want your heat emitters to have as little heat capacity as possible, that way they are most responsive to changes in the heating load. If you look at the Warmboard website they tout their "high responsiveness."

I'm not a fan of Warmboard per se, it's awfully expensive for what you get. But you can do better in terms of comfort than poured concrete.

PhilipXP

@Larry Weingarten Can I do that test any time of the year or should it wait until the rainy season (which we don't have in Colorado) ? We don't have a perimeter drainage. The house has concrete all around (patio and walkway) which I suspect has been strategically put there to seal the ground and let water run off), but I don't know how effective that is.

@DCContrarian I should have mentioned we want to install engineered wood flooring on top of the overpour. The main reasons for opting against warmboard are a) cost and b) warmboards need to be screwed into the concrete, but there is a mastic cover on some sections of the slab and this mastic contains a small amount of asbestos which we don't want to drill into.

As for responsiveness: that is a concern I had. I saw some people talk about using thermometers that take the outside temperature into account to give the radiant system a heads-up. I haven't looked into this at all though.

Larry Weingarten

Hi @PhilipXP , There can be no harm in doing the test now. It could give useful information. Have you ever seen efflorescence on the slab or perimeter foundation? That would indicate water coming through.

Yours, Larry

DCContrarian

If your house is old enough to have asbestos mastic it's not going to have insulation or a vapor barrier under the slap. I agree that the best course of action is just to bury the asbestos and not disturb it.

One approach would be to put down 2" of foam insulation and then two layers of 7/16" OSB, with all seams overlapped and the two layers joined with 1-1/4" screws. That would make a large floating subfloor that would be very flat and very solid. Then you could put 3/4" plywood strips with tubing and aluminum plates, and floor over it.

The foam is something of a vapor barrier but you could put a poly sheet under it as extra insurance.

DCContrarian

We've talked about water intrusion and rising damp, the third pillar of the basement moisture triad is condensation.

If your interior air is at 72F and 50% RH, that's a dew point of 52F. Any surface at or below that temperature will cause condensation to form. In cold climates it's very common for basements to be that cold much of the year. You need to have an insulated, impermeable layer between the cold walls and the warm interior.

Condensation is an issue in any wall assembly in cold climates, but it's particularly an issue in finished basements as there's no place for the moisture to go.

hot_rod

How much floor height can you give up? 1- 1-1/2" of foam would be nice if there is no under slab insulation, tape the seams for vapor barrier. Staple tube to the foam.

How the radon out there, might have that tested and mitigate it in the process.

PhilipXP

We have a high ceiling on the main floor, so we do have some liberty to give up floor space but still don't want to add too much to the floor, if possible. We are replacing doors anyway, so that would not be an issue. The "loftiness" of the space is really what makes it so great, so we want to preserve that as much as possible (old houses in Denver tend to have a bungalow style with very low ceilings on all floors).

Some examples: with Uponor Fasttrack 1.3 we end up at a total added floor height of 2.33" while with Createrm R5 it is 2.87". That includes the flooring. One big unknown is the overpour. I went through the radiant heating system supplier's materials and found minimum overpour heights ranging from 0.31" (that one was mentioned in the context of a fiber-reinforced compound) to 1.4". The floor is really cold in winter, so more insulation is probably better.

@DCContrarian Is what you are suggesting similar to the Rehau system? ( https://www.rehau.com/us-en/mechanicalplumbing/search-by-product/above-floor-panels ). You talked about responsiveness (which I agree is a big plus). What other advantages do you see with a system like this? And don't these systems all require mechanical fastening into the concrete?

@Larry Weingarten So I went around the house and didn't see any efflorescence. I don't know if there is any on the slab as the old floor has not been removed yet fully. In case there is any efflorescence on the slab, how would you proceed?

DCContrarian

"And don't these systems all require mechanical fastening into the concrete?"

The idea is that with a floating subfloor you don't have to do any mechanical fastening into the concrete. A double layer of plywood or OSB will end up weighing hundreds if not thousands of pounds, if fastened properly it's all one big unit with little flex. If the fasteners are slightly long they protrude a bit into the foam and provide resistance to slipping.

The laminate floor is a similar concept, they typically float and the fact that it's all held together and friction is enough to keep it in place.

The Rehau does look like what I have in mind. What I don't know is whether that could be the second layer of a floating subfloor or whether you'd need two layers of underlayment and then the Rehau.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1807788#Comment_1807788

A floating laminate floor requires a stiff underlayment. It can't go directly over foam, if it flexes too much the tongue and groove connectors wiggle and eventually snap.

DCContrarian

The other thing to look out for when raising your floor is that your stairs won't be right anymore, the last step will be too shallow.

Larry Weingarten

Hi @PhilipXP Given what you say about the cold floor, I like @Hotrod 's idea of foam board. If there is any sign of water in the concrete, perhaps putting down a layer of polyethylene first would insure that the foam doesn't have to deal with a moist environment. Even if you simply put foam under a new floor, you'd feel the difference as the floor will no longer be pulling so much heat from you. Making the assembly a floating floor as @DCContrarian said, sounds like the right way to go.

Yours, Larry

hot_rod

I guess it comes down to the need for some insulation value.

I have used the double plywood method, construction adhesive and shoot it down.

Maybe start with 1/2” pressure treated plywood, Zip sheathing or Plytanium next for a nice flat surface

Any way to get some insulation around the edge of the slab, that is a big heat loss area also.

With lofted ceilings, maybe lots of glass? I would do a heat loss calc first

PhilipXP

https://forum.heatinghelp.com/discussion/comment/1807815#Comment_1807815

I can only insulate the edge of the slab from the inside. The outside is surrounded by concrete (patio, walkway).

PhilipXP

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Thanks again for all the input. Much appreciated!! Just to confirm that I processed all the info correctly I created the attached diagram. Does it look right?

Since this is the main floor, furniture (including kitchen counters and appliances ) will be on top of the flooring, plus people. I won't put pipes under the kitchen cabinets, but are there any concerns with too much mass on a floating subfloor?

As for the edge insulation, I assume it should extend up to the upper edge of the flooring. Are there any edge insulations that look nice enough next to an engineered wood floor? I've seen photos of cork being used for that purpose, which looks acceptable.

hot_rod

Bevel a 45• edge on the foam and let the OSB and flooring go over it.

Maybe consider Zip sheathing or Plytanium instead of OSB, it holds fasteners much better for the transfer plates, especially in 7/16" thickness.

It is more $$ however.

PhilipXP

One more question: Would you think a similar approach is doable with sleepers instead of a floating subfloor? I'm thinking of insulation between sleepers → radiant on top → flooring, leaving out the OSB panel. I assume the sleepers will act as a thermal bridge which will diminish the efficiency of the system, but it would reduce the height of the whole system?

hot_rod

I've put hardwood right on top of these Roth Panels. You get some R value and the ability to use 6" on center tube.

Did you do a load calc yet?

That edge detail with an outdoor slab against the mindoor is going to be a huge heat sink, it should be calculated into the heat load calc. I like this program as you can detail the insulation options.

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