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I'm trying to figure out the most ideal way to install hydronic radiant heat in my basement over the existing concrete slab. I've looked at several products for heat distribution, like Warmboard, before settling on this: http://radiantengineering.com/tfinu

I'm trying to figure out the ideal way to install it with the minimal loss of height. I was provided a sample of the aluminum fin and its height measured to just under 3/4". In their brochures and pictures, the fins are shown attached downward but I'm thinking it may be better to face them upward. I'm also planning to go with an engineered hardwood floor atop.

My current best guess to the ideal way of doing this is to first start with a 6 mil poly sheet as a vapor barrier. Next, get ~1/2" OSB boards and cut 1/4" dado grooves 8" OC where the fins will be placed and make a floating subfloor out of it. Next, I'd cut the same kind of 1/2" OSB into sections just under 8" in width and place them in between the groves. This should provide enough height for the fins to fit in. Finally, I'll use 1" screws (still figuring out that part) to fasten the fins to the two layers of OSB. I've attached a quick and dirty picture to illustrate what I mean (screws are not to scale).

Thermofin U with subfloor

I'm hoping someone could chime and provide some feedback to this approach. Or am I totally missing something really obvious here? I don't want to lay the PEX down and pour concrete over it because that goes into a whole other topic about thermal mass and heat loss.

Thanks

  • Your only thermal break between your hydronic heating and your existing concrete slab is 1/2" OSB, routed down to 1/4" under the lines? You're going to lose a lot of energy heating the slab/ground if there isn't some better insulating layer, like foam boards. From what I've read, the usual approach is: existing slab, foam for insulation/vapor barrier, subfloor with radiant tubing paths, finish floor. – Shimon Rura Nov 23 '15 at 20:05
  • My house is a relatively newer construction and there's foam under the slab. That said, I have reservations with putting a lot of weight over foam. If some heavy weight were placed over a particular spot for a long duration (say, a loaded bookshelf), wouldn't the foam compress and thus cause the flooring to cave down a bit? The only other option I thought of was some kind of reflective layer but I'm not particularly sure if it's appropriate and, if so, what kind. – vlozko Nov 23 '15 at 20:24
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    If your foundation is actually properly insulated on the exterior, you can not worry about insulation inside (or heat loss from pouring another layer with the tubes in it.) But XPS of the proper grade does not compress (if it did, the stuff under your slab would be squished flat by your house.) XPS can be had in various compressive strengths, but even 25 pounds per square inch will hold up a lot of bookshelf... – Ecnerwal Nov 24 '15 at 19:31
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From Radiant Engineering, thanks for discussing our ThermoFin U product. The preferred method is to install the plates with the fins down. For a ThermoFin U installation on concrete, you would build a surface to be able to screw the plates into by first putting down a layer of Atlas Rboard®, recommended 1/2", which you should be able to get from your local building supplier. A Google search on Atlas Rboard® will bring up their site and the product info.

On top of the Rboard® you would put down a layer of plywood. The fasteners will hold best if you can use 3/8" plywood. It will work with 1/2" plywood if necessary.

Keep in mind that you want the heat energy to be going into the plywood sleepers and the finish floor, which is why we recommend the Rboard®.

The energy without the layer of insulation will be drawn into the concrete, and the response time would be longer in heating the floor. Heating up the mass of concrete is not necessarily a good thing when you want the entire surface floor to be warmed. The ThermoFin U heat transfer plates are easier to install if the fins are down against the subfloor and plywood sleepers on the fins. See the PDF multipage spec sheet that is linked on the U-fin page of our site. You are making a sandwich of aluminum and plywood to evenly heat the structure and provide a friendly surface for the flooring installation. You can call or email us for more help. We also have routed 180 degree return bends for the PEX tubing when using ThermoFin U.

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assuming you are going to put a finished floor over top of all of this, you are going to have to add another layer on top of the radiators to subfloor it. this is going to add another 1/2" or so, 5/8" really. so your total floor height is going to be 1.625" or so, before you put your flooring on. you could easily pour a concrete floor at this thickness if you use use fiber reinforced mpa30 or higher concrete (air entrained or not).

to minimize thickness, you can use foil insulation liner to act as a thermal reflector. just lay it on the old slab prior to pouring the new concrete.

concrete will be cheaper to install than OSB, be more stable and have far higher thermal mass (which will take longer to warm up but will radiate the heat longer so its actually more efficient from a heating/cooling cycle perspective)

however, I tell this fact customers all the time. 1 1.5kw electric space heater costs 15cents to run for an hour. even if you left it running 24 hours a day for 4 months (the typical heating season), it would only cost you $400 to heat that space for that period of time. you will spend 5 times that just to do the work you are talking about, not to mention you will still have to pay to fire the boiler to heat the water to run through the radiant floor.

its usually way more cost effective to add 3.5" foam insulation throughout the whole height of the basement wall. you will almost always save more money conserving the heat you already have then you will trying to put more heat into a space. and it will be more comfortable.

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