I’m Radiant — Radiant Floor Heating, That Is

The remodel is starting to take shape and we as homeowners have to continue to make choices about many finishes. If we don't make choices, our contractors will choose the default. So we're selecting tubs, showers, flooring, and so on.

Just as fiberglass is the default for insulation, in our house which already has forced hot air, forced hot air is the default for heating. But I've never liked forced hot air that much. I find it noisy, I think the vents are ugly, and I really don't like the temperature fluxuations during the heating cycle and the uneven heating of the rooms (warmer near the vents, cooler near the far side of the room). I know that some of these deficiencies will be lessened by insulation, but I got to thinking about radiant floor heating and began to do some research.

Radiant floor heating is prized for how evenly it heats a room and people who have it swear by it. It is, however, hideously expensive and you will never recoup the cost of installation in energy savings. But as a comfort splurge, it sounds pretty good.

It comes in two basic types: electric and hydronic (forced water). The the prices of electricity around here, electric is cost-ineffective to operate for anything more than a small room. So I was looking at hydronic. Basically, a hydronic system uses hot water that runs in PEX (cross-linked polyethelyne) tubing underneath your floor surface. You can embed the PEX in concrete (or lighter-weight gypcrete), attach it under the subfloor, or attach it to the top of the subfloor (in which case you have to have something on either side of the PEX runs to keep them from getting crushed by the floor itself).

For a renovation like ours, gypcrete is impossible -- the building can't take the weight. Also, concrete is a terrible conductor of heat and, while the prevailing thinking for years was that once the concrete got warm it would stay that way forever, modern thinking has realized that this is akin to arguing that once a concrete skillet is warm it'll stay that way: that's not how people use skillets. In general, you want your house to be cooler at night and warmer during the day and to be able to react reasonably responsively when you change the thermostat temperature. So most modern systems use aluminum.

There are a host of manufacturers that basically build aluminum tracks into which you lay the PEX that typically reside under the subfloor and radiate heat through it. Vendors of this type of system include Roth, Wirsbo, and Rehau. But, on the recommendation of Philip Desautels on the east coast, I began to look closely at a product called Warmboard. Warmboard is different: it's a structural 1-1/8" plywood T&G subfloor with grooves for the PEX and aluminum bonded to the top with epoxy. It comes in 4x8 sheets like subfloor and is installed the same way -- it doesn't take a plumber to install it, it just goes in like subfloor. At least, that's what my regional sales manager, a nice guy named Barry LaDuke, says.

Warmboard isn't cheap -- it comes ot about $6/sq. ft. and that doesn't include PEX or the new boiler we'd need. In all, I expect installing Warmboard in the house to be about 5x more than just running sheet metal from our existing furnace. At the same time, it still feels like a good idea for the comfort factor.

The price may still cause us to halt, as may the further research our framing contractor is doing. But right now, it's looking like we'll be one of the first on the block (heck, first in the city) to install Warmboard.

Comments (3)

  1. Paul Vick says:

    Our 1921 house was built with a forced water heating system (using radiators) that managed to fail, freeze and burst just before we started renovations. So we really had no choice but to redo the system, and I’m glad we did!

    In the end, we went with radiant flooring in our kitchen and bathrooms, both of which have a tile floor, and it’s worked out just great. I would *highly* recommend radiant flooring to just about anyone — I think we might have even used the Warmboard (the details sound familiar, but it’s been a long time). For the rest of the house (which has hardwood floors), we stuck with radiators which are equally as great — where they fit we still use old-timey radiators and where they don’t we installed baseboard radiators that are super-unobtrusive (the only downside being that you need a long run along the wall to get enough heat output).

    Drop me a line if you’d like to know more about our experiences…

  2. johnmont says:

    I’m particularly curious what brand and size of boiler you wound up using for how many square feet. We’re looking at a Munchkin 300K BTU boiler to power both the floor and to run a loop into a water heater.

  3. schopenhauer says:

    hi there,

    Regular concrete has 20 times the thermal conductivity of wood, so you are MUCH better pouring concrete over the pipe than tieing it to an expensive aluminium construct, which, being beneath the floor, can’t even hope to push radiant heat into the house.

    My dad built radiant heating for our house in New Zealand (we had free hot water from a backyard spring!). Some was in polyethelene, bhut most was in galvanised pipe, all bedded in a 3 inch slab.

    Wow was that house comfortable.

    The thermal mass of the concrete does mean that you keep on getting heat for 2-3 hours after you spin the system down.

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