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Are heat blocks effective without dead air space?

I'm working on installing STEP warmfloor radiant heat, and I'm looking for a material to install under the heating element that will limit the amount of energy lost to the crawl space under the house.

So far I've only been able to find products aimed at hydronic radiant heat systems, rather then the heat pads like STEP, and those are all to be installed under the house, rather than on top of the subfloor. Is there a product that i can install directly under the element?

Asked by Evergreen State College
Posted Dec 5, 2012 3:46 PM ET


1 Answer

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First, resistance electric heating of any type is a pretty inefficient not-so-green use of electricity- are you sure you're committed to that route for heating?

Sure, it's "100% efficient" between the low-voltage controller and the heating pad, maybe 95% efficient from the transformer to the pad, but far less efficient between the generator & pad, even worse when looking at source-fuel or source hydro-head potential to the pad. Heat pump solutions are far greener, and ductless air source heat pumps will hit north of "300% efficiency" in Olympia WA.

Even tied directly to the DC output of solar panels you'd be at best about half as efficient as hydronic solar solutions.

The web-info discussions seem to allow use of rigid foam directly under the subfloor, and as long as the flooring above the heating mats is sufficiently low-R even putting the mat directly on the foam would keep the localized temperatures under the max operating temp of polystyrene, but it would have to be engineered. It takes at least R10 to get good zone isolation between the radiant floor room and the space below, which would take 2.5" of EPS o 2" of XPS, and the rigidity of what ever subflooring flooring combination would have to be quite stiff to keep it from developing air gaps (a real no-no with this stuff) over time. Putting something like half-inch OSB between the mat & foam would stiffen it up some.

Putting batt insulation or blown cellulose snug to the subfloor would likely be an easier/cheaper solution that wouldn't eat up headroom in the conditioned space.

But I still can't fathom using if greenlieness is your goal. It's any thing but "green", marketing hype references to Leed IP/EQ/LT/MR notwithstanding. They claim:

" The products are designed to offer energy efficient systems and are backed up by calculations to prove it."

But somehow they neglect to spell out what those efficiencies are, nor do they provide the calculations. If the assertion is that it's more efficient than crummy 1980s vintage electric radiant based on the quality & precision of control, I'll buy that, but it's a "Yeah, so?" kind of deal. From a raw energy use point of view you'd literally be better off insulating & sealing the crawlspace walls and heating the crawlspace to 80F or more with ductless mini-split to heat up the floors. Not that I'm recommending that approach, but it would use less than half the electricity of low-voltage mat solutions.

Answered by Dana Dorsett
Posted Dec 5, 2012 4:58 PM ET

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