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Radiant Slab Retrofit

Can a slab foundation accept a radiant hot-water heating system?

Richard Trehewey holds a toilet

I'd love to install radiant floor heating to replace our existing gas forced-air system. Can our house be modified to accept a radiant hot-water heating system if it has a slab foundation with no basement?
— Evan, Toms River, N.J.


Richard Trethewey replies: Your fuel type isn't a problem; gas service can easily be set up to fuel the boiler for a hot-water — or hydronic — radiant system. The real obstacle here is the slab.
Without a basement, you can't retrofit a radiant system without ripping up the floors. And the new system installed on top of the slab will increase the height of your floor, which means you'll probably have to trim the bottoms of all your doors, at the very least.
How much height is added depends largely on the kind of system you install. A hydronic system — the type I recommend for your climate because of its efficiency — uses grooved floor panels that are about ½ inch thick. Electric
in-floor heating is thinner, about ⅛ inch, but is expensive to operate unless you happen to live in a place where the kilowatts come cheap. It's an option suited to supplemental or spot heating, but rarely used to warm an entire house. With
either system, make sure it's placed over a vapor barrier and some kind of thermal break, such as a layer of ¾-inch plywood or ½-inch Homasote. Without this layer, the slab would continuously suck your floor heat into the ground.
One more thing: Before you invest in radiant heat, consider some of the other, less expensive ways to improve your comfort, such as adding insulation and weather-stripping, or replacing your old furnace with one that's more efficient.


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