In First-Floor Living Spaces
The best way to retrofit radiant without disturbing an existing floor is to do it from below, against the underside of the subfloor. In most houses, the only place to do this is on a first floor where joist bays are open to the space below. It can go under most existing flooring, including wood, tile, vinyl, and even carpet.

The retrofit starts with aluminum tracks screwed between the joists, which hold PEX (cross-linked polyethylene) tubing carrying hot water. "Plastic like PEX is not a good transfer medium," says Richard. "So the aluminum sucks the heat out and conducts it to the subfloor." The tracks keep the tubing evenly spaced at 6 to 8 inches to avoid cold spots. The tubing snakes uninterrupted from bay to bay through holes drilled at least 2 inches from the edges of each joist. In a kitchen or family room, though, Richard takes care not to run it under cabinets or other built-ins. "You don't want to trap the heat under a sealed box," he explains.

Insulation beneath the tubing is critical to keeping the heat moving up into the floor, rather than dispersing every which way. At the project house, the crew will spray foam insulation over the tubing between the joists, forming a tight seal.

A radiant system needs a hot-water supply, a connection between the tubing and the plumbing, and a pump to move the water from one to the other. In a first-floor retrofit, all these either already exist or are easy to locate nearby.

The TOH project house has a manifold in the basement where all the tubing originates and returns. It connects to a circulator pump. Each room's tubing feeds into the manifold's supply, loops around under the floor, and reconnects at the return. Every room or zone has a separate thermostat; turning it up sends warm water to the room, raising the room temperature. Turning it down shuts the water off.

The manifold gets its hot water from the boiler, which has a special mixing valve that's set to deliver water at no higher than 140 degrees F. This limit, explains Richard, ensures that the floor's surface doesn't rise above 85 degrees, which can break down tile mastics and warp wood. At the project house, the temperature in the tubing will rise and fall with the weather—running between 85 and 90 degrees on winter's mild days and as high as 130 or 140 on the coldest days.

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