Q: Our house is over a crawl space and is constantly cold. Last year we spent a ton to have it insulated, and it did not help. Now my husband wants to run a vent into the crawl space to heat it. Is this a good idea? — Rebecca, Plymouth, ME

A: Richard Trethewey replies: You can run a duct in a crawl space, but you must take some precautions. First, the duct should be insulated for two reasons: to keep the heat in the duct until it can get to the room and to keep the duct from sweating and rotting away. A hot duct in a very cold space can cause water vapor to settle along the duct, causing rust (on metal ducts) and mold (on any duct). Neither one is a good thing.

Next, the duct must be large enough to deliver enough heat to make it worthwhile, and the room will need a return duct to allow air back to the furnace to be reheated.

If it were my house, I would add radiant floor heating, which involves attaching special plastic piping called PEX to the underside of the floor and circulating warm water through it. There is nothing like it in a cold room.
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