Dear Umbra,

We have a gas fireplace and a new baby girl, so we have decided to not use the fireplace this winter. What is the best way to keep the cold air from coming in through the fireplace?

Rochester, N.Y.

Dearest Jeff,

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First of all, congratulations! There are “hearth gates” one can use to fend youngsters away from stoves and fireplaces. A hearth gate is similar to a baby gate, but longer, articulating, and heatproof. You may wish to look into it.

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Baby, it’s cold inside.

All unused fireplaces of any sort should be sealed up to stop leaking warm air out of the house, or letting cold air in, or both. We’ll discuss gas fireplaces first, then regular old wood-burning ones.

Jeff, before reading your note I honestly had no idea that gas fireplaces could leak cold air, but a thorough surfing through the chat rooms of the virtual hearth led me to understand that it’s a common problem. Trouble on my end is, I’m not clear where your fireplace leaks, which kind of venting system you have, or how tight your house is in general. Also, I’m not a contractor. So instead of giving you a specific answer, I’m going to quickly review the potential leak causes and solutions. Actually, the very first solution is to call the manufacturer (or the installer, if you know them) and go over the problems. In general, I do not think it’s so good to mess around with a gas appliance unless you know exactly what you are doing.

As to what the problem may be, it may be the fireplace was poorly installed and is leaking around the “doghouse” (the boxy exterior), around the gas line or venting pipes, or around the glass. The glass clamps you purportedly should be able to check yourself, but I would have someone professional come look at the other two problems. Apparently in a certain installation the chimney damper itself could be the problem — here again I would at least call the stove dealer. Aren’t I helpy?

There are potential issues with having either a too-tight home or a non-airtight home; in the latter case, the house would be losing air out the attic and sucking new air in through the vent. You might want to check out my tips on insulating if you think you have draft issues.

Those of us with wood-burning fireplaces should also be taking action this winter. The EERE says [PDF] the fireplace accounts for 14 percent of air lost out of a home — more than the windows. So before we get all excited about window glazing, it pays to stop up the chimney. At the very minimum, close the damper when you’re not using the fireplace. If you know for sure you will not use it for an extended period, plug and seal the flue. It’s an opportunity to use a chimney balloon! Yay! They inflate inside the chimney and stop it all up. You can find one at a useful hardware store or on the internet. Don’t forget to put a note on the fireplace, so that no guest or forgetful resident lights a fire in the plugged chimney. I think the balloons are designed to ignite and disappear in such a circumstance, but it’s better to be safe than sorry.