Ask the Remodeler: Tips for lowering your basement’s humidity

By Mark Philben -- Globe Correspondent

Q. We moved into the first floor and garden level of a South End brownstone that has a basement under the stairs. There is a lot of humidity from that basement to the point that National Grid is coming to replace the gas line into the building due to corrosion. The mildew smell is bad when we don’t have on the heat. We are running a dehumidifier in a bathroom, but it doesn’t work that well because it is far from the basement. Now that we have moved everything out for National Grid, I’d like to do something to lower the humidity. Is there a paint/coating we could put on? Do we need to put in drainage for the dehumidifier? What kind of firm should I hire for this type of work?

J.H.

A. I am guessing that the foundation is stone, which is difficult to damp-proof. Not knowing all the parameters of your basement, there are three things I would recommend:

1. Repoint the stone foundation as needed. Often we find the mortar in many parts of an old foundation has deteriorated to dust. This is a laborious process for a mason but will hold back some moisture. If the foundation is not terribly uneven, you can apply a moisture barrier paint that will help.

2. Assuming the basement floor has a poured-concrete slab installed over the old dirt floor, an epoxy concrete floor paint will help hold back some moisture.

3. Finally, a dehumidification system installed in the basement will help immensely. Of the three things I mention here, this will be the most effective. It will have a condensate line carrying excess moisture wrung out of the air that needs to be connected in some way to a plumbing line or to a utility sink if you have one nearby. This can be tricky, and you may need a pump to draw the condensate water up into an available plumbing drain line.

Q. For 10 years now, I have had a gas heating system with an added zone for making hot water, and it is great. There is one strange thing I’ve noticed recently, however, that has me stumped. Whenever the hot water cycle is used, I find myself with warm radiators in another zone. The pump is not running in that zone, but the hot water is flowing through — and backward, of course, not in the line with the pump but in backflow. Have you heard of this before? I’m reluctant to call my plumber; he may think I’m crazy.

J.M.

A. You should absolutely call your plumber because one of your zone or check valves may be staying open when it is not supposed to be. All zones will have some way to close when not being called on, whether it is electronic or a built-in check valve. If for whatever reason that valve gets stuck open, hot water will gravity feed into the wrong zone. This is not a complicated fix, but you should call your plumber or heating contractor.

Mark Philben is the project development manager at Charlie Allen Renovations in Cambridge. Send your questions to [email protected]. Questions are subject to editing.

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