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Why You Have a Cold Basement

April is Indeed the Cruellest Month...


Downstairs basement of modern home with wood trim David Papazian/Stockbyte/Getty Images
Refinishing your basement means combating some tough enemies: moisture and cold. If you want to be able to use your basement as a playroom, guest bedroom, apartment, or theater, you need to understand why your basement is so cold before you start adding additional heat sources.

1. Basements Run 69 Days Behind in Temperature

It's the winter months that make spring in your basement so chilly.

Studies in Canada--a nation that knows about cold basements--have shown that exterior foundation wall surfaces take about 3 days to react to changes in outside temperature. At the exterior base of your foundation wall (about 6 feet down), it takes about 69 days to catch up to outside temperatures.

That's one reason why temperatures in your basement often feel so...unseasonal. Basement foundation walls can literally be one season behind, transmitting those temperatures into your basement.

2. Solar Heat Gain Upstairs Affects Basements

Do you have a single-zone heating system? With this type of system, one furnace supplies heat to both the basement and upstairs. The thermostat is located upstairs.

Solar heat gain warms the upstairs during the day but it does not warm the basement. After all, upstairs has the windows, downstairs does not.

As a result, upstairs' temperature is higher because: 1.) Heat pumping from the system rises and congregates upstairs; 2.) The added solar heat gain. So, throughout the day, your furnace is kept in check by that upstairs thermostat--to the detriment of the basement.

Cool spaces need a long time for heat to build up to become comfortable. By the time you get home at the end of the day and go down to the basement, the cold you're experiencing is the result of a day's worth of reduced heating cycles.

3. Snow Likely Not the Culprit

Snow: it's just got to make your basement cold, right? Researchers have found that snow accumulated on the ground near your basement walls have almost no impact on basement temperatures. It's only when snow exceeds 2 feet that temperatures in the basement can start to be dramatically affected.

4. Humidity Makes It Feel Colder

Basements are inherently damp places, even if you don't have active sources of water. Cool temperatures plus humid air makes one feel cold. Humidity can create other problems, as well. Cold foundation walls subjected to 20-30% relative humidity will cause condensation, which can lead to mold, mildew, and rot.

5. You've Got Imbalanced or Poorly Sealing Heating Registers

If you're the person with the single-zone heating system for upstairs and downstairs, it's a big mistake to open/close your heating registers equally.

Each room has its own temperature, and so it has its own heating needs. By shutting down registers in south-facing rooms that experience solar heat gain, and opening up registers in the basement, you can drive more of this valuable heat to the cold places. More to the point, you prevent those cold basement areas from experiencing severe temperature dips during the day. These dips are hard to correct when the sun goes down and you want to spend time in your basement den or theater.

How do you go about opening and shutting registers on an hourly basis throughout the day? Don't you have a job or school you need to tend to?

Motorized registers will do the opening and closing for you. These battery-powered units are individually controlled by a thermostat in each room. When heat rises in the room, the register shuts; when the room cools, the register opens. This helps keep the entire system in better balance. Activent (link below) is one manufacturer of motorized registers.

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