Why Is My Basement So Cold?

Refinishing your basement means combating tough enemies: moisture and cold. If you want to be able to use your basement as a playroom, guest bedroom, apartment, or home theater, you need to first understand why your basement is so cold before you take on projects to warm it up.

What Temperature Should Your Basement Be?

The expected unconditioned basement temperature varies according to your area and to the season.

One rule of thumb to help determine the basement temperature is to average out your area's temperature across the entire year. In the summer, the basement is often cooler than the outside air. In the winter, the basement is often warmer than the outside air.

The theory holds that unheated, uncooled basements generally follow the area's annual temperature. Your level of comfort is a different matter, though.

Ground Level Cold Is the Main Reason

Viewing a basement prior to finishing, you may look at the below-grade walls and flooring as the source of all of that cold. Surely these broad expanses contribute to all of the coldness that you feel, right? 

The greatest offender, though, is ground-level cold. Walls that extend above grade, vents, ducts, windows, and more act as freeways that allow cold to cascade into your basement. 

One poorly insulated dryer vent and duct will chill your basement far more than an entire below-grade basement wall.

Moisture Makes a Basement Feel Colder

Basements are inherently damp places, even if you don't have active sources of water. Cool temperatures plus humid air make one feel cold. Humidity can create other problems as well.

Cold foundation walls subjected to high humidity will cause condensation, which can lead to mold, mildew, and rot.

Running dehumidifiers is an effective way to reduce moisture in basements. Look for leaking water from pipes or from the outside. Make sure that the basement's sump pump, if any, if working properly.

Basements Run Behind in Temperature

Winter months 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 three days to react to changes in outside temperature. At the exterior base of your foundation wall (about six feet down), it takes about 69 days to catch up to outside temperatures.

That is 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.

Upstairs Solar Heat Gain Confuses Your Heating System

Do you have a single-zone heating system? With this 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, the upstairs temperature is higher because:

• Heat pumping from the system rises and congregates upstairs.
• 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.

Poor Heating Registers Turn a Basement Cold

A single zone heating system is one where the HVAC or furnace pumps heat to the entire house equally. All heating registers, at all times, receive and pump out the same heat. While this isn't the most desirable set-up, it can work for single-story, modestly-sized homes. With an upstairs and a downstairs, this will not work.

Each room has its own temperature, with its own heating needs. By shutting registers in south-facing rooms that experience solar heat gain, and opening registers in the basement, you can drive more of this valuable heat to the cold places.

More to the point, you prevent 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.

The best thing to do, if possible, is to physically open and close heating registers throughout the house, throughout the day and night, on an as-needed basis. Few of us can do this. Fortunately, there are motorized registers to do the work.

Motorized registers, such as those from Activent or Vent-Miser, do the opening and closing for you. These battery-powered units are individually controlled by a thermostat in each room. When the heat rises in the room, the register shuts; when the room cools, the register opens. This helps keep the entire system in better balance.