Monday, October 3rd, 2022 by Aaron Stull
Is the floor or room over your crawl space colder than the rest of the house in the winter? Have you tried to insulate the floor only to find it had little to no benefit? This is a common problem in homes built over a dirt crawl space or with a basement that’s unheated and uninsulated.
If you’ve been researching how to make the floors over your crawl space, or basement warmer, then you know there is a lot of information on the best solution. The key takeaway from the research is that to make cold floors and uncomfortable rooms over these areas warmer, you need a continuous air barrier and proper insulation in place to stop cold air. We’ve put together this guide to explain what causes cold floors (and rooms) over the crawl space and basement as well as solutions for making them warmer.
Why is the floor over the crawl space or basement is cold?
Houses suck in cold air
In the winter, the warm heated air inside the house rises to higher levels. This causes a positive pressure differential in the higher levels. As the warm air rises through the house and escapes out through leaky windows, the attic and roof, this creates a negative pressure in the lower levels. This is called the “Stack Effect.” The stack effect, or chimney effect as it is sometimes called, draws cold air from outside into the basement, crawl space, garage, and first-floor windows. To add to the problem, most basements and crawl spaces either don’t have insulation at all or don’t have proper insulation. If nothing is stopping the cold air from reaching your floors and the rooms above, it’s no wonder they are cold.
Problems with fiberglass insulation
Insulation works by trapping pockets of air that slow the flow of heat out of the house in the winter (and into the house in the summer). The R-value of the insulation is an indicator of how well the insulation resists this movement; the higher the R-value, the more effective it is at lowering heating and cooling costs and maintaining comfort.
Fiberglass batts are inexpensive and quick to install, which is why they are the most common type of insulation found in crawlspaces. However, challenges with properly installing fiberglass batts make it a poor choice. Batts are tested in a closed cavity, not an open bottom cavity such as the one that exists between floor joists under the floor platform. This dramatically reduces the R-Value performance. Additionally, in the summer months, moisture-laden air saturates the insulation, making it heavy and prone to sagging and dropping. This creates many thermal breaks and diminishes the performance of the batts in the winter.
Lastly, the crawlspace ceiling/first floor is not the proper placement for the thermal barrier (the line of demarcation where the outside of the house meets the inside of the house). The crawlspace floor and sidewalls are the appropriate places. For all of these factors insulating the floor joist cavity is a poor strategy.
Cold Floors Over Crawl Space
What you should know
Adding spray foam or rigid foam board insulation to the underside of the floor can help warm up the floor and rooms above the crawl space. However, this approach makes it difficult to access plumbing and electrical wires when the need arises. Another common mistake that homeowners make when repairing the crawl space is adding plastic (or a vapor barrier) to the underside of the floor joists in an attempt to make their floors warmer in the winter but in doing so can trap moisture that can lead to mold and rot.
The best approach
Sealing or “encapsulating” (insert appropriate KBS hyperlink) the crawl space is the most effective way to improve the floor temperature and much more. In addition to solving the cold floor problem, you will prevent mold and rot that is commonly found in unconditioned crawlspaces. This typically requires help from a crawlspace professional like Keystone Basement Systems. For detailed information on this process, please visit
If you’re worried about how your unheated basement or crawl space is impacting your home’s comfort and energy efficiency, check out our tips to winterize your basement or crawl space. You can also always contact Keystone Basement Systems for a free evaluation of your home's crawl space!
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