To Block... or Not to Block Crawlspace Vents during the Winter?

Non-insulated crawl of typical American home.
Non-insulated crawl of typical American home.
Slobo Mitic/iStock

Should you block your crawlspace vents? This is a very common question. Especially in areas that have both a reasonably hot summer and fairly frigid winter. There is not much question that in general these vents are a needed and necessary part of a well maintained home. These vents are placed within the crawlspace to allow the moisture that builds up underneath to have an escape route. Without such a moisture release, you could be looking at mold, mildew, and rotting wood over a matter of time. None of which are a pleasurable experience to deal with. But these vents have been known to create a cold draft in your floors, which can reduce the energy efficiency of your heating system.

So the question remains, will a few winter months with your vents blocked off do all that much harm? The problem with this question is that there are a lot of factors that create a home to be more susceptible to moisture build-up than others. Not to mention a lot of differing opinions from experts. The Answer is "No", but with Options.  The safest and most commonly accepted code for any home is to leave crawlspace vents in place year round. However, there are a few things you can do to help combat those cold winter floors and reduced heating inefficiency due to drafts. One alternative is to insulate your sub floor, another is to beef up your insulation in the crawlspace itself, but which one is best depends on the type of crawlspace you are dealing with. 

Option One and Two, Depending on Type of Crawl Space:

In simplest terms, there are crawlspaces with water pipes and those without. Crawl spaces with pipes will either have the concrete walls around the perimeter coated with insulation, or the pipes themselves will be insulated or wrapped with heat tape (sometimes both). If your crawlspace has no pipes, or the pipes are self insulated, you can get away with adding insulation to the sub floor to inhibit the cold air from reaching your living space above. If your crawl space has uninsulated pipes and insulation around its walls, it was most likely designed to allow a portion of the heat from the living space above to escape below to keep the pipes from freezing. In this case it would be highly unadvised to insulate your sub floor, as that would be taking away from the one source of heat your crawlspace has. What would be most beneficial for this scenario is beefing up your existing insulation around the concrete walls to keep the area warmer. 

Option Three, Temporary Vent Block Off:

When you know you've got a really bad cold front coming in, many folks recommend blocking off and insulating your crawlspace vents for those couple days. This can be done by taking a small trash bag, stuffing it with insulation, then placing it within the vented area. You never want to push unprotected insulation to the vent as any direct moisture contact from the outside will ruin its insulating properties. Just remember to remove the block off once the cold front has passed. To ease your mind a bit on any moisture building up during the time that your vents are closed off, it is important to make sure that your dirt floor is properly covered with a good vapor barrier. Each sheet of the barrier should be no less than 6 mil in thickness, and overlap a minimum of 12 inches for maximum protection. The natural water from within the dirt is one of your biggest culprits for introducing moisture into the crawlspace, so a good plastic barrier will reduce crawlspace humidity substantially. You can also add a dehumidifier to the area, running a hose from the water collection of the dehumidifier to your sump pump, allowing for the ultimate in maintenance free operation. 

Option Four, Seal your Floor Perimeter and Crawlspace Door:

If you check around the perimeter of floor above your crawlspace, you might be very very surprised to discover how much cold air is escaping through to your living space. This is especially common for those who have replaced carpeting, or gone from carpet to a hardwood or tile floor. To help reduce drafts, you can seal the molding running along the bottom of your walls with a silicone/latex caulk. This should reduce the air gaps leading from the crawlspace to the living space directly above. The actual crawlspace door is another big culprit for allowing cold drafts into the above living space. Many experts recommend insulating the door with a foam insulation board and then running weatherstripping around the perimeter of the opening to get a tight seal once the door is in place. If you have a hole drilled in the door to allow for its easy removal, try using a cabinet handle instead. You can then block off the old hole using the foam board mentioned above.

So the short answer to the question, should I block off my crawlspace vents during the winter, is no, if you want to be on the safe side. But using one or more of the above mentioned options will help reduce the amount of cold air introduced into your living space, thus improving both personal comfort level and energy efficiency substantially!

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