Many people are not sure they are supposed to cover your foundation vents during winter months. The floor can get very cold if you don’t cover the vents to keep the winter wind out, but some people think that this cuts off the air supply to the floor furnace, making carbon monoxide a danger. So what is the truth regarding your foundation vents?
Well, the answer to this question depends on how tightly the vent lids cover their openings. Most of the foundation vents installed in standard home construction are loose-fitting enough to allow a little air flow but tight enough to prevent rapid air flow. The formula for adequate combustion air is one square inch of open vent space for each 4,000 Btus per hour the furnace is rated, meaning the typical 75,000 Btu floor furnace would require 18.7 square inches of open vent space to operate efficiently.
A single 6-by-12-inch vent would provide 72 square inches of open vent space. A loose-fitting vent lid (assuming a quarter-inch gap between the lid and vent opening) would provide nine square inches of opening, meaning two loose-fitting vents would provide the necessary 18 inches. The typical house, with 18-20 foundation vents, would provide 162-180 square inches of open vent space for a furnace requiring only 18.
This means you could probably seal up all the vents along the north and west walls and leave 2-3 vents in a “loose-fitting” condition along the south and east walls to provide your floor furnace adequate combustion air. Homes with central heat and air systems drawing their combustion air supplies from outside normally need not have their crawl-space vents left open during the winter. For maximum protection from freezing temperatures in the crawl space, tight-sealing insulated covers could be constructed for each vent and wired or bolted into place. Homes with crawl-space dampness problems might require a little ventilation even in the winter.
Excessive Moisture In Your Crawl Space
If you have a crawl space and have noticed a lot of moisture coming up from the ground, there are certain steps you should take.
For one, you probably need better ventilation down in this space. Trapped moisture can encourage the growth of all sorts of creatures plus some crystallization of salts from the soil. Situations like this require several crawl-space vents in the exterior walls.
Such foundation vents have been required for years, but they mostly get ignored and overlooked and are considered energy wasters. However, they are vital to protect the wood on the underside of your house from dry rot fungus, and they are usually part of the combustion air scheme for your furnace, water heater, fireplaces and clothes dryer.
If you notice excess moisture in your basement, set a small fan in the most northerly opening for a week or two, blowing out the vent to help lower the humidity in the crawl space and discourage fungus and crystal growth. Rather than disinfect the soil, scrape off the top layer of foreign material and discard it. Once the soil is moderately dry, cover it with a layer of 6-mil black plastic sheeting.
Of course, you’ll need to tackle any outside sources of this excess moisture as well. Make sure there are no dripping faucets, move sprinkler heads away, divert the downspouts and switch to dry-scape landscaping near the house.
All these changes will make the area cooler than you’re used to. So you’ll probably want to insulate the “ceiling” of the crawl space – the underside of the floor. Use some 6-inch batts and, to hold them in place, stretch some fine wire perpendicular to the floor joists between partially driven Sheetrock nails.