Should foundation vents remain open?

Posted10/24/2021 7:00 AM

Q: My question involves the unwise practice of closing foundation vents around a home. As a heating contractor, I've noticed some homeowners and maintenance people close the vents in order to conserve heat during winter. Instead, they are causing serious building damage and heath problems. I often find blocked vents when working on homes, and some hardware stores even sell products specifically made for closing the vents. You've addressed this subject in the past, but the practice remains widespread. Could you please advise your readers to let the subfloor areas beneath their homes breathe?

A: Closing foundation vents is definitely a bad idea. Doing so overlooks the purpose and intent of ventilating the subarea under a building. Winter is when ground moisture under a building increases. Therefore, it is the time of year when vents are most needed to prevent dampness, condensation and moisture damage. Closing foundation vents wrongly assumes heat should be contained in the subarea. Instead, heat should be retained inside the dwelling by installing insulation beneath the flooring. The subarea needs to be dry, not warm.


The key points in a nutshell are these:

• The building code requires cross ventilation of a subfloor space beneath a building, with openings equaling at least one square foot per 150 square feet of floor area. Exceptions are allowed when a plastic membrane (vapor barrier) is installed on the ground surface or when mechanical ventilation is provided.

• Insufficient ventilation can cause moisture condensation on the structural components under a building, resulting in fungus infection, dry rot damage and rusted hardware. Excess moisture can also cause mold, which is a common health safety concern for many people.

• Insufficient ventilation can also violate combustion air requirements for fuel-burning equipment, such as furnaces.

Surprisingly, some builders overlook this common requirement, and municipal building inspectors occasionally fail to notice the omission. Fortunately, most homes have properly vented subareas, and owners are strongly advised to keep these vents open at all times.

Q: My shingle roof is about 10 years old and is gradually turning green, kind of like there's a fungus on it. What can I do to clean it and prevent worse damage?

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A: Roof surfaces that receive little or no sun exposure are prone to grow a thin layer of moss or algae. This commonly occurs on northern exposures or where roofs are shaded by tall trees. Typically, moss and algae do not adversely affect the condition of the shingles. Therefore, removing the discoloration is not essential to maintaining the integrity of the roof. However, if restoring the original color of the shingles is desired for cosmetic reasons, you can try washing the roof with a watered down solution of algicide, such as would be used in swimming pools. My advice is to have the shingles inspected by a licensed roofing contractor or home inspector before making any changes.

• To write to Barry Stone, visit him on the web at, or write AMG, 1776 Jami Lee Court, Suite 218, San Luis Obispo, CA 94301.

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