Should home's foundation vents be closed?

Posted1/29/2023 6:00 AM

Q: I'm confused about foundation vents and whether to open or close them in winter. Some articles say you should keep them open to minimize moisture condensation under a building. Others warn that foundation vents allow moist air to enter crawl spaces, causing condensation, fungus infection and mold growth. How does one resolve these conflicting opinions about foundation vents?

A: Those who recommend closing foundation vents usually reside in areas with high humidity and are concerned with preventing moist air from entering the subarea below a building. However, the primary source of humidity under a building is ground moisture: wetness of the soil due to natural ground water runoff or overwatering of landscaping. The way to prevent water intrusion into a crawl space is to improve site drainage conditions.


When ground wetness occurs under a building, it does what water normally does; it evaporates. When evaporation occurs in an enclosed space (such as an unvented crawl space), the humidity increases and condensation takes place on whatever cold surfaces are available (i.e. wood framing, insulation, steel hardware, etc.). When these surfaces remain wet for prolonged periods, hardware begins to rust, fungus and mold begin to grow and dry rot eventually happens. These conditions are commonly prevented with simple ventilation, as required by the building code.

In most cases, passive ventilation is sufficient, allowing airflow to occur at screened vents on opposite sides of a building. When passive ventilation is not adequate, mechanical ventilation may be added.

Any home inspector, pest control operator, drainage contractor or mold inspector can tell you that closing or blocking these vents promotes condensation on the exposed building surfaces, with consequential damage in many cases. Professionals see this routinely, wherever vents are missing or have been blocked. When vents are provided on one side of a building but not on the other, we typically find condensation and damage on the unvented side. When homeowners add subfloor insulation and inadvertently block the vents, condensation and fungus damage often occur.

The best advice is to make sure the subarea under your home is adequately vented and that the vents remain open.

Q: I'm writing to comment about a real estate agent who routinely recommends her husband to do home inspections for her clients. How can an agent refer a family member for this important service? Isn't that a conflict of interest? Is it even legal?

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A: Of the two agents I know whose husbands are home inspectors, both recommend other inspectors, rather than engaging in this questionable practice. Whether it is illegal for an agent to recommend a spouse for home inspection is a matter that varies according to the laws of individual states.

Homebuyers should do their own research before hiring a home inspector, rather than relying exclusively on their agent's referral. Before hiring, look for someone who has many years of experience and is known among the local agents as "very thorough." Call a few real estate offices and ask who is the most qualified inspector in the area.

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

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