How do I ventilate a basement?
I live in a 1967 house in zone 6. I need suggestions on how to best ventilate our basement as we’re finishing it. We want to ensure there’s good air quality year round. We have central air and baseboard heat throughout the house, except the basement. The temperature is generally comfortable down there w/o AC or heat.
We’ve had a musty smell down there despite having no water issues, french drains, sump pump, downspouts extended from house and a 70-pint Frigidaire dehumidifier.
A solution idea we have is to put in a duct to pull stale air out of the basement and also put in two returns and 1 supply into the existing HVAC ducts to help circulate the air. In the winter, when the AC isn’t on, we thought we could move air via the house fan mode. We will also put in 3 new hopper style windows. Suggestions?
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Replies
The musty smell is usually a function of the lack of foundation insulation. Insulating correctly would make a large difference, but it may also need summertime use of a dehumidifier.
In a zone 6 climate you'll want to put 1-1.5" of EPS under a subfloor on top of the slab, and continuous 2" EPS (or 1.5" foil faced polyiso) against the foundation wall, behind a 2x4 batt insulated studwall, or 3" of polyiso (or 4" of EPS) strapped with furring through-screwed to the foundation wall. A batt wall without the foam using an interior vapor barrier to mitigage wintertime moisture drives would load up with ground moisture, but without a vapor retarder would load up with frost on the above grade portion. The foam keeps the studs & fluff above both the wintertime interior air dew points above grade, and above the summertime outdoor air dew points below grade, and you wouldn't need vapor retarders (other than standard latex paint) in either season.
If you only insulate the walls you'll still have mold risk under a mold-susceptible finish floor, such as rugs or wood, since the deep subsoil temperatures will be below the summertime outdoor dew point, but it works fine with ceramic or asphalt tile floors. Keeping it dehumidified to below 60% RH with a room dehumidifier would fix most of it, and would be necessary during the summer.
Properly insulated you can then ventilate the basement with conditioned air from the rest of the house. Test for radon first. If the radon levels of high you may be able to fix that by ventilating the basement with a small HRV such as a Lunos rather than spreading the radon to the rest of the house with a recirculation scheme.
Masb,
Here are some links to articles you may wish to read:
How to Insulate a Basement Wall
Fixing a Wet Basement
If you are interested in ventilation (not conditioning), you may wish to read this article:
Designing a Good Ventilation System.