I have a crawl space under my main entrance of the house. It's about 3' tall. I added a heat duct because the floor at the entrance was always cold. So my question is do I need to vent the crawl space because I added the heat vent. Even if I just cut a hole in the door leading to the crawl space and put a vent cover there. Thanks

  • Is the space enclosed? Does the heat duct vent into the space?
    – wallyk
    Feb 23 '14 at 2:52
  • I have a full sized door leading into the space. It's in my basement which is fully finished. The heat vent is in the space. There is no insulation on the floor joists.
    – Tim
    Feb 23 '14 at 19:25
  • 1
    What's your climate?
    – Bryce
    Mar 25 '14 at 19:25

Best practice varies with the climate.

  • If you open up venting between the basement and the dirt crawl, cover the dirt crawl with a moisture barrier.

  • Consider instead sealing the crawl entrance from the finished basement air, and opening a vent to the outdoors.

  • In SOME areas you may be allowed to put a fan to the outdoors. Mechanically controlled by humidity, it would offer the best of both cold and warm weather performance.

For research on this topic see,

Moisture Solution Becomes Efficiency Bonanza in Southeastern United States, PDF by Bruce Davis and Cyrus Dastur, AdvancedEnergy.org -Crawl Spaces


Is there insulation in the floor? There should be. There should be a vent but that has nothing to do with adding a new heat duct. A vent is going to make the floor even colder. Vents are normally opened in spring and closed in winter. So you should add 2 on opposite walls come spring.

  • I dont mean add a vent outside. Just wondering if I need to vent in the house to keep air circulating. Just don't want mould. Thanks
    – Tim
    Feb 23 '14 at 20:03
  • No you do not want to vent a crawlspace air into the house. You want to keep that air separate from the house. If you want to circulate air in crawl space you add vents in the foundation. If you want to circulate air more in the entry you would add a return vent back into furnace.
    – Justin K
    Feb 24 '14 at 6:45
  • @JustinK that isn't strictly true, depending on the type of crawlspace. A crawlspace, like any other space in the house, is either conditioned space or it's not. If it's conditioned space, it's part of the building envelope and should be insulated accordingly (including vapour barrier). If it's not conditioned space, then it should separated accordingly and as far as the rest of the house is concerned treated as if it's "outside" (meaning: there is insulation and vapour barrier between it and the rest of the house).
    – gregmac
    Apr 25 '14 at 0:12

Under floor ventilation of wood floors over earth is required by any model code to manage moisture from the earth molding and/or rotting the wood structure. As Justin K indicated, it does make the underfloor area colder, this is why many people block off the vents in the Winter. IMO, if you have adequate floor (and duct) insulation the temperature gained only makes a small difference in energy costs. If the blockage causes moisture issues it will be a false savings.

How serious any moisture problem could be varies greatly by location, climate, soil conditions, and if the earth is covered with a vapor barrier or not. Regardless, you do need vents. Seasonally block the vents if you like, but only if you're sure it will not cause moisture issues.

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