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We just completed a 20x20 great room addition on the side of our house with a 4 foot crawl space and poured concrete floor. There is a 4’ by 4’ cut through to access this crawl space from the existing full sized basement. We live in New England. The new addition has baseboard heat and the floor is insulated with insulation bats. The new addition includes 3 foundation vents (metal with screens, we can slide them open or closed) for the crawl space. The rest of the existing basement has no venting to the outside (just a door with stairs to a bulkhead).

The contractor said these are required and wants them left open. However, in the winter - even if we slide the vent closed, cold air is making the floors cold and it just lets cold air into the normal existing basement which will be half finished.

Do we need these vents or can I just plug them with some sort of ridid foam insulation? All this cold air seams like a big waste of energy. In the summer we keep a dehumidifier running - is this enough to keep the crawl space conditioned as well?

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    Hello, and welcome to Stack Exchange. Is there a reason why you wouldn't close off and insulate that 4' x 4' cut-through from your full-sized basement to the new crawl space? – Daniel Griscom Nov 18 '18 at 17:58
  • I am open to closing this off if recommended. My thought was that if we seal the crawlspace vents, then warmer air from the basement could pass into this crawlspace to help keep it warmer (reducing risk of heating pipes freezing, keeping the living space above it warmer) – Andrew Nov 18 '18 at 19:12
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    Wait: your extension is electric, not forced water, right? If so, then what's going to freeze in the crawlspace? – Daniel Griscom Nov 18 '18 at 20:07
  • No - it is baseboard heat (non-electric). the feed for those baseboard units is under the floor. – Andrew Nov 19 '18 at 15:56
  • So, letting the crawlspace freeze wouldn't be a problem, right? – Daniel Griscom Nov 19 '18 at 15:58

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