Crawl spaces are vented for a reason. Of course this discussion always gets nipped in the butt by many saying crawl spaces don't need ventilation. Personally my belief is all crawl spaces need ventilation, winter or summer. By saying that there would not be any blocking the crawl space ventilation in the winter. This would allow proper air flow for intake or combustion air. I also am a believer that pulling air from a crawl when almost all crawls are notorious for conditions that would not be conducive for drawing any air from is not a good idea.

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Ted Menelly
Castle Home Inspection Services
www.inspectmycastle.com