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crawl space vents, again

Ken Meyer

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I agreed to do some miscellaneous work for a friend of a friend, her priority is adding insulation to save energy. There is a small basement area where the furnace is located, flanked by two crawl spaces. The basement is open to the crawl spaces.

There was no vapor barrier in the crawls, and there is no subfloor insulation. I installed vapor barriers in the crawls, and she will get someone else to install the subfloor insulation (I don't care if business is slow, there are some jobs I just won't do).

My question is whether it's better to close the crawl space vents or leave them open in this situation. It seems to me that from an energy savings standpoint, closing the vents in the winter makes sense, the entire basement/crawl space area would then become a conditioned space. Does it make much difference in energy use if the furnace is in a colder space with the vents open, or if it's operating in a warmer space with the vents closed?

Of course, I just don't think that the average homeowner would bother to open and close the vents with the seasons, so maybe it's better to leave them open all the time. The best solution, I think, would be to wall off the crawls, leaving access doors, but that won't happen.

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In these parts, there's quite a few furnaces in garages. In some of the older places, the furnaces are also in un-conditioned, cold basements. Furthermore, ducts travel through vented crawlspaces all the time.

Garages and basements are cold but maybe not as cold as the basement of which you're describing.

Make sure all the ducts are insulated and all the joints sealed with mastic.

Check with the maker of the furance and see what its limits are in cold places.

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It always makes more sense to keep the ducting and the electro-mechanicals within the conditioned envelope of the house. If the crawlspace is open to the basement, you can close the vents, insulate the foundation walls and turn it all into conditioned space and leave the underside of the floors uninsulated. However, you'll have to make sure that you have sufficient combustion air and that there's nothing nearby, like a clothes dryer, to cause that furnace to backdraft.

OT - OF!!!


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