this post was submitted on 22 Jun 2026
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Not crazy, but the cool air from the crawlspace has to be replaced, probably by warmer air from outside. So you are cooling the attic but warming the crawlspace. Heat rises. What's the insulation between the crawlspace and house? You may be cooling the attic but warming the house. The attics I know are all vented to/from outside. I would be more inclined to put the cool air from the crawlspace into the house, rather than the attic.
also cool air can not hold as much humidity as warm air. this means that the water (of the warm air in your attic) might condensate in your attic, so stuff might start to get rusty or moldy.
That's interesting! The carrying capacity of air Vs temperature is non-linear so if air in the crawlspace and attic were both near saturated, when mixed the result would probably be super-saturated / condensing.
Hence you get visible condensation from breathing and all sorts of exhausts.
That attic insulation is pretty sad, I doubt there is a vapor barrier or attic and roof venting.
Yes it's sad and old but there's about 8" of it. There's an attic vent in the roof and a vapor barrier in the crawlspace.
That would be my guess, the crawl space is cooler to begin with because there's not enough insulation from the crawlspace and the rest of the house. So the cool air from your crawl space and home is now being sucked into the attic.
Your attic is supposed to be hot, that's why you have so much insulation between your house and the attic, and not the roof and the attic. It's supposed to be closed off from the rest of the climate control.
Old houses were not designed for insulated attics. But, along with attic insulation, the attic and roof needs to be vented, to let the hot air escape. Ideally a ridge vent across the entire roof.
There is a ridge vent
The ridge vent gives the hot air some place to escape that is not your living space. Creating a draft from your living space to your attic is just going to pull your cool air into the attic.
It's from the crawlspace under my house, not a living space at all
Still might be a problem, especially if you don't have great insulation under your subflooring. You're pushing cool air from under your house to the attic. That vacuum of cold air is either going to be replaced by cooler air pulled from your house if you don't have enough insulation, or from the hot air outside.
Pulling in hot air from outside is going to make your AC work harder because that hot air will rise and warm your house. And pulling colder air from your house into the crawl space means your AC is now working harder to cool your house, your crawl space, and your attic.
The space in your attic and your crawlspace are supposed to act as insulated buffer cells between your living space and outside. Connecting the two basically creates a vacuum that vents cool air from the ground to the vent in your roof.
Plus.... Depending on how old your home is, you may unintentionally be pumping a dangerous amount of radon into your attic.
Attic is vented but it does little good without a decent wind. Crawlspace is slightly cooler than outside ambient air, call it 85° in June. The same wind that would help the attic would also remove the cool from the crawlspace I think? Attic is insulated with old old blown fiberglass and then a layer of newer rolled fiberglass - not terrible insulation actually. Also the crawlspace is insulated from the house with spray foam insulation.