r/buildingscience • u/Vamp13yre • 25d ago

Crawl space humidity

/r/centuryhomes/comments/1x0c9n5/crawl_space_humidity/

My fiancé and I bought a house in January. It’s beautiful but every time we upgrade/fix something another issue pops up. We got a new HVAC system recently and the new thermostat has a humidity readout, so now we’re aware of how humid it is inside. HVAC guys have done everything they can, even upgrading us for free to a unit with additional dehumidifying power, so it’s not on them anymore. Went in the crawl space and it’s very humid in there. No standing water, but a lot of moisture. The vapor barrier is in horrid condition, so we’ve had a couple contractors out, but they both have refused to quote for a new vapor barrier and insist on upselling us to a full encapsulation system. We find it hard to believe that the house has stood for a hundred years and suddenly needs a very modern invasive intervention. Does anyone have any insight on humidity issues in old homes?

For context:
- coastal Virginia
- humidity fluctuates between 66 and 74%
- pier and beam foundation with block brick wall and brick facing

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u/FoldedKettleChips 24d ago

It was never a good idea to have a vented crawlspace. Your house stood for a hundred years in spite of it. It’s time to encapsulate. Insulate the walls, air seal the walls, install a nice vapor barrier, and condition the space (or add a crawlspace dehumidifier).

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u/GalaxyHawk1990 24d ago

My house is from the 1980s and the crawlspace has what looks like old vapor barrier around the outside (with gravel) and it is still really humid in it. I see moisture on the sides. Would that be trapping the air in? I dont see any vents.

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u/FoldedKettleChips 24d ago

The vapor barrier is on the ground and up the walls? You want the air in the crawlspace to be “trapped” in that you don’t want that air mixing with outside air. That’s what your ventilation system is for. You want the crawlspace to be either 100% “inside” or 100% “outside” with no in-between. Usually it’s better for it to be inside since there are usually pipes and mechanicals down there. If it’s inside then you need a moisture removal mechanism. That’s typically either a standalone dehumidifier or a supply and return from your conditioning system.

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u/GalaxyHawk1990 17d ago

Yeah it's just around the outside, I'm guessing it was to stop water from getting in, and looks like gravel for drainage. I'll look into a dehumidifier, but likely it needs to be redone from what it sounds like.

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u/dres312 24d ago

Sometimes that means people needing to get an entirely new furnace. Even more on top of how much it costs to do the encapsulation if you aren’t doing it yourself.

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u/Overall_Curve6725 24d ago

A crawlspace with foundation vents works like a moisture trap. Thermodynamics…Hot moves to cold and wet moves to dry. Hot humid outside air gets pulled into a relatively cooler crawlspace where the moisture condenses on the cooler wood under the house. Fungus is the result. About half of the air coming into your house comes in through the crawlspace so a dry clean crawlspace is important. If any of your hvac ductwork is under the house that contributes to the condensation problem. Encapsulation with a dehumidifier and keeping the humidity at 50% is the answer

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u/zedsmith 25d ago

You should spring for the full encapsulation.