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Old 06-13-2016, 01:41 PM   #1
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Crawlspace Encapsulation / Basement Vents and Registers


Hello, I live in a 1957 Split Level (Philadelphia Suburbs), about 1500 Sq. Ft, 3 floors including the basement, and a crawlspace about 500 sq. ft. under the living room floor (about 3 to 4 feet from floor to ceiling. Living room is in the front, the back splits off into basement and upstairs) The crawlspace connects to the basement and is pretty much part of the same room..
(Crawlspace is totally underground, half of the basement is underground as well, the other half is not and has a door, windows, etc.)
In the basement, in the room with the HVAC unit, there is direct connection to the crawlspace.. It basically has a 1 foot open area around the ductwork directly connecting basement to crawlspace area.. The HVAC room has a door that is a large open vent style door (so it can breathe presumably) leading to the living area of the basement.

Before encapsulation:
-Rat slab concrete rough floor, no major visible water problems..
-It had about a 12in x 6in foundation vent on one side of the house..
(basically like having an open window year round right into the basement)
-Humidity sensor read up to about 77% at times
-Bad insect problem and musty smell

After encapsulation:
-Cleanspace drainage mat over entire floor
-20 mil thick Cleanspace vapor barrier covering the entire floor and foundation walls.
-Vent has been sealed off with styrofoam and caulked, etc.
-Musty smell has been slowly improving.. Humidity got high (77% again) with basement windows open, but dropped down to 55 - 60% with windows closed and AC on (register in crawlspace fully open)


Anyway after they made it a nice accessible clean space (well worth the money), I went down there to check things out, and to put the humidity sensor back down there to monitor it.. Also checked out the duct work and to my surprise there was a return air grill on the old "vent" side of the crawlspace, and an output register on the other side of the crawlspace (furthest from the now sealed foundation vent).

I'm guessing since there was only a single foundation vent, they supplemented that with the HVAC system being opened up to the environment?


My question is this.. My upstairs doesn't get nearly as much AC as the lower levels. Should I keep the basement registers closed, while leaving the crawlspace register open?

Should I close the crawlspace register? Seal the crawlspace Return grill?

Open all vents?


I'm looking to get the most AC upstairs as possible, while still dehumidifying the crawlspace as much as possible to solve the musty smell / insect/ mold issues..

Would it be best to get a dehumidifier and close the vents?

The crawlspace guys said that passive ventilation to the basement area should suffice, and it does feel weird and wasteful pumping all that conditioned air into a non-living space.

(Central Air: Fedders C1030BBD1V, I think it was installed around 2007.)

Thank you for your time

Last edited by Anomaly0; 06-13-2016 at 01:58 PM.
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