Crawlspace Humidity with Encapsulation


Old 05-23-19, 01:57 PM
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Join Date: May 2019
Location: USA
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Crawlspace Humidity with Encapsulation

I live in south GA and built a home 2 years back.
Brief facts on the home:
On crawlspace
2800 sqft.
Foamed all exterior walls and roof deck to seal the envelope
Carrier infinity with green speed serving downstairs
Full home automation system with sensors all over
Hard Wood floors with bat insulation under floors

Briefly after moving in we began having some "cupping" on our floors. I immediately started to correct some humidity problems we had under the house to alleviate the humidity and hopefully correct the cupping before it became permanent.

Humidity under the house was staying around 80%. I decided to have the crawlspace encapsulated (by my foam insulation contractor). The encapsulation consisted of very thick plastic liner being placed on the floor and ran maybe 2' up each foundation wall. Also had him do the same to all the piers. I then had closed cell foam sprayed over the poly to the walls to seal all the walls and piers. Also sealed the crawlspace vents.

After doing this I knew I would still have to deal with humidity so I installed a dehumidifier under the house (Sylvane Dual XT).

I've got the dehumifier programmed to turn on at 65% and bring it back down to 50%. I am seeing this dehumidifier cycle 5-8 times per day. After reaching 65% it takes about 1 hour to bring it back to 50% and then usually takes about 4 hours to reach 65% again.

The question I have is;;;;; Why is the humidity rising so rapidly?

I am including a couple of charts from various sensors that may help somebody in helping me understand. The house pretty much stays between 70 and 74 degrees all the time. The humidity readings labeled for the attic or identical the rest of the house for reference.

Currently I have no mechanical ventilation other than a 4" duct that is ran to my return plenum from outside. This of course is doing nothing unless the AHU fan is running. I've pondered whether or not I'm pulling a negative pressure on the crawlspace from the house and this is causing me to pull hot air into my cool crawlspace which is causing the humidity to constantly rise. Just a thought.
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Old 05-23-19, 02:02 PM
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Join Date: May 2019
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If any other data would be helpful in diagnosing the problem please let me know and I will provide if possible.
Old 05-23-19, 02:49 PM
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Water vapor coming up through the slab. I would not have the dehumidifier wait until 65% to turn on; I would try to keep a narrower band of humidity down there.
Old 05-23-19, 03:03 PM
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Can't read your graphs... too small on my phone, but I would say that at least partly, your RELATIVE humidity is changing with the temperature of the crawl space. Humidifiers vent hot air. As the temp rises your RH probably decreases. But once the dehumidifier shuts off and the temperature drops in the cool crawl space, the RH goes up again. The dew point changes based on the temperature. So the SAME AIR will have a different humidity depending on the temperature of that air.

Personally I would never want the humidity higher than 55%. So i'd set it much lower... maybe shoot for 40%.
Old 05-24-19, 05:52 AM
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Also couldn't read your charts, but some selected RH readings, outside, inside, and crawlspace, will help point to the source of the moisture. You will want the temperature readings at each location at the same time the RH reading is recorded.

Then you convert each reading to the same temperature so the RH numbers can be compared. I use this calculator and can explain further if needed.

When they foamed the CS walls did they also foam the rim at the top of the foundation, that is often a source of air leakage.

Closed cell foam?

Where does the dehumidifier drain the water removed?

In totally encapsulated house as you describe would expect to see an air exchange system such as an HRV or ERV. Not sure if the 4" feed to your system is sufficient. Do you have any supply and return ducts helping to condition the basement?


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