Crawl Space Humidity blog
Part 1
I've been concerned about the humidity in my crawl space since we moved in. In fact, I wondered about it at our former house but never did anything about it. At the current house we've had some problems with water getting under the house in heavy rains but I think I've got that whipped.
I decided the first step was measuring the humidity under there so I bought a Honeywell TM005X Wireless Thermo-Hygrometer and put the remote unit under the house. I bought it back in January 2009 and boy was I shocked. The humidity seldom got below 75% even in the winter! As the summer came on and the humidity rose to above 90% I decided I had to do something. It is so humid down there that water condenses on the cold water pipes and drips on the floor.
Completing the plastic covering on the floor (about 20% was uncovered) did nothing. So I went looking for solutions. I found the SmartVent Crawl Space ventilator and after reading their convincing argument that you can't dry under your house with wet air, I decided to purchase one of their vents. It cost me $315.
It arrived less than 1 week after I ordered it and I installed it the next weekend. I had never removed a crawl space vent before but it turned out to be a pretty easy job. Mine were installed with mortar which I broke out using a long chisel and a hammer. It took me approximately 1/2 hour to prepare the opening to receive the smart vent. The place I chose to install it had an electrical outlet nearby so I was set for power. I used clear caulk to seal around the opening.
I installed it on 20 June 2009. After 2 weeks my crawl space humidity is down to 77% from 92% on the day I installed it. I'll make a table below to record occasional readings. It has been dry here since I installed it. That, no doubt, is part of the rapid improvement but since I'm not doing any sort of controlled experiment, I'm just going to report what I see.
The SmartVent appears to be well made and runs a clever algorithm. It has two muffin fans, a small circuit card, and a thermometer and humidistat positioned near the front grill. It uses these sensors with the fans off to sample the outside air. If the outside air temperature is above 42 degrees F, it runs one fan for about 15 minutes to pull crawl space air over the temperature and humidity sensor. It then compares the dew point of the outside air with the dew point of the crawl space air. If the dew point outside is lower both fans run to pull wetter crawl space air out so that it will be replaced (through leakage) by dryer outside air. If the outside air is wetter than the inside air, the unit waits.
My sporadic observation of its habits confirm that it is behaving as expected. I've seen it running continuously for the last few days while the outside dew point has been low but last week it ran much less because the outside air was very humid.
I'll try to record some readings here so I can track how it does.
Part 2
This is an update of my earlier post.
I gave up on the SmartVent in March of 2011. I still believe it is a clever design but here in central North Carolina the outdoor dew point is simply too high for it to do much good. Now I have sealed my vents with rigid foam and caulk and I've got a cheap ($200) dehumidifier in the crawl space. It is controlling the humidity very well. I let it drain into the drain pipe for my AC so no emptying or additional plumbing required.
I believe the real issue in crawl spaces is the dew point; not the humidity. Relative humidity is very misleading. Dew point is the temperature that water condenses out of the air and the issue for mold is condensation. If the dew point under your house is higher than the temperature of the pipes, boards, etc. you'll get condensation. I had water dripping off my cold water pipes!
For the SmartVent to work for you the dew point outside should be LOWER than the dew point under your house. Check your local weather data and see what the dew point in your area can be expected to do. If it goes below 60 (I base that on my water temperature), for significant periods the SmartVent could work very well. If it goes up above 70 and stays there (as it does here) then it can't do you much good.
My dehumidifier is a DEP740EW from Edgestar. I chose it because 1) a buddy had one, 2) it had good reviews, and 3) it was cheap. I've had it since about March and it is working fine so far. I put a Kill-A-Watt power meter on it so that I can track the power consumption. It averages between 200 and 250 watts extracting about 2 gallons per day. At our electric rates its costing me about $14 per month to operate it.
I like that the dehumidifier moves the problem from the nagging thing I should do something about to the economic domain. Money tends to get my attention and I can easily measure the value of improvements I've made.
This fall I plan to improve the coverage of plastic on the ground and eliminate one place where rain water occasionally gets in.
Part 3
July 04, 2012 at 08:14 AM |
This is an update of my 2011 and 2009 posts on my attempts to control the humidity in the crawl space of my house in central North Carolina.
After running the dehumidifier from the previous post for about 15 months I decided it was time to come up with a more sustainable solution. The dehumidifier did the job, but I could hear it running (a lot) and I knew that it wasn't going to last forever.
The Advanced Energy Report showed that carefully sealing the crawl space and venting about 50 CFM of conditioned air into it would deal with the humidity problems once and for all. My friend Herman's new house was built that way, and Brian had his converted with good results. After toying with the idea of doing the work myself, I realized that it was much too much for me and decided to find professionals for the job.
I contracted with Brad Dehring of Crawlspace Doctor to do the work after getting three proposals. I'm very pleased with their work. Here is a brief description of the job.
They removed the old damp, moldy insulation from the floor joists. They cleaned mold from the wood and pipes and got up all the bits of rock and junk from the floor. They sealed every hole, gap, and crack in the house floor, sill plate and foundation walls. They had a plumber come in to temporarily remove the water heater so they could put plastic and insulation below and behind it. They covered the concrete block walls with 10 mil plastic with a 3 inch gap for termite inspection; every seam is taped. They covered the plastic on the walls with 2-inch foam insulation with foil on the surface. They neatly cut around every pipe and wire that passes through the walls. They taped all the edges, joints, and fastener heads with aluminum tape. They covered the floor with 10 mil plastic taping it to the plastic from the walls. They installed a special purpose 50 CFM crawl space vent on the HVAC plenum near the center of the crawl space; it lets air flow out when the system is running but not back in. They replaced the access door with an insulated and weather stripped wooden door. They cleaned up completely; the space is cleaner than it ever has been. They fixed several small unrelated problems. I was wise to get professionals to do the job. They had two or three young guys who knew what they were doing here for six days. Crawling around under there for one day would have killed me!
It's been two weeks since they completed the job and the dew point under the house is staying between 57 and 59 degrees. This is the same, within my measurement accuracy, as the dew point inside the house; perhaps they are coupled. Or maybe it will drop lower as the wood and concrete blocks gradually dry out. In any case, it is already low enough to prevent condensation and mold.
I hope this is the last installment in my crawl space humidity series but I will update this post if I see significant changes.
