Sealing the Crawlspace Under My House

    In the southeast, vented crawlspaces are full of humid air.  This might not be a problem if they did not also contain ducts within which there is air-conditioned (cold) air, but these days they typically do.  These ducts condense the water in the humid air and then it drips onto the vapor barrier.  More humid air is continually entering the crawlspace through the vents and condensing, while the puddles on the vapor barrier slowly evaporate and add to the humidity of the air in the crawlspace.  Essentially you have a tropical rain forest in the crawlspace.  This moisture is absorbed into any wood in the crawlspace, contributing to the growth of mold in that wood.  Unless you are comfortable with your floors and joists rotting, you need to do something to reduce the moisture in your crawlspace.

    My colleague, Bill Grossnickle, suggested that I consult with Dan Oglesby at Greenville (NC) Utilities.  Dan was exceptionally helpful.  He explained that the problem in the crawlspace was aggravated by the fact that we keep the house very cool -- my wife is Swiss and is very uncomfortable with temperatures above 65, so that is not going to change.  He explained that moisture in the crawlspace condenses on the flexible ducts in the crawlspace and then is drawn, through pinholes, into the insulation around the ducts, destroying the insulating properties and increasing the condensation -- a vicious positive feedback loop.

    The worst area in my crawlspace was under the sink cabinets of the bathrooms.  These were very poorly designed (but typical) -- the duct opened directly into a plenum under the cabinet and conditioned air was to be vented to the living space by a register right next to where my feet are when I am shaving.  I always hate having cold feet while shaving, but that is not the worst of it.  That plenum is very cold, so the floorboard is very cold too, and on the other side of that floorboard is the crawlspace full of moist air.  Duh!  Even with insulation the floor board attracts and condenses water, and mold follows.  Look at this photo of the floor under one of the cabinets -- water was just dripping from it:

    Dan suggested that I close all openings between the crawlspace and the living area, which I have been doing, with caulk and "Great Stuff" foam.  He also suggested that I get a wood moisture meter and check the moisture levels of the wood in the crawlspace.  If too high then the wood will rot.  At 18% termites become a problem and at 20% rot starts.  I bought a Delmhorst J-Lite meter for $100 and checked the moisture content of the joists -- they tested at 17% to 26.5%.  Clearly I needed to take corrective action.  Some of the joists showed evidence of mold growth known as "dry rot" (a misnomer):

      I also checked the relative humidity of the air in the crawlspace --it should be less than 75% -- mine was 85% with one meter, 99% with another.  Yikes!

     I called Russ McIntyre at Air Quality Control and spoke with him at length about the situation in my crawlspace.  He came to my house and we spent a good deal of time chatting in the crawlspace.  He presented a proposal for sealing and dehumidifying the crawlspace, an action I had already decided was a good idea.  I hired him to do the job.

    Russ sealed off all of the foundation vents and then put down a 6 mil (.15 mm) polyethylene vapor barrier covering 100% of the ground.  As you can see, it was glued to the foundation walls a few inches below the sill plate.  The adhesive was that used with dry wall, applied with a caulk gun.  Also note the use foam to seal all openings from crawlspace to the exterior, here the space between sill plate and the foundation wall.

Sealed Vent	Liner Glued to Foundation Wall

        There was no wall between my crawlspace and the area under the front porch.  Russ installed partitions between crawlspace and the space under the porch and an access door in case I need get under the porch.

    Near the southwest end of the house, where the crawlspace door is, Russ installed a dehumidifier and a drain pipe to carry the condensate to the exterior.  He also put in a light

Front View of Dehumidifier	Rear View of Dehumidifier and Drain Pipe

    Near the east corner of the crawlspace Russ hung a fan to evacuate air from the crawlspace.  This should draw dehumidified air through the crawlspace, remove any gasses emitted by dying fungi, and discourage the movement of air from the crawlspace to the living area.

    Russ also put foam between the crawlspace door and its frame to seal it better.

    Russ advised me that there was a small leak in a drain pipe where the drains from the sink the garage, the washing machine, and the kitchen sink/dishwasher enter the main drain pipe.  This is at the furthest corner of the crawlspace, of course.  He tried to seal the leaky joint with the same adhesive he uses to glue the vapor barrier to the brick foundation walls.  I went down there, located the troublesome joint, and applied Plumbing Goop to it.  I also put a dish pan below the joint to collect water if the fix did not work.  I went back the next weekend and found the dish pan dry, so the problem with the drain is apparently fixed.  I also removed the water that had collected on the vapor barrier there -- over two gallons.  Some of that water may well have come from the duct above, which still had insulation saturated with water.  I also bailed out water from a trench under the air conditioning (and heating) ducts where they exit the foundation to the gas pack outside.  There were five gallons of water there.  Although I cannot dismiss the possibility that this was surface or ground water, my hypothesis is that it was water that had drained from the insulation around the ducts.  Hauling that water, in buckets, out of the crawlspace made me discover, painfully, muscles that I never knew I had.  If I have to do this again I'll use my portable sump pump.  The following weekend I checked these trouble spots again -- no water there this time.  Bravo!  I was thinking I might have to put a drain in the trench that had filled with water.

Trench Under Ducts at Foundation Wall

    Russ set the humidifier to turn on when the relative humidity gets up to 55%.  The humidifier ran constantly for a few days and then it started cycling on and off repeatedly, never running long.  Since we can hear this, especially at night when it is otherwise quiet, I set the humidifier to run in two-hour cycles -- for two hours it is active (will run if the humidity is above the set point) and then for two hours it will not run (regardless of the relative humidity).  I expect this to results in less acoustic disturbance from constant cycling off and on.  I also lowered the threshold for activation to 50%.

    I was suspicious of this dehumidifier, an LG model LHD45EL.  It is intended for use indoors, not in a crawlspace.  I found it on sale on the Internet for $100.  It failed in the Fall of 2007, after a little more than a year's use.  It had been making more noise than usual.  I went down there and found that the fan would not run anymore.  I also found a lot of mouse feces around it and saw a mouse running along ductwork above it.  Damn mice had also chewed through the insulation of one of the ducts going to my bedroom.  I put out mouse poison and turned the dehumidifier off.  The compressor still worked.  I removed the dehumidifier and took it apart but could not find any evidence of mouse damage.  Apparently the motor for the fan failed on its own.  Eventually I will replace the fan and use this dehumidifier in the house.

    I had a new dehumidifier installed by Ken Russ Pest Control in Ayden.  It is a Sante Fe Advance.    The condensate is pumped out into our drains to the septic tank.

Here is a photo of the dehumidifier with condensate pump:

Here is where the condensate enters the drain pipe:

Links

• Knowledge Library, Buildings -- from Advanced Energy in Raleigh

• Closed Crawl Spaces

• New NC Building Code

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Dr. Karl L. Wuensch

This page most recently revised on 18. June 2008.