# crawlspace temperature, humidity

• posted on June 12, 2006, 3:48 am
Does anybody know what a reasonable temperature for a crawlspace would be? I recently put two dehumidifer units in my crawlspace (along with a couple box fans to circulate air), as I've had major problems with moisture down there. I have all the crawl space vents sealed. However, the temperature in the crawlspace is now running about 8-10 degrees hotter than it had been. It was running around 68 degrees. Now it is consistently up around 76-78 degrees. I guess this is because the dehumidifers heat things up. The humidity level in the crawlspace is now under control, the RH has been staying around 45-47% (it had been running around 80-90%). But I'm concerned about the temperature running higher. Is this anything I should worry about? What would be the ideal temperature and RH for a crawlspace?
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• posted on June 12, 2006, 4:35 am
dblho39 wrote:

RH is function of temperature. So looks like nothing really changed in your crawl space. My cabin has crawl space where furnace, water heater, well water tank are located. It's cool in summer, warm in winter down there and no moisture problem just with natural vent. Furnace draws fresh air from outside thru insulated duct. The floor of crawl space is dirt and it's dry, it;s almost dusty.
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• posted on June 12, 2006, 9:44 am
dblho39 wrote:

Close to the soil temp? I suppose most people don't care.

Where does the moisture come from? Rainwater? A floor with no vapor barrier?

So the water vapor pressure in the crawlspace air used to be about 0.85e^(17.863-9621/(460+68)) = 0.594 "Hg, and now it's about 0.46e^(17.863-9621/(460+77)) = 0.436 "Hg. That's progress.
It used to contain 0.62198/(29.921/0.594-1) = 0.0126 pounds of water per pound of dry air, and now it contains 0.62198/(29.921/0.436-1) = 0.00920. This is called the absolute humidity or humidity ratio, and it does not depend on temperature.

I'd say just keep the RH below 60% to avoid mold.
Nick
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• posted on June 12, 2006, 10:53 am
ADD COMPLETE VAPOR BARRIER! Plastic seal edges and double thickness of 10 mil plastic.
Make certain no extra water is getting dumped under home things like route downspouts well away from area! Adding a ground water drain under and away from home with gravel under house can be used for tough or standing water situations...
OPEN ALL VENTS and add more vents so air flows easily under home!
REMOVE AND FORGET about dehumidifiers, they arent needed in this application and will just increase your electric bill dramatically!
If your really intent on spending money on electric add a ventilating fan with humidistat so it only runs when the humidity is high!
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• posted on June 12, 2006, 12:16 pm
dblho39 wrote:

Temperature is up because you are adding heat. Those dehumidifiers make heat. Condensing water releases heat (A/C gets cold because it is evaporating something the part outside gets hot because it is condensing it.)
Note: I would agree with the vapor barrier and with looking for the source of the moisture.
Why did you close up the vents?
--
Joseph Meehan

Dia duit
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• posted on June 12, 2006, 12:28 pm
Joseph Meehan wrote:

health of the house, as is good drainage away from the perimeter of the house. When it rains, no water should get under the house.
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• posted on June 12, 2006, 7:11 pm

And it can do more harm than good in summertime.
Nick
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• posted on June 12, 2006, 7:23 pm
snipped-for-privacy@ece.villanova.edu wrote:

if the ventilation is adquate and a good vapor barrier is used dehumidifers are unnecessary.
moisture naturally wants to go to where its less, so dehumidifying can draw moisture to your area:(
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• posted on June 12, 2006, 10:07 pm

But the dehumidified area will still be drier.
Nick
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• posted on June 12, 2006, 11:21 pm
snipped-for-privacy@ece.villanova.edu wrote:

At the large cost of electric heat buildup.
Is it really worth it?
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• posted on June 13, 2006, 6:51 pm
The original symptoms last summer was that all of the a/c ductwork in my crawlspace were sweating like nuts. The insulation on all the ducts was soaked and dripping water. The floor insulation was also soaked and hanging down everywhere. I also was having mold start to grow on the joists.
I already had a vapor barrier, over the usual 80-90% of the ground.
I've owned the house for about 1.5 years. About a year before I bought it, it had a geothermal heat pump installed.
Had a french drain and sump pump installed, as well as 2 large ventilation fans in the crawlspace vents, at the end of last summer. I also had new vapor barrier put down. That seemed to help at the time. But this summer, I found that the ducts were starting to sweat again.
So I sealed up all the vents, disabled the vent fans, and put in the dehumidifiers. Now, everything is dry. But the temperature is up under there, and of course, I'm jacking up my electric bill. In the short term, this is probably okay, though.
Since then, I had a patio installed, and the guys installing it discovered that the underground drain pipe for all the downspouts on one side of the house had been screwed up when the geothermal heatpump was installed. The pipe was completely blocked. I think this is probably where the moisture was coming from.
I probably also need to have the insulation on all the ductwork replaced, though I have yet to find somebody to do this. The company that installed the geothermal heatpump has just been jerking me around on this for a year. I am really pissed at them. It seems like no other companies are willing to work on it because it is a geothermal heatpump.
BTW, what are peoples opinions about having the existing mold cleaned off the floor joists once the moisture problems are fixed for good? The company that installed the french drain, sump pump, and vent fans also sprayed some sort of anti-mold stuff under there. We've never noticed any mold problems in the house itself (and my family would notice this, we all have bad allergies).
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• posted on June 13, 2006, 7:00 pm
BTW, I didn't mention, I live in Knoxville, TN. This is a *very* humid area, sometimes it seems like a rainforest.
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• posted on June 14, 2006, 1:24 am
Since you said you're a resident of Knoxville TN then the results from Advanced Energy http://www.advancedenergy.org/buildings/knowledge_library/crawl_spaces /
should be the bible you work from.
I would expect that as you correct the outside drainage problems you found that should take a lot of load off your dehumidifiers. and as they work less the temps should come down.
As Advance Energy showed in their field studies I think the only alternative to a active dehumidifier is to have a supply vent from your HVAC system into the crawl space [but NO return vent]
With the dehumidifier running in a closed space we should expect the temp to be elevated above what it would have been with out the dehumidifier. I'm not sure why I would be concerned about that.
In my own case, while I may be be burning more electricity running the dehumidifier I'm saving energy because my home is quite a bit tighter and I put R-35 in the floors [net AC and heating costs are lower]. My whole house air quality is significantly better. The important thing is that the crawl space is DRY with a RH < 50% [50% is cutoff for dust mites and 70% is cutoff for mold growth].
http://healthandenergy.com/mold_prevention.htm http://www.cpsc.gov/cpscpub/pubs/425.html http://www.buildingscience.com/resources/homeowner.htm
As far as mold on your floor joists -- it really depends on how bad it is and more importantly if it is a structural issue. Most of the research I've done on the subject said that in most cases it's better to just leave it alone if it's not causing a structural problem. Read the building science references and see what you think.
If the insulation was "wrapped" around the outside of your ducts you may be able to cut it off yourself and apply new insulation on your duct work yourself. I insulated my duct work myself after first sealing all joints -- that really made a big difference in my home air quality and comfort.
I'm thinking of upgrading to a geothermal system -- what energy useage improvements did you experience? What system did you have before?

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• posted on June 14, 2006, 7:33 am

Why would you think that? A SmartVent(.net) would be another alternative.
Nick
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• posted on June 15, 2006, 3:36 am
Not an effective one. I prefer to go with Advanced Energy current research results.

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• posted on June 15, 2006, 8:22 am

It's more efficient and effective, IMO...
Nick
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• posted on June 14, 2006, 3:09 pm
Thanks for the comments. The Advanced Energy site is a good one, lots of good info there.
Incidentally, the RH in my home seems much more stable since I put the dehumidifiers in the crawl space. Seems more comfortable as well.
The goethermal heatpump was installed before I bought the house, so I can't say for sure how much energy savings I'm getting. I believe the house previously had a regular heatpump. The previous owners didn't run the air conditioning as much as I do, though (I work at my home office, so I have to keep it running through the day). But I can compare to what I had at my old house, which was about 1/2 the size of this house, and it had a gas furnace and a separate central air conditioning unit. Other than the periods when the moisture problems were occuring in the crawlspace, my heating/cooling bills have not been too much higher, maybe in the range of 10-20%.
If you do get a geothermal heatpump, make sure the installer properly fill the trenches they dig for the exchange loop. The poeple who installed the system at my house just pushed dirt over the trench and smoothed it out, without compacting dirt down into the trench. The ground keeps sinking, and there are hollow cavities everywhere. I'm having to dig up parts of my yard, and have fill dirt brought in to fix it. It's not an issue for the heatpump itself, but it sure is a pain to keep tearing up the yard.
Jay Stootzmann wrote:

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• posted on June 12, 2006, 12:55 pm
I don't think the box fans are necessary since you already have the dehumidifier.
I assumed you've already followed Advance Energy's recommendations to seal up the crawl space and have a good vapor barrier installed.
I have two crawl spaces -- each with it's own dehumidifier. the larger space has a double vapor barrier and maybe runs one or two degrees higher than before [66-68 degrees] while the other crawl space is quite a bit smaller and runs about 72-78 degrees -- the second has only a single layer 6mil plastic vapor barrier. both spaces are sealed up pretty tight though and the floors above them are also pretty heavily insulated.
I keep the RH set below 50% on both -- below the level for dust mites.
I think the higher temp is from the dehumidifier having to work harder because of the lack of a second vapor barrier and the heavy insulation keeps the heat in.

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