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?
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.
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 guess not, but you might worry about your electric bill.
I'd say just keep the RH below 60% to avoid mold.
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!
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
Note: I would agree with the vapor barrier and with looking for the
source of the moisture.
Why did you close up the vents?
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
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
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).
Since you said you're a resident of Knoxville TN then the results from
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].
Check some links about Mold at:
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?
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:
I don't think the box fans are necessary since you already have the
see: http://www.advancedenergy.org/buildings/knowledge_library/crawl_spaces /
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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