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.
| Crawl Space | Outdoor | |||
|---|---|---|---|---|
| Date | Temp (F) | Humidity(%) | Dew Point(F) | Dew Point(F) |
| 20 June 2009 | 68.5 | 92 | 66 | 72 |
| 4 July 2009 | 69.8 | 77 | 62 | 58 |
| 11 July 2009 | 69.8 | 81 | 64 | 64 |
| 19 July 2009 | 70.3 | 81 | 64 | 59 |
Note: On 14 July 2009 my Honeywell wireless hygrometer died and I replace it with an Oregon Scientific RMR500A. The calibration was clearly different. I calibrated the new one using the damp salt method.
I expect it to become gradually harder for the SmartVent to improve the situation under the house because as it lowers the dew point, there will be fewer times when the outside dew point is lower. But, I hope and expect it to be better than it was.
5 comments
In my Crawl Space I have 70F and 77% humidity in the summer time and 60^ and 60F in the winter time.
The crawl space is actually 1-2 ft deep and above it are the beams of the floor of the house.
Please send me a acomment wih your opinion on this situation.
I insulated and sealed the floor of the crawl space with bubble wrap-the crawl space is on sand actually.
I made some experiemnts and I discovered that he evaporation from the sand surface is about 0.1 mm per day of water that is: 150 x 0.0001 x 1000= 15 kg or 30 pounds of water waports per day- for an uncovered surface but mine is covered although not perfect- so i assume in the worst case is about 10 pounds of water vapors emmanating from the crawl space up in the house via cracks.
Please help send me a comment.
Thanks
Mike
Hi Mike,
Thanks for your very interesting comment. I’m no expert so my opinion of your crawl space humidity isn’t worth much. You’re clearly doing better than me! The dew point outside is so high in the summer that I don’t see me gaining much more.
I’m very interested that you measured the evaporation rate. How did you do that?
From the amount of water entering the air daily you should be able to calculate the lowest dew point you could achieve.
The folks at SmartVent argue that vapor pressure drives the process. They are trying with their controlled ventilator to rapidly follow the decreases in vapor pressure (by running the fans) and, at least, slow rises by closing off the vents.
I think your measurements of evaporation from the substrate add an interesting term to the equation.
Definitely put insulation around the cold pipes- I put the thermometer in the ground at 2 m and I found that the temperature here in Ontario at the parallel 40 is all the time 6 C winter and summer(even if outside is 40C or -40C). So in your part of the world I think is the same. The insulation on cold pipes have to be carefully done- the wrapping of the foam has to be tightly to the skin of the copper pipes.This will create a gradient starting from 6 C at the skin to what ever temperature is around without condensation.
I think you are right, in the summer time, the RH in the house is 50% or even 40% (at 24C) and underneath is also around 50%(at16C). But in the summer in the house is going to 64%(at 20C)at and underneath could reach 77%.(at 19C).
In fact, everywhere, down and up we have the same atmospheric pressure and the evaporation is directed by the temperature- I visualize that if you take a microscope and go there in the sand -you see like huge boulders with large gaps -so when you go deeper you reach the water in form of pellicle on the boulders(the sand grains) but the temperature make them to agitate and go in form of vapor gas.
The whole structure has a lot of gaps- and the process of evaporation is going on- the vapor gas is emerging from everywhere and goes up (being lighter)- The question is : THE MOLDS ARE FEEDING FROM VAPORS OR FROM DROPLETS? THIS I have to check- because I believe they feed only on droplets of water that is physically condensed water-that is humid water on the surfaces and not from vapors-which has to go anyway up.
based on this I ASSUME THAT IF YOU DO NOT HAVE CONDENSATION, YOU DO NOT HAVE MOLDS. I checked as I said at 77% and 20 C the surface of the wood and was simply dry no discoloration no humid feeling. I think this is the main question.
I’ll cut a trap in the floor in about a month -when I’ll install the servo dehumidifier and I’ll take photos.
I’ll keep you posted
All the best,
Mike
Is October 2009 and I did nothing yet there but I ‘ll do it definitely.
Therefore , the vapors are in fact a gas having the density of about 0.8 kg/cubic meter-and goes up through the cracks.
It is interesting to note that the water is evaporating but for example the Calcium Chloride is not.I had a glass with water in the basemement and evaporates freely about 1 mm per day but the calcium chloride liquid never evaporates-stayed at the same level for over a year.
Also I checked that 77% RH and about 20 C is a dry environemment without condensation in a closed insulated space- therefore in my case I had bubble wrap on the soil so the soil is insulated.
I hope immediately I finish the order in the garage
I’ll be able to do the opening trap and the servo dehumidifier I mentioned before and inspect what is in the crawl space.
All the best
Mike
Another coment: therefore an open water surface is evaporating at a level of 1 mm per day.
But a soil in genetral at a level of 0.1 mm equivalent vapors per day. I did some experiemnts with some glasses with sand and water and i timed the total evaporation of the water I put in those glasses.
But these vapors are a gas- is like air or anything elese- definiteloy is going up because the air is 1.25-1.3 kg/cubic m and the water vapor is 0.8 kg/cubic m therefore will tend to go up
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