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#1
04-14-2012, 01:25 PM
 F.Pu-du-he-pa-as Guest Join Date: Feb 2010
Calculating humidity levels

As I mentioned here a while back, I have trouble controlling the humidity in my apartment, and even if I run a dehumidifier, the place still returns to 80 or 90% relative humidity in short order.

Now I'm curious where all this humidity is coming from. Before, I'd considered sources like the humidity in the air outside. But, today I started wondering whether the humidity could just be coming from the evaporation of water in the apartment, say from toilet water or something.

Is there some equation that would let me calculate the evaporation of water and relative humidity over time like this, given the volume of space in the house, the temperature, and the starting relative humidity?
#2
04-14-2012, 02:04 PM
 F.Pu-du-he-pa-as Guest Join Date: Feb 2010
In other words, what could have possibly made the relative humidity jump from 55% to 80% over the course of an hour when the relative humidity outside is 45%?
#3
04-14-2012, 05:59 PM
 Una Persson Straight Dope Science Advisory Board Join Date: Mar 2000 Location: On the dance floor. Posts: 14,260
A couple of things sound unusual here.

First note the difference between relative and absolute humidity. That is, you have:

* absolute humidity which is how much mass of water you have per volume of dry air, and

* relative humidity which is a ratio of how much humidity you have before you reach saturation.

Meaning - if it's 90F and 45% RH outside, and your inside temperature is 70F, that same outside air, cooled to 70F, will have a relative humidity of 85% (if I'm reading my psychrometric chart properly). The air at 90F and 45% RH has about 0.0135 lbm water per lbm of dry air, and so does air at 70F and 85% RH. In fact, by my chart if you cool that outside air to about 66 F, you reach saturation.

http://www.truetex.com/psychrometric_chart.gif

So just letting in outside air under those conditions and cooling it to 70F, assuming you have no condensation, will give you a very high humidity in your house.
#4
04-14-2012, 06:01 PM
 Una Persson Straight Dope Science Advisory Board Join Date: Mar 2000 Location: On the dance floor. Posts: 14,260
...so what I'm saying is consider air coming first, as opposed to just things like your toilet water vaporizing. I mean, unless you're boiling up a huge old mess of seafood and pasta every night, where you certainly could see some short-term increases.
#5
04-14-2012, 06:06 PM
 F.Pu-du-he-pa-as Guest Join Date: Feb 2010
Ah. In my case, the outside air is about 60 degrees F, and I like to keep the air in the house around 75 degrees F, so if I kept the absolute humidity constant and just increased the temperature, we'd expect the relative humidity to go down. Which is the exact opposite of what's happened here. This is why I'm so puzzled!
#6
04-15-2012, 07:42 AM
 Una Persson Straight Dope Science Advisory Board Join Date: Mar 2000 Location: On the dance floor. Posts: 14,260
Quote:
 Originally Posted by F.Pu-du-he-pa-as Ah. In my case, the outside air is about 60 degrees F, and I like to keep the air in the house around 75 degrees F, so if I kept the absolute humidity constant and just increased the temperature, we'd expect the relative humidity to go down. Which is the exact opposite of what's happened here. This is why I'm so puzzled!
You are correct.

Without knowing an awful lot more about your house and habits it's not immediately clear to me why you would see huge humidity spikes. Even if you had a furnace humidifier it's doubtful it would get the humidity up by that much. You could try an experiment where you took a graduated measure (such as a glass or plastic measuring cup measuring ounces or ml, set it out with some water in it, and see how fast it evaporates a fixed amount of water.

Sources of humidity from within your house include:
* Furnace humidifiers
* Toilets and drain traps (likely very small)
* Transpiration from humans and pets
* Cooking/cleaning activities
* Evaporation from house plants and their soil
* Vapor seepage through foundations, and sump pump wells

Is it possible too that your meter or method of measuring is flaky? Sometimes an inexpensive meter will either have a bias, or act strange when it gets to one end or the other of its scale (usually, however, they seem to act up at really *low* humidity).
#7
04-15-2012, 10:50 AM
 F.Pu-du-he-pa-as Guest Join Date: Feb 2010
Quote:
 Without knowing an awful lot more about your house and habits it's not immediately clear to me why you would see huge humidity spikes. Even if you had a furnace humidifier it's doubtful it would get the humidity up by that much. You could try an experiment where you took a graduated measure (such as a glass or plastic measuring cup measuring ounces or ml, set it out with some water in it, and see how fast it evaporates a fixed amount of water.
I set up the experiment as you described; I'll let you know how it goes.

Quote:
 Sources of humidity from within your house include: * Furnace humidifiers * Toilets and drain traps (likely very small) * Transpiration from humans and pets * Cooking/cleaning activities * Evaporation from house plants and their soil * Vapor seepage through foundations, and sump pump wells
(1) is out, since I don't have a furnace. (2) and (3) are possible, though the humidity spikes rapidly even when I'm not around. (4) is out, since I conducted my tests yesterday when I wasn't doing any cooking or cleaning; I didn't even have any dishes drying in the dish dryer. (5) is out because I don't have house plants. (6) is partially out, since I don't have a sump pump.

(6), vapor seepage through the foundations, seems the most likely. My house is on a concrete slab which sits in the ground. Actually, it wasn't built as a house; it was built as a garage, then converted for human occupation. I've suspected that something was going on with the floor for a while, because if I leave clothes on the floor, they start to feel damp.

Is there any way I can test for vapor seepage through the foundations?

Quote:
 Is it possible too that your meter or method of measuring is flaky? Sometimes an inexpensive meter will either have a bias, or act strange when it gets to one end or the other of its scale (usually, however, they seem to act up at really *low* humidity).
It's entirely possible, since I'm just using the humidity meter on my dehumidifier. On the other hand, I can feel how muggy it gets in here whenever I come back from an errand, even if I've brought the humidity down before I left. So, the huge humidity spike is real, even if the numbers are somewhat off.
#8
04-15-2012, 06:07 PM
 Una Persson Straight Dope Science Advisory Board Join Date: Mar 2000 Location: On the dance floor. Posts: 14,260
Quote:
 Originally Posted by F.Pu-du-he-pa-as Is there any way I can test for vapor seepage through the foundations?
I've heard of people taking an accurate humidity meter and sealing it to the concrete floor inside a 5-sided plastic bag (the sixth side is the slab) but other than that, I don't know.
#9
04-15-2012, 06:47 PM
 F.Pu-du-he-pa-as Guest Join Date: Feb 2010
Quote:
 You could try an experiment where you took a graduated measure (such as a glass or plastic measuring cup measuring ounces or ml, set it out with some water in it, and see how fast it evaporates a fixed amount of water.
The results are in: it's been sitting there all day and the water level has not budged at all. I think we can safely say that evaporation of open standing water is not the cause of my humidity problem.

Quote:
 Is there any way I can test for vapor seepage through the foundations?
I had two ideas for how to tell whether the humidity is coming from the floor. The first is something like yours; seal a humidity gauge against the floor. I thought I'd keep a second humidity gauge in an identical bag or container that is sealed, but not against the floor, to control for any effects of the bag/container or anything else. A second method would be to do roughly the same thing, but with a specific amount of desiccant (silica gel or calcium chloride, I think) instead. I could then weigh the desiccant before and after to determine the mass of water that came up through the floor.

Anyway, if I try either of these methods, I'll certainly let you know how it goes. And thanks for all your help!

Last edited by F.Pu-du-he-pa-as; 04-15-2012 at 06:47 PM.

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