Another Humidity Question

Mike T., Swampeast MO

http://www.skuttle.com/humid.html

Is this chart accurate? I'm assuming it is the humidity level that would be "natural" in a space if it had no source of moisture save that in the outdoor air. Was trying to find a calculator (or even equation) to determine this but struck out as they all seem to only work with air temps above freezing...

Anyhow, if air is changing in a structure multiple times per day (naturally or mechanically in a really tight structure) how does indoor humidity level EVER manage to stay much above what it would be if outside air were warmed to indoor levels?

Is the "extra" humidity imparted by people/appliances/etc. in the structure going into the structure & contents themselves which effectively raises the humidity of incoming air?

jerry scharf

computing relative humidity

Mike,

It took me a few minutes to find what I was looking for, but here it is.

Relative humidity is defined as the ratio of the vapor pressure of water existing vs. the saturation vapor pressure at a given temperature.

My CRC handbook has nice tables of water vapor pressures over both ice and water. If you don't have a copy, you should look at picking up a used one (these numbers don't change very fast and used ones go for 1/3 the cost.)

So to go from 80% at 0F(-18C) to 72F(22c) works out as follows:

vapor pressure at -18C is .939, times .8 =.751

vapor pressure at 22C is 19.827

relative humidity of the air raised from 80% at -18C to 22C is 3.79%.

When you change the out heated air at cold temperature, the humidity goes flying out with it. That's why I use energy recovery ventilators vs. heat recovery ventilators. In my climate (No CA) the wicking type is fine. For the southeast in summer and the cold areas in winter, you may need to go to the rotating descicant wheel types (too complex for me.)

hope that helps,
jerry

Mike T., Swampeast MO

Thanks Jerry

My CRC manual (bought for Calculus in HS) is still in my bedroom at parent's house! Need to retrieve some day. That was 1981-2 right when scientific calculators and computers were becoming affordable and I kind of got lazy...


>When you change the out heated air at cold temperature, >the humidity goes flying out with it.

So...does that imply that the amount of water vapor put into the indoor air over the period of a day is the difference of amount of water in the cubic contents of the structure minus the amount contained in same amount of outdoor air times the air changes in a day?

jerry scharf

> So...does that imply that the

> amount of water vapor put into the indoor air

> over the period of a day is the difference of

> amount of water in the cubic contents of the

> structure minus the amount contained in same

> amount of outdoor air times the air changes in a

> day?




I'm not sure I exactly understand this. For me, I think about this in term of water mass. You have X water in the air to start with, as you exchange inside for outside air, the amount of water drops by the ratio of water content times the rate of exchange. If there is no moisture source, this would approach the outside moisture level (mass wise) over time.

The other part of this is the moisture introduction that occurs inside the house. Things like people breathing (I've gotten no cooperation on controlling this at my house,) showers and cooking all introduce moisture into the air.

Non of this is even close to steady state, the moisture sources, the exchanged air volume and moisture content move around in totally unpredictable way. I wouldn't even begin to know how to model this.

So you take a shower, turn on the fan and who knows whether the moisture in the envelope rose or fell and by how much.

It seems like most allergic problems minimize in the 40-50% range, with some increasing with higher RH and others descreasing with higher RH. Mold is my main battle (wife is terribly allergic to it) so I'm always working to keep it at or below 45%

jerry

Mike T., Swampeast MO

Sorry. Hard to explain.

If there were no other source of moisture than that in the outdoor air, then there would be the same amount of water in the inside as outside air...OK?

If warmer inside than outside, than the relative humidity of the warmer air would be lower than that of the outside air even though it contains the same amount of water vapor...still OK? If so, consider the actual amount of this water as the "Base Water Vapor".

If the RH of the indoor air is higher than it would be without any other source of moisture than the "extra" water is the amount at Current Water Vapor - Base Water Vapor.

Say that works out to be 8 pints. BUT, the air in the structure is constantly changing. Even a VERY tight house has 1/3 air changes per hour so there are 8 air changes in 24 hours.

So, does that mean that there are 8 * 8 = 64 pints of water being added to the space each day?

jerry scharf

in a word

yes. It's a tad more exact to talk about mass that volume. Other than that, that is exactly how I understand it.

jerry