Thursday, 2 May 2013

The right level of humidity

I found something more precise on humidity levels from Justin O'Keeffe's blog.
It looks more precise, but the reproduction leaves a bit to be desired. It's a photo of a poster from a presentation, by the look of it, but I can't find the original source online.

There was one here:
There's another more sketchy one here in the green garage, Detroit, with the temperatures in Farenheit. I know I'm prejudiced, but I don't trust temperature scales based on the body temperature of sheep, when there is a perfectly good one based on the freezing and boiling points of water. Biology is at least two steps down the fuzziness ladder in the realm of the sciences, and physics should not be borrowing measurements from there.

There's another one here.

During the one-year evaluation, the boss's son from the builder said we should aim to keep it between 40 and 60%, although he didn't have anything more scientific, and for more precision told us we should see how dry our skin and throats feels. I asked where the humidity should be as far as the wood is concerned.
The architect then started talking about wood having 8-10% humidity, so the humidity of the house should be fine.
I researched more about this later, and found that he was actually talking about the moisture content of the wood. This is not the same as the humidity.
The thing they have in common is that both are percentages.
But the percentages are very different. The moisture content of the wood is the amount of the weight of the wood that is water. The relative humidity is the amount of moisture in the air, as a percentage of the maximum moisture that the air can hold.
Obviously there is a relation between the two, since wood is somewhat permeable and moisture can get in and out.
Here are some relationships, extrapolated from
22% RH = 5% EMC
28% RH = 6% EMC
35% RH = 7% EMC
42% RH = 8% EMC
49% RH = 9% EMC
57% RH = 10% EMC
65% RH = 12% EMC
74% RH = 14% EMC
80% RH = 16% EMC
EMC is the equilibrium moisture content. In other words, the moisture content that you'll end up with if you leave wood in conditions with that relative humidity.

They give a short version too:
0 = 0 
30 = 6 
50 = 9 
65 = 12 
80 = 16

Changing the moisture content of the wood will make it shrink or expand, so if it is supposed to stay between 8 and 10%, then the relative humidity needs to stay between around 40% and 60%. Or if the relative humidity of the building stays between 40% and 60%, that's what the moisture content will be. I don't seem to be getting much nearer finding definitive recommended humidity, but I'm still recording it in my house and within the walls, where the humidity has been averaging 36% and fluctuating between 49% and 23% over the past two months, with an average temperature of 15 degrees.

I can see some kinds of trends in the humidity within the walls, with some differences between the North and the South. Back in July the middle of the north wall was averaging 58%, fluctuating between 51% and 68% at an average temperature of 27 degrees. Over twenty days in the middle of February it was averaging 31%, fluctuating between 23% and 40%, at an average of 11 degrees.
In the South wall, the July humidity was slightly lower with slightly larger fluctuations. In both May and October, the humidity in the North wall was around 10% higher than the south wall. Roughly averaging 50% against 40%.