The Secret Life of Wall Humidity

Not to KEEP bringing it up, but if you’ve new to my site, this is regarding “the controversy.” (You can read other people’s comments and my comments below her review.)

It IS good that I learned some risks so that we can check this out and not have something bad (mold) develop without my noticing.


Today’s Stats

Date:  November 17, 2016

Time:  2:45 pm – 3:30 pm (takes a while to get ALL the measurements)


Indoor temperature:  64 degrees Fahrenheit

Indoor relative humidity: 33% relative humidity


Outdoor relative humidity (not that it matters for our purposes, but to be thorough): 94% relative humidity (It’s snowing.)

Outdoor temperature: 31 degrees Fahrenheit


This means that the dew point temp for the indoor air has been reached by the outdoor temperature. In other words, air at 64 degrees with a relative humidity of 33% has a dew point of 34 degrees.

Anything 34 degrees or below should cause the water vapor in the indoor air to condense (if the indoor air and complete load of vapor is reaching that surface).

So, if the siding is 34 degrees or colder on the interior side in the wall, then there is risk of condensation on the siding in the wall.


Insulation temp (in the “middle” of it, with no plaster over, where we are checking):  58 degrees Fahrenheit

Insulation humidity (in the “middle” of it): 30% relative humidity

(Insulation is sheep’s wool from Oregon Shepherd.)


Siding (interior side, in wall) “temp” (measured with temp/humidity monitor probe touching the siding, but it reads all “around” the probe, so basically measuring the air temp “between” the insulation and the siding): 41 degrees Fahrenheit

Humidity right next to the siding (interior side): 55% relative humidity


So then Steve reached his hand through the insulation to the siding to see if he could detect any moisture or condensation at all.

Drum roll please…………

DRY.  Cold, but dry.

Insulation there probably isn’t as good as most places, because the hole where he was checking is a hole for the water pipe bringing water into the house. Since it is loose fill, and this section is open with no lath or plaster to hold it in place, it’s probably not as well-packed or thick. (We cover it with a bit of foil when not measuring or feeling behind for condensation.)

But this was with the outdoors “barely” at dew point temp for indoor air, so we will need to keep checking as it gets colder.

If the interior of the siding gets colder, or the humidity level goes up inside the house (though not likely to go up much because we are so careful), this will increase risk of condensation on the interior surface of the siding.

If the insulation keeps the siding only 10 degrees warmer than outside (and I have no idea if this will turn out to be true or not), the “next level” where risk would be reached is outdoor temp 10 degrees below dew point. That would make the interior surface of the siding AT dew point if all the interior vapor load gets to that interior surface.

So, assuming we stay around 64 degrees and 33% relative humidity inside, that means the next major time to try this experiment is when the outdoor temperature is 26 degrees or lower outside.

Other thoughts—

Thermal Bridging: As Steve was disturbing the insulation, he probably created somewhat of a thermal bridge (letting heat and vapor through), so this probably makes things worse at that spot temporarily. And as such, we are not going to do these tests too often because that in itself could cause risk of mold.  We are considering an infrared monitor to be able to gather some data without disturbing the walls. An area of cold (which the infrared monitor should show) would show an area of thermal bridging, which means much higher risk of condensation.

Diffusion through walls: The place where Steve tested and another secondary test (near the washer)—neither place has plaster over top. This likely increases diffusion. In other words, the plaster on the rest of the walls probably decreases diffusion of air and vapor through the wall, decreasing the humidity and condensation risk inside walls. This is one drawback of an infrared monitor, because even a cold area has to have enough vapor coming through to condense, so finding cold areas doesn’t necessarily mean mold. Knowing “how cold” an area makes it risky is tricky, and opening up a wall (at least while its cold outside) has risks in itself! I wish there were a way to measure humidity inside a wall without letting humidity in the wall!  😀

EVEN MORE thoughts for those of you who are still with me. 😀 As long as the walls remain dry, this stuff is “kinda” theoretical, but my main goal is to understand all this so that people can begin to make more of the science of building and build many great houses with little mold risk. And so I will know what to do if my walls begin to get condensation.

The numbers: The interior air was at 64 degrees with a relative humidity of 33%. If my calculations are correct (ha! I sound like some kind of scientist), when you take that air and cool it down to 41 degrees (as was in the wall), the humidity “should” be 76% relative humidity.

But the water pipe hole only had a humidity level of 55%, even though the temperature was 41 degrees.

The hole by the washer was even colder (36 degrees) but still had a humidity level of only 55%.

This seems then, that one or more things are happening.

  1. The total vapor in the house is not diffusing through the insulation. I do not know if this is because of insulation period or THIS insulation (wool) in particular. I think both probably play a part, because of what I’ve read in the articles I mention in our book.
  2. More vapor was diffusing through at the water pipe hole than the washer hole. Could it be because at the water pipe hole, Steve sat down there with the monitor breathing hot, humid air near it? Probably. Like one might fog up a cold window. At the washer hole, he found a way to let the humidity monitor sit by itself while it re-adjusted to the siding temp/humidity.

The amount of variables is so high, and without a lab, I can’t really do a “control” for this experiment. I’m also not going to raise humidity (risking my walls) to see what happens.  😀 I feel there are probably also other things I’m not taking into consideration, but I don’t know.

I think this is the longest post I’ve ever written.

So will we end up with condensation as the weather hits lower temperatures?!? Stay tuned and find out! 🙂

Christa Upton    Black Hills Picture Books    Edgemont, SD   57735



About Christa Upton

I am a wife and mother of three children ages 11, 14, and 18. I used to be a stay-at-home mom (teaching piano & dance, volunteering, etc). From 2007 to 2010, I suffered accidental Toxic Injury (also called Multiple Chemical Sensitivity or MCS). MCS has had major impact on our family, but the forced time in bed has given me time to write. So far, I have published 4 children's books (2 in e-book format on Kindle, one in Print-on-Demand at CreateSpace, and one printed by a local printer). Sometimes I miss my old life, but I love writing for children!
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