Giving it away 1st this time. 🙂 There is no condensation in our walls nor any mold. Whew. Extremely happy & thankful.
The first test (The Secret Life of Wall Humidity) went well, but the temperature at the siding inside surface was 10 degrees warmer than outside.
So, we decided to wait until the outside was at least 10 degrees below dew point of interior temps to test again. Theoretically, if the temperature at the siding gets cold enough to reach dew point of the interior air, there could be condensation on that surface—the interior surface of the siding inside the wall.
We have found tonight that there is not.
Why is this?
Well, let me give you the stats and then my guesses.
Date: December 5, 2016
Time: 7:40 pm – 8:30 pm (takes a while to get all the measurements)
Indoor temperature: 62 degrees Fahrenheit
Indoor relative humidity: 33%
Outdoor temperature: 20 degrees Fahrenheit
Outdoor relative humidity: 72%
This means that the outdoor temp is 12 degrees below the dew point temp for the indoor air. In other words, air at 62 degrees with a relative humidity of 33% has a dew point of 32 degrees (and 20 degrees is 12 degrees below that).
Insulation temp (in the “middle” of it, with no plaster over, where we are checking): 42 degrees Fahrenheit
Insulation humidity (in the “middle” of it): 33% 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): 19 degrees Fahrenheit
Humidity right next to the siding (interior side): 67% relative humidity
Like last time, 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.
However, we have now reached the point that if condensation happens, it will be ice or frost (on the interior side of the steel siding). Of course mold cannot grow on ice.
Mold also cannot grow in temps of 19 degrees, so the outside of the wood framing (where it meets the metal) is safe even though the humidity is “relatively” high at 67%.
But IF we get condensation that is ice, of course at some point it will melt. Will it be enough to cause mold? I doubt it, since the steel itself is a drying plane (moisture can rise up the steel ribs and out the top or run down into the metal flashing at the bottom). Also, as the ice melts, this means the temperature is heating up, which means the air can now hold more moisture, which means it should “grab” some of that frost and turn it to vapor in the heat. Am I missing something? (Serious question.) (Update: very cold weather has not caused any problems.)
Once again, the numbers: The interior air today was at 62 degrees with a relative humidity of 33%. If my calculations are correct, when you take that air and cool it down to 19 degrees (as was in the wall), the humidity “should” be 100% relative humidity (condensation because it has reached dew point).
But the test hole only had a humidity level of 67%, even though the temperature was 19 degrees.
It now seems clear that the total vapor in the house is not diffusing through the insulation. This is what was predicted in various articles. Diffusion is a weak and slow force. Some of these articles are mentioned in our book.
I would love to know how other insulations perform with metal siding and no interior vapor barrier, or even better with metal siding, no vapor barrier, and plaster interior walls.
Christa Upton Black Hills Picture Books Edgemont, SD 57735
One reader asks how the humidity is at the bottom of the wall (could be the coldest place in the wall since heat rises, thus possibly the most humid place as well). Steve took the measurements inside the wall about 12 inches from the floor, so that is pretty close to the bottom! 🙂 (He also took this on the north wall, the wall which will get the least sun and theoretically the least drying action during the day.) I would like to check even lower sometime, maybe even touching the concrete slab inside the wall.
She also asks if there were any ice crystals in the insulation near the siding. No, there were not!!! 🙂 I am very happy about this as our framing is wood. Wood is more susceptible to mold than steel framing, so our goal is to really keep moisture out of the walls.
For those who use steel framing, moisture in the walls does not seem like it would be as big a deal. Mold can still form if there is dust for it to “feed on,” but also steel also dries out much quicker than wood does.
I have several friends who are using steel framing. One lives in the southeastern United States, where the laws say she would have to use pesticides (for termites) if there were any wood framing!!! This is completely not a possibility for her chemical sensitivity.
I am eager to see my friends’ houses come together (both are building right now). I have a feeling both of their houses will work very well also.
Hey, thank you so much. Condensation is something I’ve had to worry about too. I like the way you went about it. It does take us longer to finish our projects but it’s better to get things right. Well done. I look forward to linking to this post tomorrow when I post about how we solved one condensation issue.
Condensation is so hard to fight, isn’t it? I think there hasn’t been a lot of research on this. Yes, better to get things right, when we can! 🙂 Thank you. I look forward to reading your post also!!!