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 somewhat of 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 seems it should “grab” some of that frost and turn it to vapor in the heat. Am I missing something? (Serious question.)
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