I see this occur commonly in our climate north of Kamloops. I have pondered it for a while as there is a home I drive by everyday that has it. I believe it’s caused by a few different things. Obviously there is thermal bridging taking place along the studs of the home. On the inside of the home it’s easier to understand… when the studs transfer the house heat to the exterior they become cooler than the drywall beside them. The moisture content on the surface of the wall is higher over the stud itself as the warm moist house air slightly condenses on it. Dust or smoke particulate stick to the moister surface and create a stain over time.
It seems the same phenomenon is taking place on the exterior of the home, but in this case why would the staining take place over the studs? You would think it would form over the colder part of the exterior wall which would be in between the studs where the insulation is best. If heat is transferring through the studs to the exterior wall, the stud area is warmer and would not form condensation.
But what I think may be happening is that when relative humidity is just right and when the temperature outside quickly freezes over night FROST forms on the entire exterior wall surface (over the studs and the stud cavity). As the exterior temperature gets close to zero the heat from the house helps melt the frost at the studs first. This melting takes place FASTER and PRIOR TO the rest of the wall surface which melts and evaporates slower. I think the difference is the speed of melting causes the melted frost over the studs to drip down the wall surface (before it can evaporate) smearing any particulates that had previously attached themselves to the wall during the summer.
So my theory is that if the wall surface was perfectly clean and there was no heat transfer at the studs, this wouldn’t happen. If the house was insulated perfectly and had a perfect vapor barrier on the inside wall this wouldn’t happen. Notice in Bob’s picture you can see that about within 2′ of the roof line there are no marks. This is because the dew falling from the air downward isn’t able to hit the wall because of the roof overhang.
I don’t think anything grave is at at stake here. It just means imperfect insulation and vapor barrier. The only concerns being a little less efficient home and unsightly appearance on the outside wall.
Again this is just my own opinion. I could be wrong but that’s what I think is happening.
November 29, 2017 at 11:13 am
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Yard view
