I am trying to figure out the reason why I am getting wet rim joists on my late 70’s bungalow in only three of the four corners.  I live in southern ontario canada and we get a fair bit of snow in the winter.

 

I discovered a couple years ago that I had water getting in through the siding and around the patio door and soaking the drywall in a closet and cupping my hardwood floors on the back corner of my house.  I also found that under the area where the wall was wet, the rim joist had frost on it and appeared to be soaked.  The joist bays had 2×4 fiberglass insulation and no vapor barrier.

 

I already planned on replacing the siding, doors, windows and improving the insulation in the walls so I did all the work that summer. The year before that I also improved the attic insulation from 2×4 fiberglass to 18″ of blown in cellulose and installed all new rafter vents and put on a new roof the year before that.

 

so I had assumed that the rim joist was getting wet from water intrusion through the old siding and patio door, but now after fixing those issues, Its still getting wet and I noticed its not just in that one corner of the house but three of the four corners are getting wet.

the three wet corners all have a down spout near them.  I figured it had to have something to do with the downspout locations, but the rim joists only get wet in the winter time so I don’t know if that’s it since there is no liquid water.

so my theories as to why the rim joist is getting wet are:

 

1. snow is building up against the concrete foundation and somehow soaking through the concrete and getting through the vapor barrier?  I don’t see how this could be it but I do often have snow piled against the house.  Its not unusual to have 2-3 ft of snow outside in the winter.

 

2. The foundation is getting cold enough to cause condensation where the air flow in the basement is poor and unable to dry or warm those areas. (I think this makes sense as I found frost there the first year.)

 

3. Water is somehow getting through the screws that go through the siding to attach the down spouts to the house and making its way through the wall until it gets to the rim joist. I don’t think this is it, as I checked the back of the sheathing from inside the house on one corner recently, and it looks nice and dry with no evidence of getting wet.

I think I am okay to just air seal and insulate my basement walls and joist bays as that will take care of the cold air condensing on the rim joist but If that’s not the issue than I could just be locking the moisture in and i’ll have major problems down the road.  so I wanted to ask for your opinions!