moisture above foundation, anyone know what's going on here?

[brimic]

I'm doing some renovating in my basement right now and have found that along the north wall of the house, there is a treated rimjoist that sits edgewise on top of the foundation. The joist is wet, and has some mold growing on it. I found this when I pulled the insulation away from it to give it a quick checkup while installing ceiling tile.

My foundation is concrete block. I sealed it with several coats of Drylock a few years ago and have never had a problem with any kind of moisture in the basement before or after this application. The other walls in the basement look fine, and the Block wall on the north side is dry too.  I pulled off a few rows of siding on the north side of the house today and the OSB underneath looks dry without any sign of present of past dampness. My house does not have a vapor barrier wrap such as Tyvek.  

On the North side of the house, I assume I have good drainage- I did the grading myself and made sure there was plenty of slope away from the house, I also have about 10" of foundation sticking above the ground on that side of the house.

I have 2 ideas of what might be happening.

1. The too much moistue int he air, and the North side of the house gets little radiant heat from the sun and the water condeses and sits there.  The only flaw to this logic is that my basement is a rectangle with another square attached- I have a second North wall in my entry way thats about 12' long and no moisture problems there.

2. This is a stretch but maybe water is traveling up through the block in a capiliary action and going into the wood? The builder did not properly install a moisture barrier betreen the foundation and rim joist (I could just strangle them  )

[Matthew Carberry]

Any pipes or penetrations in the wall above that spot?  Condensation on a vent pipe or cold water pipe that flows above that spot might hit it with dripping at occasional intervals.  How did the insulation look?

Is the spot centered on a joint in the block below or square on one block?

[grampster]

Any ice buildup on the eaves this winter.  If so, water may have backed up and seeped down behind the siding and interior walls and saturated the plate?

[brimic]

Any pipes or penetrations in the wall above that spot?

Yes. I have a vent pipe for my clothes dryer going through the joist at one end, and the intake/exhaust for the furnace going through in the middle of the wall. I suspect that some of the moisture at one end of the wall is due to the dryer vent, I'm thinking about wrapping the pipe with metal tape to try to seal it up to keep residual moisture from leaking out of the pipe, but I'm not sure that this is the main problem. I had sealed the hole around the vent pipe a few years ago with expanding foam insulation.

Is the spot centered on a joint in the block below or square on one block?

I'll have to take another look at it, but the rimjoist seems to be wettest at each corner of the wall.

The rim joist runs paralell with the floor joists but is tied into the floor joists by short pieces of joist: something like this if you viewed it from the top:

Rim joist    -------------------------------------------------------------
                  I <-24" interval->I                    I                    I
floor joist   -------------------------------------------------------------

                 ------------------------------------------------------------

Any ice buildup on the eaves this winter.  If so, water may have backed up and seeped down behind the siding and interior walls and saturated the plate?

That's an angle that I might have to investigate. Is there a way to determine by inspection if this is the cause? I didn't notice any ice on that side of the roof. The house is a ranch style, with the north end being one of the ends of the house (peak of roof on that side). The very edge of the subfloor where it rests on the joist is damp in a few places.

How did the insulation look?

The insulation looked fine with the exception that it felt damp.

[280plus]

Is it possible there's a hole in the dryer vent letting it vent to the inside of the house? That'll get you some moisture going. Got mice around? Mice LOVE to chew into the dryer vent cause it has such good nest making materials inside...

[brimic]

Entire dryer vent is 4" furnace pipe.   I wouldn't consider it to be airtight because of the folded seams.

[280plus]

Sounds like it's condensing and dripping out a seam or a joint. I strongly suspect that it involves the dryer vent. But that's without actually seeing the whole situation so take that with a grain of salt.

If it IS that get some 4" corrugated alminum duct and rerun it with that. We call it "buc duct" around here. It comes in 2' sections that stretch out to 8'. Make sure you get a watertight seal to the hood with it. The buc duct is one solid piece so there is nowhere for water to drip out. As long as you don't put holes in it.

[grampster]

If you had ice dams, you would have seen it, it is very obvious.  It can get 4 or 5 inches thick at the drip edge and go back up the roof at least the width of your overhang.  The snow is melting on your roof and freezing as it passes over the eaves.  Once it dams up, the water gets pushed back up under your shingles and can enter behind the siding or even into the interior of your house.

[280plus]

Grampster is right with that idea too. I lived in a place once where there was a lovely waterfall right down the wall in my living room because the ice had built up in the gutter and backed up under the eaves. Couldn't do a dang thing about it until spring thaw either.

So for a while I lived like a rich person with a water feature right in my living room AND I got a discount on the rent to boot!

[brimic]

I had ice dams asy you guys described a few years back on the sunny side of my house. I ended up taping a scoop to a long pole and using it to dump rocksalt on the affected areas- it prevented damage to the interior of the house, but probably didn't help much with the longevity of my shingles.

I didn't notice anything out of the ordinary this last winter on my roof though.  

I think I'll try the dryer vent angle and try to seal the pipe up. The one joint in the pipe is taped up with tin tape, I'll try caulking the seam on the pipe.