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I live in a 1940s house with uninsulated double brick wythe exterior walls. The interior wythe has some kind of masonry skim coat. The front wall of the house has a bay window. Last spring, I built a bench seat in the bay window nook using 3/4” ply bolted to the walls, and 1/4” ply for the floor, on top of a plastic vapor barrier (concrete subfloor underneath). The inside is used for shoe storage.

In the fall, I noticed some mold developing inside the bench along the lower exterior wall / floor. I used a moisture meter and concluded that that moisture was wicking through the lower portion of the exterior wall, because moisture readings were highest in this area. There are no window leaks or sill leaks.

There’s a garden on the other side of that wall. In one spot, the soil was mounded up against the wall, above floor level, with low spots next to the mounded soil and visible water pooling near the brick during rain. I suspected the poor grading was the cause of the issue, so I dug a trench along the entire bay window wall, installed a French drain, sealed the brick with a couple coats of Henry foundation sealer, and backfilled the soil making sure it was floor level. The floor level soil level isn’t ideal, but it’s a couple brick courses lower then it was before, plus there’s a French drain now.

I thought for sure this would fix the problem, but alas, after a few winter months, the mold has returned in the same spot as before.

Did I misdiagnose the issue? Is this a condensation problem due to no insulation and poor ventilation? The bench seat is full of shoes and has no ventilation, aside from a row of tiny holes I drilled (probably not effective).

I also just learned of a “damp course”. I went back through photos and the exterior wall does not appear to have a damp course, nor a buried one. Could it be that the bricks are wicking water from below the French drain?

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Yes your french drain helps, but french drains do not “whisk” every drop of water away as it rains. Often the pipe and soil around the pipe hold the water for sometime after it stops raining.

Your problem is that the pipe and surrounding soil is too high, which allows moisture to wick into the house.

Your double wythe brick wall is not waterproof and allows moisture to wick in through cracks in mortar joints, etc. Also, I wouldn’t be surprised if you covered up weep holes with your garden dirt. This keeps moisture in the wall until it seeps into the house. (There’s more moisture at the bottom of the wall.)

Look around your house and see if there isn’t weep holes on other sides of your house.

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