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White mould in insulation
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I own a four year old house. Last weekend I started construction on a bathroom in my unfinished basement. I was installing the air duct through the rim joist where the floor joists sit on the foundation wall. So I removed some insulation in one of the spaces between the floor joists and noticed the rim joist and the insulation were wet and growing a white mould. Then I checked around the whole rim joist perimeter. It was wet and mouldy behind the insulation almost everywhere I checked. I can’t figure out where the water is coming from because the house has gutters, it sits high up off the ground and the roofing is almost new. What’s your best guess?
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This is a more common problem than most folks suspect. The only reason we don’t here more about it is that most homeowners don’t get up on a step ladder and check this location in their basements. The insulation that is installed in-between the floor joists against the exterior rim joist is required by Code (for good reason) to be sealed with a vapour barrier. Often, this vapour barrier is omitted by contractors because it is one of those fiddly details at the end of the construction journey that just gets forgotten or postponed. Often, the municipal or regional building inspector will note the lack of vapour barrier at the rim joists as a deficiency on his final inspection report, but the new home owner won’t recognize the potential problem and leaves the insulation exposed.
Here’s what happens. The warm, moisture laden air in your basement rises up to the underside of the main, plywood sub floor and rolls out to the rim joists. This moist air leaks through the insulation and starts to cool as it reaches the rim joist. When the moist, cooling air hits the rim joists, it finds the condensing surface it needs to become water. The rim joist and the insulation can become damp and mouldy as a consequence of this missing vapour barrier. However, this problem does not occur in every unprotected rim joist. A lot depends on the humidity level in each house. If your basement struggles with moisture issues because it is dug into a damp environment then there is a higher risk of dampness, mould and eventual rot at the rim joists.
Installing small, well sealed 6 mil UV rated polyethylene rectangles over the insulation in each of the floor joist "bays" is the most common way to seal the rim joists. Where the floor joists are parallel to the foundation walls, seal the vapour barrier with black acoustic caulking to the wood sill plate and then seal it to the first floor joist running parallel to the concrete wall. I prefer to cut tight fitting SM high density blue Styrofoam into rectangles and squeeze them into each floor joist bay. If you are really fussy about a good vapour seal you can then caulk the seams of the Styrofoam to the joists and sub floor. This way you gain additional R value and you get a better vapour tight seal than polyethylene.
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Good Question!
This article was written by Lynch Building Inspection Services Ltd. of Nelson, British Columbia. It originally appeared in The Nelson Express.
Lynch Building Inspection Service offers residential, commercial, institutional building and construction inspection.
Our territory encompasses the Kootenay/ Boundary Region of BC, and includes Ainsworth, Balfour, Burton, Castlegar, Creston, Christina Lake, Fauquier, Fruitvale, Grand Forks, Greenwood, Midway, Montrose, Nakusp, Nelson, New Denver, Procter, Rossland, Slocan Park, Salmo, Slocan, Kaslo, Silverton, South Slocan, Trail, Warfield, Winlaw, Wynndel, and Ymir.
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