Condensation

Published on May 31st, 2013 | by Henri

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Dark line around rug next to outside walls

Q.    We have a house built on a cement slab about 4 to 6 inches above the ground at which point the vinyl siding starts. There is a dark line all around the edge of the rug that butts up against the outside walls. Is there a way to eliminate this problem? Vacuuming doesn’t cut it, and washing doesn’t seem to do much good either. Is it because the dirty air blows under the siding where it meets the slab and comes into the house with the dust?  Someone told me that we cannot caulk under the siding where it meets the cement slab because it would cause mildew. Is that true? When they put the vinyl siding on five years ago, I watched them wrap the house with Tyvek first.

I hope you understand what I’m trying to tell you; my description might leave a lot to be desired. Could we caulk from the inside before we get the new carpeting so we don’t repeat the same process with the new rug where it gets dirty along the outside edges? Or can we caulk under the siding on the outside to keep this dirty air from entering, if indeed this is what is causing it?

A.    Although it is possible that cold air is infiltrating at the joint of the concrete slab and wall framing, I think the most likely cause of the dark stain on the edges of the carpeting is due to the formation of mildew because the perimeter of the concrete is cold.

I see no reason why you can’t caulk the joint of the bottom plates of the walls and the concrete slab from both inside and out if there isn’t a sill sealer between them, as there should be. You should be able to tell if there is one by pulling the carpeting up along the edges and looking for it; it may be a black, shiny plastic covering cellulose; a strip of fiberglass insulation; a white or blue foam strip — or whatever is commonly used in your area. You can also check for air infiltration by holding the back of your hand or a lit candle as close as you can to the joint to see if you can feel cold air or see the flame flicker. Just be sure to pull the carpeting up and put something on the concrete to catch the drips from the candle.

If I am correct and the dark stains are caused by condensation and mildew formation because the slab is cold, the solution is to install rigid insulation against the outside of the slab. Dig down a couple of feet along the entire perimeter of the slab. Clean the concrete with a stiff brush, and water if needed; let it dry. Run a strip of polyurethane caulking or StyroBond at the top of the slab and put daubs of the same adhesive every few inches along the concrete all the way to the bottom of the excavation. Press in place a 1-inch thick rigid extruded polystyrene (Styrofoam blue board, pink Foamular or whatever is commonly used in your area).

Cover the insulation with half-inch thick pressure-treated plywood power-shot into the concrete or aluminum coil stock adhered with caulking or Styrobond, making sure that the insulation and its cover are properly flashed under the vinyl siding to keep water, dirt and other foreign objects from getting behind them. Backfill the trench, sloping the grade away from the slab and plant grass. An experienced carpenter should be able to do this easily.

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