What can we do about sky-high summertime humidity in our house?

What can we do about sky-high summertime humidity in our house? The building is 35 years old and sits entirely on a concrete pad. Humidity is fine in winter, but in the warm months we smell mildew in the carpeting if we don’t run a dehumidifier constantly. My plan is to dig around the perimeter of the house and install 2″-thick x 12″-wide pieces of rigid foam against the concrete foundation, and then put the earth back. Will this work?

concrete_floor_insulationI’m sorry to say that your plan probably won’t do much good. That’s because the source of trouble is almost certainly the cool temperatures of the soil underneath your floor. I suspect this is responsible for causing warm, moist summertime air to condense on the concrete, forming droplets of liquid moisture within the carpet. My theory is especially likely given the age of your home. When contractors installed slab-on-grade floors like yours in the early 1970s, they rarely put meaningful amounts of insulation underneath. Many homes have no insulation under the slab at all.

So what can you do? Short of running your dehumidifier all the time, the only thing I can recommend is troublesome: the addition of rigid floor insulation over the slab before installing new floor coverings. Your situation certainly highlights the need for plenty of insulation underneath every slab-on-grade floor.


Posted on February 19th, 2013