Basement insulation


Old 04-10-03, 02:38 PM
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Question Basement insulation

I'm finishing my basement - masonry block wall which is painted with oil-based block-seal paint. I want to insulate. I've read of every suggestion imaginable...
(I did have a sheet of rigid foam insulation against one wall for several years and when I removed it, there was a liitle bit of mold near the floor and the paint was slightly eaten away, from moisture I imagine.)

I'm concerned about moisture causing damage behind the insulation and drywall. What's the safest way to go here?
I'm leaning toward rigid foam against the painted wall, but not sure.
Thank you very much, Al
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Old 04-10-03, 07:25 PM
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The moisture and mold illustrates what occurs under these conditions when you have 2 different materials that have different rates of absorption and expulsion towards moisture. This is also common with wood and cement, especially when heat transfer occurs. An example is the insulation that is usually laid down between the foundation wall and wood sill. Though there are other reasons for the insulation to be placed there, this is a major one.

Understanding that all heat has moisture in it and insulation and vapor barriers do not stop heat and moisture flow, they only slow it down, is the way you prohibit the condensation from occurring. You also have to consider that basement water-proofing prohibits the water molecule from permeating the wall but does not stop the water vapor molecule from permeating the wall. What this explicitly implies that the source of the condensation in this application can come from the outside regardless of the water-proofing. Coupled with humidity in heat transfer from inside the basement, the probability of condensation occurring where the 2 materials touch is high.

Though in some cases condensation does not occur, it would suggest the soil outside the basement walls is dry and well drained and there are no combustion appliances in the basement, like a furnace.

To finish and insulate a basement wall, I would recommend building the wall one inch away from the masonry. The reason for this is this dead air space separates the 2 materials and at the same time the air in the dead air space can absorb moisture either from the heat transfer or the moisture permeating the wall. This ability to absorb moisture provides the time needed to make up the difference between the different rates of absorption and expulsion of the 2 materials. In other words it reduces the probability of condensation considerably verses if the 2 materials touched each other.
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