XPS and EPS do not have the same thermal resistance and are impacted differently by the presence of moisture. R-values for XPS and EPS insulation are listed in Table 1. The “dry” EPS and XPS insulation values are what is shown on the insulation label and literature. The “wet” insulation values for EPS and XPS are building code approved reduced R-values that assume long-term water absorption over the life of the building. The “wet” insulation values for XPS includes the effects of both moisture and aging.
Preventing condensation of interior moisture in the basement
Once the basement wall is protected from liquid water penetration, it is also necessary to protect the basement wall from the condensation of water vapor in the air space. Figure 3 shows various basement wall materials. If the basement wall has already been compromised by a failure to provide protection from bulk water, then such additional measures will not be very effective (Figure 4). In this sense, the basement wall assembly is a system of components, and each component must be free from defects. No one element is independent of the others.
Basements may be prone to high humidity; interior moisture often is removed using dehumidifiers but it is not a complete solution. If thermal conduction between the masonry or concrete walls and floors to the surrounding soil is excessive, then the masonry or concrete may become cold enough for moisture condensation even at moderate humidity levels.
When moisture condenses on or is absorbed into concrete walls and concrete floor slabs, basements can become cold, dank, and musty (Figure 5). The most reliable long-term solution to prevent condensation is to maintain concrete wall and floor temperatures well above the dew point of the ambient air in the basement space. In this manner, condensation on the concrete wall and floor slab can be avoided.
Depending on the types of insulation and the quantity and locations of it on the basement walls and floor, insulation can have a positive effect on managing the condensation of interior moisture.
Exterior or interior insulation?
Considering all the above thermal challenges, a good question to ask is, “where should the insulation be placed relative to the basement walls and floors?”
