The military “two lines of defense” analogy in the IRC Technology Update is apt. The outer lines of defense are expected to protect against liquid water penetration. This outer layer is sometimes referred to as “effective environmental separation.” If the first line of defense allows liquid water to reach the second line of defense, then the second line must provide some protection against liquid water penetration. In this case, the basement certainly could not be considered a Class A basement.
Drainage does more than just helping in preventing leaks through the basement wall. It helps to address potentially significant reductions in R-values of insulation (up to 50 percent) due to water absorption and retention; refer to the section “Getting practical” later in this article. However, water drainage does not address the negative effects of water absorption on R-value entirely.
If liquid water and water vapor are not properly managed, the insulation R-value is reduced.
To summarize, the defense against liquid water for the structural foundation is an applied layer of damp-proofing or waterproofing. The defense against liquid water for exterior insulation is to assume a reduced R-value and increase the thickness of the insulation accordingly and provide proper drainage of water. Highly water absorbent insulation materials are impractical for this application because of the dramatic drop in R-value (due to the presence of water) and reduced longevity after repeated freeze-thaw cycles over the life of the building. Lower absorption insulations, such as polystyrene foam are favored exterior insulation materials, and polystyrene foam (i.e. extruded polystyrene [XPS] or expanded polystyrene [EPS]) should have their thickness increased based on reliable data from long-term, in-service studies. Facers could be used to mitigate water absorption in the insulation, but the facers may eventually be compromised over the life of the building, and the insulation thickness adjustment, nonetheless should be applied.
Keeping temperature extremes outside, especially cold temperatures
Structural materials typically perform rather poorly in keeping heat and cold outside. On the contrary, insulation plays a significant role in decreasing heat transfer through basement walls and floors. Heat from the interior of the basement can quickly transfer through basement walls in contact with the surrounding soil if the basement is not sufficiently insulated on the walls below grade.
Insulation of basement walls above-grade and insulation beneath the floor slab are also important in managing basement temperatures, especially when the intention is to create a habitable space. Basement insulation moderates the surface temperatures of basement walls and floors. As for sustainability objectives, insulation reduces the environmental footprint and energy costs by decreasing the heat transfer which occurs in the heating and cooling of basements.
The placement of the insulation on either the interior or exterior of the wall (or both) makes a difference in human comfort. When the interior surface of the basement wall is warm, it feels much more comfortable to the occupant. Placing the insulation on the exterior side of the basement wall keeps its wall surface temperature closer to the overall interior temperature of the room.
