Insulation’s crucial role in creating habitable basements

To summarize, the defence against liquid water for the structural foundation is an applied layer of damp-proofing or waterproofing. The defence 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 favoured 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.

Using insulation to keep temperature extremes outside, especially when it is cold

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.

XPS and EPS do not have the same thermal resistance and are impacted differently by the presence of moisture. R-values are listed in Table 1. The “dry” insulation values are shown on the insulation label and literature. The “wet” insulation values 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 vapour 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 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 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?”

The basic challenge for insulation installation is to meet the required or desired R-value. However, this simple challenge is complicated by the presence of moisture. It makes a big difference where the insulation is installed on the basement wall.

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