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During the cold season, insulation on the interior side of the basement wall will be colder on the side facing the exterior of the building, and the interior side of the insulation will be facing the warmer moist air of the basement’s interior. This is a perfect recipe for moisture condensation on the interior side of the basement wall, and moisture condensation within the insulation. Moisture condensation typically leads to mould and mildew (Figure 6).
Conversely, insulation on the exterior side will be exposed to exterior moisture, as well as the compressive forces of soil around the building. It may also be exposed to ultraviolet (UV) radiation if the insulation is left exposed above grade.
The advantage of locating insulation on the exterior side of basement walls usually outweighs any advantage of locating insulation on the interior side for thermal control. The basement wall will stay much warmer during the winter when the insulation is on the exterior. Since the basement wall is warmer, moisture condensation is far less likely within the basement, and mould and mildew can be prevented from forming—provided the interior relative humidity is controlled.
Getting practical
If the goal is to (1) facilitate drainage, (2) to keep temperature extremes outside, and (3) to prevent condensation, then what practical measures must be implemented during basement design?
Start with the polystyrene foam insulation thickness. Table 1 (page 2) includes R-values for polystyrene foam insulations. Based on recognized R-values for these, special considerations for insulation thickness adjustments for habitable basement designs are as indicated in Table 2.
In general, the thickness of EPS insulation should be increased by 24 per cent to achieve the desired thermal performance and to prevent condensation when EPS is placed outside below-grade walls. Further, EPS thickness must be increased by 50 per cent in below-grade horizontal applications (i.e. under the floor slab). Remarkably, the thickness of XPS insulation needs to be increased by only 11 per cent to achieve a desired thermal performance and to prevent condensation when outside below-grade walls. The thickness needs to be increased by only 25 per cent in below-grade horizontal applications.
More specifically, the following design examples apply to insulation beneath a basement floor slab that requires an insulation design R-value of R-10, and on the exterior side of a basement wall which requires an insulation design R-value of R-20.
Figure 7 shows the thicknesses required to obtain R-10 design R-value for insulation of a below-grade concrete floor slab. For XPS insulation, 10/5.0 equals 50.8 mm (2 in.) of R-5.0 XPS; adding 25 per cent (2.0 + 0.5) gives an adjusted thickness of 63.5 mm (2.5 in.) for an R-10 design thermal resistance. For EPS insulation, 10/4.2 equals 60.45 mm (2.38 in.) of R-4.2 EPS. Adding 50 per cent (2.38 +1.19) gives an adjusted thickness of 91.44 mm (3.6 in.) for R-10 design thermal resistance.
Figure 8 shows the thicknesses required to obtain R-15 design R-value for exterior below-grade exterior wall insulation. For XPS insulation, 15/5.0 equals 76.2 mm (3 in.) of R-5.0 XPS; adding 25 per cent (3.0 + 19.05 mm [0.75 in.]) gives an adjusted thickness of 95.25 mm (3.75 in.) for an R-15 design thermal resistance. For EPS insulation, 15/4.2 equals 90.67 mm (3.57 in.) of R-4.2 EPS. Adding 50 per cent (3.57 + 1.78) gives an adjusted thickness of 135.89 mm (5.35 in.) for R-20 design thermal resistance.
These thickness adjustments are based on design R-values derived from field data on polystyrene foam insulation in cold climates, as per the design standard American Society of Civil Engineers (ASCE) 32, Design and Construction of Frost-Protected Shallow Foundations.4 The user is responsible in determining whether these thickness adjustments are applicable for the local climate zone, rain exposures, and other moisture exposure from vegetation or runoff from the building rooftop. While these design values are for frost protected shallow foundations and may not apply to all climate zones, they provide some insights into how moisture absorption affects R-values in below-grade applications, including basement insulations.
One must not assume foundation drainage protects thermal control. Rather, one must use the right amount of thermal control and plan for the presence of moisture leftover after drainage. Design redundancy with thermal control keeps the basement habitable.
