Insulation’s crucial role in creating habitable basements

by arslan_ahmed | May 22, 2023 4:00 pm

[1]By Mike Fischer

One of the first questions to be asked when designing a basement foundation is “what is the desired functionality of the basement?” On one end of the spectrum, a basement may provide comfortable, fully habitable spaces comparable to the above-grade portions of the building (Figure 1). In the middle of the spectrum, the basement may be less habitable than the main living areas. In other words, it will be cold in the winter and damp during much of the year. On the far end of the spectrum, the basement may serve as a structural foundation only and nothing more.

The Swinton and Kesik study, a Canadian research paper, shows precedence with respect to classifying basements. Although a complete review of the classification system is beyond the scope of this paper, this article is focused on Class A basements as defined by Swinton and Kesik. In their study, Swinton and Kesik list service criteria and limitations/allowances for different classes of basements.1 According to their classification system, Class A basements provide a below-grade living space comparable to the living space of the floors above. To ensure long-term performance, the design and construction of Class A basements must be virtually defect-free and provide critical, redundant moisture control measures. They emphasize the construction of a comfortable and habitable basement (i.e. a Class A basement), which requires much more attention to the components and the system design compared to the design of basements not intended to be used as living spaces. There may also be health and safety requirements, as well as HVAC requirements, which go beyond the scope of the 2005 report by Swinton and Kesik.

Thermal control challenges

Aside from the structural challenges, the thermal challenges for basements include the following functions: (1) facilitating moisture drainage at the face of the exterior insulation down to the foundation drainage system, (2) keeping temperature extremes outside the basement wall, especially cold temperatures, and (3) preventing condensation of the basement air onto the interior of walls in the basement. Insulation strategically placed on the exterior side of the foundation plays an important role in each of these objectives (Figure 2).

 Facilitating movement of exterior moisture to the foundation drainage system

In an NRC Institute for Research in Construction (NRC-IRC) Technology Update,2 Swinton et al. describe their approach to managing water exposure to basements is in terms of “two lines of defense”:

The successful realization of a habitable basement requires careful attention to design details. Photos © Adobe Stock/courtesy Extruded Polystyrene Foam Association (XPSA).

The most effective strategy for managing water is to provide two lines of defense. When exterior basement insulation is used, the first line of defense is the exterior surface of the insulation, which supplies a continuous means of managing water from the ground surface down to the gravel and drainpipe at the footing. The second line of defense is the outer face of the foundation (cast-in-place concrete, concrete block, or wood sheathing in a permanent wood foundation), which can handle the incidental quantities of water that may get by the first line of defense.

It is important to understand this concept because even the best below-grade insulations benefit from some degree of environmental separation.

Since 1999, much research has been dedicated toward accumulating valid experimental data on how insulations perform when placed on the outside of a basement wall in contact with the earth, and when insulation is placed beneath floor slabs.  For example, the continuous monitoring of the thermal performance of 13 different basement insulation systems by the NRC-IRC throughout two heating seasons provided some answers.3

Figure 2 Exterior insulation and damp-proofing or waterproofing along with effective drainage helps prevent water intrusion into the basement. Photo courtesy Kingspan.

The military “two lines of defense” analogy in the IRC Technology Update is apt. The outer lines of defence are expected to protect against liquid water penetration. This outer layer is sometimes referred to as “effective environmental separation.” If the first line of defence allows liquid water to reach the second line of defence, 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 per cent) 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 vapour are not properly managed, the insulation R-value is reduced.

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.

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.

 R-Values Explained
The R-value of snow keeps the colder temperatures outside—762 mm (30 in.) of snow performs about as well as 50.8 mm (2 in.) of dry extruded polystyrene (XPS). Most igloos do not survive a long freeze-thaw cycle, but snow blocks are readily available in some climate zones.

R-value is the resistance to heat flow and is expressed as rate of heat loss per hour, per square foot, per inch of thickness of material, per degree, Celsius (Fahrenheit). The higher numbers indicate lower heat flow and better insulation.

R-values can be expressed in metric units (SI units), as well as imperial (or inch-pound) units. The metric thermal resistance is sometimes referred to as the “RSI value.”

The R-value in I-P units per inch is obtained from the RSI value, by multiplying the RSI value by 5.678 / (W/m·K) and then by 0.0254 m/in. to obtain the R-value per inch.

For example, the thermal conductivity of ice at –1 C (30.2 F) is 2.24 W/(m·K). The RSI value of thermal resistance is (1/ 2.24) = 0.446. R-value per inch in I-P units = 5.678 (0.0254 m/in.) * 0.446 RSI = 0.06.

Values of thermal conductivity and R-value per inch for select materials.1

1 Values adapted from John Straube, “High Performance Building Enclosures,” Building Science Press, 2012, Appendix A.

Multipurpose insulation materials

As can be seen from the previous discussion, there are two sides to a habitable basement design strategy:

A livable basement mus that meets multiple design criteria. It needs to use wall assemblies made with multipurpose materials providing multiple lines of defence and performing multiple functions.

The first line of defence separates the basement wall from the outdoor environment and the second line of defence manages the moisture of the indoor environment in a way to provide a livable basement.

These multiple lines of defence need to be carefully modelled in basement wall assemblies for habitable basements that will last. The most workable moisture control strategies will include attention to the selection of quality foam insulation. The foam insulation board needs to allow for drainage, provide high R-value, resist moisture absorption, and retain R-values in below-grade applications.

Even with a well-designed drainage protection system, water, and water vapour are likely to be present throughout the life of the foundation. Interior basement condensation can be avoided through the proper selection and installation of insulation if the reality of water and ice in the assembly is acknowledged.

According to the table in the sidebar on page 2, the R-values of water, ice, and XPS insulation are 0.24 per inch, 0.065 per inch, and 5.0 per inch, respectively. Minimizing the moisture absorption provides the greatest chance of retaining the highest R-value regardless of the presence of exterior moisture. Interior moisture due to condensation also can be minimized by blocking heat loss through the basement wall to the surrounding environment.

Choosing an inexpensive water-permeable insulation and relying solely on drainage to keep it dry risks a potential moisture absorption scenario that may be unintended, but nonetheless will be  expensive to fix.

For these reasons, XPS foam board insulation is recommended for use in habitable basement designs, especially when it is installed exterior to the foundation walls and floor slabs. The principles in this article are applicable to a range of different insulation types. A range of products may be able to meet the technical challenges, depending upon the local conditions such as climate, soil and drainage. The scope of this paper is limited to polystyrene insulations for this application.


1 Read the report, “Performance Guidelines for Basement Envelope Systems and Materials: Final Research Report” by Michael C. Swinton, IRC/NRC and Dr. Ted Kesik, University of Toronto (Institute for Research in Construction / National Research Council Canada), October 2005.

2 Refer to the study, Swinton, M.C.; Bomberg, M.T.; Kumaran, M.K.; Normandin, N.; Maref, W. “Performance of thermal insulation on the exterior of basement walls,” NRC Construction Technology Update, Number 36, Institute for Research in Construction (1999-12-01).[6]

3 Refer to note 2.

4 See the paper by Rob Brooks et al., “Effects of Moisture Absorption Mechanisms on In-Service Design R-values of Polystyrene Insulation,” XPS Insulation Performance, Below Grade Series ID: IP-BG-02.

5 Learn more about, SEI/ASCE 32-01, Design and Construction of Frost-Protected Shallow Foundations, by visiting American Society of Civil Engineers (ASCE),[7].


Mike Fischer is the executive director of the Extruded Polystyrene Foam Association (XPSA). Fischer is a 40-year veteran of the building products industry, and has held positions in logistics, management, sales, technical marketing, and regulatory advocacy. For more than 20 years, he has been on the association side of the building products industry, focusing on exterior products including insulation, roofing, fenestration products, and cladding. He is a frequent speaker and noted author as a subject matter expert on issues affecting the building products industry. As XPSA executive director, Fischer serves as the XPSA staff lead, as well as the spokesperson and advocate for XPSA members and the industry.

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