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.
Multipurpose insulation materials
As can be seen from the previous discussion, there are two sides to a habitable basement design strategy:
- First, keep the outside environment on the outside.
- Second, prevent condensation of interior moisture on the basement walls.
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). https://nrc-publications.canada.ca/eng/view/ft/?id=2a7aded0-f4cd-4718-9de8-51b9ba9b17ec
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), www.asce.org.
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.