| R-VALUES EXPLAINED |
| The R-value of snow keeps the colder temperatures outside. Thirty inches (762 mm) 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, Fahrenheit. The higher numbers indicate lower heat flow and better insulation. 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.
Note 1 Values adapted from John Straube, “High Performance Building Enclosures,” Building Science Press, 2012, Appendix A. Refer to |
Even with a well-designed drainage protection system, liquid water and water vapor 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 section titled R-Values Explained, 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.
Notes
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). 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.
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