Thermal Properties Of Building Materials

Previous Technical Data columns have covered thermal properties for many of the materials that are common to electronics packaging. The Technical Data for this issue is broader in scope and addresses common building materials, some of which are used in heat transfer laboratory environments, in addition to their usual construction applications. Knowledge of thermal conductivity and heat capacity of items used to construct or support a test set is often required to understand and interpret the results (or at least understand why thermal equilibrium required such a long time to achieve).

Table 1 lists a few construction materials and their thermal properties at nominal room temperature. Metals and alloys were not included because they have been previously addressed. It should be noted that these values are approximate and are representative of the particular type of material. Some materials absorb water which in turn changes their properties. For example, the thermal conductivity of wood can increase by 15% when wet. Materials used as insulators that rely on air, such as fiberglass blankets, exhibit a greater change in properties when wet. It is worth noting that the range of thermal conductivities for these materials is rather modest (about two orders of magnitude).

Table 1. Construction Material Thermal Properties at Room Temperature [1-4]

Increasing energy costs and the renewed realization that minimizing unwanted heat transfer is beneficial continues to provide incentives for lower energy use construction methods and materials. The benefits of efficient thermal management of indoor electronics should also be coupled with thermally efficient room construction. The use of insulating (low thermal conductivity) materials may be desirable but nature did not provide true thermally insulating materials, at least when compared to the range of materials choices for electrical conduction. Researching thermal properties for these types of materials will result in data with significant variation, due to composition differences and different test conditions.

For many materials, data may be found in terms of an R value. The R value represents the inverse of thermal conductance and has units of ft²��F�h/Btu (occasionally data is shown with SI units of K�m²/W and is usually referenced as RSI). A larger R value indicates a more restrictive heat flow path. Provided that the thickness is given, extracting an approximate thermal conductivity is possible. However, confusion and disagreement over extrapolating R values to a per thickness value and the fact that most of these materials are used in environments with moisture and moving air, and incur aging, have forced standards on how they should be measured, reported, and advertised [5,6]. If more than approximate values are required then further testing is usually needed.

References

1. Incropera, F., De Witt, D., Introduction to Heat Transfer, 2nd Edition, John Wiley and Sons, 1990.
2. www.goodfellows.com
3. Comfortable Low Energy Architecture website (http://www.learn.londonmet.ac.uk/packages/clear/index.html )
4. www.coloradoenergy.org/procorner/stuff/r-values.htm
5. ASTM C1303, “Standard Test Method for Estimating the Long-Term Change in the Thermal Resistance of Unfaced Rigid Closed-Cell Plastic Foams by Slicing and Scaling Under Laboratory Conditions.”
6. Federal Trade Comission “Labeling and Advertising of Home Insulation 16CFR460”, {www.ftc .gov/bcp/rulemaking/rvalue/16cfr460.shtm#content#content}