A material’s ability to absorb heat is separate from conduction


By CCHRC Staff

The “Ask a Builder” series is dedicated to answering some of the many questions Fairbanks residents have about building, energy and the many other parts of home life.

Q: I hear that log and concrete walls have the added benefit of “thermal mass.” How much of a difference does it make?

The basic concept of thermal mass revolves around a materials ability to retain heat. Logs, stone, and concrete all have the capacity to store significant amounts heat. When denser materials such as logs are used to build exterior walls, they will perform differently than your standard “light frame” construction which uses wood studs and fiberglass, as the logs will store more heat. It is not that simple however, as other material properties come into play as well.

Large logs also have some insulating value, whereas a material such as concrete will store heat well, but will also readily conduct heat. This changes when concrete is combined with insulation, such as an insulated concrete form (ICF) which sandwiches the concrete between two layers of rigid foam board. The extra insulation improves the ability to of the concrete to retain heat dramatically.

This is an important concept. A material may be able to absorb large amounts of heat, however if its insulative properties are low, it will just as readily conduct that heat through the wall.

In a Fairbanks winter, the path of heat flow is always in one direction, from inside to outside. This means that the heat absorbed by the mass in the exterior walls is from the inside and is still heat you are paying for. On the other hand, in climates where there is a lot of solar heat during the day, properly applied thermal mass can store some of this heat during the night. On the bright side, if the power goes out, or you are heating with wood and the fire dies down, then the room will stay warmer for a longer period as the walls provide a buffer due to the extra heat energy they contain.

Q: I noticed the screws holding down my metal roofing have backed out in places, why is that?

If the screws were not over tightened or stripped when they were put in, and are of the right type, then it is possible there are other causes. Metal roofs experience large, fast shifts in temperature, much more so than the wood underneath.

As the metal expands and contracts it exerts stress on the fasteners, particularly on a large roof. Sometimes with temperature swings, you can hear it moving. If the screws are small, this can also play a role. Upsiz-ing to a larger screw in the troublesome area usually solves this problem.

Alaska HomeWise articles promote home awareness for the Cold Climate Housing Research Center (CCHRC). If you have a question, email us at akhome-wise@cchrc.org