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Thermal Mass vs. R-value


Log-home owners have proclaimed the warmth and comfort of their homes for centuries.  Log homes are naturally energy efficient due to Thermal Mass properties.  Thermal Mass can improve your family’s comfort because it moderates temperature swings inside your home year-round and consequently reduces monthly utility bills.

During the winter, moderating the indoor temperature swings can result in less heat being lost to the outdoors.  This helps in rooms where heat is generated indirectly, such as cooking in the kitchen or bathing in the bathroom.  It also helps in rooms with south-facing windows which receive passive solar heating. Instead of a low thermal mass room overheating and losing heat, a high thermal mass room overheats less and stores the heat.  If your house is heated with a heat pump with electric-resistance backup heat, additional thermal mass is a definite plus.  Heat pump efficiency and its heat output are greater when the outdoor air is warmer, such as during a sunny afternoon. With adequate thermal mass, enough heat may be stored indoors so the backup resistance heating does not have to come on or run as long when the outdoor temperature drops at night.

During the summer, this thermal mass can delay the need for air conditioning until later in the day.  Once the outdoor temperature drops, the windows can be opened and the thermal mass cools off overnight.  Another option is to run your central air conditioner or heat pump in the morning to cool the home while the overall electricity demand is lower. This can reduce the peak electricity demand for your utility company during mid-afternoon.

These are the basic concepts that keeps log homes more comfortable in all weather conditions compared to conventionally constructed homes.  Even though walls constructed of solid logs have a low insulation R-value relative to an insulated framed wall, log homes stay more comfortable.  Furthermore, Thermal Mass properties inherent in solid logs, typically result in an energy savings of 5% - 15% as compared to a similar stick-built R-value home.


One question that is consistently asked by potential log home dwellers is, "What is the R-Value or insulation value of your log?" Most sales representatives will stumble on this question even though they have heard it before. Why?  Because there is no quick answer.  For nearly 30 years, the log-home industry has invested heavily in the concept of thermal mass to explain the warmth and comfort experienced by log-home owners.  Thermal mass (the ability of a material to absorb, store and later release heat) has blended into the energy codes, and those codes
continue to change.

First of all, for every one inch of wood between you and the outside, there is an R-value of 1.5.  So if you have an eight-inch log, the R- value will be 12, but only across the full diameter. On a typical framed house the insulation in the wall will be rated at R-19.  So, how is it that a 4" wall filled with fiberglass insulation has a better R-value than a solid log?  Simply stated is that air is the ultimate insulator. An eight-inch thick log is in effect the better of the two choices. Fiberglass insulation traps air in thousands of individual pockets preventing heat from conducting through your wall. A 2x6 wall with an R-30 insulation will give you almost the same benefits as an eight- inch log wall.  (However keep in mind that in the real world, stud framed walls and insulation have many voids where materials meet and settle thus creating paths of less resistance and ultimately reducing the insulating value.)  This is not the case with solid logs.  To understand this concept you must understand two different ideas, R-value and
Thermal Mass.

Heat will always travel from a warmer area to a cooler area by one of three ways, conduction, convection and radiation. Conduction is the direct transfer of heat through a material such as a pan on a stove. Convection is the movement of heat through the air. When air is heated it becomes lighter and less dense than the cooler air which being denser tends to sink, the currents produced by the flow of warm air up and cool air down is the basis of convection, much the same as your oven works. Radiation is similar to convection, however heat is transferred through the air and absorbed by another material.  Much like standing in front of a fire and feeling the heat on your face, even though you are beyond the convection area.  The rate of flow depends not only on the material but also the difference in temperature on opposing sides of the material.  R-value is based on its Thermal Resistivity and refers to unit thickness and is defined as the reciprocal of thermal conductivity. Thermal Conductivity specifies the rate of heat transfer of a given material.

When you consider that fiberglass has a Thermal Resistivity of .050 and wood has a Thermal Resistivity value of .144 then why is a solid log more efficient than an insulated framed wall? This is where thermal mass comes into play. Thermal mass is a materials ability to absorb and hold heat. In building terms, thermal mass is the ability of a material to absorb heat during the day when you prefer to be cooler and then radiates this heat inside at night keeping the building at a more steady temperature.  Ideal materials for thermal mass are those materials that are high density and have low thermal conductivity, which makes logs a perfect candidate. To give you another example, consider a concrete basement that is surrounded by earth.  Basements such as these are usually cool as they are not only built of thermal materials but the earth around them are also excellent thermal materials. However in the case of such a basement, this is too much of a good thing. If you were to try to keep a basement such as this at 78 degrees, you would find that any heat source would have to work overtime to achieve this result.  The reason for this is that the concrete and the earth are constantly absorbing the heat and storing it away from the area you want to heat.  Like most things, too much of a good thing is bad.  If a thermal material is too thin versus too thick, the heat from outside would sink right through the material and be radiated into the home.

So then, why is there still so much confusion concerning R-value vs. Thermal Mass?  Because there are no government standards on measuring thermal mass.  It is a highly variable measurement.  When you have an average daily temperature change from hot to cold, or cold to hot, thermal mass is more efficient.  When you live in an area such as Alaska where the temperature stays at or below zero for most of the winter, thermal mass is negated and your logs work more like a standard R-value rating.

Several organizations are trying to produce meaningful test results that formulate Thermal Mass characteristics found in log homes.  In fact, evolving energy-conservation standards such as this were a factor in the formation of the Log Homes Council. These standards focused on limiting heat loss from buildings by prescribing minimum insulation levels, an approach that continues today. The standards emerged as the Model Energy Code (MEC) in 1983. That’s the same year that the National Bureau of Standards (NBS) tests documented the effect of thermal mass by comparing the performance of six buildings whose only difference was the wall assembly (wood-frame with and without insulation in the cavity; masonry without any insulation, insulated on the interior and insulated on the exterior; and a log wall).

The tests demonstrated that thermal mass does have a tempering effect on the enclosed air space, moderating change in the interior as outside temperatures change. The report shows that the 7-inch-thick log wall outperformed the 2-by-4, R-11 insulated wall.  Based on the testing, it became evident that solid-wood walls could be analyzed in terms of both their static R-value and thermal mass. With R-value controlling the energy code requirements, a new “dynamic R” measure was needed to represent log-wall performance.  The energy-code changes seem to be getting closer to what log-home owners have been saying for centuries.  All log homes have warmth from natural resistance to heat loss, radiant comfort from thermal mass and further reduction of energy consumption by low air-infiltration rates.


James Dulley, Cooperative Living, 6906 Royalgreen Dr., Cincinnati, OH 45244
Rob Pickett of Rob Pickett and Associates, Hartland, Vermont
Zgrinch, Silver Hill, Texas