Log Homes R-Value Vs. Thermal Mass

When shopping for your new log home, one of the first questions that you are sure to ask is, “What is the insulation value of a log?” Most sales representatives will stumble on this question even though they have heard it before. Why? Because there is no simple answer. 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 a eight inch log, the R value will be 12. On a typical framed house the insulation in the wall will be rated at R-19. Now, this is where people become confused. How is it that a 4″ wall filled with fiberglass insulation has a better R value than a solid log? The simple answer is that air is the ultimate insulator. However, 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. If you were able to afford it, a 2×6 wall with an R-30 insulation will give you almost the same benefits as an eight inch log wall. To understand this concept you must understand two different ideas. One, what is R value and what is 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 transfered 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. First of all lets start with R value and how it is determined. R value is based on K value. (1/k) Which is its Thermal Resititivy and refers to unit thickness and is defined as the reciprocal of thermal conductivity. The K value or Thermal Conductivity specifies the rate of heat transfer of a given material. If a material has a K value of 1, it means that for every 1cubic meter of material it will transfer heat at a rate of 1 watt for every degree of temperature difference between opposite faces. This is expressed as 1W/mK. The lower this value is the more resistance to heat transfer. There is also a C value which is Thermal Conductance, which for our purposes we will not go into.

When you consider that fiberglass has a K value of .050 and wood has a K 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 a better example, 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, there is a balance to these things.

Determining your wall thickness can be calculated by your architect or even by your budget. However a good rule of thumb would be a minimum of 6″ anywhere. If you live above 45 degrees north it would be a good idea to have a minimum of 8″. If you live in the extreme north, such as Alaska, I would recommend at least 12″ of log between you and the outside world. So then, why is there still so much confusion concerning R value vs. Thermal Mass?
The best and most definitive answer is this. There are no government standards on measuring thermal mass. At Oak Ridge National Laboratories, they are currently trying to resolve this. However, while researching this column (even though I am a log home advocate) it came to my attention how much controversy there is over the subject. When you have an average daily temperature change from hot to cold, 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. So, have I answered any questions, or are you just as confused as ever. If so, you may search the links provided as sources and maybe, just maybe you will have a better understanding of this controversial subject.

On a final note, there is little doubt that log homes offer a more natural and soothing home than any man made or fabricated material. So, if you are trying to decide on your choice of home based on R values and thermal mass alone, then you have made a mistake before you have even started. With all of the information on this subject, and all things being considered, none of this matters if your doors and windows are not properly installed. The largest loss of heat in any building besides the roof is around window and door jambs. There are a thousand different methods for installing doors and windows in log homes. If you would like to know the best methods, then read my next article on window and door installation in log homes….Thanks and if you need more information, feel free to contact me through Associated Content.

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