Monday, April 03, 2006

R-value of log homes

In today’s energy conscious environment, the question of R-value is always on the home builder’s mind. Energy consumption is such an important consideration that everyone has done studies to determine the best building materials for new homes. A U.S. Department of Energy study compares the different materials and building techniques.

Your typical stick-built home has a 2”x4” construction (although some are opting for a 2”x6”construction to increase the R-value of the outer wall) that produces an R-value of approximately R-14. A solid wood wall of eastern white pine (a softwood) such as that we will be using produces an R-value of 1.41 per inch of wood. Our walls, constructed of 8”x8” milled logs, will have an R-value of approximately R-11. (Beaver Mountain offers logs that are 6”, 8” and even 10”in thickness.)

R-value is not the only thing to consider in a log wall, however. The solid wood of a log wall acts as a thermal mass. It slowly absorbs heat during the day and slowly gives it off during the night. As the study says, this can increase the “apparent R-value of a log by 0.1 per inch…in mild, sunny climates that have a substantial temperature swing from day to night.” Our building site is at the northern edge of the zone the study cites. It gets sun during the day despite its northern slope and will have those swings.

The R-Value of Wood

An R-value (Btu/ft2/hour/oF) is the rating of a material's resistance to heat flow. The R-value for wood ranges between 1.41 per inch (2.54 cm) for most softwoods to 0.71 for most hardwoods. Ignoring the benefits of the thermal mass, a six inch (15.24 cm) thick log wall would have a clearwall (a wall with no windows or doors) R-value of just over 8. Compared to a conventional wood stud wall [3? inches (8.89 cm) insulation, sheathing, wallboard, a total of about R-14] the log wall is apparently a far inferior insulation system. Based only on this, log walls do not satisfy most building code energy standards. However, to what extent a log building interacts with it's [sic] surroundings depends greatly on the climate. Because of the log's heat storage capability it's [sic] large mass may cause the walls to behave considerably better in some climates than in others.

Logs act like "thermal batteries" and can, under the right circumstances, store heat during the day and gradually release it at night. This generally increases the apparent R-value of a log by 0.1 per inch of thickness in mild, sunny climates that have a substantial temperature swing from day to night. Such climates generally exist in the earth's temperate zones between the 15th and 40th parallels.

There are other concerns with log walls. How one log sits upon another can greatly affect the air tightness of the home.
Air Leakage

Log houses are susceptible to developing air leaks. Air-dried logs are still about 15%-20% water when the house is assembled. As the logs dry over the next few years, the logs shrink. The contraction (and expansion - see below) of the logs opens up gaps between the logs, creating air leaks and causing drafts and high heating requirements.

To minimize problems like these, logs should be seasoned (dried in a protected space) for at least six months before construction begins. The best woods to use to avoid this problem, in order of effectiveness, are cedar, spruce, pine, fir, and larch. Since most manufacturers and experienced builders know of these shrinkage and resulting air leakage problems, many will kiln dry the logs prior to finish shaping and installation. Some also recommend using plastic gaskets and caulking compounds to seal gaps. These seals require regular inspection and resealing when necessary.

Round logs must be either cupped on their under side to nest one atop the other or depend solely upon a lot of chinking between logs to seal out the weather. This can be a problem if the logs shrink over the years. Chinking has to be replaced and patched. The color of the chinking may change over time. Air (and energy) can pass through the walls.

Stick-built homes are now wrapped with a fiber sheet (such as the white and blue Tyvek you see everywhere homes are being built) that serves as an air/vapor barrier to reduce drafts. You can’t wrap a log home so the logs better fit tightly. This need for a tight home has encouraged manufactures to move to milled logs with tongue and groove interlocking edges. Kiln-dried logs, tongue-and-groove edges, foam sealant tape, and caulking will ensure that wind and weather will have a very, very difficult time moving through the walls.


Gun Trash said...

So, about R-12ish, more or less. That is good.

I like that idea of thermal mass and I'm guessing the trees around your homesite are deciduous, so you get shade in the Summer and sun in the Winter. That is good also.

Contrary to the tightly-wrapped house zealots at the DOE, I think you need a small degree of air leakage in a home. I'm of the stale house air is not all that good for your health group.

Anonymous said...

Hi, I'm lovin' your blog! I'm a rater for Energy Efficient homes and I get a lot of people commenting on the "tightly-wrapped house" thing. The current standard and what I have to test to is to make sure the thermal boundaries are tight enough as to not allow more than 5 air changes an hour under a slight negative pressure. Even in a small home this is still quite a bit of air moving through the home. Since moisture intrusion is one of the greatest factors contributing to home deterioration it is most important to design and build the home as to not allow moisture intrusion as well as ensure the movement of any moisture that does get in to the structure (and it always will even through condensation at cold/warm air points) will run or wick in the proper direction. This in the case of condensation is done through proper use of wall covering and vapor barriers depending on climate zones. Much different here in NY than what I would do in say Alabama.

Hope everyone is having a great day!


Anonymous said...

Oh, what I did fail to mention and is very important is that we also promote proper ventilation of homes. That is to say that instead of random air exchange we strive to control the air movement therefore control the movement of the moisture within the home. It is our best intention to create even safer homes for families to live in.


Anonymous said...

It's much better to tightly wrap a home and ventilate it mechanically. Check out Passive House standards. Heat Recovery Ventilation retains the energy of ventilated air at 92% efficiency. It's definitely the way of the future and, yes, ventilated.