It may come as a surprise to some, especially owners, but many log houses should be insulated to make them energy efficient. A look at the R-values will tell one reason why. Dry wood can have an R-value of up to 1.25 per inch. Even low quality insulation has a value double that and more.
Another problem not often considered is moisture. Where does the moisture in the house go when the outside wall is solid wood? It still tries to get out, and might if there are cracks in the logs.
The propensity for moisture-laden air to move outside has been utilized in the past to make a log house better insulated. In the far north, where winter temperatures may stay below freezing for months, it was a common practice to seal the house from the inside. This was done by putting several containers of water on the stove and bringing them to a boil. The vapor traveled to the outside through the cracks. It immediately froze on contact with the outside air, effectively scaling the house until the outside air temperature again was above 32 degrees Fahrenheit. This cut down on drafts and made for a snug house, at least until the weather warmed up.
Logs have another property that helps explain why log houses don't seem to be as cold as their R-value would indicate: They are capable of storing heat. Logs in a house wall will absorb heat up to the temperature of the air in the house. When the house temperature is lowered — at night, for example — they give back heat into the rooms until the temperature is again equalized. Unlike a block or a stone wall, logs can be two temperatures simultaneously, one on either side.
Where basic insulation for a log house is needed, even if none is required elsewhere, is between the logs. No matter what type of log construction is used — round fitted, flattened, three-sided, or round with chinking — a thin layer of insulation should be put between each log and those above and below it. This fills irregularities between the logs and closes cracks open to the outside. Caulking or “chinking” is then done on either side of the insulation.
The size of the logs makes a difference. Many modern log houses are made of logs 8 inches or less in diameter. Two-sided and three-sided logs are often only 6 inches thick. When placed in a wall, there may be only 3 or 4 inches of contact surface between the logs. This contact is the thickness that should be considered when thinking of additional insulation. Not only is the joint between logs thinner than the logs, but that is the place where there will be leaks in the wall if any exist.
Even houses made of turned logs (logs turned to a given diameter in a large lathe, and then grooved with a machine to give a very tight fit) need insulation and caulking where logs meet. This type of construction is a machine-made variation of the old Scandinavian system of building log houses. Most of these houses use logs turned to 20 inches or less in diameter, with only about 8 inches of thickness where the logs rest on each other.
Some log-house builders still hand-fit the logs in the same Scandinavian style. They usually use logs from 20 to 36 inches in diameter and will have a thickness of 12 or more inches where they join. This type of construction is usually done only on contract.
Factory-built log houses are most common today. The logs may be only 6 to 8 inches thick, and are often squared on two or more sides for ease in construction. One maker of log-home kits even starts with 8-by-8 inch timbers, running them through a machine that rounds one side (for the outside of the cabin), and puts a tongue and groove in the top and bottom for a tight fit. The inside surface of the log is dressed smooth so that it looks like lumber. This construction needs a strip of insulation between the “logs” and caulking in the crack between the logs on both the inside and the outside.
Some log-home builders use hand-peeled logs without doing anything else to them. The bark of the log is removed by a man wielding a large drawknife. Then the logs are fitted into the house by notching the ends so there will be as small a crack as possible between each set of logs. Usually the logs are no more than 8 inches in diameter on the small end and no larger than 14 inches in diameter on the large end. As a wall is built, the large end of one log is placed atop the small end of another to get a close fit between the logs. Once the logs have been fitted for a house at the factory, they are numbered and trucked to the home site, where they are reassembled.
When the house is up, the gaps between the logs are stuffed with fiberglass insulation; then a vinyl material that won’t shrink away from the logs is applied, sealing the crack. The wall between the logs may be only 3 inches thick and filled with a material that has a lower R-value than the logs themselves, so the insulation value is low.
Log houses often differ from frame houses in the roof. In order to get the most rustic feeling from a log house, the roof is often open beam. In that case the insulation must be put on top. In the good old days, sod was often the material used, with up to a couple of feet being placed on top of the house. This entailed a heavy roof structure, as sod can weigh up to 75 pounds a cubic foot. Even though sod has approximately the same R-value as fiberglass, nowadays it will probably be more expensive.
What we might term as rustic or old-fashioned was, more often than not, done for reasons of sheer necessity by our forebears.
Log houses were built because there were more than enough trees, and the trees needed to be cleared from the land to grow food. Sod was also plentiful, especially in places where there was some prairie. It was there for the taking. A couple of men with a saw and axes and a flat shovel, often carved from a split log, could build a solid house in a couple of days, with the expenditure of nothing more than sweat.
They also weren’t concerned about fuel to keep the place warm. If it was cold, they built the fire up more. “Energy efficiency” wasn’t in their vocabulary. Neither was “insulation,” except in the far North Country where tundra moss was used to stuff the cracks between logs or stones and was often applied in a thick layer on the roof.
During experiments on the insulation value of indigenous building materials, University of Alaska engineers discovered that dried tundra moss has a higher insulation value than most manufactured insulations. Of course, unless you live on the tundra, the cost of obtaining the moss puts it out of reach. It is also very flammable and would probably be banned by most fire codes.
Most log houses use insulation only between the logs. However, you must do something to increase the insulation along the logs to make your cabin truly an energy-efficient house. This means building a wall on the inside of the log wall. This should be thick enough to provide energy efficiency. It should also have a vapor barrier on the warm (room) side to reduce moisture flow into the log wall as much as possible. Once the vapor barrier is in place, the wall can be finished as desired. The barrier is an extra expense, but if the log walls of the house are as thin as some described earlier, it may be necessary for the conservation of energy.
A roof on a log house has the same problems as one on a frame house, and should be treated the same way. In order to keep the rustic effect and utilize the efficiency of modem insulation, some log-home owners have insulated their roof with urethane foam and then added a layer of sod to complete the illusion of a rustic dwelling. The same problems and benefits apply here as they do for a regular framed roof. The point is to get the highest possible insulation value for the money spent.
Reprinted from Home Insulation: Do It Yourself & Save As Much As 40% (Storey Communications).
To save even more energy around the house, try these do-it-yourself projects: