January 14, 2001
for Double Wall Cordwood
All materials conduct heat, some more than others. Materials such as copper, steel, aluminum, glass and concrete are good conductors of heat; whereas wood, paper, fiberglass, cellulose and mineral wool are poor conductors. Materials that are poor conductors of heat are good insulators. Insulating materials have a high level of resistance to conduct heat. This resistance is measured in R-value. The higher the R-value, the better it is as insulation. Here's a list of some insulation materials that could be used as loose fill insulation for our double wall cordwood project:
(Values are from the University of Purdue Agriculture Department and the U.S. Department of Energy. All materials are expressed in terms of R-value per inch.)
|Material||R-Value per inch|
|Cellulose||3.13 - 3.70|
|Glass or Rock Wool||2.50 - 3.00|
|Shavings or Sawdust||2.22|
|Expanded Polystyrene (Molded Beads)||3.57|
vs. Unnatural Insulation
During the course of building the house, I have received letters from some asking if I have strayed a bit off of the path of building a natural house. There's no doubt about it, our house will not be built out of 100% naturally found materials. I do make an effort to use natural products where it makes sense. There are always tradeoffs to consider.
Our goal is to build the house with as many natural materials as possible, wherever it makes sense to do so and to keep the impact upon the earth to a minimum. There are instances in which a less earth-friendly material may become the best environmental option over the life expectancy of a house. For example, I used extruded polystyrene under the floor of the house to insulate the heated floor from the subsoil. Polystyrene is about the only material suitable for insulating under a floor. If I had not used the insulation, quite a bit of heat would be lost to the subsoil. This would increase the need for burning more fuel, which in turn would have a greater impact on the environment over the long-run by using more nonrenewable energy and adding to air pollution. The less nonrenewable energy we use to heat and cool the house will far surpass the impact of using some unnatural materials to build the house.
Insulation comes in both natural and unnatural forms. You can insulate a house with dried leaves, straw, feathers, wool, cotton, saw dust or just about any other organic product that does a reasonable job of preventing heat flow. You can also insulate a house using fiberglass, rock wool, and different forms of foam insulation.
Fiberglass insulation is made using high-temperature gas furnaces and uses 20 to 30 percent recycled glass. Rock wool is made from the scum of molten metal and although it is a byproduct that would normally get pitched, it requires lots of energy to be produced along with the pollutants that the factories generate. Polystyrene and polyurethane are made from petroleum byproducts and other chemicals. Although it's R-value is exceptional, it's as about as unnatural of a product as you can get.
Down the Options
A double wall cordwood wall by itself will increase the R-value of the wall by creating a thermal break between the interior and exterior walls. Taking this into consideration, I decided not to get "hell-bent" over the R-value of different forms of insulation. As a matter of fact, I am excluding any unnatural or "Earth-Unfriendly" form of insulation and just focusing on natural products.
Cellulose is certainly an option to consider. It's made out of shredded newspaper that is treated with boron and borax to make it fire resistant. It is commonly used in attics, but it can be used in walls as long as it is densely packed to prevent settling problems. This decreases the R-value slightly, but it is still a very good option. The only disadvantage I have found is the cost. Although it is not as expensive as some other forms of insulation, it would probably cost about $700 to dense-pack all 32 walls. I actually have made my own cellulose using a leaf shredder and newspaper. It works, but is extremely messy and time consuming. Too much work for too little of a gain.
Out of all the other natural options, sawdust seems to be the best option. It's got good R-value and when mixed with hydrated lime (Thank You Jack Henstridge!), it creates a insect resistant insulation that sets up like beadboard if any moisture gets into the mix. This was confirmed by Ed McAllen who told me in a recent conversation that he had to remove a log from a wall to install an electrical line. When Ed finally got the log out, the sawdust-lime insulation stayed in place and did not fall out of the cavity. It's texture was similar to beadboard.
This sounds like a winner to me. There's plenty of sawdust available from sawmills in the area for little or no cost. Lime is cheap too. So, after deliberating all the factors, sawdust is the winner.
So, what kind of R-value should I be able to get out of the wall? The wall will consist of 8" of logs and mortar on the interior, followed by 8" of sawdust and finally 8" of logs and papercrete on the exterior (providing I find a good source of paper pulp or newspapers).
Pine is a fairly airy wood, so the logs themselves should give about an R1.5 per inch. Mortar is about R.5 per inch and papercrete (Jim Juczak version) is about R2 per inch. Sawdust is about R2 per inch. With all of the R values known, we can compute an estimated R value for the cordwood wall:
Interior wall = 8" of wood and mortar (80% wood, 20% mortar). 8" x 1.5 = 12 | 8" x .5 = 4 | 80% of R12=9.6 | 20% of R4=.8 | 9.6 + .8 = R10.4
Wall cavity = 8" of sawdust @ R2.2 = ~R17
Exterior wall = 8" of wood and papercrete (80% wood, 20% mortar). 8" x 1.5 = 12 | 8" x 2 = 16 | 80% of R12 = R9.6 | 20% of R16=R3.2 | 9.6 + 3.2 = R12.8
Total Wall R Value = ~R40
At this point, I am still considering using papercrete mortar for the exterior wall. If I can find a good, local source of paper pulp or if I'm ambitious enough to slurry newspapers, I'll probably stick with the above configuration. If I were to just use regular old cordwood mortar on both walls, the R value would be approximately R38.