Insulating a Wooden House

The general idea about wooden houses is that they are very well insulated, and a lot better insulated than brick houses. Is this correct? A short explanation.

maison-bois-toulouse

Lambda, R-, U- and k-values

In order to calculate the thermal insulation of a wooden house we have to go back to high school. In Science class we had to deal with Ohm, Volt and Ampère. Thermal insulation works just the same only Volt is now Kelvin, Ohm is now R-value, The energy we lose via a wooden wall can be calculated with a U-value. The principles are all the same.

insulated laminated timber homes

To calculate the U-value of a double wooden wall of two times 14 centimeters pine with 9 centimeters mineral wool in between, we do the following: mineral wool has a lambda-value of 0,043 pine has a lambda-value of 0,17 So for the double wall we calculate as follows:

  • outside wall, 14 cm. pine, R = 0,14/0,17 = 0,82
  • inside wall, 14 cm. pine, R = 0,14/0,17 = 0,82
  • mineral wool, 9 cm., R = 0,09/0,043 = 2,09
  • total R-value: (0,82+0,82+2,09) = 3,74 m²K/W.

The U-value is 1/3,74 W/m²K, that is 0,27 W/m²K. That is a very good value. In most European countries municipalities will require 0,4 W/m²K for outside walls and with a traditional brick wall such values have become impossible.

insulating a wooden house RT2012 TEK10

The U-value can easily be improved by adding mineral wool. If we increase the 9 centimeters to 14 centimeters, then the resulting R-value would be 4,90 m²K/W and the U-value would be 0,20 W/m²K. Instead of mineral wool we could use glass wool, we would then come to R = 5,85 m²K/W and U = 0,17 W/m²K.

Your Energy Bill

What about your energy bill when you have a U-value of 0,27 W/m²K? The theory is simple. Let’s assume your house has a circumference of 40 meters and a wall height of 3 meters. Total surface of your outside walls is then 120 m2. Outside we have 5 degrees Celsius, inside we have 21 degrees Celsius. The amount of energy that disappears via the walls is then: 40 x 3 x (21-5) x 0,27 = 518 Watt. At a U-value of 0,17 W/m²K that would be 326 Watt, or about three light bulbs or one-third of a coffee machine.

But watch out. We still have not calculated for the doors and windows, the floor and the roof. Also when there is a lot of wind we will use more energy. In practice these things are a bit more complicated than a simple theory. But at least you get an idea.