A closer look at a wood wall

Figure 5. An example of a Minergie wall module. This one is by Isover.

I was not completely satisfied with the wall discussion from three days ago so I spent a little more time looking through the PDF of the Bauteilekatalog I had linked to. The big challenge of calculating the U-value for the type of construction I've shown here is that the layers making up the sandwich don't extend uninterrupted through the entire length. There is a lattice made of wood that runs through the assembly holding things in place and providing stability. Wood is a better conductor of heat (λ in W/(m·K) is between 0.13 and 0.18) than insulating materials such as glass wool (λ in W/(m·K) runs from 0.031 to 0.048) and these areas where there's wood instead of insulation constitute a leak for heat flow (a thermal bridge) from the warm side to the cold side of the wall. In poorly insulated walls they don't make a big difference but in well insulated ones they do. Unless these regions are taken into account, the U-value that is calculated is lower than the true U-value (remember that a lower U-value is better).

The walls for our house are currently being designed. In the meantime let's consider a Minergie-certified wall module. There are a number of these units from different manufacturers that are guaranteed to satisfy the requirements. This one is made by Isover[1], a glass wool manufacturer. It has a total thickness of 35.75 cm and a thermal-bridge corrected U-value of 0.15 W/(m²·K). (According to the product description the non-corrected value is 0.12 W/(m²·K) though my own calculations give 0.13 W/(m²·K) – I must be using wrong λ values for some of the components). Anyway, the breakdown of the components is as follows. I've looked up the λ values when not provided by Isover (the numbers in red) :

1. 12.5 mm at 0.25 W/(m·K) | Gipsbauplatte (gypsum board of some sort).
2. 40.0 mm at 0.032 W/(m·K) | Support lattice; installation space embedded in Isotwin.
3. 0.05 mm – too small to matter | Vapor retarder/air sealer Vario KM Duplex.
4. 15.0 mm at 0.12 W/(m·K) | OSB (oriented strand board is 95% wood and 5% binder).
5. 200 mm at 0.035 W/(m·K) | Isofix.
6. 30 mm at 0.06 W/(m·K) | Fiberboard.
7. 40 mm – not counted | Ventilation gap.
8. 20 mm at 0.14 W/(m·K) but not counted | Wood siding.

Isotwin has a λ = 0.032 W/(m·K) and Isofix has a λ = 0.035 W/(m·K).

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[1] Here is the page where I found the information: Isover.ch 24-201