Limits on the overall heat transfer coefficient through the building envelope

I've mentioned before that the first step in reducing energy consumption is to invest in a well-insulated building shell. Just like choosing a thick and warm winter jacket. I've also stated before that in the end what is important for the Minergie specification is that the weighted (I'll talk about this weighting very soon) energy consumption per unit area be less than 0.38 kWh/m² per year. It is possible to achieve this by adjusting the different factors that play into this equation and to simplify the matter the Minergie group has come up with a set of "standard solutions". The limits given in the table below are those for the building elements. It is possible to build a Minergie house with components that exceed these values but make up the difference in other ways. In fact, not only it is possible, it seems to be often the case. This clarifies some of the confusion I had in earlier posts.

Table 1. Limits on the total heat transfer coefficient for building components.

	Separation from exterior	Separation from unheated interior	Separation from ground
	W/(m2·K)	W/(m2·K)	W/(m2·K)
Roof	0.15	0.20	-
Wall	0.15	0.20	0.20
Floor	0.15	0.20	0.20
Window	1.00	1.60	-
Door	1.20	2.00	-

Limits on the overall heat transfer coefficient through the building envelope

Figure 3. A simplified representation of the heat transfer coefficient limits for a few specific building components.

In general, the first step towards reducing energy consumption in a house is to use a lot of insulation. The lower the heat transfer through the building envelope, the better. One limitation to this is that the thicker you make your shell, the smaller you make your interior area. In the case of our house, we're probably looking at a minimum of 35 cm (≡ 14") thick exterior walls. This is for timber construction[1], construction in masonry or concrete usually ends up being thicker for the same insulating properties.

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[1] Actually, it is more appropriate to say "mixed" construction. About a quarter to a third of the shell will probably be of concrete because our building land is on a slope.