049. Assembling the house. Part II.

Figure 43. A peek into the installation, upper floor before they started to put the roof on. View towards the east. Figure 44. One of the roof elements being placed.

048. Assembling the house. Part I.

Figure 42. A peek into the installation, upper floor. View towards the east.

The first batch of wood elements were trucked to our site yesterday and the woodworkers started their work. This photo is from the end of the working day today, after the upper floor was mostly put together. This stage should be completed by Thursday.

045. Concrete foundation

The concrete foundation and retaining walls were completed before the winter holidays. Here are a couple of photos showing them at different parts of the process.

Figure 37. Concrete foundation/shell. The black stuff is a bituminous paint. Figure 38. Most of the backfilling had been done by the time I took this photo. In the foreground the top of the rainwater reservoir tank can be seen. A little patio will be built on top of the area.

044. Wood wall elements at Hecht Holzbau

No, I haven't abandoned this blog project despite the evidence so far. When I started the blog I wasn't working and I had lots of time to spend on the write-ups. But now that I'm gainfully employed, I can't seem to be able to find a decent chunk of time to work on this anymore. I'm going to try harder: let's see how it goes. So, here's a little report from our trip to Hecht in Sursee to check out the wall construction process.

Figure 34. The upstairs west wall can be seen in the back. The opening is where the door to the patio will be installed. The piece in the foreground is the downstairs south wall. More photos of it below. Figure 35. One part of the downstairs south wall being loaded onto the trailer. At the bottom left of the photo you can see a wall element edge-on with one layer of insulation. Figure 36. The same wall, seen from the other side. The rectangular openings are for the ventilation system, as are the round holes. When the thing is assembled, the round holes will be inside the floor and the vents for fresh air will be connected to them.

037. Design consideration: wall finish

This is a case where we've had a really big change in our opinion, catalyzed by the suggestions of WA. Our original thought was plaster walls painted white (or in shades somewhere between white and grey in some rooms) with a dark industrial-type floor (more about the floor in another post).

WA suggested that this combination is a dime a dozen and that a much better choice for our house would be to leave the wood of the construction elements visible. Our first reaction to this was quite negative as the examples we had seen so far were too rustic and we didn't really like them except maybe in small chalets surrounded by fields of snow and pine trees. This discussion kept coming up and we kept shaking our heads and finally we went to see a house with an application similar to ours. Actually, if I understand it correctly, ours will be one grade higher than what we saw. The manufacturer, Pius Schuler[1], has recently started producing panels with knot-free, A-grade wood on the visible side which is quite smooth and we were pleasantly surprised and agreed that it does look very nice and airy. The company also has a planning service and they're (re)designing the walls for our house right now. I'll put up detailed drawings once the design is decided. Quite possibly the energy performance will be much better than that of the version I had written about before. More on that topic later.

Figure 28. A close up of a AB-grade Pius Schuler panel in spruce and fir. One half of the panel has been treated with a UV protectant to prevent darkening. Figure 29. Interior of a house built using Pius Schuler panels in larch. Our version in spruce or fir should be lighter, and have less variation in the strips. More images can be found at the Schuler website.

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[1] Pius Schuler is well-known in the Swiss low-consumption wood construction circles → Examples of houses built using Pius Schuler wall elements

016. An air-tight building envelope

We've already seen that the first step in making energy efficient buildings is a thick layer of insulation. Once the heat loss via conduction through the building shell is minimized, the amount of heat that is lost via air leaks in the shell becomes notable. Not only that, there's also increased risk of condensation around the areas of the leaks and consequently increased possibility of mold growth. With current technology, it turns out to be energetically favorable to make the shell 'air-tight' and incorporate a mechanical ventilation system equipped with a heat exchanger to replace the indoor air. More about this in the next post.

The air tightness of a building is quantified by measuring how many times in one hour the entire volume of air in it is replaced when the building is subjected to a pressure difference of 50 Pascals (1 Pascal ≡ 0.0003" of mercury). The test is called the Blower Door Test[1] and the quantity measured is called the air exchange rate and is denoted as n_L50. This test is mandatory for Minergie-P houses where n_L50 must be shown to be less than 0.6 [1/h]. In other words, it takes about 2 hours for the air in the house to be replaced by air from the outside. A mechanical ventilation system is required to refresh the air and I'll talk about in the next post. For plain Minergie houses this test is not required but it is expected that the level of air-tightness be less than 1 [1/h]. For comparison, consider that a 1989 survey of 35 Swiss timber homes done by EMPA[2] showed that they had an average air exchange rate of about 7 [1/h]. Current construction without special consideration to air tightness is somewhere around 3 [1/h].

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[1] About the Blower Door Test on Wikipedia → Blower Door Test

[2] Kropf F., et al. Luftdurchlässigkeit von Gebäudehüllen im Holzhausbau → EMPA Bericht 218, 1989

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	-

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

Walls of wood and concrete compared

Figure 4. Two different above-ground wall constructions. Numbered elements are: 1 ‐ Interior wall surface. 2 ‐ Support structure. 3 ‐ Vapor retarder. 4 ‐ Insulation. 5 ‐ Post. 6 ‐ Wood fiberboard. 7 ‐ Ventilation gap. 8 ‐ Exterior cladding. 9 ‐ Exposed concrete.

In my last post I claimed that we were expecting the exterior walls of our house to be a minimum of 35 cm (14") thick to achieve the required heat transfer coefficient (U-value). I had also remarked that massive construction – masonry or concrete – would result in thicker walls, a few things being equal. Well, I ran some numbers through for a couple of wall types[1] I found in a compendium[2] of such things.

First I picked out the wall types seen in the figure. Then I chose the same material (Steinwolle[3] of the fiberglass persuasion) for insulation in both cases and I increased the thickness of this until the U-value dropped to 0.15 W/(m² K). The total thickness of the wall in the case of the wood construction turned out to be 35.5 cm and 40.0 cm in the case of exposed concrete.

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[1] There must be a couple of hundred wall element combinations possible! If you think I'm exaggerating, you can download a PDF of the Bauteilekatalog by clicking here.

[2] There are some decent free online tools with which one can calculate the U-values of different building components. I used this one here. Available in German and French. But wait, there's more! The tool also estimates the grey-energy embodied in the structures and 'environmental impact points' to help in the decision making process. More about these at some later date.

[3] Steinwolle is made from mostly natural mineral raw materials while Glaswolle has about a 70% recycled-glass content.