The kWh/m2 and the kgh/m2 1/2

Strange, really, that calculating in kWh seems normal for everyone. Especially in this time, because kWh does say nothing about the origin of the energy, which can be anything, from labor, wind turbines, or coal to nuclear energy. Currently it is an important measure of energy use in buildings, the kWh / m2: the amount of energy used to deliver a certain function, like warming people. Our new legislation in the Netherlands is even based on that kWh / m2: a maximum number of kWh per m2. However, that does not say anything about the origin of that kWh, and therefore about fossil fuel or CO2 emissions impact. Nonetheless, it is a good indicator to start with: the less energy demand, the better. In Principle that is, but before you eat it, there are some flies in the soup, about that later.

Thus, the kWh is a quantity. A rounded amount. Just like Joule. No continuous flow, that would be the Watt, or joule per second. But, that is the same with the kg That is a rounded amount for material consumption. No continuous flow, which would be kg / second, the growth speed of trees for example.
It is fairly easy to see that for materials the same simple approach can apply as for energy: the kg / m2 versus the kWh / m2, when we talk about buildings. The kg is an amount that is used to deliver a certain function: for example, to carry a floor and roof so that people stay dry. And the less kg / m2 is a good indicator for the material impact: in principle again.

My question to students with a design is therefore standard: What does your building weigh per m2 ? Then they first look a bit weird at you. A kg calculation, no one has ever heard of that. Which is strange of course ? When I show the examples, things start to dawn: an average Dutch apartment complex weighs 1000 kg per m2. Pretty much actually. That weight increases as you build higher. The Taipei 101, until 10 years ago the highest building in the world: weighs about 1850 kg / m2. But then I show Calatrava: The Turning Torso in Malmö, only 100 meters high, weighs 3500 kg / m2: due to a double rotating steel construction. The price of architecture ….? Well, that is not fair for architects, Calatrava is not an architect but an engineer. So that explains a lot. Not?

The other way also works: when building lighter than 1000 kg / m2, you have soon to forget brick and concrete and switch to wood for example. Halving the weight per m2 is possible: 500 kg / m2. Especially if you combine that with straw-bale construction. A good example of this is the home of Rene Dalmeijer In Ijburg in Amsterdam: built entirely of wood and straw bales, 5 storeys high: and weighs approx. 650 kg / m2. But thats more then 500…? Yes, that’s right, but then the following is interesting: in the calculation per m2, everything counts, the finish, but also the foundation. And in this case the house stands on concrete piles. That was not necessary, the house was light enough to be built on a ring foundation. But Amsterdam sold the construction sites in Ijburg, ( on a new island made of sand, exclusively including the poles. Without those poles the house would have come out below that 500.
Another good example is the Agrodome house, in Wageningen. Built as an example for Biobased construction. And indeed a good example, however, it weighs heavily over 1000 kg / m2. How is that possible? The main reason is the foundation: the houses are located near the river, and are very light. As a result, a heavy concrete slab had to be placed underneath, in order to prevent the house from buoying in times of high waters. *

This kind of bad choices only come to light when you start calculating the kg’s. But apart from that, what these examples show is that with lower kg / m2, firstly less material is naturally consumed, but secondly there is automatically a shift from non-renewed to renewed materials, from inorganic to organic or biobased materials. From materials with a high impact to materials with a lower impact (with the same function, with some exceptions but more about that later), in contrast to the kWh / m2 measuring , that does not have that effect. That is why in the next Dutch energy standard (BENG) a second condition is included, namely a minimum of renewable energy, to bring about that change, one of the flies that were in the soup.

But: energy is a measure per m2 per year, and kg / m2 is not? At first sight that’s right. But you can, for example, accumulate that energy over the lifetime of a building, say 50 years. In that case, you have the same total value for the service function for both: the energy and materials quantity for the habitability of 1 m2 for those 50 years. Or vice versa, of course, you can also divide the materials in kg / m2 by those 50 years and then the annual load is 1/50 th of the value at delivery: say the annual material input to keep 1 m2 available. The kg-year, or even the kgh, similar to the kWh: The amount of material to provide a function for 1 hour ….

It is clear that the longer a building will last, the less the material load will be per year: after 100 years it will only be half of the 50 years average. While the annual energy burden remains the same. (apart from changes in between). In fact, the kg load will also increase slightly, due to wear and maintenance. It is the same process as energy, only on a much longer timescale. Over thousands of years, material is also a stream that must be constantly supplied.
This is another indication that demolition and new construction should be avoided as much as possible. The use of material only becomes more effective, while demolition and new construction again causes a high initial load and you still have to wait and see how long that new building or house will last.
Currently, a material performance is mandatory in the Dutch regulation for building permits. However the calculation is packaged in a black box full of categories and weighing factors. [1] I would argue for the same approach as with the kWh, so a direct kg approach, as with energy. . We still might need to set an additional criterion, just like with the kWh, for the remaining flies in the soup. Such as the coherence and dependence between energy and mass.

More about that next time.

* All materials are included for both the Ijburg and the Wageningen homes, not just the construction.



Author: ronald rovers