The Number Song

(DJ Shadow, 1996)

I’ve mentioned before that we’re aiming for a ‘Low Energy’ house. So here’s what this means in practice. I warn you, this is going to be geeky. Fear not, we’ll be back to nice pictures and plans soon.

Apart from the obvious energy savings and environmental benefits, being official 'Low Energy' also means that we will be entitled to a grant from the Brussels Region, which is heavily incentivising house-owners to invest in measures to reduce energy consumption over the long term. This year’s budget was 12 million euro for such projects.

The pinnacle of low-energy achievement is the Passive House, which consumes less than 15 kWh per m² per year. That’s about 90% less than the average house in Northern Europe. Your house is so well-insulated that you hardly need heat it at all: it stays cosy essentially through the warmth given off by the inhabitants and rays of sunshine. Good insulation also requires the house being basically air-tight. If this sounds about as pleasant as living in a teenager’s shoe, you’d be right. This is why you’ll need a super duper ventilation system. And to make sure that fresh air does not mean cold air, you’ll need a ventilation system with a heat exchanger, so that the warm, stale air going out will heat the fresh air coming in. That’s the basic idea. The cherry on the cake is solar panels to produce what energy you do need, or solar heating panels to heat your water.

Doing this in old houses is a particular challenge, since this kind of insulation/air-tightness/ventilation system may require demolishing many lovely original features. Unsurprisingly, real passive houses tend to be new-builds. The next best option is a ‘Very Low Energy House’ which consumes less than 30 kWh per m² per year, and then the ‘Low Energy House’ which consumes less than 60 kWh per m² per year. Given that our house has some very lovely original features, and having discussed with a number of people who have done eco-renovations to old Brussels townhouses, we went for the pragmatic ‘Low Energy’ target.

How to go about this? To start with, you’ll need a good advisor. If your architect is not into this, there are plenty of specialist consultancies who can advise on the energy performance during the planning phase.

The tool they use to work out the energy consumption of your house is the Passive House Planning Package (“PHPP” in the lingo).

It’s a sophisticated model (developed in Germany and now used widely) into which you plug in detailed information about the thickness/type of your walls, technical specs of the materials used, orientation of the house, etc. In fact, not every wall and window, just the ‘envelope’ or outer shell of the main part of the house where you’ll be living. Because within that envelope, air and heat circulate freely. Once you have plugged in all this information, out pops a number. If the number isn’t right, you make a few tweaks and try again.

Sounds simple, but sitting through Friday’s meeting with the consultant and the architect, with thermal bridges, lambdas and U-values being bandied about, I can tell you it is anything but.

However the outcome of the discussions I do understand. We have three scenarios:

1. Exclude the basement from the ‘envelope’ (which is sensible, as you would not normally waste energy heating your garden tools). This means an 'envelope' with a total surface area of 167 m². With basic insulation and double glazing, normal air-tightness of 7.8 vol/h, we would reach an energy consumption of 64 kWh per m² per year. Ok, but not brilliant, and above the magic 60 kWh which we need to be Low Energy.

2. So, we beef up the insulation (with things like multipor and unilit 20 if that means anything to you). We get our air-tightness down to 4 vol/h (which will have to be tested through a Blower Door Test). We add a heat exchanging ventilation system. This takes us to 43 kWh per m² per year. Woo hoo, we’re there!

3. Still with me? Ok, so now comes the cunning part. And also the key flaw in the system, which is that it is based on a ratio between surface area and expected energy consumption. The first two scenarios assume that the envelope excludes the cellar, which gives us a total surface area of 167 m². If we include the basement (and the staircase which was also previously outside the envelope), owing to the weird way in which the surface area is calculated, we get to total surface area of 244 m². Weirdly, the assumed thermal loss of the basement floor is not that much (because of its depth, and the neighbouring houses on either side, which are assumed to be heated). So with some good insulation, and without even having to get the air-tightness below 7.8 vol/h, we end up with energy consumption of 46 kWh per m² per year - still well within the Low Energy category.

The bottom line is that we can save on the costs of insulating the basement ceiling, on messing around with air-tightness and ventilation systems AND get even more of a subsidy (which is calculated according to the surface area of the envelope - so we get about Eur 3,000 more). How? By heating our basement. Nuts.

As I say, we’ll be back with the nice pictures next week…