Our solar Passive House - a summary

The words "Passiefhuis" (Dutch) and "Maison Passive" (French) are starting to become well know here in Belgium. Mainly because the federal government (you know, the one that keeps resigning ;) ) gives significant tax benefits for owners of such houses. And with all the focus on green technology it's regularly in the news and on TV.

Wikipedia has all the details and history, so I'll just focus on what we are doing specifically.

Our plot was a plateau of 2.5 meters high, and the restrictions limit the roof line to 4.5 m which is not enough for a full 2 floor house on top. While some neighbors decided to keep their houses one level we did not do this. Compactness is important to limit the heat loss surfaces so the ideal shape is a perfect cube.
In addition to that we digged down, putting the sleeping quarters (partially) underground and the garage completely underground at street level. The garage is placed outside our "perfect cube" but is not part of the heated space anyway. This earth sheltering further reduces heat loss and buffers the rooms from summer heat. This is the architect's first sketch, some refinements were made later but it does show the concept very nicely.
It's quite a radical design but definitely got what we asked for: a unique design. The concrete pit plays an important part: There are different levels of landscaping and various sided holes are the eye catcher on the street side. Large open sections in the East and West walls allow for windows right above ground level in the bedrooms.
Rainwater recuperation is mandatory when building or renovating a house here. Our 6500 liter tank is placed very close to the concrete wall and had to be placed by hand. The rainwater will be used for flushing toilets, cleaning, watering plants and garden and it can also be used for the washing machine.
Heating, ventilation and hot tap water will all be supplied by one machine that will use no more then 60 cm x 120 cm of floor space. It's a compact unit specifically designed for passive houses. It holds 200 l heated water, recovers heat from the vented air at 85% efficiency and has a build-in heat pump for both the water and heating the fresh air to max. 50 degrees C. Required heating power for the whole house: less then your average electric iron.
It's not to common but we do get strong winters sometimes here in the west of Europe. Especially the past 2 years. Oh the irony of global warming ;). In order to prevent freezing air entering the ventilation unit there will be an additional heat exchanger pre-heating the outside air. This uses thermal energy from the soil captured with a 150 meter long pipe 1.5 to 4 meters underground. We placed this conveniently around the outside of the concrete, before the ground was filled up again.
Common building practice is Belgium is an inner load-bearing brick wall then an insulating layer and finally an outside brick. Not being fans of brick walls we never intended to use this but also the insulation thickness needed for a passive house make this completely unpractical. Using a wood frame we easily get 36 cm of insulation inside the wall without losing to much floor space. The walls were produced in a workshop and delivered on site with 2 flat loaders.
The slightly more complicated balloon frame construction is used to prevent the floor from forming a "cold bridge" like is common in the platform construction. It's also a lot easier to ensure air tightness because all the seams can be taped inside the house. An engineered timber joist (called Finnjoist) forms both walls and floors, a significant reduction in material use compared with regular constructed frames of this thickness.
Since this year the government went even further and doubled the tax benefit for a passive house if it supplies it's own energy for room and water heating. This has convinced us to make our house completely zero-energy. 4600 watts of photovoltaic solar panels will make our roof a net-coupled power plant that will produce more then our complete yearly power budget.