Designing the layout and choosing windows

I haven't posted for a while but the project has been continuing. We have been busy working with our architect to design the internal layout of the house; focussing on stuff that clients have to consider in the design stage of every such project. We are trying to make the best use of the space for our current and expected future needs and, to a lesser extent, for the needs of a potential future buyer - although we are not planning to sell for many years. One point to consider during the design of the layout is how to minimise the hot water runs from the hot water tank or thermal store and the various point of use.

At this early stage in the project, it is so easy to add to our original requirements, so-called project scope creep. So far, I've added a smallish (3.5m x 9.7m) green or living roof on what would have been a plain, flat roof! Nothing to do with Passivhaus but it will give us a bit more green space and a nice view from two of our bedrooms. It should also help to moderate temperatures in the summer.

We have not yet specified exact window sizes, just their approximate positions, which will often be where the existing windows are located. I am also looking at window and door manufacturers. This is easier in one respect in a Passivhaus project because there are relatively few products that have been certified by the Passivhaus Institute as meeting the necessary performance standard. The U-value of the whole window, i.e. the glazing and the frame, must be 0.8W/m2K or lower. Most new windows in the UK are in the range 1.5 to 2 and are double glazed. Passivhaus certified windows are always triple-glazed but triple glazing alone is not enough to reach the PH standard. The design of the frames and the spacers (the bit between each pane in the window). Frames and spacers must not create any thermal bridging between inside and outside. Triple-glazing has three properties relevant to Passivhaus: a very low U-value for the glazed area, an inner surface temperature within two or three C of room temperature (in winter), this helps with thermal comfort; lastly, triple glazing lets less sunlight through than their double glazed equivalents. The best Passivhaus windows try to address this last point by using glass that has the highest "G-value" - a measure of solar transmittance. Passivhaus windows are also designed to ensure that the 0.6 air changes per hour standard is not compromised. They have multiple seals to ensure this.

Even if you find your 'perfect' Passivhaus window, their real world performance will be determined by how well they are installed. Passivhaus window manufacturers are based in the countries where there is a significant PH market and only the larger ones have a UK presence. Two of these are Internorm and Nordan. Internorm have an agent for our part of the country and their installation teams have been trained in a two day course by Internorm. However, I doubt that their installation teams have ever installed windows for a Passivhaus project, where airtightness and avoidance of thermal bridging are both so important.