A lot has happened since the last post. I have been working on the Passive House Planning Package (PHPP), the very sophisticated Excel based spreadsheet used to model how your proposed building will perform. I had spent a lot of time getting to grips with the basics of entering our proposed building's parameters - areas, materials, glazing, window frames; together with the climate and any shading - which is a bit tedious but which has to be done accurately if you want the PHPP to predict how the real life building will perform. It was only when I started getting initial estimates of how the building would consume, especially whether we were going to make it down to the magic 15kWh/m2/annum, that the power and usefulness of the PHPP really became apparent.
Since then, I have been refining the information going into the PHPP and it has allowed us to pinpoint the problem areas and tweak the design further to optimise performance. The floor is still our biggest obstacle. It definitely does not not make sense to remove the slab, as this would also mean removing all the internal supporting walls, all of which we would otherwise want to keep. My earlier modelling of the floor-wall junction may have been too optimistic, as I had only assumed a temperature of zero outside, when -10C would be a more realistic worst case. Also, the outside walls will likely perform better, as they will have insulation outside, right down to the footings. The inside walls won't have this insulation and are therefore probably a bigger thermal bridge.
We have added south facing glazing in the roof, following a visit to Ecobuild earlier in the month. Velux now do a triple glazed window with an overall U-value of 0.82. The window has an integrated external blind (essential in a south facing roof window) with remote controlled operation - even though I am not keen on excessive complexity and gadgetry built into the fabric of the house, when the window is 4m or 5m up, I'm willing to make an exception. I had wanted to avoid using roof windows, as I think it is very hard to avoid thermal bridging around the frame. How a window is installed makes a huge difference to its performance. Even with good double glazing, the way it is installed can boost or degrade its rated performance a lot. In a Passivhaus, as well as avoiding thermal bridging, the window has to be fitted to create an air-tight seal around the frame and set in the wall so that the wall reveals and overhang do not create additional shadowing on the window. In a standard block or brick built house, this means wrapping the external insulation around the window frame on the outside so that the window frames will look quite slim from the outside. Also, the rough opening, into which the window is to be installed, needs to be plastered accurately and smoothly, and let to dry out fully, before the window is installed. This allows the specialist tapes, used to make the building airtight, to be fixed between the window frame and the plasterwork to form a good air-tight seal. This changed sequencing of work is quite different to conventional builds but is essential in a Passivhaus. These tapes should be used around the frame, even when air-tight bands are placed between the frame and rough opening. Such bands are designed to expand once in place, so making a good seal but the tape provides a second layer of airtightness and it is easier to patch it if the building fails it's airtightness testing during the build (more of this later).
Visiting Ecobuild, it was possible to compare triple glazed windows. Many only have a single rubber gasket to maintain the airtight seal between the opening section and the frame. Although it may perform adequately when the Passivhaus is first commissioned, it is likely to fail over time. Better designs have double or treble gaskets and use engineered wood, which does not warp nearly as easily as its non-engineered counterpart.
On airtightness testing, we will need to test at two points during the build: after the windows go in but before the 'first fix' and again after first fix. This is essential because there is not much point in identifying any failures in the building's airtightness after it is all finished. It is much easier and cheaper to resolve problems before all the finished surfaces and fittings are put in.
On a wider note, new passive houses are between to get formal certification from the Passivhaus Institute. One of the latest was a house in Kent featured on Grand Designs. Congratulations to them! We will kick off the process for getting certification after we have got through planning.
We want to get our planning application off this week. In a conventional build, we could have sent our application off some time ago but in a Passivhaus this is much more risky as we had to do a lot more detailed design work, aided by the PHPP, to get the building working right first, before we could apply for planning permission. Otherwise we might have found ourselves unable to change something that is crucial to the building's energy performance. This might be a real barrier for some people, as it means spending more on architect's fees while the project is still not certain to go ahead.