[quote="henno"]as far as draughts are concerned, the u value of the window has no relationship to its air tightness. Windows should be rated in accordance with the BFRC http://www.bfrc.org/
. Airtightness is specifically dealt with in this classification (The effective heat loss due to air penetration is measured) and an A rated window should be selected.
The BFRC (and our NSAI celtic clone) should be treated with caution. Bear in mind that the scheme was only ever designed for replacement windows and the ratings are about as useful as a NAMA valuation ie there is a value but its relative to put things mildly. Iâ€™d put it on a par with the â€˜dynamic u-valuesâ€™ of supply air windows, or the â€˜effective u-valuesâ€™ beloved of Rationel, orâ€¦. go-faster stripes on a Nissan Micra.
â€œThe actual energy consumption for a specific application will depend on the location of the window in the building, the building parameters such as insulation and occupancy, the building geometry and orientation, the local climate and the indoor temperature set by the occupants. The BFRC Rating allows accurate comparison of window performance under identical conditions.â€
The BFRC u-value data is accurate â€“ but only
for the stated size and construction of window. The airtightness figure is useless for an assessment of the windows actual performance sitting in a wall. Itâ€™s derived from the result of a test conducted at 50Pa and dividing the result by a factor of 20 - making just about any window appear to be â€˜airtightâ€™. (Real world testing is undertaken to 600Pa). The solar factor (g value) is for the whole window construction whilst DEAP specifically requests g factor of the glazing only. The â€˜Energy Indexâ€™ is a nonstarter for trying to simulate the actual performance of a building â€“ as its not designed to do that. Obviously a north facing â€˜Aâ€™ rated window is going to have a completely different energy balance then the same window in a south facing elevation.
There are BFRC â€˜Aâ€™ rated windows with a u-value of 1.6W/m2K. Not exactly what I would be looking at if I was building a low energy building.
OP. Forget the A rated nonsense and pursue the triple-glazed option. Without going to any great trouble you can get glazing with a u-value down to 0.5 and a g value in the region of 0.35 or lower (which would be a pretty good double-glazed solar-control specification). This will keep your internal glass temp at a comfortable temperature during cold winter nights and most summer days. You donâ€™t need â€˜Passiv Hausâ€™ windows to achieve this. Download a copy of Calumen (St Gobain) or Spectrum (Pilkington) and you can easily play around with glazing specifications. Give the window supplier a specification that is specific to the building and break it down to individual windows.
PS. The u-value of a window is certainly affected by its air-tightness. Hot-box testing shows this up unlike software modelling which assumes the thing is built as per the CAD dwg.