The existing house has a huge amount of glazing, 61m² in total. This will reduce slightly, even with the addition of the extension and its roof lights, to 54m². This is still a lot of new/replacement glazing and the cost, well, is eye-watering, particularly for high performance timber, triple-glazed units that we are looking to use.

After receiving some quotations upward of £35K(!) we have a re-think. Our core requirement is for high quality windows that perform well, both thermally and functionally, including security. So, along with Ruairi, we go window shopping and obtain a few more quotes. Ruairi suggests that Green Building Store’s EcoContract windows with a U-value of 0.9 W/m².k. We suspect our existing windows range from between 2.8 and 4.0 W/m².k. and, comparing these values to standard double-glazed units that require a minimum performance standard of 2.2 W/m².k, the EcoContract windows at 0.9 W/m².k is a good performer. Timber framed 4-16-4-16-4 argon-filled units with two low-e coated panes, and at a fraction of the price of its nearest competitor.

But, EcoContract do not meet the Passivhaus standard. For Passivhaus Institut certification, we would need to purchase certified triple-glazed windows with a U-value of ≤ 0.8 W/m².k. The payback for the difference between the PHI and EcoContract specifications of 0.1 W/m².k. would never be realised in the lifetime of the windows and therefore the Passivhaus standard, in this instance, was not considered economical. This is one of two key reasons why we are not able to achieve certification on this project (the other likely to be thermal bridges)