Sash windows in eco-houses: two things that you might not automatically associate with one another. Here's the rationale:

Firstly, in the UK we design for winter-time - and rightly so, because this is where the majority of domestic energy consumption occurs. Insulating the building envelope not only stops heat getting out into the cold of winter, but it also reduces heat from outside coming in through the building fabric in summer. Intrinsic in good thermal performance is a high level of air-tightness and significantly reduced levels of ventilation, compared to the previous condition of the building. This means that, when rooms do get uncomfortably hot due to solar gain on the hottest days of the year, additional ventilation (called purge ventilation in Part F of the building regulations) is required - and sash windows are uniquely good at achieving this, as the following diagram shows:

Secondly, we're doing an eco-refurbishment of an Edwardian house, and the majority of the windows were originally sliding-sash type; as estate agents will tell you, the British public has a love affair with Victorian/Edwardian houses, and most people would pay a price premium for an 18th/19th century property over a 20th century one. As a proof of concept project, the Withington Co-operative Eco-house will have more appeal - and further reach - if we can produce a workable model that preserves those elements of architectural heritage that people cherish (if the refurb programme isn't going to be sympathetic to the architecture of the building that we've inherited, then we may as well have saved ourselves a lot of headache and done a new-build instead). Here are two neighbouring properties - which do you think adds the greatest aesthetic value to the streetscape?