Lessons from history

Bill Bordass warns about the ‘forgetting curve’ and looks again at the benefits of the 2+1 window

The recent article in CIBSE Journal by technical editor Tim Dwyer reviewing old research papers from BSERT got me thinking (‘Buried treasure,’ February 2019).

In a world increasingly focused on the here and now, could we learn more from the past – and not just from research papers? It resonated with two events I went to last November: a session reviewing the work of around 30 research students on energy and buildings; and the CIBSE Build2Perform conference.

Domestic overheating came up in both, with people advocating external solar protection – something the latest report1 by the Committee on Climate Change also says is needed.

At both events, I suggested that 2+1 windows might be a cheaper and more practical option, particularly for refurbishments – but nobody had heard of them, apart from one or two older attendees. A 2+1 window has a sealed double-glazed unit on the inside, a protective sheet of glass on the outside, and a space big enough to contain a blind (often venetian) in between.

The academics also complained that practitioners didn’t read their papers, and I countered that the academics also missed a lot by seldom referencing anything that hadn’t been peer-reviewed.

A 2+1 window – a cheaper and more practical solution for domestic overheating?

At the research event, two PhD students told me that their supervisors didn’t want to see references more than 10 years old. Since then, I’ve heard the same from other recently qualified PhDs. Shocked, I confronted their head of department, who said he would remind all the supervisors. The building industry has always been poor at learning from experience – is academe now enforcing this ‘forgetting curve’?

Recently I reviewed a paper from a research institute, which didn’t even remember its own past work – perhaps because much of its library was thrown in the skip as soon as its privatisation was complete. PDFs disappear too, particularly when public funding for projects and bodies ceases. And research is often funded in three-year chunks, with no continuity.

Where is the legacy? Where is the institutional memory?

The 1971 Rothschild Report foresaw government outsourcing more while remaining an ‘intelligent customer’. The Civil Service used to have administrators who moved around, and scientists and professionals who didn’t much. Now there are very few scientists and technical professionals in government and, in my experience, those that remain are seldom listened to.

Instead, we go off in search of blue-sky innovation, while forgetting history and ignoring the evidence under our noses. For example, information from case studies of buildings in use tends to be dismissed as anecdotal: ‘We need more evidence,’ they say, ‘a statistically significant sample.’ Often we don’t2 – this is just a delaying tactic; and statistics seldom capture the context of the outcome.

What are the benefits of a 2+1 window?

They originated in continental Europe, and are often made by relatively small, specialist firms but, in recent years, many of these have been taken over by larger manufacturers and closed down. The last remaining UK manufacturer of any size ceased trading in the early 1990s recession. UK suppliers of continental products have also dwindled away; today only one remains, importing high-quality Austrian open-in, tilt-and-turn windows. But open-out casements, top-hung windows and bottom-hung, open-in, hopper fanlights suit many British buildings better.

A 2+1 window no longer offers better heat retention than today’s coated, gas-filled double- and triple-glazing units, not to mention vacuum glazing.

However, 2+1s come into their own when managing heat gains: mid-pane blinds are much more effective than internal ones and are much cheaper, requiring less maintenance than external ones, while 2+1 performance can be enhanced further by incorporating heat-rejecting glass.

External shading may also attract birds and obstruct window opening and cleaning – and, if one small piece blows off in the wind, facilities managers may rip the lot off the entire building for health and safety reasons.

The industry has always been poor at learning from experience – is academe now enforcing this ‘forgetting curve’?

Frames for 2+1s cost more to make than for sealed units alone, but not that much more, and there are advantages: the sealed units last much longer because their perimeter seals don’t get wet; the inner frames can be made from timber for better appearance, insulation and sustainability; and the external frame is usually aluminium for weather protection, but lightweight because the stiffness comes from the timber frame and the glass.

If there was a proper market for them, costs would go down. How about making them ‘deemed-to-satisfy’ in the Building Regulations?

Further improvements can be made by ventilating the blind cavity, which is normally only partially sealed to avoid condensation. For example, the cavity can be:

  • Cooled further if ventilated naturally from bottom-to-top, or used for exhaust air in a pressurised building
  • Used for heat recovery into the building, either by natural transpiration or drawn through by an extract system; this works particularly well in airtight flats that have kitchen/bathroom extracts running 24/7.

In fact, if used as a supply air window, an effective U-value of less than 1W.m-2K can be achieved by using a single internal pane, not a double-glazed unit,3 so reducing capital and maintenance costs – important in social housing.

Now we are more aware of the need to limit overheating in the UK, will the 2+1 window come back into fashion? It has proved itself in UK projects, including the Elizabeth Fry Building4 at the University of East Anglia and the Open University’s Design Studio refurbishment.5 We talk about innovation, hoping for the next big thing. But much innovation is purposeful improvement, based on feedback on what works and what doesn’t. Sometimes this means going back to the future.

I am encouraging British manufacturers to produce 2+1 windows and researchers to explore them, while examining the products on the European market.


  1. UK housing: Fit for the future? Committee on Climate Change, February 2019.
  2. This is argued convincingly by Professor Bent Flyvbjerg, now Chair of Major Programme Management at the Said Business School, Oxford in Five misunderstandings about case study research, Qualitative Inquiry 12(2) 219-245, April 2006.
  3. Improving air quality in homes with supply air windows, BRE Information Paper IP 6/03, 2003.
  4. Probe 14: Elizabeth Fry Building, Building Services Journal, 20-25, April 1998.
  5. Naturally comfortable offices – a refurbishment project, Good Practice Case Study 308, BRECSU, BRE, March 1997.
  • Bill Bordass FCIBSE is research and policy adviser to the Usable Buildings Trust bill@bordass.com