Government plans to deliver thousands of new energy-efficient schools in England could be under threat after recent studies showed they are leading to drowsier and less attentive pupils in the classroom.
Two reports by Reading University and University College London have shown that new schools designed to be more air-tight to reduce heat loss are also poorly ventilated, resulting in higher carbon dioxide levels.
According to the studies, high CO2 levels in classrooms lead to children becoming sleepier and less capable of taking in information.
It will come as a blow to schools secretary Ed Balls' target of every new school being zero carbon from 2016.
In Wales, there are no such tight targets, but the Assembly government is also working towards making schools more energy-efficient, including conservation ventilation and health and safety, according to an official spokesperson this week.
Dr Dejan Mumovic, a lecturer at University College London, told The TES Cymru: "Although the school rebuilding initiative in England is a fantastic one, the Government has rushed its sustainable schools programme a little bit.
"We monitored 10 schools that were built 50 years ago and nine schools built under the Government's Building Schools for the Future programme and nothing had changed, the ventilation rates were equally appalling. The CO2 levels are exceeding targets and that can affect kids' learning performances."
Professor Derek Clements-Croome, a research director at Reading University, ran similar tests at eight primary schools. "We tested reaction times and memory of pupils, and when the CO2 was very high the reaction times would slow and memory would be affected, the kids would also get drowsier," he said.
"There is no point pushing for energy reduction if the kids are falling asleep."