Homes in urban areas are most at risk from problems during summer heatwaves.
Prof Chris Goodier, of Loughborough University’s department of civil and building engineering, told the BBC that the risk of overheating had been overlooked in the “big rush to insulate and make homes airtight”.
He said: “Overheating is like the little boy at the back of the class waving his hand. It is forgotten about because the other challenges are so big.
“If you are in the wrong type of house, facing the wrong way, in the wrong street and you don’t deal with heat in the right way, it is a problem.
“Particularly for the elderly. They are going to suffer. Suffering means they are going to die from overheating.”
A report by the professor’s team warned: “Green Deal measures could create new problems in the future, with inappropriately insulated properties experiencing poor indoor air quality and significant summer overheating.”
It said the increased likelihood of summer heatwaves could lead to a rise in heat-related deaths from 2,000 to 5,000 per year by 2080 “if action was not taken”.
The Department for Energy and Climate Change said it has now issued fresh guidance to Green Deal suppliers to help reduce potential risk from installing energy efficiency measures.
In a statement to the BBC, the department said: “The real problem facing our nation’s draughty homes is a lack of adequate insulation and energy efficiency in the colder months.
“The Green Deal is giving households a new way to fund improvements, helping them protect themselves against rising energy bills and keep homes warm and cosy in the autumn and winter.
“If energy efficiency measures are installed appropriately, overheating should not be a common problem and there’s guidance available for those involved in the Green Deal.”
It added: “Decc is working with experts and other government departments to understand the potential risk of overheating in retrofitted homes and ensure that the energy efficiency supply chain, including those working within the Green Deal, are aware and guidance is provided on homes which are most likely to be vulnerable and what steps could be taken to minimise any risk of overheating.”
Wetherby Building Systems, a leading provider of external wall insulation systems, has hit back at the overheating claims.
Bob Deane, managing director of Wetherby said: “The way insulation works is very simple; as well as keeping heat in, it also keeps heat out.
“Homes that have been fitted with external wall insulation are actually in a better position during hot spells, as the heat will not be able to transfer through the walls to heat the property, so these homes will in fact stay cooler.
“This BBC report has only added to the problem of the slow take-up of Green Deal initiatives.”
Only four people have signed up to the Green Deal since it was launched six months ago but Decc says that number is expected to increase with more finance in place.
In total it said there had been 38,259 Green Deal assessments, where customers are given initial advice about what energy improvements they might be eligible for.
Of those, 241 households have confirmed they would like to proceed with work.
