As winter approaches, home owners are being warned over stick-on window films, which promise to drastically reduce heat loss.
A number of adhesive film products on the market claim to reduce internal heat loss through windows by up to 44 percent, at a cost of thousands of dollars.
But, the Building Research Association, on behalf of the Energy Efficiency and Conservation Authority, has found that in some instances, the products might only reduce heat loss through single glazing by up to 25 percent and can also create problems with condensation.
The research was prompted by ongoing questions from the public, asking for the product's claims to be verifed.
John Burgess, a senior scientist at Branz, said the film reflects heat, making the window colder and creating more condensation.
"Because it's reflecting heat away from the glass... it's actually stopping that single glazing from getting warm from the heat inside the dwelling, inside the room," he said.
They found there was 50 percent more condensation on the film, than on single clear glass, he said.
The film's performance is ruined by the condensation, he said.
"Rather than the film being the thing the warm air sees, now the warm air is trying to get out through the condensation which is basically water hanging on the top of this film," Mr Burgess said.