Drying washing indoors on rails and radiators is bad for people's health and can contribute to fuel poverty, according to new research.
The study, from architectural experts, suggests one solution could be a return to the days of communal drying areas, and says housing associations and other providers of social housing should take heed of the findings.
The problem is caused in part by modern housebuilding, which tends to create homes which are small, sealed boxes for reasons of cost and energy efficiency. It can lead to poor indoor air quality and researchers say householders should be aware of the potential health impacts.
The Mackintosh Environmental Architecture Research Unit, based at Glasgow School of Art, surveyed householders in the city between September 2011 and May 2012, and discovered that 87% had dried domestic washing indoors, and 64% had done so on or near heat sources. Some (23%) turned the heat up to make clothes dry faster and 37% opened a window while drying.
The problem, researchers point out, is that indoor drying increases moisture in the air, encouraging dust mites and leading to increased concentration of mould spores, as well as reducing the indoor air quality. Dust mites and mould spores have been linked with poor health, particularly asthma and other respiratory conditions.
Turning up heating meant residents spent more on fuel, as did opening a window for ventilation. By avoiding indoor drying, residents could reduce the moisture in their homes by an average of 15% or by 30% on wash days.
While indoor drying spaces used to be specified in building regulations in the 1960s, this is no longer the case, partly because of technological solutions, particularly tumble dryers.
However researchers found many residents were conscious of the high energy bills associated with tumble dryers and avoided using them for lengthy periods.
One of the report's authors, Professor Colin Porteous, said the findings were a particular problem in Scotland, where the weather limits the opportunities for outdoor drying and air is already frequently moisture laden.
He added: "Levels of fuel poverty in Scotland are already excessive and increasing the domestic heating load during the drying process makes a bad situation worse. It is a catch-22 situation because tumble drying is more energy intensive than open window passive drying.
"People are sealing their homes more tightly to be energy efficient and more comfortable, but this is not accompanied by ventilating them adequately."
He called for dedicated drying spaces and argues that housing providers could consider returning to the model of tower blocks which were often built with communal laundry facilities on the ground floor, or could explore the use of covered drying areas outside. New homes could be built with drying cupboards, isolating drying from the rest of the house, which contain a heating element and ventilation to remove the moisture safely, while older homes could be retro-fitted with a variety of solutions.
Prof Porteous added: "Building regulations currently require new homes to have a one-metre cubic space for drying which is not enough, and can be above the bath. That is only likely to increase the problem of moisture in bathrooms."
The report was published with a design guide aimed at government, private and social housing providers and architects, providing technical guidance on design upgrades.
It was launched at multi-storey blocks in Wester Commons, in Glasgow, managed by Queens Cross Housing Association, which include indoor drying areas for the use of residents.
Shona Stephen, chief executive of Queens Cross Housing Association, said: "We're keen to encourage designs to reduce and manage moisture in the home, improve indoor air quality and reduce energy consumption for the benefit of the health and income of our tenants.
"We would be mindful of the proposals for indoor washing/drying cupboards in line with the GSA's research recommendations for new projects but current cost constraints are a challenge."