The surveys, carried out in partnership between owners the National Trust for Scotland and Historic Environment Scotland, shows the extent of the damp and water damage to the Helensburgh building.

Infra-red thermographic (IRT) imaging records differences of surface temperature, which gives an indication of where moisture from decades of almost constant wet weather is retained within the building fabric.

A previous IRT survey was carried out in 2003 at the site and these fresh images have been combined with new 3D digital survey and microwave moisture readings. These three surveys allow building conservators to pinpoint areas of damp, and further understand the declining condition of the property.

Since it was completed in 1904, years of wind and rain have caused significant issues with water ingress. In efforts to protect Mackintosh’s domestic masterpiece, the conservation charity is in the process of surrounding the building with an innovative mesh structure to protect it from the weather.

Richard Williams, general manager for Glasgow and West at the National Trust for Scotland, said: “By combining the infra-red thermographic survey, the 3D scan and the microwave readings we have a very powerful tool to aid our technical understanding of the complex problems at The Hill House, and a robust baseline before we surround the building with its protective shield.

“These surveys reinforce what we already knew about the house, which is that it is very damp and has considerable issues that need to be overcome. Due to the design of The Hill House, there are many ledges, wall heads and chimneys that have had a history of many attempts to remedy, yet this problem continues.

“We’re also now have additional areas of concern, such as large sections of harling that have become disengaged from the walls where damp is accumulating, and internal walls we hadn’t realised were so damp.

“We have also been able to see the direction that the water is travelling in some of the rooms, in particular in the exhibition room, where there was already clear damage.

“The works to create the ‘box’ are now well underway and we are grateful to the many individuals who have generously donated to help us to tackle these problems. The intention is that the structure will provide a temporary respite for The Hill House pending a long-term solution to the water ingress being found, in conjunction with our Mackintosh partners.”

Dr Ewan Hyslop, head of technical research & science at Historic Environment Scotland (HES), said: “We’re very pleased to offer our skills and expertise to support the National Trust for Scotland in this important project.

“Hill House is a Mackintosh masterpiece, and this project is a great example of how we can use innovative technology to better understand the risks to historic sites such as this, and inform work to conserve and protect them.”

The house and gardens are currently closed to the public but are expected to reopen in late spring this year, complete with raised walkways around the exterior of the house and over the roof.

Donations to the appeal to build the ‘box’ shield around the house can be made online.