Innovation and investigation at the Hill House

Innovative digital and scientific methods of documenting the condition of one of Charles Rennie Mackintosh’s best buildings is a prelude to tackling its dampness.

A 3D model generated from laser scans of the exterior of the Hill House.

1 Introduction
2 Infrared thermography
3 Microwave moisture measurement
4 Digital documentation
5 Related articles on Designing Buildings Wiki

Introduction

The Hill House in Helensburgh, Scotland, is one of Charles Rennie Mackintosh’s greatest and most complete works. However, since its construction at the beginning of the 20th century, the house has been subject to damp ingress, threatening its very fabric.

In an effort to further document and understand the complex story of the Hill House, the digital innovation, documentation, and conservation science teams at Historic Environment Scotland (HES) have teamed up with the National Trust for Scotland (NTS) to digitally document the site, and complete moisture mapping and thermographic surveys. These surveys will inform our understanding of how to care for the Hill House, while providing virtual access for professionals and visitors alike.

The Hill House, built and designed by Charles Rennie Mackintosh and Margaret Macdonald Mackintosh in 1904, was a bespoke home for the Glaswegian book publisher Walter Blackie and his family. Because of its cement roughcast, chosen by Mackintosh for its aesthetic, price and seemingly waterproof properties, rainwater has entered the fabric through cracks and defects, causing the house to suffer from damp for decades. As the wind-driven rain in Scotland increased over time, the house was in growing danger of becoming irrevocably damaged if no action was taken by those who protect it.

To stall the damage to the building, in 2019 the National Trust for Scotland built a protective box round the house. Made of chain-linked steel, the box lets the house dry out slowly without being pelted by wind-driven rain. The teams at HES were called to perform several surveys, building on some earlier investigative work that the NTS had previously commissioned.

The science team at HES used two methods to investigate moisture distribution at Hill House: thermography and microwave moisture measurement. The first provides information about the near surface of the building and the second looks deeper below the surface.

Infrared thermography

Thermography uses thermal cameras to detect infrared light emitted by objects. The intensity of infrared emissions is related to surface temperature, effectively providing a temperature distribution map of a surface.

Several aspects of a wall’s internal features can affect its external temperature, including heat flow, structural elements, voids and moisture content. To detect these features, we need heat input to the wall to drive the temperature variations. In an unheated building, thermography is ineffective because all surfaces would be equilibrated at virtually the same temperature. Fortunately, the Hill House provides both internal domestic and external solar heating. Normally, solar heating only affects sunlit surfaces (mainly south facing), but the metal roof of the box is strongly heated by the sun and re-radiates this heat on to the building beneath, illuminating surfaces in all orientations in infrared light. Radiated heat from the roof of the box improves the data we can obtain from thermal images of external elevations. Specifically, we can observe the distribution of moisture trapped beneath harling and detect areas of detached harling.

Rainwater can penetrate the building envelope through shrinkage cracks and other defects. Its evaporation is inhibited by the impermeable nature of the cement roughcast. Thermal imaging can observe this trapped moisture because it affects surface temperature in several ways. Although inhibited by the relatively impermeable cement roughcast, moisture does still evaporate, especially where there are cracks and microfractures which penetrate the depth of the harling. Evaporation of moisture at the surface produces a chilling effect on the substrate as the energy for evaporation of water is taken from the surface itself. The result is a relatively cold patch on the surface.

Thermography can also be used to detect detachments of harling, depending on heating of the wall. Where there is a detachment, there will be a void between the harling and the underlying substrate. Heat flow is significantly reduced across air gaps, being most efficient in solid structures. When the surface is warmed, where the harling is firmly attached, heat will flow easily into the bulk of the wall, but where there is a void, heat will be trapped at the surface, resulting in a warm patch.

Microwave moisture measurement

A microwave moisture measuring instrument works through a sensor head placed in direct contact with the surface. The head contains an emitter that generates a microwave signal which is projected a certain distance into the substrate. Different sensor heads allow investigation of depths from near-surface to up to 80cm deep.

The device uses the principle that there is a significant difference between the relative permittivity of water and most building materials. A low-energy microwave signal is projected into the substrate, and a receiving unit in the sensor head measures the proportion of microwave energy that is reflected (the reflection coefficient) and converts this into a measure of the relative moisture content. One advantage of this method of moisture detection is that it has low sensitivity to dissolved salts, although it is sensitive to the presence of embedded metal.

The HES science team used two sensors at Hill House: a near-surface sensor that gives information about moisture concentrations within about 2–3cm of the surface, and a depth sensor that detects moisture to depths of 20–30cm. Readings were taken on selected wall surfaces on a 15 or 30cm grid. Data processing allows us to interpolate the moisture levels across the grid and produce a 2D mapping of sub-surface moisture distribution. The ability to track changes in subsurface moisture concentrations year on year will allow the science team to monitor the progression of drying deep within the walls. Although the box had only been in place for a few months prior to the 2019 moisture measurements, some drying was already apparent, especially near-the surface.

Digital documentation

The digital documentation and digital innovation teams have undertaken several digital surveys of the house. This project has allowed us to expand on our usual workflow of digital documentation, taking on exciting new experimental work, including the use of thermography.

These digital surveys were done through laser scanning, a technique that records the geometry of a surface in 3D space, creating a digital model of buildings, sites or landscapes. It works by emitting a laser beam which scans a target surface up to a million times a second, reflecting back to the laser scanner, which computes its distance to the scanned surface. These coordinates, or points, come together to create a point cloud, accurately defining the surface geometry with up to millimetre accuracy. Scans are then taken from multiple positions with enough overlap to allow them to be joined together, creating a point cloud with complete coverage of both the interior and exterior of the site.

For this project, we used the terrestrial 3D laser scanner Z+F Imager. This scanner has the ability to record HDR colour information to layer on the point cloud. The box allowed us to scan from unique vantage points of the exterior, including providing excellent coverage of the roof. What was also novel for us in the workflow at the Hill House was the addition of a thermal camera attached to the scanner, which allowed us to record a 360-degree panorama of infrared information and apply it to the point cloud.

Recording thermal data in 3D space allows us to visualise how damp could be moving throughout the house with a more holistic point of view. By being able to directly visualise how the damp from the chimney, for example, speaks to a patch of damp in the corner of a room seems simple enough, but when visualised together is incredibly powerful in communicating how the house functions as a single unit. Once a second comparative survey is completed in the coming months, the data can be taken forward to inform conservation, interpretation and site management decisions. The plan is to return to the house in 2021 for the next round of survey, so that we can assess how the box has affected the damp ingress of the house.

We are still very much in the midst of this project. We have produced a baseline dataset, documenting how the house was faring in the summers of 2018 and 2019. With this baseline, we can create visualisations, and provide virtual accessibility and education about the house. Behind the scenes, it has allowed us the opportunity to collaborate with novel and innovative workflows of documentation and investigation, which can be applied to all the sites we care for and document. Through the combination of techniques and disciplines, we are uncovering new ways to tell the stories of our heritage.

At the Hill House, the newly appointed research and development team, led by LDN Architects, will begin to dig deeper into this information over the coming months to help shape our long-term approach of conserving this vital piece of Scottish heritage.

This article originally appeared in Context 168, published by the Institute of Historic Building Conservation (IHBC) in June 2021. It was written by Sophia Mirashrafi, a project officer on the digital innovation team and Maureen Young a conservation scientist on the science team, both at Historic Environment Scotland.