Context 168 - June 2021

C O N T E X T 1 6 8 : J U N E 2 0 2 1 21 DIGITAL TECHNOLOGY Several aspects of a wall’s internal features can affect its external temperature, including heat flow, structural elements, voids and moisture con- tent.To detect these features, we need heat input to the wall to drive the temperature variations. In an unheated building, thermography is ineffec- tive 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 distri- bution 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 imag- ing can observe this trapped moisture because it affects surface temperature in several ways. Although inhibited by the relatively imper- meable 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 per- mittivity 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 coef- ficient) 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 sensi- tive 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 concentra- tions 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 mois- ture measurements, some drying was already apparent, especially near-the surface. Digital documentation The digital documentation and digital inno- vation teams have undertaken several digital surveys of the house.This project has allowed us to expand on our usual workflow of digital docu- mentation, 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 emit- ting a laser beam which scans a target surface up Moisture distribution to depths of: (a) 3 cm, June 2018 (b) 30 cm, June 2018 (c) 3 cm, June 2019 (d) 30 cm June 2019