BRANZ FACTS SUSTAINABLE CONSTRUCTION #3 Trombe walls This factsheet promotes and provides the information needed to design, build and make proper use of a robust solar space-heating technology that can significantly enhance your home’s indoor comfort year round – Trombe walls. THE DEFINITION of a robust technology is the concrete wall. This heat slowly conducts There are several key considerations that one that provides significant comfort benefits through the wall to be reradiated out many must be followed to ensure the success of for homes in New Zealand while also being hours later into the internal space, reducing Trombe walls in the field. This factsheet, both easy and low cost to design, build, use the amount of conventional space heating based on the findings of four New Zealand and maintain. needed. During summer, Trombe walls need examples, provides much of the information Trombe walls were shortlisted by BRANZ to be shaded carefully so as not to overheat needed to successfully implement Trombe from a wider set of less commonly used tech- the inside. walls in New Zealand. nologies that provide a more comfortable Trombe walls are particularly well suited to indoor climate using natural means. homes in sunny climates that have few clouds Eight less common technologies were and large day-night temperature swings in investigated, with only three making the the cooler months. In New Zealand, the ideal robust technology shortlist. regions are all of the North Island and Nelson Trombe walls work best when integrated into and Blenheim in the South Island. However, homes that have been passive solar designed the rest of the country, including Invercargill, – where building elements are used to collect, will still get significant benefits from this type store and distribute the sun’s thermal energy of technology. to keep the internal spaces at a comfortable temperature all year round, with no mechanical assistance. Passive solar integration is funda- mental to good home design. What is a Trombe wall? A Trombe wall is a heavyweight, sun-facing, glazed external wall that absorbs heat from approximately 200 mm thick the sun during the day in order to slowly concrete wall release it to the interior at night. A typical overhang sized for summer shading Trombe wall cross-section is shown in Figure 1. The idea was developed into a work- radiated heat into room WALL able architectural element in the 1960s by French engineer Félix Trombe and architect Jacques Michel and has been successfully exposed (uncovered) finish single or double glazing unit TROMBE used around the world. approximately 15-50 mm air gap dark painted or coloured surface A Trombe wall takes advantage of the concrete slab stop flashing natural energy characteristics of heavy- weight building materials (which are also protective surface to insulation fairly conductive) that have been exposed to direct sunlight. Typically, a Trombe wall consists of a darkly coloured, north-facing DPM 30–60 mm XPS insulation masonry external wall covered by glazing on the outside. Solar energy transmitted through the glazing heats the external face of Figure 1. Schematic cross-section of typical Trombe wall, showing key components. January 2018 v2 | Sustainable Construction #3 branz.nz 1 Why use this technology? as-built houses along with their respective required for the return on an investment to Trombe walls are an all-round winner – Trombe walls were also thermally modelled repay the sum of the original investment. provided they are properly specified and in a range of climate zones to examine their Although it doesn’t account for the risk or constructed. They can greatly benefit transferability to other climates. the financing of money, it is a useful indicator indoor comfort by providing a very pleasant The modelled thermal impacts were when used carefully to compare similar radiative heat, are very reliable and simple assessed quantitatively across a range of investments. to construct, require very little in the way of useful indicators: On average, for the four Trombe walls maintenance throughout their lifetime, have ● Trombe walls reduced the winter space- examined: a reasonable build cost and are applicable in heating requirements of the living areas ● the benefit-cost ratio is 2.1, making for all New Zealand climates. They are especially of the individual homes by an average a very good investment indeed – this good where a portion of a home’s northern of 35%, which equates to savings of is based on a 5% discount rate over 20 view may need obscuring or the views are to approximately $170 annually. This was years, which is the rate that is typically other orientations. In addition, they are also based on a temperature set point of used for these types of energy-saving useful where too much natural light or glare 20oC for the hours between 7pm and technologies is a concern. 11pm and an energy cost of 25c per ● the simple payback period is 5.4 years – kilowatt hour (kWh). this is very appealing and far better than User experiences ● Trombe walls worked well in different the expected return if that money were Trombe walls were recommended by all the climate zones. The averaged space- to be invested in shares or in the bank. New Zealand users surveyed by BRANZ. A heating reduction percentages for The assumptions made and details of the typical quote: “The quality of the heat is Auckland, Wellington, Christchurch and equations applied are listed in the confer- really enjoyed – not stuffy like artificial Invercargill walls were 45%, 21%, 20% ence paper by Jaques (see ‘Supporting heating. The subtle radiative heat of the and 18% respectively. information’). wall seems to be better than other forms ● On average, homes with Trombe walls of heating.” were more comfortable for 10%, 12%, Strategies More survey responses from New Zealand 14% and 15% of the time in Auckland, This section provides the detail required users on key performance issues are found Wellington, Christchurch and Invercargill for a Trombe wall to be planned, designed, in Table 1. respectively compared to those homes specified, constructed and utilised correctly All users were asked about the extra initial without the technology. Comfortable in a home. The way in which the Trombe installation costs. In all cases, the response temperatures were defined as those wall is constructed and the occupants’ was that the Trombe wall was not separately between 18oC and 25oC in winter. behaviour will impact on the effectiveness priced, as it was integrated with the rest of of the system. the house. Financial implications A financial examination of the Trombe wall Planning Quantified thermal benefits technology was undertaken. Two common Talking the client through the benefits of the In addition to interviewing users, BRANZ economic indicators were chosen: bene- wall, especially in terms of the quality of the modelled the Trombe walls using a thermal fit-cost ratio and simple pay-back period. heat provided, is suggested. The low-intensity simulation program called SUNREL. The Benefit-cost ratio summarises the overall radiative heating provided over a very large thermal implications of using a Trombe value for money of a project: the higher the surface is found to be a considerably more wall were examined in detail for four actual ratio, the better the investment. As a rule of pleasant form of heating than fan-forced houses. Comfort provision, the impact on thumb, if the ratio is higher than 1, the project heating such as from electric heaters/heat conventional space heating and the financial is a good investment. pumps. Emphasising its very low mainte- savings resulting were investigated. These Simple pay-back period refers to the time nance requirements, reliable performance Table 1. Key performance indicators of Trombe walls – from BRANZ interviews in 2011/2012. House location Q. What do you really like about Q. What is the annual maintenance Q. How reliable is the system? Q. Would you recommend the system? cost? the system to others? Northland It’s effective and how it seasonally None. Very occasional painting of black It’s a pretty bulletproof system. It Yes – have no reservations about recom- self-adjusts – the gradual adjustment exterior concrete needed though. behaves today exactly the same as it mending it. is so perfect – the way it warms up as did 40 odd years ago! winter approaches. Wellington Its passivity, its robustness and its None. Very reliable in providing comfortable Yes – because of its low dweller involve- multifunctional nature. temperatures. ment. Nelson No artificial heat required. The subtle Wash windows occasionally. Very. Yes, absolutely. radiative heat of the wall. Seems to be better than other forms of heating. Nelson Simplicity. Feeling of all-year-round None really. After a couple of years, Relies on the sun – so reliable as the Yes. But extra concrete and engineering comfort. Aesthetic: pebble face interi- cleaning the cobwebs in the air space sun! calculations required. or surface came out really, really well. is needed. January 2018 v2 | Sustainable Construction #3 branz.nz 2 over time and safety as a space heater are recommended in order to reduce heat loss. – the fewer air gaps the better to provide all good selling points. See Figures 3 and 4 for one solution. Using consistent conductivity. Autoclaved aerated The Trombe wall will need to be oriented extruded (XPS) insulation rather than concrete does not have sufficient density to between 330° and 30° of true north (rather expanded (EPS) on the external face of the warrant being used as a Trombe wall. than magnetic north) to provide a reasonable footing will reduce the amount of moisture The size of the thermal inertia must be in amount of thermal benefit. This accounts for that can be absorbed.