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. Additionally, XPS proportion to the living space it is heating – a both the practicalities of shading non-north- insulation provides better insulation for a very small Trombe wall will not be beneficial facing walls as well as the reduction in their given thickness. Both polystyrene types need to a very large room. The ratio of the Trombe effectiveness. To understand how the benefit some form of surface protection though – for wall’s volume (in m3) to the adjacent affected reduces with non-ideal orientation, see example, protective plaster. living area volume (in m3) should be a Figure 2. The material chosen for the Trombe wall minimum 1:80 for concrete-based materials. For the operational lifetime of the Trombe mass has implications for how much heat These figures assume that the home is an wall, the external northern aspect of the can be stored and therefore later released otherwise lightweight construction. wall will need to remain unobstructed in to the adjacent internal living area. This heat The thickness of the Trombe wall the colder months from landscaping, hard- storage ability is commonly called thermal determines the time delay for the heat scaping and vegetation. Ideally, it should be inertia. The higher the thermal inertia of to conduct through from the exterior wall positioned so that any neighbours cannot a material, the longer the time needed to to the interior, which is proportional to its obstruct it in any way. change its temperature. thermal conductivity. The rate at which the The use of computer tools to assist the The thermal inertia values for poured/ heat travels through concrete is about 25 design process is recommended. A 3D visual- tilt slab concrete and grout-filled concrete mm/hour. Thus, for a 200 mm thick Trombe isation tool to examine year-round external blocks are similar to cast concrete. Alternative wall, heat absorbed by the exterior will take shading – such as SketchUp (which is free) building materials, such as rammed earth about 8 hours to reach the interior space of – is recommended as a minimum. A thermal and adobe, are thermally different. Rammed the building. simulation program such as SUNREL can earth/compressed earth blocks can store A thickness of around 200 mm is usually be used for examining the thermal aspects about 25% more heat than concrete does used for concrete-based wall materials, as of the Trombe wall but needs expertise to even though it is less dense for a given the time delay means that the occupants will use. There are a few energy consultancies volume. Adobe can store considerably less benefit from it in the evening. The thermal in New Zealand that are able to provide the than concrete for a given volume. If concrete time delays for rammed earth/compressed expertise required. blocks are used, they need to be fully grouted earth blocks is around 40 mm/hour, while for

Design and specification The glazing used can either be single or double. Computer modelling shows that there are only marginal benefits to be gained by double glazing (in terms of heat loss) for most circumstances. 100% More important is to look for high trans- 99% 93% mission glass – glass with a solar heat gain coefficient (SHGC) of 0.7 or more. SHGC is the total fraction of available solar radiation that 92% is transmitted through the window as heat 81% gain. Having a high SHGC ensures that a high percentage of solar energy gets transmitted OPTIMUM through the glazing onto the wall surface. If 330

o the glazed wall is difficult or inaccessible to o o 76% 300 0 30 reach from the outside, consider self-cleaning o 66% o 60 glass for long-term transparency. 270o 90o The size of the air space between the outer wall surface and the glazing should West East be between 15 mm and 50 mm and can be either vented or unvented to the outside. Keeping the gap to 20 mm or less means that the convective losses will be minimised. The benefit in venting is to reduce the risk of overheating during the warmer months by allowing for a breeze to exhaust the heated air outside. Thermally decoupling the footing from the ground using insulation is highly Figure 2. Percentage thermal effectiveness for differently oriented Trombe walls (bird’s eye view).

January 2018 v2 | Sustainable Construction #3 branz.nz 3 adobe, it is about 35 mm/hour. Exact figures In terms of maintaining the integrity of the are minimal, can the outside of the are very dependent on composition. wall, very little is required. The occasional glazing be cleaned easily? If the glazing Some early passive solar references coating (whether painting or staining) of isn’t fully sealed, access to the wall for suggested vents at the top and bottom of window mullions/framing may be neces- cleaning cobwebs should be considered. the wall to vent warm air from the front of sary. Ideally, occasional accessing of the air the wall into the room for early morning heat space should be provided for to clear out any Minimum criteria checklist for gain. It is now recommended that Trombe spiderwebs or repaint the dark wall surface. designers walls aren’t vented to the interior so they The operation of the vents will need to be For this technology to work properly, some heat up more during the day to provide checked and lubrication and maintenance key design issues need to be addressed. These more heat at night when needed and that provided as necessary. key issues are set out below and are minimum early morning heat gains are harvested from requirements for the viable application of direct sunlight entering windows adjacent Questions for the homeowner and Trombe walls in New Zealand homes. to Trombe walls. To assist the ability of the designer to discuss external wall to absorb the sun’s radiation, ●● It is extremely important that the Externally o it should be a mid-dark colour. exterior northern aspect of the ●● Trombe wall orientation between -30 Summer and shoulder season shading is Trombe wall remains unobstructed. and +30o of true north (not magnetic strongly recommended. This is especially What implications will this have for north). True north can be measured by important where good summer whole-house landscaping and vegetation over the using either a compass (corrected for ventilation is either difficult or not possible. next 10 years, 20 years and 30+ years? magnetic north) or by using the shadow External shading for midday direct summer What is the risk from neighbouring cast by a stick in the ground at noon sun is critically important, but mid-morning buildings (or trees) causing obstructions during winter (non-daylight-saving time). and mid-afternoon shading is important as during this time, and how can the risks ●● Winter shading: no shading of the well. Ideally, shading and ventilation should be successfully mitigated or managed? external Trombe wall for at least 70% be employed in the warmer months. However, ●● How has the seasonal shading of the of the winter day compared to an shading is more beneficial than ventilation. external wall been addressed – shading unobstructed horizontal skyline so that Fixed shading, where users don’t have in the summer to reduce overheating, sunlight is falling directly on the wall. to do anything, will be more reliable than and full exposure in winter to maximise ●● Summer shading: external Trombe wall movable shading devices. 3D shading studies heating gain? What about the shoulder fully shaded for at least 70% of the day, can easily be carried out using SketchUp or seasons – autumn and spring? preventing almost all direct sunlight. similar 3D design tools to ensure appropriate ●● What implications does ensuring an ●● Single glazing units installed with solar shading year round. unobstructed internal wall have on the heat gain coefficient (SHGC) ≥ 0.7. The Trombe wall interior surface must not circulation routes and adjacent furniture ●● A cavity space between the glazing and be covered with plasterboard or insulation layout? the external wall surface of 15– 50 mm. of any type to ensure that its radiative ●● What are the possible surface finishes ●● A darker-coloured external face. properties are not compromised. If it does to the internal wall to ensure good ●● The ratio of the Trombe wall’s volume (in need to be covered, use a conductive looks that last a long time and are m3) to the adjacent affected living area surface like shotcrete or solid plaster/stucco. easy to clean while complementing the volume (in m3) should be a minimum Alternatively, the surface can be painted, adjacent internal walls? (Due to cleaning 1:80 for concrete-based materials. plastered in cement-based plaster or even difficulties, uncoated concrete block is ●● The wall thickness to be around tiled as long as the material is directly fixed. If not recommended.) What opportunities 200 mm depending on time lag required painted, the colour is incidental to its thermal does this present in terms of polished and construction material desired. performance – the same amount of heat will concrete, exposed aggregate, painting or ●● The foundation footing must be be emitted whether the wall is light or dark. other methods of surface treatment? insulated from the ground with at least Finishes such as gypsum plastering, lining ●● What strategies have been adopted to edge insulation. with timber, cork, wallpaper or felt and/or ensure that good ventilation is possible objects such as pieces of artwork, flatscreen within the house for the warmer months? Internally TVs, whiteboards and the like should be For example, are there substantial ●● A wall that is at least 90% uncovered avoided. openings available on both the windward by items such as flatscreen TVs, artwork, and leeward side of the house to promote internal linings and so on. Living cross-flow ventilation? ●● A minimal (≤10% wall area) amount One of the real benefits of a Trombe wall is its ●● Although Trombe walls are usually of furnishings (couches, shelving units) low need for user adjustment to function well. constructed of concrete (whether tilt slab, pushed up against the wall. The only control needed is for shading and poured in situ or block), other massive ●● Located within living areas or even venting in the warmer seasons, if the glazed materials such as brick, rammed earth and bedrooms. wall incorporates adjustable vents. There are adobe can be used. What opportunities do many other ventilation options beyond the these other materials present in terms of Other considerations simple ones suggested in this factsheet, which aesthetics, cost, flexibility of construction ●● The Trombe wall is part of a wider are expanded on in the paper by Sadaatian et and maintenance? passive solar design strategy that al (see ‘Supporting information’). ●● Although maintenance requirements encompasses the whole house.

January 2018 v2 | Sustainable Construction #3 branz.nz 4 Figure 3. Cross-section of Trombe wall (not to scale). Figure 4. PlanTrombe wall (not to scale).

●● Use a 3D visualisation tool to examine independent and relevant information on Conference, Hong Kong, November 2013. year-round external shading of the Trombe walls and passive design strategies. ●● Saadatian, O., Sopian, K., Lim, C. H., Asim, Trombe wall. ●● Torcellini, P. and Pless, S. 2004. Trombe N., and Sulaiman, M. Y. 2012. Trombe ●● Use the opportunity presented by the walls in low-energy buildings: Practical walls: A review of opportunities and large wall for decorative purposes, experiences. Presented at the World challenges in research and development. such as exposed aggregate, unusual Renewable Energy Congress VIII and Renewable and Sustainable Energy aggregate types, coloured concrete, Expo Denver, Colorado August 29– Reviews, 16(8), 6340–6351. painted and so on. September 3, 2004. National Renewable ●● Consider the use of computer thermal Energy Laboratory. Colorado, USA. This factsheet is part of a BRANZ series on modelling to better understand the NREL/CP-550-36277. the best-performing, unusual space-condi- year-round comfort implications of the ●● Computer tools: SUNREL (www.nrel. tioning and micro-renewables technologies whole house. gov/buildings/sunrel), AccuRateNZ, for the New Zealand situation. and SketchUp (www.sketchup.com). Construction details Thermal modelling providers include The construction details in Figures 3 and 4 www.snughome.co.nz, www.settlement. detail a Trombe wall that has successfully co.nz and www.righthouse.co.nz. been providing free space heating for a ●● On the principles and strategies of number of years to its very satisfied owners. passive solar design: www.level.org. The house uses a concrete tilt slab Trombe nz (more technical focus) and www. wall covered with double-glazed windows. smarterhomes.org.nz (more general The external face is cast and painted flat focus); BRANZ Bulletins 540 Solar gain black, while the interior face is finished with in housing and 494 Thermal insulation an exposed aggregate. This is one viable of new houses available at www.branz. solution of many and is provided courtesy co.nz; the free Designing comfortable of architect Peter Olorenshaw. homes, by CCANZ and EECA at www. ccanz.org.nz/files/DCH_Book_WEB.pdf. Supporting information ●● Jaques, R. 2013. Uncommon energy These references are specifically chosen to technologies in NZ homes. Conference ensure that the designer and user can gain paper for 4th International ASA

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