Application

Passive solar design

The beautiful south

The interaction of south-facing windows, shading and thermal mass can reduce a home’s energy demand by up to 40%. Tom De Saulles explains how

Passive solar design (PSD) is a simple technique that helps capture the sun’s energy, reducing the need for space heating from autumn to spring. It works by combining appropriate building orientation and window size with thermal mass in the building fabric, which collectively enable sunlight to be absorbed and used as a source of heat. Studies have calculated potential energy savings of around 11% in conventional masonry and concrete homes, with greater savings of around 40% (35% in Scotland) where more specific design features are applied, such as the use of sunspaces.

In spite of its recognised benefits, uptake in mainstream UK housing has been few and far between, partly due to building regulations that do little to encourage passive design, partly due to a housing industry that prioritises density and generic design over orientation and spacing, and partly due to the absence of appropriate shading on south-facing windows, which has resulted in the common misconception that a southerly aspect automatically increases overheating risk.

However, this may change over the coming years, as renewed focus is turned to passive performance. Revisions to Part L and the introduction of the Futures Homes Standard are likely to increase emphasis on dwelling orientation in response to greater uptake of photovoltaic panels, which should ideally be south-facing. Meanwhile, the need to reduce overheating risk should lead to a regulatory requirement for proper shading on south-facing windows, ideally in the form of deep roof eves, balconies and overhangs – all capable of keeping out the high summer sun without limiting solar gain during the heating season.

Design considerations

In its simplest form, PSD can be implemented by increasing the level of glazing on the south elevation so it is roughly twice that on the north elevation. North-facing windows have a net heat loss over the year, so should be sized to just provide adequate daylighting. Conversely, south-facing windows experience a net heat gain over the year, so should be sized to take advantage of this.

Designing to take advantage of PSD requires an integrated approach to find the best overall balance of glazing, orientation, and thermal mass. However, the general approach can be summarised as:

  • A southerly orientation to allow passive solar gains from autumn to spring
  • A sufficiently clear view of the sky from the south
  • A high standard of insulation and airtightness
  • A medium to high level of thermal mass
  • Well-insulated double glazing that combines an inner pane designed to reduce heat loss and an outer pane of extra-clear glass to increase the amount of free heat from the sun
  • Windows or an alternative means of passive ventilation that provide cooling on summer nights, while taking account of any security or noise issues
  • A compact rectangular plan with the main living area on the south side of the house.

Orientation and shading

Orientation is the most critical factor in determining the amount of sun that a dwelling receives from autumn to spring. Ideally most of the window area should face within around 30° of south, with the south elevation enjoying a relatively clear view of the sky, to allow radiation from the low winter sun to pass directly inside.

In midwinter, the sun reaches a maximum altitude above the horizon of about 17º in southern England. During the height of summer it will reach about 64º, which can be particularly useful when designing a shading strategy, as a simple overhang will block the sun during the hottest part of the day. This very simple form of shading requires no user control or maintenance.

Dwelling location and spacing

The further north you are, the lower the solar gain. However, the heating season is longer, so the benefits of PSD could be more significant. A house in Scotland, in an average year, will require nearly 45% more energy to maintain a given temperature than in south-west England. Greater emphasis could be placed on winter performance in the north and more effective solar shading in the south.

To avoid overshadowing, the spacing of dwellings also varies between northern and southern Britain. Based on average house height, minimum spacing is 20m in Southampton and 25m in Leeds, increasing to 35m in Inverness. Where an obstruction is likely to reduce the amount of direct solar radiation, some heat is still obtained from diffuse and reflected radiation.

Thermal mass

A medium to high level of thermal mass is most easily and cost effectively provided by concrete and masonry floors and walls with a suitable finish that does not impede heat flow. As a rough guide, the surface area of the fl oor/soffi ts and walls providing the mass should be at least six times that of the glazing in the room, although this will to some extent be infl uenced by the particular thermal capacity and conductivity of the material. So, as the area of south-facing glazing increases, more thermal mass is required to maintain a stable temperature during the summer.

The position of the insulation is also very important, as the thermal mass needs to be located inside the insulated building envelope. In practical terms, a masonry cavity wall already satisfies this basic rule, as the insulation is located in the cavity, allowing the inner leaf of blockwork to be room side. For solid masonry walls, the insulation should be located behind the waterproofing layers on the outer surface of the structural wall. The insulation for solid ground floors should be located under the slab, although screed placed on top of insulation will also provide useful thermal mass.

Internal finishes

It is important that the surface of heavyweight walls and floors remain as thermally exposed as practicable. For walls, this is best achieved with a wet plaster finish, which conducts heat relatively freely, as well as providing a robust air barrier that will help minimise air leakage. Dry lining will reduce heat flow, but its impact will depend on the thermal mass potentially available in the wall. For an aircrete block inner leaf (which has relatively low thermal conductivity), plasterboard is less of a thermal bottleneck than for heavier aggregate blocks. These have higher thermal conductivity (and thermal mass), and if their full potential is to be unlocked, the choice of finish will have more impact. With some forms of concrete wall and floor construction, it is possible to achieve a high-quality, visual finish with little more than a coat of paint.

Wherever feasible, the thermal mass of the ground floor (particularly in south-facing rooms) should be optimised, with carpet avoided. Stone, ceramic or porcelain tiles are useful, as is exposed concrete. Shiny or glossy floors will absorb less heat than a dull finish; however, this must be evaluated alongside daylighting requirements and the tendency of such a surface to absorb light. These surfaces also work well with underfloor heating.

Layout

Where practicable, the most frequently used rooms should be on the south side, with bathrooms, utility rooms and halls to the north. In southern England, bedrooms are usually best located on the north side to help maintain comfortable night-time conditions during summer. Where possible, the layout of bedrooms should also enable cross-ventilation or stack ventilation, particularly effective in lowering the internal temperature during summer and removing heat from the thermal mass.

Tom De Saulles is a building physicist at The Concrete Centre

Photo caption Gusto Homes’ Woodlands Edge development near Lincoln. The majority of glazing is on the south side, maximising solar gain and minimising heat losses from north-facing elevations

Photo Peter Segasby