Working with Nature
The fundamental principle of sustainable home design is to work with nature rather than against it. In particular, this means harnessing the sun’s energy to help heat the house in the cooler months whilst also preventing it from overheating in the summer. This can be achieved through the careful siting of the building, balancing passive solar gain from glazing, screening to reduce overheating and to help even out the temperature across the day, incorporating thermal mass in the building’s structure and high levels of insulation and airtightness.
The characteristics of the plot and planning situation will play a big part in informing the type of sustainable home design that is most appropriate.
If it is possible to have the majority of the glazing facing due south, then there is the potential to build a passive solar house, which uses the sun’s energy to provide most of the space heating requirement. North-facing elevations should include minimum glazing and maximum insulation. If the best views are from a different direction, then a super-insulated airtight house might be a better approach.
If the site has a slope, the building could be sited so that the north side is partially or fully built into the slope to protect it from the elements. The earth’s thermal mass and constant temperature can balance out the house’s climate. The distinctive sloping, south-facing glazed atriums of many passive solar houses arises because the angle of 45° is the optimum for maximising solar gain across the year. But large areas of glazing can be incorporated into homes of other styles — even a classical façade.
With super-insulated houses, the approach to orientation is different: the heat requirement may be so low that cooling becomes more of an issue than heating, and so too much southfacing glass is avoided.
Overheating can be reduced by screening out light. The sun is at its highest in the south, so for summer heat it’s straightforward to design a small roof overhang, or brise soleil — horizontal slats over the windows. Low, powerful light from the east or west can be harder to screen out, so think about using planting, blinds or shutters.
The ‘Mass and Glass’ Approach
Though a net source of energy gain during the day, even with the most energy-efficient glazing units available, the large area of glass in a passive solar house is a net source of heat loss at night. Passive solar design overcomes this by incorporating a high level of thermal mass in the building — the so called ‘mass and glass’ approach.
Building using solid concrete floors and walls using concrete block, stone, brick or similar heavyweight materials creates thermal mass that can store heat, helping keep the house cool during the day and stay warm at night.
An alternative to building the entire house from dense materials is to build a lightweight, airtight house, but include a solid floor laid with a material such as stone or tiles, and a solid internal wall facing due south to capture and store the sun’s energy.
Buildings with a high thermal mass lend themselves to low-temperature heating systems such as underfloor heating. The downside is that the building is kept warm all day whether or not the building is occupied.
A household with an irregular occupancy pattern might better suit a building with a very light thermal mass and a very low heat requirement — such a building will heat up very quickly when required and cool down when unoccupied.
Creating a Comfortable Environment
Warming or cooling rooms by just a few degrees can make a huge difference to comfort levels, but can cost a lot of energy to achieve, so designing the room plan to ensure that rooms which require little or no heating are in the north, east or west side of the house, and those that require most heat are in the centre is a good design principle.
Colder rooms include adult bedrooms, storage areas, utility rooms, larders, and integral garages. Bathrooms do not have to have natural light, but need to be warm so can be located in the centre of the house, although principal bathrooms probably need a window and view.
The main living rooms must be warm and need the light and views, so are best facing south. Woodburning stoves, chimney stacks and the kitchen cooker are best located against a solid wall in the heart of the house where their warmth will be stored rather than lost to the outside.
Controlled ventilation is an essential design feature in maintaining a comfortable temperature and a healthy damp-free environment. There are two principal approaches to ventilation for eco homes: passive or powered.
The low-tech approach is to use passive ventilation — harnessing the principle that warm air rises through convection to remove stale damp air, controlled via humidity-sensitive ducts. The system needs to be carefully designed (Passivent is the big brand name) and uses plastic ducts that have to be near vertical.
If the building is sufficiently tall and airtight, heat recovery can be incorporated. Some houses, especially passive solar designs, also incorporate a ventilation tower, which can harness convection to provide a simple form of air conditioning in hot weather, controlled by opening and closing vents to draw in cool air.
The hi-tech approach is to make the building shell as airtight as possible, and to have mechanical ventilation with heat recovery. Low-tech equates to low cost and has no ongoing running cost — although some research has suggested that passive stack ventilation will not work on days when there is absolutely no breeze; however, this can be overcome by adding a small solar-powered fan.
Mechanical ventilation is more expensive to install and requires constant power, but this is mitigated by heat recovery in the colder months via a plate heat exchanger and the ability to control air quality.
The heat requirement of a sustainable house will be very low, so the impact of the heating system is greatly reduced in terms of CO2 output. Space heating options include ground-, water- or air-source heat pumps powering underfloor heating, solid fuel stoves with a balanced flue, a biomass boiler, or the most simple approach — a small gas condensing boiler. Domestic hot water is likely to be the biggest heat load requirement, and this can be met using solar panels, plus a heat pump, or a biomass or gas condensing boiler.
The Right Materials
A truly sustainable home does not look solely at energy and water conservation, but also takes into account the environmental impact of the materials used in its construction. As a general rule, the less energy used in the production and transport of products and materials the better.
However, energy consumption can be mitigated in products such as solar panels and other renewables, and also where materials have a very long lifespan, and can be reused or recycled, such as lead, copper and other metals, and even concrete and clay brick and roof tiles.
Timber must be sourced from managed forests and should be certified as such, and not be from endangered species.
With all materials, the less waste and pollution created in the manufacture the better. The principles of sustainable construction are closely aligned with those of the healthy home, whereby materials are free of harmful chemicals, such as volatile organic compounds (VOCs).