We are all seduced by bling, and eco-bling is no exception. Hence the first thing many people will want to talk about for their new ecohouse is what renewable energy system to use, with the expectation of shiny solar panels on the roof and high-tech gizmos for the heating system. But the most important design feature of an ecohouse is not ostentatious, in fact you won’t even see it, it’s the energy efficiency of the building fabric. Hence the expression “fabric first”.
The concept of “fabric first” applies to the building materials, the “fabric”, that will create the building envelope, its external skin of walls, floor and roof. It means maximising the energy efficiency of the building envelope before considering the use of mechanical or electrical systems, with three key issues: insulation, airtightness, and thermal breaks.
When asked how much insulation is required for an ecohouse the simple answer is lots more than you are used to. The insulation value of a wall, floor or roof will depend on the whole assembly, not just the dedicated insulation, running from the plaster on the inside wall through to the cladding on the outside. It is calculated as a measure of thermal resistance, often referred to as U-value or R-value. To achieve the same U-value, you might need twice the depth of a natural insulation material like sheep’s wool as you would for a man-made insulation material like PIR foam.
Buildings have historically been very “leaky” structures, with airflow through construction joints and around door and window openings allowing for significant heat loss as heated air escapes and cold outside air gets in. Sealing the building envelope to minimise air infiltration is crucial to limiting this uncontrolled airflow. The solution is to ensure all construction joints are well sealed and that doors and window can be closed tightly shut. Airtightness is measured in air-changes per hour and an ecohouse should aim for less than 1 compared to 3-5 for a typical new-build house.
When building components with poor thermal resistance extend through different layers of the building envelope, especially where they run unbroken to the outside, they act as a thermal bridge allowing the heat energy to bypass the insulation. Steel and concrete are some of the worst examples of thermal bridges – just think about how cold steel is to the touch in cool weather. Thermal bridges occur at many different scales, right down to nails and screws, but are especially bad where the structure has to extend out to support balconies or cantilevered walls. The solution is to create thermal breaks, which just means a discontinuity of the poorly performing material – you can use a cavity, encase the item with some insulation, or insert special thermal barrier pads in structural joints. One of the best ways to understand this is that the whole building envelope should be wrapped in a continuous layer of insulation.