Homeowners looking to build their own house, or substantially renovate an existing property, might assume that meeting the building regulations applicable to their location is the best result they can aim for.
In truth, however, building regulations are a minimum standard – and that often shows in how buildings perform in use.
The Passivhaus standard was developed by a German physicist, Dr. Wolfgang Feist, who wanted to understand why buildings continued to perform poorly despite the addition of more and more insulation. He recognised that something must be failing in the way buildings were designed and constructed, and set out to find some answers.
Although it remains a relatively niche area of interest, awareness of the Passivhaus standard – and its retrofit counterpart, EnerPhit – is growing. Becoming mainstream can only be a good thing, not just because these standards deliver comfortable, energy efficient buildings, but also because there tends to be a lot of misconceptions.
The standard Dr. Feist developed is both a comfort standard and an energy efficiency standard. By constructing a highly insulated, airtight building with carefully controlled mechanical ventilation, that uses solar gains, it aims to:
● keep surface temperatures throughout the building at a minimum of 17 deg.C;
● keep the heating demand at a low and predictable level.
Because of the airtightness and controlled ventilation, there are no draughts to cause occupants discomfort. Windows can be opened if desired – a popular misconception! – but building users generally find they don’t need to, because the internal environment is a healthy, comfortable and pleasant one.
Evaluation studies of Passivhaus projects – including the first one ever constructed several decades ago – show they achieve their designed levels of performance, and continue to do so. The standard is the ultimate example of a fabric first approach, with a demonstrable ability to eliminate identified performance gap issues.
Because of the performance levels required, there are only a limited range of Passivhaus-certified roof windows on the market. As well as exceptionally low U-values, they feature airtightness and windtightness skirts, and insulated flashings to make sure they contribute to the overall quality of the building fabric – but there is an issue to be aware of.
Doors and windows form part of a building’s insulation envelope. To maintain continuity of that envelope, it is recommended to install them along the line of the insulation. In a pitched roof, the frame of a roof window is installed to the outside of the roof – and, in many cases, outside the line of the insulation.
As a result, the heat losses around a roof window can be up to ten times greater compared to a standard window installation in a wall. From a Passivhaus point of view, that makes roof windows a thermal weak point, but one that can be accommodated with the right planning and modelling of the proposed construction.
Whatever energy standard is being worked to, and whatever level of construction quality is delivered, it is essential that insulation in the building fabric is detailed and installed appropriately around a roof window. More information can be found in the NARM (National Association of Rooflight Manufacturers) Technical Documents.