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A Simple Approach to Passivhaus

"The beauty of Passivhaus is that it's simple," says Gary Bundy, Technical Director at Sto Ltd. "It is possible to build a low carbon building by adding expensive renewables and hope that this technology performs as well as you'd anticipated. Or you can take the simple option and build a house that has excellent thermal performance, exceptional airtightness with mechanical ventilation that saves carbon simply by using less energy."

What is Passivhaus?

The definition of a Passivhaus is a building that can be heated or cooled solely by the small amount of fresh air needed to keep the occupants healthy. Since it is not possible to introduce a lot of heat into a building via the ventilation system alone, Passivhaus buildings have a very low heating demand. A building certified as complying with the Passivhaus standard will have a minimal space heating requirement of no more than 15 Watts per m² of floor area per year.

The Passivhaus standard was developed in Germany in the early 1990s by two professors. The first dwellings completed to the standard were constructed in Darmstadt in 1991.

The Passivhaus standard is sometimes confused with more generic approaches to passive solar architecture, with which it shares some common principles. But a Passivhaus is designed to have a heat demand which is as low as is practically achievable.

Despite its name, the Passivhaus "fabric-first" approach to energy efficient design can equally be applied to commercial, industrial and public buildings, as well as to residential properties. It can even be used on retrofit projects.

External wall insulation for Passivhaus projects

As the leading supplier of external wall insulation systems, Sto has worked with many of the UK and Ireland's Passivhaus pioneers on both domestic and commercial schemes. "There has been an exponential growth in Passivhaus construction in the UK and Ireland," says Bundy. "Two years ago it was individuals building one-off homes, but now the more innovative housebuilders, building perhaps 50 homes a year, are adopting the standard as a marketing attribute."

Bundy says that in the future, mainstream housebuilders will be looking to the Passivhaus fabric-first approach to help meet more onerous carbon targets proposed in changes to the Building Regulations. "Even for projects not aiming for Passivhaus standards there will be a massive swing towards the use of external wall insulation. If you've got to put a 250mm depth of insulation on a wall, designers will struggle to get it to fit in a cavity."

Materials & Design

Sto can supply external wall insulation up to 400mm thick for Passivhaus projects. However, the thickness of insulation is only part of the solution: "Passivhaus construction is not simply about sticking a big wodge of insulation on the outside of a building. What makes the system work is getting the detailing right around the windows, doors and balconies where significant amounts of heat can be lost," says Bundy.

Sto's Passivhaus details were developed in Germany and are Passivhaus Institute approved. On a typical Passivhaus scheme, Bundy says Sto would work with the designer or architect to develop the details specifically to take account of the insulation type, its thickness and the type of finish for a particular project. "We will provide the interface details between the insulation and other components."

For developers wanting a Zero Carbon building, rather than a low energy Passivhaus scheme, elements of Passivhaus principles can still be useful as a basis for the design. Rather than throw expensive renewable technologies at a scheme, it will often be significantly cheaper to focus on reducing the building's energy demand by getting the fabric right. This is achieved by applying Sto external wall insulation so that only modest amounts of renewables are needed to make a building Zero Carbon.

Other material solutions for Passivhaus projects

In addition to the external wall insulation system, Sto also supplies internal finishes that are ideal for Passivhaus projects. "For projects that have high levels of exposed thermal mass and hence a lot of exposed hard surfaces, we can supply acoustic finishes, such as the StoSilent acoustic ceiling, to absorb sound and reduce reverberation times," Bundy says.

Sto also manufactures a silicate based interior finish, StoColor Sil In, which is particularly useful in helping to prevent condensation at times of peak use in areas like kitchens and bathrooms. Because the ventilation in a Passivhaus is so carefully controlled, the paint acts as a buffer and temporarily holds water vapour until the building's ventilation system has the capacity to remove it.

Bundy says his advice for anybody considering a Passivhaus building is to use a Passivhaus expert to develop the design. "We provide an important part of the solution and we do have the knowledge to develop appropriate details to comply with Passivhaus standards. For a scheme to be successful, that insulation has to be part of an appropriate, holistic design solution."


Glencullen House

Glencullen House is a certified Passivhaus located in the Wicklow Mountains.