How to build a passivhaus vet surgery

We need to be building and retrofitting our houses and buildings to the highest possible standard and reducing energy use in them to as close to zero as possible. The highest building standard is passivhaus, a low energy building standard developed in Germany 30 years ago. Spreadsheet based software is used to calculate all the energy losses and gains from a building in order to design a structure that has incredibly low heating and cooling requirements and is a healthy, comfortable space in which to live and work. We decided to build our new surgery to Passivhaus standard and also to use the most eco-friendly materials available.

An old pub, The Oddfellow Arms, stood on the site of the new Vet Surgery. After initial attempts to find a way to use this old building and retrofit it to Passivhaus, it was realised that there were too many issues with the structure; it was decided to demolish it, reclaim where possible, and use the crushed material from the building to provide a solid base for the new one.

Passivhaus Planning Package Software was used early on in the design of the new surgery. The building was designed with a good form factor – a compact shape to reduce energy use – and the software was used to decide on insulation requirements and orientation and specification of windows and doors. To meet Passivhaus standards a building has to demonstrate use of only 15 watts/m2 heating demand and must reach airtightness levels twenty times better than UK building regulations. The planning package tells you if you are meeting this energy requirement.

A base known as a passive slab was laid down on the well compacted and levelled sub-base made from crushed demolition material. This consisted of 30cm of polystyrene with a lip. Reinforcing mesh and heating pipes are laid next and then eco-concrete was poured in to form the floor and provide a base for the building. This system enables 360 degree insulation and avoids the need for strip foundations thus reducing the amount of concrete used by fifty percent.

An off-site manufactured wooden frame was delivered in modular form and craned into position and fixed down by the team from MBC Construction. The twin-wall construction was made up of two connected timber stud frames with airtight smartply on the inside and a breathable board outside. This left a cavity of 35cm.

The roof was formed using timber I-beams and a glulam ridge: engineered timber products with exceptional strength.

A standard tiled roof was put on with 10kw of integrated photo-voltaic panels, which should provide the vast amount of energy this building requires.

High spec timber Green Building Store triple glazed windows and doors were fixed in place. As more heat is lost from roof windows, quadruple glazed Fakro roof lights were used. These have to be sealed in place with special airtightness tape.

The inside of the walls floor and roof were sealed with airtightness tape and any penetrations of the external walls for services were sealed. A blower door test has been performed which measured air leakage at 0.33Airchanges/hr, well within the 0.6ach requirement for passivhaus. A service void runs internally to allow cables and pipe runs without penetrating this airtightness barrier.

A heat recovery system (MVHR) was fitted by running ducting through the internal ceiling void to supply fresh air to op, prep, waiting and staff rooms. Ducting, extracting stale air, was run from kennel, consult, kitchen and bathrooms. The heat is transferred from the warm stale air to the fresh incoming air in a heat exchanger in the loft.

Mechanical ventilation (MVHR) - Thermix UK

Once services had been installed external walls were filled with recycled cellulose insulation, a product known as Warmcel, with excellent thermal and acoustic properties. It also helps to avoid summer overheating. Internal walls and external walls were also filled with this material.

Walls were lined with a board called Fermacell: made with recycled paper and gypsum. These boards are much tougher than plasterboard so are very suitable for a Vet Surgery.

The outside of the building was lined with wood fibre boards and then either rendered with lime mortar or clad with Scottish Larch. The building is heated by an airsource heat pump, this uses one unit of electricity to make 4 units of heat. The new surgery is predicted to use about 7000kwh of electricity while generating around 8000kwh per annum. In the last six months we have generated 3500 kWh and used 1650 kWh. So far so good!

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