Our enthusiasm for sustainability and low energy buildings has lead Keri to become a certified Passivhaus Designer. The main benefits of following the Passivhaus criteria are the very low energy bills and a high level of occupant comfort. When combined with sensible material choices, the Passivhaus standard can also go a long way to address the need for reducing carbon emissions – it could even provide a viable option for a zero-carbon building.
Our ‘Energy’ service is modelled on the Passivhaus Planning Package (PHPP). Whether you intend to build and certify your new build project to the Passivhaus standard / or retrofit your existing building to the EnerPHit or AECB Retrofit standards – or use the Passivhaus principles to achieve a low-energy building – we can support you with advice, design and specification.
Passivhaus is a rigorous voluntary energy-based standard. If you’ve not heard about Passivhaus before, it is a construction standard developed by the Passivhaus Institute in Germany for highly energy efficient buildings – the basic principle is to reduce the heat losses of the building to the point that it hardly needs any space heating at all. The sun, occupants, appliances and the warmth from extracted air cover most of the heating demand. High levels of insulation with minimal thermal bridges combined with an airtight envelope and the calculated design of windows and services are all critical to the buildings performance. The standard can be met using a variety of design strategies, construction methods and technologies and is applicable to any building type.
If additional heat is required, a small efficient backup system (ideally using a renewable energy source) can be used to boost the temperature of the fresh air supplied to the house. It is not possible in a Passivhaus building to disguise an inefficient building behind renewable technologies; but renewable technologies may be added to the low energy building to further enhance performance.
If you’re thinking…why should this be relevant to me, I’m only extending my home. Well, then we encourage you to think more about retrofitting your home, or at least plan for retrofitting over a period of time if it doesn’t make sense to do it all at once. A deep retrofit follows the Passivhaus principles and is something we encourage all clients to strongly consider.
Achieving the Passivhaus Standard in the UK typically involves:
The design must be looked at holistically to incorporate these five fundamental principles to ensure the energy efficiency of a Passivhaus or low-energy building.
The building envelope is what separates the inside of the building from the outside – these are the outside walls, roofs and floors. Where inside air is heated to keep the building comfortable, some of that heat will be lost as it moves through the envelope. To reduce this heat loss, we use more insulation in the wall, floor and roof assemblies. We go a good way beyond the thermal performance that is required by the current Building Regulations.
Insulating to Passivhaus levels has the added advantages of greater soundproofing, improved durability, and greater building resiliency—including the ability to maintain interior comfort for extended periods even if there is a power failure.
Achieving Passivhaus levels of thermal performance is not just about how much insulation you have, but whether that insulation is used effectively. Insulation is most effective when it wraps the building uninterrupted by other materials, but there will always be areas where this is not possible, such as around structural components. When a material bypasses the insulation, it is known as a thermal bridge and can significantly reduce the effectiveness of insulation, especially if that material is very conductive, like metal.
Minimizing repeating thermal bridges and aiming for continuous insulation where possible, helps to make the most of the insulation within the building envelope.
Heat can also be lost through the envelope via air leakage. A building’s air barrier is a layer of material around the inside of the envelope that restricts the movement of air in and out of the building. Gaps in the air barrier can allow air to move in and out of the building uncontrolled; they occur when there is insufficient detailing during construction, when there are numerous ducts or other penetrations in the air barrier, or when construction is of generally poor quality.
High volumes of uncontrolled air leakage can lead to a whole host of problems, including increased energy use from having to repeatedly reheat the air, discomfort from cold air drafts near the walls and potentially localised moisture and condensation problems. While air exchange is necessary for ventilation and providing fresh air, it is far more effective to control air exchange by tightening the envelope and using mechanical ventilation.
There are strict design and construction requirements for a Passive House project to be certified airtight. Quantitatively, this means that when tested the building needs to have less than 0.6 air changes per hour, at a test pressure of 50Pa, to achieve Passivhaus certification. This allowance is significantly lower compared to the Building Regulations, which is about 10 air changes per hour. A visual comparison: imagine half a parking space (5sqm) with a 5p coin – that 5p coin is the biggest hole a Passivhaus is allowed to have for that amount of area // Building Regulations allows a hole that is 5x 20p coins in size in that same area.
Achieving this degree of airtightness requires very careful planning in the design stage and once on site, quality control in the installation of the air barrier is critical.
While the walls typically make up the largest area of a building’s envelope, the glazing systems (windows and glazed doors) can play an even bigger role when it comes to contributing to space-heating energy. Due to their function (providing light and visibility), glazing systems cannot be insulated to the same degree as a wall, resulting in the windows being the weakest areas of the envelope in terms of resisting the flow of heat. Therefore, it is very important that high-performance glazing systems are used to reduce that heat flow as much as possible.
A well designed building will have windows appropriately placed to take advantage of free passive heating from the sun. Solar heat gain through these windows can help offset the amount of heat a building needs during colder months. During the summer months, this needs to be counteracted with shading to prevent overheating. For each project, there will be an ideal number of windows that can balance the advantage of free heat from the sun with minimising the heat loss from having too many windows.
Due the highly insulated envelope, thermal bridges need to be minimised. Where they are not, heat is allowed to flow through the envelope, reducing the benefits of the super insulation. Thermal bridges create cold surfaces which increases the risk of condensation and mould growth at their locations.
The easiest way to avoid thermal bridging is to avoid complicated junctions and forms. Where there are balconies and canopies, they should be self supporting. Where such details cannot be designed out, the affect needs to be carefully calculated and additional insulation may need to be designed in to counter balance the heat loss.
Eliminating or minimising thermal bridging on Passivhaus projects helps ensure the effectiveness of the envelope performance in reducing space-heating energy use.
Since Passivhaus projects are airtight, a controlled ventilation system is needed to bring in fresh air and exhaust out built-up pollutants, odours, CO2, and moisture. During winter, this means dumping out warm air and bringing in cooler air that needs to be heated up again, which increases the heating energy. Mechanical ventilation with heat recovery (MVHR) is used to continuously remove stale or moist air and deliver fresh air. During this process, it extracts heat from the exhaust air and puts it into the incoming air without directly mixing the airstreams together. This way, all the heat in the exhaust air is not completely lost to the outside.
For warmer summer months, the ventilation system diverts air around the heat recovery, so that fresh, unheated, air is still brought into the building.
While Passivhaus projects can still be fitted with a heating system (such as air source heat pumps, or boilers) having heat recovery in ventilation can greatly reduce the size, capacity, and maintenance needs of this equipment, shifting project costs from the mechanical systems to a superior building envelope.
Read more on the Passivhaus Trust website, or call us to chat about it!