Thorne Wyness Architects

Passive House

What is a Passive House?

Passive House is an advanced low energy construction standard that provides excellent comfort with minimal heating or cooling requirements.

What are the main advantages of Passive House design?

  • A high level of comfort is the main aim of a Passive House design. Indoor air temperature is maintained at a minimum of 20°C and surface temperature at 17°C. This is achieved by using high quality windows and highly insulated walls, roof and floor slab to keep the building warm in Winter, and cool in Summer.

  • Heating demand is minimised because Passive houses are super insulated buildings with a continuous 200mm(min) layer of insulation with no gaps, and triple glazed windows.

  • Mould growth on internal surfaces is avoided as the surface temperature is more consistent with room temperatures. Mould can grow at temperatures of 12 degrees and below, while in a Passive house building internal surface temperatures should be in the region of 17 degrees, even in corners.

  • The heat losses of the building are reduced so much that it hardly needs any heating at all. Passive heat sources like the sun, human occupancy, household appliances and heat recovered from the extract air using a Mechanical Heating and Ventilation System (MVHR) cover a large part, if not all of the building’s space heating demands.

  • The Mechanical Heat recovery and ventilation system (MVHR) supplies constant fresh air, making for superior air quality without unpleasant draughts. Passivhaus buildings have fully openable windows, but provide excellent winter ventilation without wasting heat. An energy-efficient fan draws the used and humid air from the kitchen, bathroom and toilet and blows it towards the outside. A second fan brings fresh air into the living room and bedrooms. The two air flows are conducted past each other in a heat exchanger so the heat of the used air can pre-heat the fresh outdoor air “passively”.

  • By concentrating on fabric-first measures to reduce energy consumption, alternatives to fossil fuels become possible, meaning that a zero-carbon building is relatively easily achievable. The European Energy Performance of Buildings Directive requires all new buildings to be nearly zero-energy by the end of 2020.

A video explaining Passive House in 90 Seconds can be found here.

Passive House Key Principles

Passive House Criteria

  • The building envelope: insulation

    To minimise heat loss, a super insulated, continuous thermal envelope without breaks, acts like a wooly jumper around the building. All the elements of this envelope must be high performance insulation materials and external walls, roof and floor slab need to achieve U-values of up to 0.15W/(m².K) to minimise heating demand.

  • Airtightness

    The airtight layer has an important role in Passive House buildings. To reduce heat losses from draughts and protect the building elements from condensation, a continuous airtightness layer should be maintained. This air tight layer must be tested under pressure and achieve a maximum of 0.6 air changes per hour for new build and 1 air changes per hour for retrofit.

  • Windows and solar gain

    High quality windows play an essential role in the overall success of the building fabric to ensure thermal comfort for occupants. Passive House windows and doors require U-values of up to 0.8 W/(m².K) and have a high total solar energy transmittance factor of 50-55 % which contributes to the reduction of the space heating demand.
    In the UK, this is achieved by using high performance triple glazed windows with low-e or equivalent glass, an insulated glass edge bond, and highly insulated frame. It is important that attention is paid to the installation detail of the windows to reduce thermal bridges.

  • Solar gain and Solar shading

    Generous glazing areas facing South are recommended as high quality triple glazed South facing windows can provide a net heat gain over the heating season. In some climates, windows to the South, East and West should have sun protection positioned on the exterior of the building so that summer solar loads, which can cause overheating, can be minimised. Internal temperatures greater than 25 degrees are limited to less than 10% of the year in order to avoid overheating, that can also be reduced through cross ventilation and night ventilation. The design of the MVHR system is key in the success of a Passive House build.

  • Thermal bridge free detailing

    A thermal bridge is a localised area in the thermal envelope where heat flow is increased compared to adjacent areas of the building resulting in localised reduced surface temperatures which lead to increased heat losses, mould and even condensation. Passive House buildings, require thermal bridge-free design and construction to prevent condensation and reduce heat losses. This means paying attention to details and common building faults and avoiding penetrating the insulation layer.

  • Fresh air with heat recovery

    Proving optimal indoor air quality for the comfort and health of occupants is a key principle. Opening the windows twice a day, as it usually happens in traditional buildings, does not allow sufficient air changes and can be very uncomfortable when the outside temperatures are extreme, and causes an increased energy use. Trickle vents are not a good solution as usually they are not used or maintained correctly and result in a significant heat loss. Passive House buildings require constant supply of fresh air and extract of stale moist, which is achieved through a highly efficient Mechanical ventilation with heat recovery (MVHR) system. The two air flows are conducted past each other in a heat exchanger so the heat of the used air can pre-heat the outdoor air “passively”. The heat recovery rate must be at least 75% so that the efficiency as well as room comfort can be guaranteed. The total ventilation electricity demand should not exceed 0.45 Wh/m². A high performance unit can save 90% of winter warmth.

  • Indoor air

    Indoor air temperature is maintained at a minimum of 20°C and surface temperature at 17°C. Keeping internal surface temperatures similar to indoor air temperatures, even in the face of extreme outdoor temperatures, this prevents mould which grows on internal surfaces with temperature below 12°C.

  • Renewables

    A Passive House building obtains a large proportion of its low energy requirements passively from solar gains and internal heat gains. However, in addition, a significant proportion of its water heating or space heating requirements can be derived from renewables as solar thermal panel.

  • Appliances

    Electrical appliances and lighting are designed as energy efficient.

  • Detailing and construction

    An extremely precise and detailed approach to design and construction along with quality workmanship is crucial at all stages to achieve thermal bridge free, airtight construction.

Passive House misconceptions

  • “You can’t open the windows”

    This is the most common Passive House myth. The mix-up seems to be that, with a Passive House, you don’t NEED to open the windows. The ventilation system works to bring the perfect amount of fresh air in so that there is no stale air in the home. You can open the windows in the winter if you want, but this bypasses the MVHR unit so you will need to turn the heating up to compensate. However, opening the windows can be helpful in summer to avoid overheating. Comfort is the main aim of Passive House criteria.

  • “Passive houses have no heating”

    A Passivhaus, as any other building, needs to get the heat from somewhere. The difference is that Passivhaus buildings retain heat much better than traditional buildings so the heating systems required are much smaller than in traditional buildings. Sometimes, adding a post-heater to the MVHR system is enough to cover the heating demand.

  • “Passive House system isn’t well developed yet”

    The ideas at the heart of Passive House date back to the 1970s. The first true Passive House was built in Germany in 1991 and more than 40,000 buildings have been built to Passive House standards since then.

  • “The MVHR system is noisy and creates drafts”

    This is not a problem if a high quality system is used and it has been designed and installed correctly.

  • “You can suffocate if the MVHR stops working”

    It would get a bit stuffy if you didn’t open a window, but the house is not that airtight.

  • “Passive Houses are too expensive”

    Passive House walls, roof, floor, windows and ventilation system cost more than traditional houses while heating them costs less. On balance, a Passive House costs around 10% more than a typical house but requires an average of 90% less heating energy.

  • “Heat recovery ventilation costs more to run than the energy it saves”

    When HRV systems are properly installed, the ratio of electricity required to heat loss prevented is 1:10 or better. In other words, the ventilation system saves more than 10 times the energy that it requires for operation.