“A building in which energy demand is reduced by: highly insulated fabric, airtightness and Mechanical Heat Recovery Ventilation”;
“Uses the free thermal gains delivered by solar irradiation externally plus thermal gains from appliances and occupants internally”;
“Achieves economic thermal comfort in summer and in winter without needing a conventional perimeter heat distribution system”;
“The fresh air supply distributes the remaining space heating required solely by post heating (or post cooling) of the fresh air mass, which is required to meet regulatory indoor air- quality conditions”.
It is essential that under EU climatic conditions the building does not exceed:
(a) 15 kwh/(m²a) space heating requirement;
(b) 10 w/m² constant heat load;
(c) 42 kwh/(m²a) annual total amount of active energy;
(d) 120 kwh/m²a total energy required for space-heating, domestic hot water and household appliances.
The simplified explanation
If physics is not your thing, then the following explanation might be easier to follow:
Passive Houses offer a status whereby a comfortable interior climate can be maintained without active heating and cooling systems. They assure comfortable indoor climates all year round, without needing a conventional heat distribution system. To permit this, it is essential that, under climatic conditions prevailing in Central Europe, the building’s annual space heating requirement does not exceed 15 kWh/(m²a).
The amount of heating required is so small that it is said that a 4 x 5 mtr room could be heated with 10 tea lights in winter, but the reality is that the individual characteristics should at least be optimised to allow for local conditions.
It is still a voluntary code
It is sometimes hard to remember that Passive House standard is still a rigorous voluntary standard for energy efficiency in a building, and is connected to the reduction of ecological footprints, but it is worth noting that local governments and municipalities have the option to add the standards into their building codes, especially as many have signed up to the Net Zero Ready standard of building and construction.
The Passivhaus Institute set the standard of insulation for walls at a U-value of 0.15.
The definitions of standard vary so much that they have become known as: Official, Semi-official, and unofficial.
Lines have also become blurred between Passive schemes and low energy schemes. In Germany for example, they have RAL Passive and Passive House, the latter including measurements and accepted standards for building components such as heat exchangers and pumps.
In Germany they have produced a chart rather like our energy ratings charts on domestic appliances, and the A++ rating is for a wall U-value of 0.1 and the A+ rating is for 0.15.
So what is the standard in the UK now?
Homes built to the Passive House standard in Ireland or the UK typically include a wall U-value of 0.15 W/m2K or better. A 2016 analysis by Passive House Plus of data from SEAI’s National BER Research tool revealed that the average U-values for new Irish homes have been dramatically improved as a consequence of tightening building regulations – with respective average figures for walls, roofs and floors of 0.17, 0.13 and 0.14 W/m2K, though the backstops stipulated in the regulations are less ambitious. U-value requirements in building regulations in the UK lag some way behind at present.
In the Passive House premier designer’s guide they state that “although the recommended standard is 0.15 in a North facing / increased shading aspect it is better to be nearer 0.1 in order to maintain the HLP (Heat Loss Parameter)”. If this is not possible then they recommend glazing with installed U values of 0.85 and good solar transmittance “g” values of 0.5.
The International Passive House Association say the U-Value should not exceed 0.15 but would recommend aiming for a U-Value of 0.1 as ideal.