Passive House Building Fabric

A Passive House buildings fabric has the following components and design considerations incorporated to ensure that it performs in accordance with the standard.

Super Insulation - The building fabric is one of the most important factors of a successful Passive House construction. The insulation surrounding a Passive House building should be applied without gaps, creating a continuous thermal envelope around the entire heated area of the building, similar to a giant sleeping bag. These super insulated wall, roof and floor elements play a large role in reducing the amount of heat loss, making it possible to heat the building without a conventional heating system. These components have a recommended U-value of less than 0.15 W/m²K (sub-criteria) and can vary in size with typical Passive House wall and roof constructions being up to 450-500mm thick (with typically 300mm Insulation or more).

Air-tightness - The building envelope in a Passive House is surrounded by one continuous airtight layer with all components and connections fitted in an airtight manner. Air-tightness tapes and seals are applied around all areas where components meet, with special attention paid to the installation of electrical and mechanical services. Air-tightness in a Passive House is verified by conducting a pressure test (blowerdoor test) to measure the amount of air changes the building experiences within one hour. In order to achieve certification, the building’s pressure test result (measured at 50 Pascal difference between inside and outside) must not exceed more than 0.6 ac/h־¹. If the buildings air changes per hour are more than 0.6 ac/h־¹ then too much heat will be lost through leakages and the reduced energy targets of less than 15kWh/(m²a) will not be reachable.

Thermal bridge free design - The third way in which heat loss can be reduced through the building fabric is to eliminate the thermal bridging effect. Windows and door frames are insulated over, and the building is detailed in such a way that areas where solid components meet are reduced, or eliminated, so that heat does not have a clear path from the inside of the building to the outside. Thermal bridge calculations are conducted (commonly using the therm or heat programmes) in order to accurately detect and eliminate any area that may create a thermal bridge. Thermal bridging within a Passive House building is recommended to be below 0.01 W/(m²K) (sub-criteria) as calculated on an external dimensions basis.

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