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INSIGHTS SITE LINES


Roofing elements of designing to Passivhaus


As designing to the Passivhaus standard continues to grow in popularity, not only for residential builds but also increasingly for commercial constructions, architect Nicolas Tye shares insights on the specific considerations a Passivhaus-accredited roof requires


O


ver the last few years, our practice has seen a significant increase in the number of both residential and commercial projects it commissions, with energy efficiency as the predominant motivation for the building’s construction. Initially considered something of a ‘trend’, the Passivhaus standard is now far from it, with over 30,000 buildings worldwide currently holding the Passivhaus accreditation.


While we originally received requests for one-off private dwellings looking to achieve Passivhaus to primarily cut heating bills, there has been a significant rise in recent years of projects within the local authority environment, such as schools and other commercial buildings, whose clients want to build to Passivhaus. Among the residential projects, there’s an interesting divide between those who are striving to officially achieve the standard, and those who are simply influenced by the approach and are wanting to create a thermally efficient home without having to go through the procedure of an official accreditation. This is indicative of the positive impact the Passivhaus standard is having on both Europe and specifically the UK, as homeowners and local authorities increasingly look to construct buildings led by ecological designs to provide a result with very low energy use.


As the standard supports the construction of extremely energy efficient buildings that minimise heating demand by using energy sources from inside the property, such as the body heat from residents or solar heat that enters through windows, it tends to eliminate the requirement for heating and manual ventilation systems. This dramatically reduces energy use and carbon emissions, therefore decreasing energy costs, while also supporting both the building’s ongoing health, and overall indoor air quality.


The Passivhaus standard requires a different approach to be taken with regard to the building’s overall design, reconsidering the quality of the building components specified. There are five main areas that architects should consider when undertaking a Passivhaus build: airtightness, thermal bridge free design, thermal insulation, Passivhaus windows and adequate ventilation with heat recovery.


The Passivhaus Institute (PHI) outlines that the recommended maximum U-values for walls, floors and roofs is 0.15 W/m2


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With regard to roofs in particular, manufacturers are proactively and proficiently creating high quality building materials that


ADF SEPTEMBER 2018


23


The Passivhaus standard requires a different approach to be taken with regard to the building’s overall design, reconsidering the quality of components


offer maximum energy efficiency, while also diminishing the loss of heat via the roof due to radiation. These products, which must of course be Passivhaus certified, are listed on the PHI website.


Roof design With regard to the design and build of a Passivhaus roof, the structure itself must be entirely airtight, with high levels of insulation and the elimination of all thermal bridges to ensure the maximum permitted U-value of 0.15 W/m2


K isn’t exceeded.


The roof’s specifications will of course be dependent on the build itself, but they must also ensure a maximum of 0.6 air changes per hour at 50 Pascals pressure (ACH50) to ensure adequate airtightness is achieved.


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