How to Specify: Passivhaus Windows

For those who haven’t yet heard of Passivhaus, you soon will.

Some of the principles found in the Passivhaus standard will become very useful in the UK beyond 2013 Part L as the Fabric Energy Efficiency Standard (FEE’s) is introduced for the first time. For the specifier, working on a Passivhaus project can seem like a daunting task.

In this post I will briefly cover the subject of window specification in a Passivhaus and how this is likely to differ from what we are used to specifying under Part L.

What is Passivhaus?

Passivhaus is a fabric first approach to building design and specification based on building physics developed over the past twenty years by Prof Wolfgang Feist and the Passivhaus Institute in Germany.

At the moment, Passivhaus is perhaps seen as yet another ‘box ticking’ exercise, akin to the Code for Sustainable Homes, or BREEAM. However, there is much more to Passivhaus than bike shelters, water butts and garden sheds! It is underpinned by building physics and has proven to address the ‘performance gap’ normally associated with buildings assessed using SAP.

Passivhaus Windows

Windows serve three primary functions.

Firstly they allow sunlight and diffuse skylight to enter our buildings to provide passive solar heating and lighting. Secondly, they give the occupants of a building a view out and a vital connection with the external environment. Finally, windows are often the primary means of ventilation in natural or mixed mode buildings. However, one of the biggest problems with windows (in the context of the UK climate) is heat loss.

In the context of new dwellings, Part L1A 2010 states that windows should have a maximum thermal transmittance (U-value) of 2.0W/m2.K. It should be noted this limiting value is expected to be reduced to 1.60W/m2.K under Part L1 2013.

This may seem like a step in the right direction, but Passivhaus certified windows must achieve a maximum whole window U-value of 0.80W/m2.K, (0.85W/m2.K installed). The installed U-value includes additional heat loss through thermal bridging at the head, sill and jamb based on calculated Ψ-values (Psi-values). Such a low U-value means that low-e argon/krypton filled triple glazed units are inevitable, with a maximum centre-pane U-value of around 0.75W/m2.K.

Windows insulated to this level not only drastically reduce fabric heat loss, but also ensure human comfort is achieved by maintaining glass surface temperatures within 4.2k of the internal operative temperature. Above this and occupants are likely to feel discomfort from radiant asymmetry when sitting close to windows in cold weather (down to -16°C).

Under the Building Regulations, window U-values may be given based on that quoted for one of the following:
1. The smaller of two standard windows described in BS EN 14351-1:2006
2. The standard configuration set out in BR443 2006
3. The specific size and configuration of the actual window

In the majority of cases option 1 or 2 will be chosen, as project specific window U-value information can be difficult to obtain for most projects. In this case manufacturer quoted U-values for the standard window configuration will be entered in SAP, regardless of the actual window size and configuration.

However, in the Passivhaus Planning Package (PHPP), all window U-values are calculated individually within the ‘windows’ spreadsheet, based on following simple equation:

For specifiers embarking on their first Passivhaus project, a good place to search for Passivhaus compliant windows (termed glazings and transparent components) is the Passivhaus Institute website.

Alternatively, Rationel, Internorm and Munster Joinery all supply products that are suitable for use in a Passivhaus. The Green Building Store also supply a range of UK made timber windows suitable for Passivhaus where locally sourced materials is a consideration.

It should be borne in mind that it is not always necessary to specify Passivhaus certified windows in order to gain certification. However, using certified components will help avoid any problems with the rigorous certification process at the post construction stage.

For the avoidance of doubt, it is best to specify certified products wherever possible.

About the Author: Leigh Caller is an Environmental Design Consultant, Certified Passivhaus designer and Chartered Architectural Technologist. You can find him on Twitter, his website or drop an email to [email protected]