Specification Tips Around U-Values for Passivhaus Certified Windows
When designing homes there are many things architects and specifiers must consider to ensure optimum energy efficiency and the greater comfort and wellbeing of occupants. To help achieve this goal the PassivHaus Institute created standards for a building’s thermal efficiency. A critical factor within those standards is the specification for the PassivHaus certified windows U-value.
It’s clear that Passivhaus buildings are growing in popularity in the UK. Or, at least, the use of materials and design principles that underpin them. There are now 1,500 certified homes in the UK, with a further 7,000 in planning, under construction or almost complete. Membership of the UK Passivhaus Trust has also doubled between 2020 and 2022 alone.
There are several reasons why interest in Passivhaus is increasing. For those working in the built environment there’s legislative pressure in the form of the Future Home Standard, which proposes changes to Part F and Part L of Building Regulations.
Today’s building designs will need to evolve in line with these updates although thought will also need to be given to futureproofing, not least because there is a huge volume of legacy building stock and further changes to building law can be expected as the demands of net zero begin to accelerate.
Window U-Value in Context
U-value is the key indicator of a window’s thermal performance, which means taking glazing and frames together as a single unit. For Passivhaus standard windows there is no particular prescribed construction system. Most construction systems can be adapted to attain the Passivhaus standard, with individual projects presenting their own mix of benefits and challenges, whether new build or retrofit.
There is potential confusion around Passivhaus windows and UK window energy rating (WER). For example, Passivhaus windows may only achieve a B rating on the WER scale due to the amount of solar gain through the window. This might lead some to assume that an A rated standard window is better.
This is however not true when it comes to thermal performance. Using this particular metric, Passivhaus standard windows will outperform non-Passivhaus windows when it comes to heat retention. Therefore, architects and specifiers should work to whole-frame U-values rather than the WER system when specifying windows for Passivhaus.
It’s important to note that it’s not necessary for windows to be certified as long as manufacturers can provide data to EN 10077. Separate frame U-values will be needed for all junctions (head, cill and jamb). For example, REHAU GENEO System is fully certified to Passivhaus standards.
Establishing Boundaries for U-Value Calculations
For Passivhaus U-value standards, heat loss in a home is measured to the outer edge of the thermal envelope. This could differ from the standard assessment internal calculation, as where the outer boundary of the thermal envelope begins may not be immediately clear.
The edge of the thermal envelope lies at the last element used in a U-value calculation. So for a masonry wall with filled cavity the last element is the exterior brick or block. For a timber frame wall with a ventilated cavity and rain-screen, the thermal envelope will end on the outside of the timber frame wall.
Counting Windows Individually
Unlike Simplified Building Energy Model (SBEM) and Standard Assessment Procedure (SAP) calculations, where each rough opening in a room can be considered as one window, for Passivhaus standard windows each window casement should be considered individually. Counting two adjoining windows as one risks frame losses being underestimated.
Additionally, opening windows usually have thicker frames compared with fixed windows. It’s therefore best practice early in the design stage to treat all windows as opening.
Air Leakage Targets
It’s worth noting that buildings with Passivhaus standard windows have an air leakage target of 0.3 air changes per hour, based on the average German home. UK homes are generally smaller and with a higher occupancy rate, meaning this is likely to be higher for British properties.
Architects and specifiers should factor this in at an early design stage to avoid U-value miscalculations and potential problems down the line.
Triple glazing gives the best energy balance and is ideal to meet occupant comfort and wellbeing requirements in avoiding condensation and mould. The glazing g-value, or solar gain factor, is just as important as the U-value in the optimisation process.
Note that building regulations only considers losses, so high performance glazing will often have a low U-value but also very low g-value, which is likely to not meet the Passivhaus standard. The certified Passivhaus U-value for windows should be below 0.8 W/m²K.
When installing horizontal windows, such as rooflights, glazing performance changes. This must be taken into account in the U-value. For example, glazing with a stated U-value of 0.60 W/m2K should be entered as 0.90 W/m2K for Passivhaus when it is installed horizontally—EN673 has a calculator to determine this accurately.
Frames vs. Glazing
Generally frames perform worse than glazing for heat transference, so smaller windows and those with lots of mullions and transoms should be minimised. When specifying a window frame the dimensions should always be considered, as well as frame U-value. While it can be tempting to choose the frame with the best U-value, there is often a trade-off between performance and thickness.
A thinner yet poorer performing frame can sometimes be the better option, as the amount of glazing (which has a better U-value) is optimised and G-value is increased. Certified Passivhaus windows include an A to C rating which rates the balance between frame and glazing, with gains being considered as well as losses.
The best approach is to model the various options for a specific project. For example, inward and outward opening windows can perform equally well. Although outward opening windows are more common in the UK, inward opening windows allow external shutters to be fitted, and are usually easier to wrap in insulation, helping achieve better thermal performance.
To achieve Passivhaus U-values for any home building or retrofitting project there are some good practices that architects and specifiers can follow, which include checking window manufacturer or supplier’s advertised thermal efficiency. Being mindful of damp, as conductivity of porous elements such as open-cell insulation or lightweight blocks can vary significantly with moisture content. And also being conservative with window U-values, as thermal performance rarely gets better during design development.
For more information about Passivhaus window standards and specifications visit: www.rehau.co.uk/passivhaus