What Passive House certification for windows involves
Passive House Component certification is not a marketing badge, but rather a rigorous scientific process to guarantee the fabric and systems performance of a Passive House building. To certify windows, the Passivhaus Institut (PHI) in Darmstadt assesses the window to a defined method, setting pass or fail thresholds for the cool-temperate climate zone that covers the UK.
For a window to be certified for this climate, the headline requirements are:
- A whole-window U-value (Uw) of no more than 0.80 W/m²K
- An installed U-value (Uw,installed) of no more than 0.85 W/m²K, so the thermal bridge at the junction with the wall is counted
- A frame U-value (Uf) of no more than 0.80 W/m²K
- A minimum internal surface temperature – also known as the hygiene factor – expressed as a temperature factor (fRsi) of at least 0.70, so the inside of the frame and glazing edge stays warm enough to avoid condensation and discomfort in cold weather
Behind those headline figures, the Passive House Institute also assesses glazing edge thermal bridge (the spacer bar, ψ-spacer) at each side of the frame and the installation thermal bridge (ψ-install) for defined installation details. Certified components are then graded into efficiency classes so specifiers can compare like with like.
Norrsken’s certified P41A window achieves whole-window U-values from 0.67 W/m²K, with installed U-values certified at approximately 0.84 W/m²K for validated installations. It achieves a high phB rating on the Passive House efficiency class scale.
When is re-certification necessary?
When a product changes, calculation standards are updated, or the certificate expires, the manufacturer must repeat the certification process. For Norrsken to re-issue the P41A, the full frame and glazing specification, updated thermal calculations, and installation details had to be submitted to the Passive House Institute to check.
This process is valuable for specifiers and energy consultants who must have confidence that the performance data they use is verified rigorously by a third party, to a published method, frequently.
Certified window vs “Passive House suitable” window
It is perfectly acceptable for triple-glazed windows to be sold as suitable for Passive House without holding a Passive House Institute certificate, providing they meet the performance requirements for the standard. The table below sets out the differences between each approach.
|
Factor |
PHI certified window |
“Passive House suitable” window |
|
Whole-window U-value |
Independently checked, must be 0.80 W/m²K or better |
Manufacturer’s own figure |
|
Installed U-value |
Assessed for defined wall types, must be 0.85 W/m²K or better |
Depends on installation quality, not defined or validated |
|
Frame U-value |
Must be 0.80 W/m²K or better |
Often quoted, not always to a uniform standard |
|
Glazing edge and install thermal bridges |
ψ-spacer and ψ-install values published on the certificate |
Often not available |
|
Comfort criterion |
fRsi of 0.70 or better confirmed |
Not usually assessed |
|
PHPP entry |
Values can be entered directly from the certificate |
Certifier may require evidence |
|
Currency |
Certificate has a validity period and is re-issued |
No expiry, no re-check |
None of this disqualifies an uncertified window from being used in a certified building: the Norrsken P31A triple-glazed window has been used in several certified Passivhaus buildings, including the UK’s first Passivhaus Plus certified school built using traditional construction, despite being non-certified.
Using non-certified products simply means performance isn’t guaranteed in the same way as for a certified product.
Why the installed U-value matters
The number most people look at is Uw, the whole-window value. Often overlooked is the installed U-value (Uw,installed), which includes thermal bridging introduced where the frame meets the wall. Fitted poorly, any well-designed frame will be significantly compromised.
A Passive House certified component includes validated installation details to prevent this situation. Norrsken consults early on in any project – especially Passive Houses – on wall build-up to ensure our products are installed in the optimal position for thermal performance. Positioning the frame within the insulation zone, insulating over the frame externally, taping the airtightness layer to the frame and getting the sill detail right all contribute to the installed figure. A certificate covers the performance within the frame; installation determines whether this is achieved.
For self-builders in particular, the questions to put to a supplier are not just “what is the U-value?” but also “can you offer installation and consultancy that protects the U-value?”
Using manufacturer PHPP data in a certification submission
If you are working towards certification, your certifier will use component data alongside the Passive House Planning Package (PHPP) to create the energy model for your building.
Certified components make this easy: frame U-values, glazing edge ψ-value and installation ψ-values for the assessed wall details map directly to the PHPP.
Norrsken publishes PHPP 10 data for its certified P41A window and Passive House suitable range, so that architects and Passive House consultants have accurate data early on.
Frequently asked questions about Passive House certified windows
Do I have to use certified windows to get Passive House certification?
No. The building is certified, not the components. But certified windows provide more evidence to satisfy the certifier. Where evidence is thin, certifiers usually err on the conservative side – certified windows remove that risk.
Is triple glazing enough on its own?
No. Triple glazing is necessary in the UK climate to reach the required U-values and comfort criterion, but the frame, spacer, installation and airtightness details affect fabric performance as much as the glass.
Do certified windows cost more?
Yes. Certification adds cost to the manufacturer, and high-performance frames and glazing cost more than standard units. A Passive House certified product will usually cost more than a non-certified alternative.
Where to go next
Norrsken’s guide to Passive House windows covers the standard, EnerPHit for retrofit, when certified components make sense, and the projects we have supplied, with PHPP 10 data available to download. For a live example of installed performance, the Ravine House EnerPHit retrofit in Sheffield and Two Rivers School, the UK’s first Passivhaus Plus certified school, both show what the detail looks like on site.