Constructing to Passivhaus standards is becoming increasingly commonplace in the UK, but by focussing purely on this standard are we missing out on other important factors when it comes to building product specification? Sonia Travis from NorDan UK Ltd discusses.
As more people become climate aware interest in either building or retrofitting to the Passivhaus green building standard is increasing.
According to the Passivhaus Trust, the non-profit that awards the rigorous, energy-based construction standard in the UK, in particular Passivhaus buildings exploded when local authorities started declaring climate emergencies and setting carbon neutral targets.
Why? Because Passivhaus is a way of ensuring that a building or home is very energy efficient, keeping operational carbon emissions to a minimum by reducing the energy demand down to very low levels while also making sure the building is comfortable inside and has high levels of good indoor air quality.
The building’s fabric plays a critical role in achieving this, and so Passivhaus design centres on the five core principles of insulation, airtightness, mechanical ventilation, reduced thermal bridging and the use of high-performance windows.
Those developments wishing to achieve Passhivhaus certification must undergo a rigorous compliance process, with elements of the building’s fabric required to achieve a minimum performance criteria.
What does this mean for windows?
For windows this can be broken down into two criteria: solar gains and solar losses.
Designing to optimise solar heat gain is a fundamental principle of successful Passivhaus design, with the size and orientation of the window on the site an important factor.
Just as gains are important, so are heat energy losses. The measure of a window’s heat loss is its U-value, calculated as an average for the whole window – glass, frame, spacer and installation.
The glass unit in northern European climates is required to have a u-value of 0.7W/m2K or lower to meet Passivhaus Standards.
What does Passivhaus miss?
In order to guarantee that they meet the specific criteria needed for a development to achieve Passivhaus status, some specifiers are choosing Passivhaus certified products.
And this may seem like an obvious choice, however, certainly when it comes to doors and windows, focussing purely on this standard could mean that you’re missing out on other important factors.
And, just to be clear, it is not necessary to specify Passivhaus certified products in order for a development to achieve Passivhaus certification.
What about Embodied Carbon?
Passivhaus does an excellent job of minimising operational carbon, however it doesn’t take into consideration embodied carbon, the carbon in the building products and materials and in construction processes.
I read a recent quote from the Timber Research & Development Association (TRADA) that said: “CO₂ released now is worse than CO₂ released in the future. There is not actually much point in getting to the future unless we do a good enough job at controlling CO₂ emissions now.”
In other words, it makes no sense for the industry to generate huge amounts of unnecessary carbon making both existing and new buildings operationally carbon efficient, as it risks making climate change worse sooner.
When it comes to windows, the materials used to manufacture the frames has a significant impact on the quantity of embodied carbon in the product.
For example, timber is carbon negative, making products made of this material significantly lower in embodied carbon than aluminium or PVCu.
If you want a clear picture on how much carbon is in a product, then manufacturer Environmental Product Declarations (EPDs) are a good place to start, as they are objective, third party reports that give clients, architects, specifiers and contractors the ability to accurately assess the data.
Performance & Cost
Frame material also has a significant impact on window performance and U-values.
As a rule, aluminium and composite windows can only achieve a u-value of 0.7 W/m2K by being quadruple glazed, and PVCu struggles to meet that figure full stop, however high-quality timber windows can achieve a U-value as low as 0.65 W/m2K when just triple glazed.
As a general rule, the more glazing the higher the cost, and when you consider that building to Passivhaus standard costs between 5-10% (a conservative estimate) more than a standard build, this is an important commercial consideration. Extra glazing also increases the embodied carbon content of the product.
There is also an implication on design, as more glazing results in a very heavy window which places particular requirements on the frames and can limit unit size commercially, and impact on solar gain.
This means that, in general, high-performance timber windows can accommodate a larger aperture size, whilst also meeting the 0.7 maximum u-Value.
Security implications
Passivhaus doesn’t take into consideration the security, an important factor when choosing windows and doors, especially for the ground floor of developments.
PAS24, the standard of testing that measures the security performance of windows and doorsets to ensure they can resist a level of attack from an opportunist burglar, isn’t just a nice to have, it is required in order to comply with current building regulations.
It’s clear that there’s lots more to consider than simply the u-value when choosing a window or door, and after 30 years of working in the windows industry I would recommend taking a more holistic view that encompasses all the factors mentioned above.
If you don’t, you could be paying more for a product that doesn’t perform optimally.
At NorDan UK we’re in the very fortunate position of having developed high-performance, sustainable, alu-clad timber windows and doors that not only meet the Passivhaus standard when triple glazed, but are also low in embodied carbon, have a 60-year life-span and are PAS24 certified.
With NorDan windows, there are no compromises, and we are becoming the supplier of choice to those taking a fabric first and whole life carbon approach to design and build.
Our team of specification experts is available to discuss your project requirements, and will ensure that you specify the right products for your project, so please get in touch we’re happy to help.
Sonia Travis is NorDan UK’s Commercial Sales Manager for the North of England.
If you would like more information on NorDan’s high-performance alu-clad timber windows then email Sonia on Sonia.travis@nordan.co.uk.
