Windows for passive houses and class A: what specification do you actually need

Anyone building or renovating with a passive house, class A or nZEB standard in mind arrives at the windows chapter with one right idea and one wrong one. The right one: in an ultra-high-efficiency building, windows aren’t a minor detail — they’re one of the points where the building envelope can win or lose. The wrong one: that it’s enough to “fit the most insulating window in the catalogue” and the job is done. In a building designed to lose almost no heat, the window becomes part of a system — frame, glass, air tightness, installation — where every link counts, and a top-of-the-range glazing unit installed badly undoes everything else.

This article exists to explain what characteristics an ultra-high-efficiency house genuinely needs from its windows, honestly: which figures matter, what changes compared with a “good-quality normal” window, and — the point too many articles skip — when it makes sense to push to these levels and when it’s over-specified. Let’s set a boundary straight away: this isn’t about which Uw values are needed zone by zone in general (that’s covered in the dedicated guide to Uw, Ug and climate zones), nor the double-versus-triple-glazing question in general (for that, see when triple glazing is genuinely worth it). Here we focus only on the passive house / class A / nZEB context.

Passive house, class A, nZEB: three different things that often get mixed up

Before talking about windows, let’s get the terminology straight, because these three terms get used as synonyms and they aren’t.

Class A (and its sub-classes A1, A2, A3, A4) is a rating on the Energy Performance Certificate (EPC) scale: it indicates a building with very low energy demand. It’s an Italian label, defined by national energy certification legislation.

The nZEB standard (nearly Zero Energy Building) is a European-derived definition adopted into Italian legislation: it describes a building with very low energy demand, covered to a significant extent by renewable sources. For new-build construction it’s now the statutory reference.

The passive house in the strict sense is something else entirely: it’s a voluntary standard defined by the Passive House Institute (Passivhaus). It isn’t a law and isn’t a requirement: it’s a certification a building can achieve if it meets very stringent requirements on heating demand, envelope air tightness and the absence of thermal bridges. Passivhaus certification is therefore voluntary, issued by a third-party body, and should be referred to correctly as such: there’s no such thing as a “statutory passive house”.

Concept	What it is	Who defines it
Class A (EPC)	The building’s energy label	Italian energy certification legislation
nZEB	Nearly-zero-energy building	European legislation adopted in Italy
Passive house (Passivhaus)	Voluntary performance standard	Passive House Institute (voluntary certification)

Why does this distinction matter for windows? Because the strictest requirements by far are the Passivhaus ones: there, the window has to meet precise thresholds. In a statutory class A or nZEB house, the requirements are significant but softer, leaving more room to specify proportionately. Understanding which of the three scenarios you’re actually in is the first step to avoiding over-specification.

The figures that really matter: Uw, but not only

The figure everyone quotes is Uw, the thermal transmittance of the whole window — frame and glass together — expressed in W/m²K: the lower it is, the less heat it loses. It’s the figure the product standard EN 14351-1 uses to declare a window’s performance.

To give an order of magnitude — and I stress order of magnitude, purely as an example, not a certain value — the levels involved can be read as follows:

Type of building	Indicative window Uw (W/m²K)	Notes
Good modern “standard” window	order of magnitude 1.1-1.3	Already well below many statutory thresholds
Class A / nZEB building	order of magnitude 0.9-1.1	Depends on climate zone and design
Typical Passivhaus reference	often quoted around ~0.80 (as an example)	Stringent threshold of the voluntary standard

These figures are indicative and not to be taken as a specification: the actual Uw depends on the window’s dimensions, the type of glass, the frame, and must always be verified for the specific configuration, in accordance with EN 14351-1. The general rule is simple: the more efficient the building, the closer the window’s performance needs to come to that of the wall it sits in — otherwise the window becomes the thermal “hole” in the façade.

But Uw alone doesn’t fully describe a passive-house-grade window. At least three other figures matter:

• Uf, the transmittance of the frame alone. In an ultra-high-efficiency window, the frame needs to insulate as well as the glass, otherwise it becomes the weak point. Multi-chamber profiles with multiple gaskets exist precisely to keep Uf down.
• Ug, the transmittance of the glass, which drops significantly with low-E triple glazing (covered below).
• Air tightness of the window — how little air it lets through at the joints — which is crucial in a passive house and covered in a dedicated section below.

The frame: 7+ chamber profiles, 82 mm and beyond

In a passive house, the frame does half the work. To accommodate thick triple glazing and keep Uf low, you need profiles with many chambers (typically 7 chambers or more) and a substantial construction depth, on the order of 82 mm and above. The chambers are the internal air voids within the profile: the more there are, the better they “compartmentalise” the passage of heat through the frame.

We work with German Salamander systems, CE certified under the Construction Products Regulation CPR 305/2011. Among the deepest systems — those designed for triple glazing and high-efficiency buildings — Uf values drop to keep pace with the most insulating glass. If you want to see how chambers, thicknesses and rebates vary across the different profiles, the comparison of Salamander systems goes into detail on the individual frames and their application ranges.

Two honest points that anyone selling “the lowest number” tends to gloss over:

• An 82+ mm profile with triple glazing is thicker and heavier. The hardware has to be sized to bear the sash weight without failing over time. It’s not a minor detail: a heavy sash on undersized hardware sags and needs frequent adjustment.
• A deeper frame takes up more of the opening. For the same rough opening, slightly less light gets in. It’s a physical trade-off inherent to ultra-high-insulation windows, to be accounted for at design stage, not discovered afterwards.

The glass: low-E triple glazing and warm-edge spacers

In a passive house, the standard glass is low-E triple glazing: three panes, two gas-filled cavities (argon or, for the most demanding performance, krypton), with low-E coatings that reflect heat back inside. Ug drops to very low values as a result — order of magnitude 0.5-0.7 W/m²K, as an example — against 1.0-1.1 for a good double-glazed unit.

I’ll stop there, because the general comparison between double and triple glazing, and when one or the other makes sense, is a subject in its own right: it’s covered in full in the article on when triple glazing is genuinely worth it. In a passive-house context, triple glazing is barely up for debate: it’s the starting point, because without the third pane the Uw rarely reaches the standard’s stringent thresholds.

There is, however, a lesser-known component that makes a real difference in a passive house: the warm-edge spacer. It’s the frame that separates the panes of the glazed unit around the perimeter. Traditional ones are aluminium, which conducts heat: along the edge of the glass this creates a thermal bridge that lowers the internal surface temperature and encourages condensation right at the edges. The warm-edge spacer, made from a low-conductivity material, reduces this perimeter thermal bridge. In an ultra-high-efficiency building, where every degree of surface temperature counts towards avoiding condensation and improving comfort, warm-edge is essentially mandatory, not an optional extra.

Air tightness: where the passive house is genuinely won or lost

If there’s one thing that sets a passive building apart from one that’s merely “well insulated”, it’s the air tightness of the envelope. A certified passive house has to pass a very strict air tightness test (the blower door test): uncontrolled air entering and leaving through the joints has to be reduced to a minimum, otherwise all the heat retained with such effort escapes through the draughts.

Windows are one of the critical points for air tightness, for two reasons:

• The window itself needs good air permeability classes, classified under EN 14351-1. Multiple gaskets and the frame geometry exist precisely for this.
• Even more important is the joint between window and wall: it’s there, at the perimeter where the window meets the wall, that most of the air tightness is won or lost. A perfect window fitted with a joint that lets air through is a wasted window.

Which brings us straight to the most underrated point of all.

Installation: in a passive house, it matters as much as the window

In an ultra-high-efficiency building, installation isn’t an accessory: it’s part of the performance. You can buy the window with the lowest Uw on the market, but if it’s fitted badly — without continuity of air tightness and the vapour barrier, without managing the thermal bridge at the sub-frame — the envelope loses heat at the edges and the blower door test fails.

Correct installation of windows and doors has a regulatory reference in Italy, UNI 11673, which sets out the requirements for on-site installation (we cite it here as a reference: this isn’t an installation manual). In a passive house, installation needs to be designed together with the window, with materials and sequences that guarantee continuous sealing between window and wall. That’s why, in these buildings, it makes little sense to think in terms of separate components: window, glass, sub-frame and installation are a single system.

The practical, if uncomfortable, conclusion is honest: between an excellent window installed carelessly and a very good window installed to a proper standard, in a passive house the second one nearly always wins. The quality of the product matters, but without installation to match, the standard isn’t reached, and the premium paid for the “extreme” window is wasted.

Summary: what to look for in a passive-house window

Putting the pieces together, here’s the technical profile — with values always indicative and to be verified against the actual configuration under EN 14351-1:

Component	What’s needed in a passive house (indicative)
Whole-window Uw	Very low (order of magnitude 0.80-1.0; ~0.80 often quoted as the Passivhaus reference)
Frame (Uf)	7+ chamber profile, 82+ mm depth, low Uf
Glass	Low-E triple glazing, two gas-filled cavities, Ug order of magnitude 0.5-0.7
Spacer	Warm-edge to reduce the perimeter thermal bridge
Air tightness	Good permeability class (EN 14351-1) + continuous joint to the wall
Installation	To a proper standard, referencing UNI 11673, designed together with the window
Certification	CE marking (CPR 305/2011); Passivhaus component if required (voluntary)

If this sounds like a lot, that’s because it is: a passive-house window is a top-tier product, and it needs to be treated as such at installation stage too. For the full picture of how profiles, glass and insulation come together in PVC windows, the complete guide to PVC windows ties the threads together.

When it’s NOT worth it

And here’s the honest point too many articles avoid. Pushing to passive-house levels only makes sense in certain cases. In others it’s over-specified — you’re paying for performance the building won’t give back to you.

• The building isn’t designed as a passive one. Passive-house windows perform best within an envelope designed to lose almost no heat: heavily insulated walls, thermal bridges dealt with properly, air tightness attended to everywhere. If you fit windows with Uw ~0.80 into a house with mediocre walls and thermal bridges throughout, the window becomes by far the most insulating part of the building and the heat still escapes through the walls anyway. You’re paying for an excellence the rest of the house doesn’t make use of.
• A mild climate zone, with no certification target. In warmer zones, if you’re not chasing the Passivhaus standard or a specific statutory target, extreme Uw levels don’t pay for themselves: the climate doesn’t generate enough winter heat loss to justify them. There, solar control on the glass matters more than the last decimal point of Uw. To work out what values you need in your zone, start from the guide to Uw and climate zones.
• A partial renovation with no work on the envelope. Replacing only the windows with passive-house-grade units, while leaving the walls and roof as they are, is wasteful: the performance of the extreme window gets swallowed up by the rest of the building. Better to fit a good, proportionate window and invest the difference in the envelope.
• A limited budget that would eat into the installation quality. If buying the “extreme” window means cutting corners on installation quality, you’re doing the opposite of what’s needed. In a passive house, poor installation undoes the product. Better a window one step down, installed to a proper standard.
• There’s no stated performance target. If no one has set a target — energy class, nZEB, Passivhaus certification — chasing “the maximum” blindly leads to random over-specification. First the design target, then the window sized accordingly.

In all these cases, the intelligent choice isn’t “the most insulating window that exists”, but the right window for the actual building, with the difference invested where it pays off.

FAQ

What Uw value do you need for a passive house? For the Passivhaus standard, a reference of around ~0.80 W/m²K for the whole window is often quoted, but that’s an indicative value, given as an example: the actual Uw depends on the dimensions, the glass and the frame, and needs to be verified for the specific configuration under EN 14351-1. For statutory class A or nZEB the levels are often a little softer, on the order of 0.9-1.1. These are orders of magnitude, not specifications.

Is the passive house a statutory standard? No. The passive house (Passivhaus) is a voluntary standard defined by the Passive House Institute: it’s achieved through third-party certification, not a legal requirement. Class A and nZEB are different — they’re references within energy legislation (Italian and European). They’re often confused, but they’re distinct things.

Does a passive house always need triple glazing? In practice, yes: low-E triple glazing is the starting point for reaching the stringent Uw thresholds required. With double glazing, it’s difficult to reach those thresholds. The general comparison between double and triple glazing, though, is a separate subject: you’ll find it in when triple glazing is genuinely worth it.

What is a warm-edge spacer and why does a passive house need it? It’s the “warm-edge” frame that separates the panes of the glazed unit around the perimeter, made from a low-conductivity material instead of aluminium. It reduces the thermal bridge at the edge of the glass, raises the internal surface temperature, and limits condensation at the perimeter. In an ultra-high-efficiency building it’s essentially mandatory.

In a passive house, does the window matter more, or the installation? Both matter, but installation is where performance is most often lost. An excellent window fitted without continuity of air tightness loses heat at the edges and causes the blower door test to fail. Installation needs to be designed together with the window, referencing UNI 11673 (cited here as a standard, not as an installation guide).

Does it always make sense to push to passive-house levels? No. If the envelope isn’t designed as a passive one, in mild zones with no certification target, or in a partial renovation that doesn’t touch the walls and roof, extreme windows are over-specified: you’re paying for performance the building won’t give back. Better a proportionate window, investing the difference where it pays off.

In summary

A window for a passive house, class A or nZEB isn’t “the most expensive window in the catalogue”: it’s a window sized for an ultra-high-efficiency envelope, where a 7+ chamber, 82+ mm frame, low-E triple glazing, warm-edge spacer, air tightness and installation to a proper standard work together as a single system. Uw values need to be verified case by case under EN 14351-1, and Passivhaus certification remains a voluntary standard, not a requirement. The honest rule is one: the window should be chosen for the actual building and its design target, not for the desire to have the lowest possible number — because an extreme window in a house that can’t make use of it, or one that’s badly installed, is wasted excellence.

Are you designing or renovating with class A or a passive house in mind, and want to understand what windows your building genuinely needs? Request a quote: we assess the performance target, the climate zone and the type of project, and propose the right configuration, with CE-certified German Salamander profiles and fully Italian-run management — without selling you the “extreme” window where an excellent, proportionate, properly installed window is all you need.