UNI 11673 window and door installation: what it means and why you should ask for it explicitly

It’s often said that installation accounts for 50% of a window’s performance. It’s a simplification, but not far from the truth: even the best Salamander BlueEvolution 82 profile with triple glazing at Uw 0.80 loses up to 30-40% of its declared thermal and acoustic performance if fitted poorly. The standard governing how an external window or door is installed in Italy is UNI 11673, published in 2017 and updated in 2024. It isn’t mandatory in itself, but it’s cited in every technical decree on tax incentives and in every serious sworn technical statement. In this article I explain what it actually requires and how to tell a properly done installation from a “quick” one.

The three parts of the standard

UNI 11673 is structured in three parts:

• UNI 11673-1: design criteria and responsibilities (who does what, what must be documented)
• UNI 11673-2: requirements for professional roles (what the installer must know)
• UNI 11673-3: operational installation procedures (how it’s done)

For practical purposes, what matters to the customer is part 3: how the installation is carried out. The core principle is a system of three concentric sealing planes, running from the outside towards the inside.

The three sealing planes

Plane 1 — External water/wind seal

Function: to prevent rainwater and wind from entering the cavity between the window and the masonry.

Correct materials: pre-compressed self-expanding tapes, impregnated with water-repellent synthetic resin. The reputable brands are:

• illbruck Compriband TP600 or ME500 (Tremco illbruck)
• Tescon Vana (for wind-tightness applications, exterior side)
• Tescon Profil (for corner joints)
• Klober Wallint or RAUEXPAND Hannoband
• Soudal SWS System (integrated system)

The tapes are applied to the sub-frame (or directly onto prepared masonry) before the window is positioned. Once the window is installed, the tape expands over time (24-48 hours), perfectly filling the imperfections in the masonry. It lasts 25-50 years while retaining its elasticity.

Common mistake: structural silicone as the sole external seal. External silicone dries out within 5-10 years when exposed to UV and rain, detaches from the substrate, and water gets in. It’s the number-one symptom of 1990s-2000s-era installation.

Plane 2 — Intermediate thermal and acoustic insulation

Function: to fill the cavity between the window and the masonry with material that limits thermal and acoustic losses.

Correct materials:

• Polyurethane foam, density ≥30 kg/m³, single-component or two-component, closed-cell. Reputable brands: Soudal Soudaframe, illbruck FM330, Hilti CP670.
• Alternatively, high-density mineral wool for specific cases (very wide joints or extreme acoustic requirements).

The intermediate layer must be continuous around the entire perimeter of the window, with no gaps.

Common mistake: low-cost plumber’s-grade polyurethane foam, density 15-20 kg/m³. It absorbs moisture, compresses, and loses 80% of its insulating value within 2-3 years. It costs 30% less but “spends” 100% of the performance.

Plane 3 — Internal vapour seal

Function: to prevent moisture in the internal air from migrating into the cavity (where it would then condense on the cold side, causing hidden interstitial mould).

Correct materials:

• Vapour barrier tapes or membranes applied continuously around the internal perimeter
• illbruck ME220, Klober Wal-int, Tescon No1 are the standard brands
• Alternatively: structural silicone applied internally in a uniform thickness

Common mistake: no internal sealing, justified by “it’s dry inside anyway”. In winter, with showers, cooking and drying laundry, internal humidity is 50-70%. Vapour migrates through any unsealed joint due to the pressure gradient, condenses at the coldest point (usually within the reveal), and after 3-5 years you have mould that doesn’t come from outside, but from inside.

The specific tapes in detail

A proper UNI 11673 installation uses a combination of pre-compressed tapes and vapour barriers. The reference brands on the Italian market are:

Brand	Key product	Function
illbruck (Tremco)	Compriband TP600	External water/wind seal
illbruck	ME500	Multi-function seal-and-insulation
illbruck	ME220	Internal vapour barrier
Tescon	Tescon Vana	External wind seal
Tescon	Tescon Profil	Complex corner sealing
Klober	Wallint	Internal vapour membrane
Klober	Wal-int	Internal sealing fabric
Soudal	SWS System	Complete foam-and-tape system
RAUEXPAND	Hannoband	Premium German multi-function
Knauf	Innenanstrich	Integrated German system

An intelligent first question to ask when getting a quote is: “which brand of tape do you use for the three sealing planes?” If the answer is generic (“foam and silicone”) or names DIY-store products (Pattex, Geocel, generic Saratoga), it isn’t UNI 11673.

Thresholds and sills: the critical point

The most difficult points of an installation are lower thresholds (French doors, balconies) and sills. They are critical for three reasons:

1. Drainage: rainwater collected by the sill must be channelled outwards with a minimum 5% fall and a drip groove beneath the threshold.
2. Thermal bridging: a traditional marble or stone threshold is a substantial thermal bridge. UNI 11673 solutions: an insulated sub-threshold with rigid XPS or EPS (Knauf PB80, Foamglas), or dedicated technical thresholds (illbruck SP025).
3. Water/wind seal under the threshold: the joint between threshold and frame must be sealed with specific products (Soudal Fix All Crystal, Sika, MS-Polymer).

On external sills, it must be checked that the stone has the correct slope and that the drip groove underneath is present and not blocked by mortar. On balcony sills, the underside of the screed below the sill must be thermally insulated (XPS).

Sub-frame and preparation of the opening

A proper installation prepares the opening before the window is fitted. The mandatory checks are:

Condition of the reveals:

• Masonry moisture <15% (verifiable with a masonry moisture meter)
• Soundness: no crumbling lime, friable cement, or detached plaster
• Flatness: maximum tolerance 3 mm over 1 metre
• Cleanliness: no residue from old windows, old foam or crumbling mortar

If these conditions aren’t met, preliminary levelling (a fine 5-10 mm render coat) or milling of the weak areas is carried out. This costs an extra €50-150 per window, but without this preparation the subsequent installation is unstable.

Sub-frame:

• In new builds: installed during construction, in treated timber or galvanised metal. Chemically fixed to the masonry.
• In replacements within an existing opening: the existing sub-frame may be reusable (rare in buildings predating 2000), will need replacing (the most frequent case), or may be absent (the worst case, requiring one to be created from scratch).

Sub-frame replacement: €50-150 per window, but essential if the old one is deteriorated.

Fixing the window

UNI 11673 specifies:

• Mechanical anchor fixings: Fischer ULTRACUT FBS-II, Würth WIT-EP, Hilti HUS, sized for the type of masonry
• Maximum distance between anchors: 700 mm
• Distance from corners: 150-250 mm
• Minimum number: 6 anchors per sash on a standard window, 8-10 on a French door
• Distance from the handle: an additional anchor within 250 mm of the main locking point

Frequent mistake: low-cost plastic wall plugs or screws driven directly into the wall. On hollow-brick walls (perforated bricks with internal air chambers, typical of the 1970s-90s), poor fixing fails after 5-10 years of thermal stress and expansion/contraction cycles.

Testing and post-installation checks

A UNI 11673 installation includes documented checks:

Standard checks (always):

1. Visual inspection: seals complete, uniform, with no interruptions
2. Opening/closing test: all sashes operate smoothly, the tilt function engages, the handle locks fully
3. Check of the closing line: all sashes meet the fixed frame uniformly (gap at closure ≤ 1 mm)
4. Hinge adjustment: perfect alignment in all positions

Advanced checks (for major projects or on client request):

• Blower door test: pressurising the building to 50 Pa to measure the n50 (air changes per hour). For nZEB buildings, n50 ≤ 0.6 is required. A test on an average flat costs €300-600.
• Infrared thermography: infrared photos of the windows from outside with the heating on, identifying residual thermal bridges. Cost: €150-400 per flat.

For Ecobonus and Superbonus schemes, the technical sworn statement may require some of these tests, particularly thermography, for substantial works.

The connection with tax incentives

To access tax deductions on window and door replacement, the work must meet precise technical requirements: maximum thermal transmittance by climate zone, declared performance, and — crucially — UNI 11673-compliant installation documented in the technical sworn statement.

The sworn statement is produced by a qualified professional (surveyor, architect, engineer) who signs it under their own professional responsibility. To sign it with confidence, they need to see:

• Technical datasheets for all materials used (tapes, foams, sealants)
• Site photos taken during the installation stages
• Visual verification that the three sealing planes have been respected

Without this documentation, the professional won’t sign. Without the signature, the incentive can’t be obtained. For up-to-date terms and rates for the incentives — including the rules on the “bonifico parlante” (the specifically worded bank transfer required by the Italian tax authorities) and the documents to keep — read our guide to 2026 window and door tax deductions and incentives and always refer to ENEA and the Italian Revenue Agency.

How to spot an installer who doesn’t follow UNI 11673

Practical questions to ask when getting a quote:

1. Do you document the installation with photos of each stage? Correct answer: “yes, we always have them for every site, we’ll send them to you by email too.”
2. Which brand of tape do you use for the three sealing planes? Correct answer: specific names (illbruck, Tescon, Klober, etc.). Wrong answer: “foam and silicone.”
3. Do you replace the sub-frame when needed? Correct answer: “yes, we assess it during the site survey, it’s a line item in the quote or an extra if necessary.” Wrong answer: “we always reuse the existing one.”
4. How long does a job take for a flat with 8 windows? Correct answer: 2-4 working days. Suspicious answer: “half a day” or “one day.”
5. Is there a warranty on the installation? Correct answer: a minimum of 5 years, in writing, on the installation, in addition to the manufacturer’s warranty on the product.

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This UNI 11673 checklist is meant to help you assess the installation work carried out by the installer you choose: if the answers don’t add up, change installer before signing. We manufacture the made-to-measure window, ready for a standard-compliant installation. Request a quote: we’ll assess measurements, profile and glazing best suited to your opening together, and provide you with a complete quote.