How to specify uPVC windows
What a PVC-U window clause has to fix, the regulatory decisions behind each opening light, the standards to cite, and how to write it in CAWS and in Uniclass.
How do you specify uPVC windows?
To specify uPVC windows, name the profile system and fabricator standard (BS EN 14351-1 with BS 7412), then set the performance the opening needs: whole-window U-value for Part L, PAS 24 security for accessible windows in new dwellings, BS 6375 weathertightness classes for the exposure, and safety glazing in critical locations. Record escape windows, restrictors and trickle vents per window type, and leave sizes and locations to the window schedule.
PVC-U windows are usually bought as a fabricator's product built from a profile system house's extrusions, so the specification is the only place the two are tied to one set of performance figures. When the clause just says "white uPVC casement, double glazed" the tender returns will vary in U-value, security, hardware and trickle vent area, and nobody can tell which one complies.
This guide covers Uniclass Pr_30_59_98_92 Unplasticized polyvinyl chloride (PVC-U) window units. Windows with an aluminium cladding over a PVC-U or timber core are a separate code, Pr_30_59_98_15 Composite window units, and PVC-U roof windows have their own code too, so check the product before you pick the heading.
What does a uPVC window clause need to cover?
Some properties are specific to uPVC windows. Others belong in every product clause. A clause that leaves either out gives the contractor room to substitute.
Properties of uPVC windows
- Frame profileSystem depth and sightlines; flush or lipped sash.
- Opening configurationCasement, tilt and turn or sliding sash per type.
- Operation methodManual in almost every case.
- Thermal performanceWhole-window Uw for Part L, not centre-pane.
- Glazing specificationIGU build-up, low-e, gas fill, warm edge spacer.
- Glazing beadsInternal beads for security and weather.
- Structural performanceNever guessWind load class to BS EN 12210 for the site.
- Weather resistanceAir, water and wind classes to BS 6375-1.
- Category of useOperation and strength to BS 6375-2.
- Acoustic performanceRw + Ctr where the noise assessment sets one.
- Free ventilation areaTrickle vent equivalent area for Approved Document F.
- Security ratingNever guessPAS 24 for accessible windows in new dwellings.
- Means of escapeNever guessEscape windows: clear opening and sill height.
- Window opening restrictionNever guessRestrictors and guarding where sills are low.
- Safety glazing classificationNever guessBS EN 12600 class in critical locations.
- Reaction to fire classificationNever guessOnly where the fire strategy asks for a frame class.
- IronmongeryMultipoint locking, lockable handles, hinge type.
- FixingsFrame fixings or straps at the stated centres.
- AccessoriesCills, cill ends, trims and add-on profiles.
- Third-party certificationKitemark or equivalent for fabricator and security.
What every product clause needs
- Manufacturer and product referenceProfile system and fabricator, or a performance description.
- Product standardBS EN 14351-1, with BS 7412 for PVC-U.
- Installation methodFabricator's instructions and BS 8213-4.
- Technical submittalsDoP, U-value calculation, PAS 24 certificate before ordering.
- Operation and maintenance informationHardware adjustment, gasket care, restrictor release.
- MaterialPVC-U profile, steel reinforced where the system requires.
- FinishWhite, foiled or sprayed; inside and outside colours.
Uniclass Pr_30_59_98_92 uPVC windows. Not every property applies to every project; where one does not, say so rather than leaving it blank.
Fields marked Never guess depend on the project and should never be filled from a default or a previous job. If the information is not available yet, the clause should say so with a TBC and a reason.
Which decisions does the clause depend on?
Most properties follow from the product you choose. These depend on the project, so they need a decision from the designer before the clause is written.
- 01
Whole-window U-value
Part LPart L limits the U-value of the whole window, frame and glass together, not the centre-pane figure printed on the glass unit. Approved Document L gives a limiting value of 1.6 W/m²K for windows in new dwellings, and new or replacement windows in existing dwellings need 1.4 W/m²K or a Window Energy Rating of band B. The new-build SAP calculation usually assumes something better than the limiting value, so the figure that matters is the one in the energy assessment. Frame width counts: a chunky PVC-U frame on a small window can lose a lot of the glass unit's advantage.
How to decide
- New dwellings: take the window U-value from the SAP or energy model, which is normally tighter than the 1.6 W/m²K limit.
- Replacement windows: 1.4 W/m²K or WER band B at minimum.
- Ask for Uw for the actual window size or the standard reference size, calculated to BS EN ISO 10077.
- Check the glass unit, spacer and gas fill in the submittal match the unit the U-value was calculated for.
Write it asThermal performance: Whole-window U-value (Uw) not more than 1.4 W/m²K, calculated to BS EN ISO 10077. Submit calculation. - 02
Security rating and Part Q
Never guessPart Q applies to new dwellings in England, including those formed by a change of use. Approved Document Q asks for easily accessible windows to be secure, and names PAS 24 as one way to show it. A window is easily accessible if any part of it is within 2 m vertically of a level you can stand on, or within 2 m of a flat or low-pitched roof that is itself within 3.5 m of the ground. PAS 24 is a test of the complete window, so a PAS 24 profile system made up with different hardware or glass is not a PAS 24 window.
How to decide
- New dwellings: mark every easily accessible window on the schedule and require PAS 24 for those types.
- Ask for a certificate for the window as made by the fabricator, not only the profile system house's test.
- PAS 24 windows are often tested with a laminated pane; keep the tested glass in the glazing line.
- Secured by Design projects: check the current PAS 24 edition the police scheme accepts.
Write it asSecurity: PAS 24 certified to windows identified as easily accessible on the window schedule. Submit fabricator's third-party certificate. - 03
Escape windows, restrictors and guarding
Never guessThree regulations act on the same opening light. Approved Document B needs an emergency escape window, where one is required, to give an unobstructed opening of at least 0.33 m², at least 450 mm high and 450 mm wide, with the bottom of the opening not more than 1100 mm above the floor. Approved Document K wants guarding where the sill is lower than 800 mm, and Approved Document O raises guarding to 1100 mm in new residential buildings where windows are opened to limit overheating. The window opening restriction you fit must not defeat the escape window: stays on escape windows need a release catch.
How to decide
- Flag escape windows on the schedule and ask for egress hinges that give the full clear opening, not just the sash size.
- Low sills: decide between a guarding rail, laminated glass below 800 mm as part of the guarding, or a fixed light.
- Where Part O relies on opening windows, set the guarding height to 1100 mm for those windows.
- Use releasable restrictors on escape windows; never permanent restrictors.
Write it asMeans of escape: Where shown as escape windows, egress hinges giving clear opening not less than 450 mm wide and 0.33 m², restrictors releasable without a key or tool. - 04
Wind load and weathertightness classes
Never guessBS 6375-1 is the UK guide that turns site exposure into classes for air permeability, watertightness and resistance to wind load, tested to the BS EN 12207, 12208 and 12210 classifications. Exposed coastal or high-rise sites need higher classes than a sheltered two-storey terrace, and large PVC-U lights need steel reinforcement in the profile to meet the wind class. The structural performance is the line most often copied from a previous job, because the site wind pressure needs a calculation someone has to own.
How to decide
- Get the design wind pressure from the engineer, or ask the window supplier to calculate it to BS EN 1991-1-4 and its UK National Annex.
- Select classes using BS 6375-1 for that pressure and the exposure.
- Check the largest opening light in each type, as that sets the reinforcement.
- State the classes; do not write "to suit exposure" and leave it there.
Write it asWeathertightness: BS 6375-1, air permeability class 4, watertightness class E1200, wind resistance class C4 for design wind pressure stated in the window schedule. - 05
Safety glazing and fire
Never guessApproved Document K treats glazing in a window within 800 mm of the floor as a critical location, and BS 6262-4 sets out where safety glass is needed and what class. The glass must then meet a class from the BS EN 12600 pendulum test, which is printed on the pane mark. On reaction to fire classification, regulation 7(3) lists window frames and glass among the exemptions from regulation 7(2), so PVC-U windows can be used on relevant buildings, but check the fire strategy for anything it says about window frames near boundaries or on escape routes.
How to decide
- Mark critical-location panes on the elevations or schedule.
- Use toughened or laminated glass in those panes, with the class stated.
- Where low glazing also acts as guarding, specify laminated glass so it stays in place if broken.
- Ask the fire engineer whether any window needs a declared class; normally none does.
Write it asSafety glazing: Panes in critical locations to BS EN 12600 Class 1(C)1 or 1(B)1, laminated where acting as guarding.
The values under "Write it as" are examples. The right value depends on the location, the use and the fire strategy.
Which standards should the clause cite?
Cite the product standard the declaration of performance refers to, the code of practice for installation, and the test method behind each value you set. A value without its test method cannot be checked against a submittal.
- BS EN 14351-1
- Windows and doors. Product standard, performance characteristics. Windows and external pedestrian doorsets
The harmonised product standard. It sets the characteristics a declaration of performance covers, including U-value, air permeability, watertightness, wind resistance and acoustic performance.
In the clause Cite as the product standard with the characteristics you need: "Standard: BS EN 14351-1, performance declared".
- BS 7412:2024
- Windows and doorsets. Unplasticized polyvinyl chloride (PVC-U) extruded hollow profiles. Specification
The UK specification for PVC-U windows and doorsets made from hollow extrusions, covering construction, reinforcement, glazing and performance.
In the clause Add it next to the product standard: "Manufacture: to BS 7412".
- BS EN 12608-1
- Unplasticized poly(vinyl chloride) (PVC-U) profiles for the fabrication of windows and doors. Classification, requirements and test methods. Non-coated PVC-U profiles with light coloured surfaces
Classifies the extruded white or light-coloured profiles themselves, including climate zone, impact and wall thickness class. Foiled or coated profiles are covered by other parts of the series.
In the clause "Profiles: to BS EN 12608-1, as the system holder's declaration".
- BS 6375 series
- Performance of windows and doors
Part 1 gives weathertightness classes and guidance on choosing them for exposure, Part 2 operation and strength, and Part 3 additional characteristics such as security and durability.
In the clause "Weathertightness: to BS 6375-1, classes as schedule. Operation and strength: to BS 6375-2."
- PAS 24
- Enhanced security performance requirements for doorsets and windows in the UK
The attack test for complete windows. Approved Document Q names PAS 24:2016; the current edition is PAS 24:2022+A1:2024.
In the clause "Security: PAS 24, current edition, third-party certified for the window as fabricated".
- BS EN 12600
- Glass in building. Pendulum test. Impact test method and classification for flat glass
The impact classes (for example 1(B)1, 2(B)2) used to show glass is safety glass.
In the clause "Glass in critical locations: BS EN 12600 Class 1(B)1 or better".
- BS 6262-4
- Glazing for buildings. Code of practice for safety related to human impact
Where safety glazing is needed and which impact class to use, including low-level glazing that acts as protection from falling.
In the clause "Safety glazing: locations and classes to BS 6262-4".
- BS EN ISO 10077 series
- Thermal performance of windows, doors and shutters. Calculation of thermal transmittance
How the whole-window U-value is calculated from frame, glass and spacer values.
In the clause "Uw not more than 1.4 W/m²K, calculated to BS EN ISO 10077".
- BS 8213-4
- Windows and doors. Code of practice for the survey and installation of windows and external doorsets
Survey of openings, fixing, packing, sealing and the interface with cavity closers and damp-proof courses, for new and replacement work.
In the clause "Installation: to BS 8213-4 and the fabricator's instructions".
Confirm the current edition of each standard at every issue. Avoice's State of UK Specifications 2026 found a withdrawn standard cited in 40% of specifications at contract issue.
For the regulations behind these choices, see Part L and the Future Homes Standard, Approved Document K explained, or search the Avoice Regulations library.
How do you write it in CAWS?
In CAWS, PVC-U windows sit in work section L10. Unlike a tiling or render section, L10 does not bundle the window into a "type of" system clause. Each window type gets its own product clause, usually numbered in the 200s and 300s, which names the system, the standard, the performance classes, the glazing, the hardware and the finish, and points to the execution clauses for fixing.
General requirements sit at the front of L10 (evidence of performance, site dimensions, control samples) and installation sits at the back (protection, building in, fixing of PVC-U frames, sealant joints, ironmongery). The window schedule is referenced from each type clause. If a project has a casement type and a tilt and turn type, that is two product clauses sharing the same 700 and 800 series clauses.
- Manufacturer
- Submit proposals
- Product reference
- Submit proposals
- Standard
- BS EN 14351-1 and BS 7412
- Configuration
- Side hung and top hung casements, as window schedule
- Exposure category/ design wind load
- BS 6375-1, classes and design wind pressure as window schedule
- Operation and strength characteristics
- BS 6375-2
- Thermal performance
- Uw not more than 1.4 W/m²K, calculated to BS EN 14351-1 and BS EN ISO 10077
- Glazing details
- Double glazed insulating glass units to BS EN 1279, low-e, argon filled, warm edge spacer
- Safety glazing
- Critical locations to BS 6262-4, BS EN 12600 Class 1(B)1 or better
- Beading
- Internal
- Security
- PAS 24 certified to windows marked easily accessible on window schedule
- Background ventilation
- Trickle vents, equivalent area as ventilation strategy, controllable by occupant
- Escape windows
- Egress hinges, clear opening not less than 450 x 450 mm and 0.33 m²
- Restrictors
- Releasable restrictors to all opening lights above ground floor
- Ironmongery/ accessories
- Espagnolette multipoint locking, lockable handles, as clause 820
- Finish as delivered
- Internally white
- External colour
- TBC Planning condition on external colour not yet discharged. Resolve before ordering.
- Fixing
- As clause 783
Illustrative, not from a real project. One property is left as a TBC with a reason so it can be tracked and closed before issue.
Clauses the type clause relies on
- L10/110
- Evidence of performance Third-party certificates, DoP and test reports for the window as fabricated.
- L10/120
- Site dimensions Measure openings before manufacture; no sizes taken from drawings alone.
- L10/710
- Protection of components Protective film and storage until the end of wet trades.
- L10/765
- Window installation generally Plumb, square and packing, with the frame in the specified position in the wall.
- L10/783
- Fixing of PVC-U frames Fixing type and positions from corners and hinge points.
- L10/810
- Sealant joints Perimeter seals inside and out, and the sealant type.
Clause numbers vary between NBS versions and practice masters. The numbers here follow typical NBS L10 specifications; what matters is that the type clause points to clauses that exist in your L10.
Photo: W.carter, CC BY-SA 4.0, via Wikimedia Commons.
How do you write it in Uniclass?
In a Uniclass specification the window system clause, Ss_25_30_95_95 Window systems, describes the installed result: the window type in its opening, fixing, perimeter seal, cavity closer interface and the performance of the finished window in the wall. The PVC-U window itself is a separate product clause with its Pr code.
This split works well for windows because the same product is often installed in different ways: a new-build cavity wall with a closer and a replacement into an existing solid wall, for instance. Each installation gets its own system clause, and both point at one PVC-U window product clause, so changing the window spec changes it everywhere.
- Description
- PVC-U casement windows in masonry cavity wall
- Window
- Pr_30_59_98_92 Unplasticized polyvinyl chloride (PVC-U) window units (W01)
- Position in opening
- Frame overlapping cavity closer, as head, jamb and cill details
- Fixing
- Frame fixings to BS 8213-4, positions as fabricator's instructions
- Perimeter seal
- Air seal internally, weather seal externally, sealants to BS EN 15651-1
- Cill
- PVC-U cill with end caps, falls to outside
- Installed performance
- Weathertightness classes as window product clause, no reduction at perimeter
- Manufacturer
- Submit proposals
- Standard
- BS EN 14351-1 and BS 7412
- Opening configuration
- Side hung and top hung casements, as window schedule
- Thermal performance
- Uw not more than 1.4 W/m²K
- Weather resistance
- BS 6375-1, classes as window schedule
- Glazing specification
- Double glazed, low-e, argon, warm edge spacer, internally beaded
- Security rating
- PAS 24 where marked easily accessible
- Free ventilation area
- Trickle vents as ventilation strategy
- Window opening restriction
- Releasable restrictors above ground floor
- Finish
- TBC External colour awaiting planning approval. Resolve before ordering.
Illustrative. Performance of the finished system sits in the system clause; properties of the product sit in the product clause.
In NBS Chorus the PVC-U window product clause also carries the NBS reference 45-25-99/330. Use whichever your practice's masters use, but do not mix CAWS and Uniclass references in one specification.
What goes in the spec, and what goes in the schedule?
The specification defines each type once. The schedule and drawings say where each type goes.
In the specification
What the product is and how it performs and is installed. Written once per type.
In the schedule or drawings
Where each type goes and how much of it. Changes with every plan revision.
Write each window type once as a product with a type mark (W01, W02) and keep sizes, handing and per-opening flags in the window schedule. Escape and Part Q flags belong on the schedule because they change with every plan revision.
For coordination between these documents, see Why your window and door schedules don’t match your specification.
What are the common mistakes?
Most problems are a missing property, a property filled with the wrong kind of value, or a decision left silently open.
- A centre-pane U-value. The 1.1 or 1.2 W/m²K printed for the glass unit is not the window. Part L and the SAP use the whole-window figure, which is worse.
- PAS 24 claimed on the profile, not the window. Security is tested on a complete window. Ask for the fabricator's certificate for the hardware and glass you are actually buying.
- Trickle vents left out of replacement windows. Approved Document F expects replacements to keep vents at least as large as the old ones. Removing them to look tidier is a compliance problem.
- Escape window sized by sash, not opening. Standard friction hinges can leave less clear opening than the sash size suggests. Ask for egress hinges and check the clear dimension.
- Permanent restrictors on an escape window. They stop the window being used for escape. Fit releasable restrictors and guarding where the sill is low.
- No reinforcement check on big lights. Large PVC-U sashes and mullions on exposed sites need steel reinforcement to meet the wind class. State the class so the fabricator has to prove it.
How Avoice helps
Avoice's spec writer drafts PVC-U window clauses from the window schedule and drawings, and shows you which openings still lack a decision.
- Drafts from your schedules and drawings
It reads window types (W01, W02) from the window schedule and elevations, picks the L10 or Uniclass heading, and fills in configuration, glazing and performance where your information supports it.
- Leaves a TBC where the decision is open
If the external colour is waiting on a planning condition, or nobody has confirmed the design wind pressure, the line says TBC with the reason and goes into the project TBC register to close before tender.
- Writes in your format and your style
Choose CAWS or Uniclass. Avoice adapts your practice's own L10 masters rather than replacing them with generic text.
- Checks the clause against the drawings
QA/QC flags a W04 on the elevations that the specification never defines, a clause still citing the superseded BS 7412:2007, or a type clause pointing at a fixing clause that was deleted.
- Keeps product data and regulations to hand
Declarations of performance and PAS 24 certificates live in the Product Room, linked to the clause, and Approved Documents B, K, L and Q are in the Avoice Regulations library for checking each window flag.
Related guides: How to specify aluminium windows, How to specify timber windows, How to specify cavity trays.
Software can draft and check. The designer still chooses the product and is responsible for the specification.
Draft your uPVC window clause in Avoice
Upload your drawings and schedules. Avoice drafts the clause in CAWS or Uniclass, cites the standards and flags anything your information does not settle.
This guide is for general education and is not professional advice. Check current editions of standards and Approved Documents, the manufacturer's data, and the requirements of your project before relying on any clause.