The thermal performance of a window depends on every layer in the assembly, the glass panes, the gas trapped between them, the coating applied to the glass surface and the edge spacer holding the panes apart. Each element contributes to the overall U-value (the rate of heat loss per square metre per degree of temperature difference, measured in W/m²K). Understanding how these components interact allows you to specify a window that meets Building Regulations, controls solar gain under Part O and delivers long-term energy savings.

Glass Build-Ups: Single, Double and Triple

Single glazing (one pane, 4–6 mm glass) has a centre-pane U-value of about 5.7–5.8 W/m²K; AGC, for example, gives 5.7 W/m²K for its clear float glass.1 It is now only found in older properties and would not meet the Part L limit for a replacement window.

Double glazing consists of two panes with an air or gas-filled cavity between them, typically 12–20 mm wide, sealed within an Insulated Glass Unit (IGU). A basic 4-12-4 air-filled unit (4 mm glass, 12 mm air gap, 4 mm glass) achieves around 2.6–2.8 W/m²K centre-pane. A soft-coat Low-E pane and an argon fill bring the centre-pane figure down to about 1.1 W/m²K. Pilkington, for example, quotes a Ug-value of 1.1 W/m²K for its Optitherm S3 low-E glass in a 4-16-4 unit filled with 90% argon.2 Cavity width matters: the optimum for air-filled units is 14–16 mm, wider cavities allow convection and reduce performance.

Triple glazing adds a third pane and two gas-filled cavities. A typical 4-16-4-16-4 argon-filled Low-E unit achieves 0.5–0.8 W/m²K centre-pane. The additional weight (with three 4 mm panes instead of two, the glass alone is about 50% heavier than equivalent double glazing) requires heavier frames and stronger opening mechanisms. Triple glazing adds cost of roughly £60–£100 per m² over equivalent double-glazed units.

Low-E Coatings

Low-emissivity (Low-E) coatings are microscopically thin metallic oxide layers, typically silver or tin oxide, applied to one surface of the glass inside the sealed unit.3 They work by reflecting long-wave infrared radiation (heat) back into the room while allowing short-wave solar radiation (visible light and near-infrared) through from outside.4

There are two principal manufacturing routes:

  • Pyrolytic (hard coat) Low-E: baked into the glass surface during manufacture. More durable and can be cut after coating.4 Lower performance than soft coat: in an argon-filled double-glazed unit, about 1.5 W/m²K centre-pane against about 1.1 W/m²K for soft coat.3
  • Sputtered (soft coat) Low-E: applied in a vacuum deposition process after manufacture. Achieves a lower emissivity than hard coat but is delicate, so it must be sealed within the IGU, on surface 2 or 3.3

The coating is identified by its position within the unit. Surfaces are numbered from the outside in, so in double glazing surface 2 is the inner face of the outer pane and surface 3 is the cavity face of the inner pane. For double glazing in the UK, the coating normally goes on surface 3, where it is marginally more effective at keeping heat in.3 For triple glazing, coatings are commonly placed on surfaces 2 and 5.

Solar factor (g-value): Low-E glass also has a solar factor, the proportion of solar energy that passes through. A high g-value (e.g. 0.60–0.72) admits more solar heat gain (desirable on south-facing elevations in winter) while a low g-value (0.30–0.45) limits solar gain (used in Part O calculations to prevent overheating). Specifying the wrong g-value on south or west elevations can lead to overheating failures under Part O (the 2021 edition, in force from 15 June 2022).5

Gas Fills

The gas fill between panes reduces convection and conduction across the cavity. Air (conductivity ≈ 0.024 W/mK) is the baseline.

Gas FillThermal Conductivity (W/mK)Approximate U-value Improvement vs AirCost Premium
Air0.024BaselineNone
Argon0.016~0.15–0.2 W/m²K improvementLow (~5–10%)
Krypton0.009~0.3–0.5 W/m²K improvementModerate (~20–40%)
Xenon0.005~0.5+ W/m²K improvementHigh (rarely specified)

Argon is by far the most common fill in UK residential glazing. It is inert, non-toxic, cheap relative to krypton, and the optimum cavity width for argon is 14–16 mm. BS EN 1279-3 is the standard behind the gas retention claims, and it is worth knowing what it actually says: it sets a maximum leakage rate, not a lifetime retention figure, and the limit is 1% of the fill per year.6 A unit at exactly that limit is down to about 78% of its original argon after 25 years, so the 90 to 95% over 25 years that gets quoted is a manufacturer’s performance against the standard rather than something the standard promises. Ask for the tested leakage rate rather than the retention headline.7

Krypton is used where thin cavities are required (e.g. secondary glazing retrofits or slim-profile triple-glazed units) because it performs best in narrower cavities than argon. It is much more expensive than argon, which is why it is kept for slim units where the space saving matters.

Warm-Edge Spacers

The edge spacer separates the panes around the perimeter of the IGU and is bonded into the hermetic seal. Aluminium was historically the standard material, still found in older units, but aluminium is highly conductive (conductivity ≈ 160 W/mK) and creates a thermal bridge around the window edge, increasing heat loss at the edge of the glass and making cold-edge condensation more likely.

Warm-edge spacers replace aluminium with lower-conductivity alternatives:

Spacer TypeConductivity (approx.)Material
Aluminium (legacy)~160 W/mKExtruded aluminium hollow section
Stainless steel~15 W/mKThin rolled section
TPS / silicone foam~0.2–0.4 W/mKThermoplastic (polyisobutylene-based) spacer, or structural silicone foam
Swisspacer / polymer hybrids~0.2–0.3 W/mKGlass-fibre reinforced polymer

The linear thermal transmittance of the edge (the psi-value, ψ, in W/mK) determines how much the edge spacer affects the whole-window U-value. Warm-edge spacers reduce ψ from approximately 0.08 W/mK (aluminium) to 0.04–0.05 W/mK (stainless) or 0.03–0.04 W/mK (polymer foam). On a small window this may improve the whole-window U-value by 0.1–0.2 W/m²K, meaningful for compliance calculations.

Whole-window U-values, such as those behind BFRC (British Fenestration Rating Council) ratings, include the frame and the edge effect; a centre-pane U-value makes no allowance for the edge spacers or the frame.8 Always request the whole-window U-value when specifying for Part L compliance, because the Part L limits apply to the entire unit, including the frame.8

Part L (Thermal) and Part O (Overheating)

Approved Document L Volume 1 (2021 edition, in force from 15 June 2022, so often called the 2022 rules) sets a limiting U-value of 1.4 W/m²K for a replacement window in an existing dwelling, or Window Energy Rating Band B as an alternative route.8 In a new dwelling the limiting value is looser at 1.6 W/m²K, because new build is controlled by the target rates rather than by the limit: the notional dwelling that sets those targets assumes 1.2 W/m²K, which is why 1.2 is the figure a new build usually has to reach in practice.8 Both are whole-window values. New homes then move to the Future Homes Standard, whose regulations come into force on 24 March 2027.9

Standard compliance routes:

  • Single window replacement: must achieve ≤1.4 W/m²K whole-window, or Window Energy Rating Band B8
  • Extension: new windows should meet 1.4 W/m²K, and the total area of windows, rooflights and doors should not exceed 25% of the extension’s floor area plus the area of any windows and doors the extension removes or covers; the alternative is to show that the extension’s area-weighted U-value is no worse than that of a compliant extension of the same size and shape8
  • New build: set by the SAP target rates rather than a single limit, and the notional dwelling’s 1.2 W/m²K is the usual window specification

Approved Document O (2021 edition, in force from 15 June 2022) applies to new dwellings.5 Its risk category is not something glazing can push you into: under the simplified method the category is fixed by location (a high risk location is urban and some suburban parts of London, with guidance for parts of central Manchester; everywhere else in England is moderate risk) and by whether the dwelling has cross-ventilation, meaning openings on opposite façades.5 What glazing decides is whether you then pass, because the category selects a maximum glazing area for the building and for the most glazed room, taken on the most glazed façade’s orientation. The mitigation options include reducing window-to-floor-area ratio, specifying lower g-value glass, adding external shading, or relying on thermal mass. Large south-facing rooflights and bifold/sliding door walls are common triggers. A dynamic thermal model (CIBSE TM59 methodology) may be required if the simplified method fails.5

Safety Glass: Part K

Approved Document K protects against impact with glazing in what it calls critical locations, and the list is narrower than the one that gets repeated on merchant websites:10

  • In a door or a door side panel, from floor level up to 1,500 mm
  • In a side panel within 300 mm of the edge of a door, up to 1,500 mm
  • Elsewhere in a wall or a window, from floor level up to 800 mm

Being in a bathroom is not itself a critical location. What matters is the height above the floor and the proximity to a door, and a bathroom window below 800 mm qualifies for that reason rather than because of the room it is in. Safety glazing is also only one of four ways Part K allows you to satisfy it: glass that breaks safely, glazing that is inherently robust, glazing in small panes, or glazing that is permanently protected.10 Where the safe-breakage route is used, the classification to look for is Class 3 of BS EN 12600 or Class C of BS 6206, or Class 2 / Class B in a door or side panel wider than 900 mm.10 BS EN 12150-1 and BS EN 14179 are the product standards for toughened and heat-soaked toughened glass, which is a different question from whether the pane passes the impact test.

Safety glass must be either toughened (tempered) or laminated. Laminated glass (PVB interlayer) is generally preferred for overhead applications and rooflights where broken fragments must remain in place; toughened glass shatters into small blunt pieces but falls freely.

Typical Glazing Specifications and U-values

SpecificationWhole-Window U-value*Suitable Application
4-12-4 air, no Low-E2.6–2.8 W/m²KLegacy; does not meet Part L
4-16-4 argon, Low-E soft coat1.2–1.4 W/m²KReplacement windows (compliant with a good frame)
4-14-4 argon, Low-E, warm edge1.1–1.4 W/m²KStandard new-build specification
4-16-4-16-4 argon, 2× Low-E, warm edge0.6–0.8 W/m²KTriple glazing, high fabric spec
4-8-4 krypton, Low-E (slim unit)Usually higher than a 4-16-4 argon unitSlim-profile heritage or secondary

*Indicative whole-window values based on typical PVC-U casement frames. Timber and aluminium frames vary.

Where windows cannot meet the Table 4.2 limit because the character of a historic building has to be kept, Approved Document L instead accepts glazing with a centre-pane U-value of no more than 1.2 W/m²K, or low-emissivity secondary glazing over the existing single glazing.8

For most UK residential replacement projects, a 4-16-4 argon-filled unit with a soft-coat Low-E and a polymer warm-edge spacer in a uPVC or timber frame typically achieves about 1.2–1.4 W/m²K whole-window, depending on the frame, and represents the best value-for-money specification at roughly £80–£130 per m² supply-only for the IGU, plus frame and installation costs.

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.

References (10)

Figures in this note were checked against the sources below on 2 October 2026. Superscript numbers in the text point to them. Every source the site cites, by topic.

  1. 1technicalsupplier figureAGC Glass Europe, guidance.RN-QPEIGSAGC's UK product page for its Planibel clear float glass. It lists a Ug-value of 5.7 W/m²K for the single clear pane, the baseline for single glazing.
  2. 2technicalsupplier figurePilkington (NSG Group), a product specification.RN-586FDUPilkington Optitherm S3, a neutral off-line (magnetron sputtered) coated low-emissivity glass. Ug-value 1.1 W/m2K in a standard 4-16-4 insulating glass unit with 90% argon fill. This is the centre-pane Ug, not the whole-window Uw, and the two are routinely confused. Light transmittance 82%, light reflectance 11%. Available from 3mm to 12mm thickness. Optitherm S3 Pro T is the toughenable version, colour matched after toughening.
  3. 3technicalsupplier figureGlazing Vision, guidance.RN-FDS2APA rooflight maker's technical article on low-e glass. It explains that hard coats are tin oxide applied on the float line and soft coats are a sputtered silver layer, applied to one face of the glass.
  4. 4technicalsupplier figurePilkington United Kingdom Limited, guidance.RN-S1296BPilkington's product page for K Glass, a hard-coated low-emissivity glass. It describes how a Low-E coating reflects long-wave heat back indoors while admitting solar energy.
  5. 5technicalvery strongMinistry of Housing, Communities and Local Government.RN-R7QZ7MSupports Part O of Schedule 1 to the Building Regulations 2010 (requirement O1 and regulation 40B); the 2021 edition took effect on 15 June 2022 in England, with transitional relief for work notified before that date and started before 15 June 2023. Critically, paragraph 0.3 limits the guidance to NEW residential buildings only (dwellings, residential institutional and residential other, plus shared communal rooms, common spaces and live/work units), and paragraph 0.7 applies it only to the residential parts of mixed-use buildings. Compliance is by the simplified method (Section 1) or dynamic thermal modelling to CIBSE TM59 (Section 2); the simplified method splits England into a high risk location (urban and some suburban London listed in Appendix C, with guidance for parts of central Manchester) and a moderate risk location (the rest of England). Table 1.1 (cross-ventilated) caps glazing as a percentage of floor area at north 15 high risk / 18 moderate, east 18/18, south 15/15, west 18/11, with the most glazed room capped at north 37/37, east 37/37, south 22/30, west 37/22. Table 1.2 (no cross-ventilation) caps glazing at north 15/18, east 11/18, south 11/15, west 11/11, most glazed room north 26/26, east 18/26, south 11/15, west 18/11.
  6. 6technicalsupplier figureSparklike, guidance.RN-JN88QWA summary of EN 1279-3:2018 by a maker of insulating-glass gas analysers. It states the standard's limits: average gas leakage of no more than 1.0% a year, no unit above 1.20%. Used because the standard itself is paywalled and the BSI page gives no figures.
  7. 7technicalstrongBSI, guidance.RN-ONRM0Qthe standard that sets the maximum gas leakage rate for a sealed insulating glass unit
  8. 8technicalvery strongMinistry of Housing, Communities and Local Government.RN-L0FCJXThe in-force U-value tables a DIY insulation or door note must quote, with the exact numbers. Table 4.2, limiting U-values for new fabric elements in existing dwellings, sets roof 0.15, wall 0.18, floor 0.18, swimming pool basin 0.25, window 1.4 or Window Energy Rating Band B minimum, rooflight 2.2, doors with more than 60 per cent of the internal face glazed 1.4 or Doorset Energy Rating Band C minimum, and other doors 1.4 or Doorset Energy Rating Band B minimum. Table 4.3, for existing elements, gives a roof threshold of 0.35 and an improved value of 0.16, which is the figure a loft insulation note needs: paragraph 4.12 says a retained element whose U-value is higher than the column (a) threshold, for example through a loft or garage conversion, should be upgraded to the column (b) value. Paragraph 4.13 allows a lesser upgrade only where the improved value is not technically or functionally feasible or would not achieve a simple payback of 15 years or less. Paragraph 4.8 requires a replacement element to be no worse than the one it replaces as well as meeting Table 4.2. Paragraph 4.10 gives the heritage exception: where character must be maintained, a centre pane U-value of no more than 1.2 or low-emissivity secondary glazing. Note 9 records that the timber window concession of 1.6 or Band C ended on 14 June 2023. The 2021 edition came into force on 15 June 2022; a 2026 edition exists at https://assets.publishing.service.gov.uk/media/69c122a6cfa346b9d4704a55/ADL1_2026.pdf.
  9. 9technicalvery strongMinistry of Housing, Communities and Local Government, guidance.RN-WMRZGFThe government's circular letter to building control bodies announcing the Future Homes and Buildings Standards: the Building Regulations etc. (Amendment) (England) Regulations 2026 and the 2026 editions of Approved Documents L and F. It gives the purpose (new buildings built with low carbon heating and high energy efficiency), the in-force date of 24 March 2027 (24 September 2027 for higher-risk building work), and the transitional arrangements, under which work notified before 24 March 2027 and started before 24 March 2028 stays under the previous rules.
  10. 10technicalvery strongMinistry of Housing, Communities and Local Government.RN-XH6E39The standard that decides whether a loft ladder is legal and when a raised deck needs a balustrade. Published 3 January 2013, it covers protection from falling, collision and impact including stairs, ladders, ramps, guarding and vehicle barriers. Paragraph 1.31 states that retractable ladders must not be used as a means of escape. Paragraph 1.32 permits a fixed ladder, with fixed handrails on both sides, only for access in a loft conversion containing one habitable room, and only where there is not enough space without altering the existing space for a compliant stair, which is the rule that catches most loft ladder installations. Paragraph 1.29 restricts alternating tread stairs to loft conversions. For a private stair the maximum pitch is 42 degrees (Table 1.1 note 1), the normal relationship between rise and going is twice the rise plus the going between 550mm and 700mm, and for dwellings external tapered steps forming part of the building need a going of at least 280mm. Paragraph 3.1 requires guarding wherever it is reasonably necessary for safety at the edge of a floor, gallery, balcony, roof, light well, basement or similar sunken area. Paragraph 3.2 requires guarding to be at least the height in Diagram 3.1, allows any wall, parapet or balustrade to serve as guarding, and requires it to resist the loads in BS EN 1991-1-1 with its UK National Annex and PD 6688-1-1, with BS 6180 for barrier and infill panel design. Handrails should be positioned 900mm to 1000mm from the pitch line or floor.

  1. Planibel Clear, AGC Glass Europe, accessed 2 October 2026. agc-yourglass.com

  2. Pilkington Optitherm S3, Pilkington (NSG Group), accessed 30 September 2026. pilkington.com

  3. Improving glass performance with low e coatings, Glazing Vision, accessed 2 October 2026. glazingvision.co.uk

  4. Pilkington K Glass, Pilkington United Kingdom Limited, accessed 30 September 2026. pilkington.com

  5. Approved Document O, Overheating, 2021 edition, Ministry of Housing, Communities and Local Government, accessed 14 September 2026. assets.publishing.service.gov.uk

  6. Why Gas Concentration Matters: Key Takeaways from EN 1279-3:2018, Sparklike, accessed 30 September 2026. sparklike.com

  7. BS EN 1279-3:2018, Glass in building. Insulating glass units. Long term test method and requirements for gas leakage rate and for gas concentration tolerances, BSI, accessed 14 September 2026. knowledge.bsigroup.com

  8. Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, Ministry of Housing, Communities and Local Government, accessed 14 September 2026. assets.publishing.service.gov.uk

  9. The Future Homes and Buildings Standards: Building Circular 01/2026, Ministry of Housing, Communities and Local Government, accessed 29 September 2026. gov.uk

  10. Approved Document K, Protection from falling, collision and impact, 2013 edition, Ministry of Housing, Communities and Local Government, accessed 14 September 2026. assets.publishing.service.gov.uk