Reference · advanced · 3 min read
BS EN 1279 and insulating glass units
BS EN 1279 governs insulating glass units (IGUs) — sealed multi-pane assemblies with gas fill and spacer systems. It defines durability, moisture control and conformity assessment for glazing used in roof lights.
The sealed insulating glass unit (IGU) at the heart of a modern roof light is a precision product. BS EN 1279 is the European standard series that defines how IGUs are constructed, tested and conformity-assessed before they reach site. For specifiers and building control officers, it is the reference behind CE/UKCA marking of factory-made double and triple glazing.
This article summarises the standard’s scope and practical implications for roof light procurement — it is not a substitute for the full normative text.
Definition
Insulating glass unit (IGU) — A glazing assembly of two or more panes separated by one or more cavity spaces, the edges of which are sealed so that the cavity(s) form a closed space filled with air or gas, manufactured as a single unit.
Structure of BS EN 1279
BS EN 1279 is published in several parts:
- BS EN 1279-1 — Generalities, definitions and requirements for the complete standard.
- BS EN 1279-2 — Long-term test method and requirements for moisture penetration (water vapour transmission into the cavity).
- BS EN 1279-3 — Long-term test method and requirements for gas leakage and gas concentration tolerances.
- BS EN 1279-4 — Methods of test for edge seal materials and components.
- BS EN 1279-5 — Factory production control and periodic tests for conformity assessment.
Together they ensure an IGU maintains its thermal and optical performance over service life — not merely on the day it leaves the factory.
IGU components relevant to roof lights
Glass panes — Usually toughened outer panes for impact and load, with laminated inner panes for safety where BS 6262 requires containment. Heat-strengthened or annealed panes appear in some builds per structural design.
Low-E coatings — Microscopic silver-based layers on a cavity-facing surface reflect long-wave radiation, improving U-value. Coating position (surface #2 or #3 in a double unit) is fixed at manufacture.
Spacer bar — Maintains cavity width. Warm-edge spacers (thermoplastic or stainless) reduce heat loss at the perimeter compared with plain aluminium.
Primary seal — Butyl-based, restricts moisture vapour.
Secondary seal — Structural silicone or polysulphide, bonds the assembly and retains gas fill.
Gas fill — Argon or krypton replaces air in the cavity, lowering heat transfer. Concentration and retention are verified under Part 3 long-term testing.
Conformity and marking
IGUs sold into UK construction fall under the Construction Products Regulation. Manufacturers operate factory production control (FPC) audited by a notified body where required, and apply CE or UKCA marking with a declaration of performance (DoP).
The DoP states essential characteristics — including thermal transmittance when declared — referenced to test methods in BS EN 673 and product standard BS EN 14351-1 for windows and rooflights incorporating the unit.
On site, verify:
- Marking on the unit or batch documentation matches the order.
- No edge damage before installation — crushed seals compromise gas retention.
- Compatible setting blocks and edge cover per BS 8213 so the seal is not stressed.
Failure modes
The visible symptom of IGU failure is mist or droplets between panes — not wipeable from either face. Seal breach admits moist air; the low-E coating may oxidise; gas fill dissipates; U-value drifts from the declared figure.
Replacement of the IGU — or complete roof light unit — is the durable remedy. Prevention lies in correct handling, supported edge clearance, and avoiding chemical attack on seals during cleaning.
Relationship to other standards
- BS EN 12150 / BS EN 14449 — toughened and laminated glass pane properties.
- BS EN 14351-1 — windows and rooflights product standard using IGUs.
- BS 8213 — installation code of practice for rooflights.
Thermal performance of the installed roof light still depends on the frame as well as the IGU — see thermal breaks and U-value reference.
Every Vant Glass roof light is made to order in Britain, backed by a 20-year guarantee and free UK mainland delivery. Configure frameless or framed sizes in the online calculators or call 03330 902 592.
Frequently asked questions
What is an insulating glass unit (IGU)?
An IGU is a factory-sealed assembly of two or more glass panes separated by a spacer bar and hermetic seal, with dry air or inert gas in the cavity. It is the glazed core installed into a roof light frame — not individual panes fitted on site.
Is BS EN 1279 mandatory for UK roof lights?
IGUs placed on the UK market must meet the Construction Products Regulation requirements, demonstrated through CE or UKCA marking against the applicable harmonised standard — BS EN 1279 for sealed units. Specifiers should request conformity documentation.
What gases are used in IGUs?
Argon is the most common fill because it is inert, economical and improves thermal performance over air. Krypton is used in some narrow cavities. Gas concentration and retention are tested under BS EN 1279 long-term methods.
How long should an IGU last?
BS EN 1279 conformity assessment includes accelerated ageing for moisture penetration and gas loss. Manufacturer warranties vary; misting between panes before end of life indicates premature seal failure and warrants replacement.
Can IGUs be reglazed on site?
Site-reglazed or hand-assembled cavities are not IGUs under BS EN 1279. Roof lights should use factory-sealed units from an audited production line with third-party conformity assessment where required.
Related articles
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Condensation on roof lights
Condensation on roof lights forms when warm, moist indoor air meets a surface below the dew point — often at cold frame sections or poorly ventilated cavities. Thermally efficient units and correct detailing reduce risk.
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Part L and roof light thermal performance
Approved Document L sets energy efficiency requirements for buildings in England. Roof lights must meet applicable U-value limits and are modelled as part of the whole building envelope in SAP and SBEM.
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Heat escapes through roof glazing via conduction, radiation and air leakage. Lower U-values, thermally broken frames, quality IGUs and careful installation all reduce losses over heated rooms.
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A U-value measures how readily heat passes through a building element. For roof lights, the declared value usually covers the whole unit — glazing, frame and spacer — and lower numbers mean better insulation.
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Light transmission through roof glass
Light transmission describes how much visible daylight passes through glazing. For roof lights, it affects room brightness, colour rendering and whether solar-control or privacy coatings are needed.
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