Choose glazing as a complete roof-and-wall system, not by the glass label alone. Balance winter heat retention (U-value), summer solar gain (g-value), daylight and appearance, safety construction, ventilation, shading, maintenance, orientation and the rules that apply where you are building. A south- or west-facing room often needs more solar control than a shaded north-facing one, while insulation without a way to reject heat or ventilate can still produce overheating.
Before accepting a quotation, ask the supplier to state whole-element performance values, identify the safety glass in every pane, explain any roof-angle adjustment, and confirm that the proposed units fit the existing or new roof system. The figures in this guide are not universal building-code limits: regulatory requirements depend on jurisdiction, project type and date.
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Start with the heat flows you need to control
Glazing affects a conservatory in two opposite ways. Heat moves out through the glass on a cold day, while sunlight entering through the glass can raise the internal temperature on a bright day. The two principal specifications address different problems:
- Low-emissivity (low-E) glazing uses a coating intended to reduce heat transfer from the warm interior, improving winter comfort. It does not, by itself, control all incoming solar energy.
- Solar-control glazing uses a coating or tint to reduce solar heat entering the room. Its value depends on orientation, external shade, desired daylight and how much winter sun you want to admit.
The Glass and Glazing Federation (GGF) explains these functions separately in its consumer and good-practice guides. Ask the designer to show how both choices work together in your particular conservatory rather than treating “energy efficient” as a complete specification.
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Use orientation, shade and room use to set the brief
South- and west-facing roofs
The GGF consumer guide specifically identifies south- and west-facing conservatories as situations in which solar-control glass should be considered. These orientations can receive strong sun for long periods, especially when nearby buildings or trees do not provide shade. A lower g-value transmits less solar energy; the GGF good-practice guide states, “The lower this figure, the less solar energy it transmits.”
North-facing or naturally shaded rooms
With limited direct sun, winter heat retention and daylight may carry more weight than aggressive solar rejection. A very strongly tinted or reflective product can reduce useful light, so compare its visible-light transmission and appearance with the room’s intended use.
Occupancy and daily pattern
A dining room used at midday, a home office occupied all afternoon and a plant-filled winter garden have different comfort priorities. Tell the supplier when the room is occupied, whether doors open to the house, and whether blinds or external shade will normally be deployed. The desired balance between winter gain, glare control and summer temperature cannot be selected from orientation alone.
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Compare the main glazing choices
| Choice | What it does | Questions to ask | Important qualification |
|---|---|---|---|
| Double glazing | Two panes separated by a sealed cavity; the GGF consumer guide describes it as a contemporary baseline for comfort. | What is the declared U-value for the complete unit, including frame assumptions? | “Baseline” is consumer guidance, not a universal code requirement. |
| Low-E glass | Reduces heat transfer from the warm side, helping winter heat retention. | Which pane carries the coating, and what whole-element U-value results? | Low-E does not automatically provide adequate summer solar control. |
| Solar-control glass | Reduces solar heat entering through a coating or tint. | What is the g-value, and how will visible-light transmission and colour look from inside and outside? | Specify it in relation to orientation, external shade and winter comfort. |
| Safety glass | Toughened or laminated construction can address breakage and falling-glass risks where required. | Which construction is used in each wall pane, roof pane and door, and why? | Requirements depend on location and jurisdiction; obtain a project-specific answer. |
| Self-cleaning coating | An optional surface treatment identified by the GGF consumer guide. | What cleaning remains necessary, and is the coating suitable for this roof pitch and exposure? | It does not establish that all cleaning is eliminated. |
A supplier may propose a different pane combination or coating. Evaluate it using the same evidence: whole-unit thermal data, solar transmission, visible light, safety construction, roof compatibility and maintenance instructions.
Read the performance numbers correctly
U-value: winter heat retention
Ask for the declared U-value of the complete glazing element, not only a centre-of-glass figure. The quotation should explain whether the number covers the frame, spacer and sealed unit, and what assumptions were used for the roof. A lower U-value indicates less heat flow through the element, but the value is meaningful only when the compared products are calculated on the same basis.
g-value: solar energy transmission
The g-value, also called solar energy transmittance, expresses how much solar energy passes through the glazing. Lower values transmit less solar heat. Request the exact value for the proposed build-up and ask the designer to relate it to your orientation, shade and ventilation strategy. The GGF guides do not set one universally correct g-value for every conservatory.
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Visible-light transmission and appearance
Solar-control coatings and tints can change the amount of daylight and the colour seen through the glass. Compare the manufacturer’s visible-light transmission figure and ask to view a sample in daylight. The GGF consumer guide notes that some modern metallic solar-control products are neutral in appearance; “neutral” is a product description, not a guarantee that every coating will look identical from every angle.
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Words such as “climate,” “energy,” or “comfort” do not tell you the U-value, g-value or light transmission. Require those values in writing, together with the glass make-up, coating position, frame type and roof-angle assumptions. If a supplier cannot provide the figures, you cannot make a like-for-like comparison.
Account for the roof angle and the whole roof system
Roof glazing is not installed vertically, so the angle can affect the thermal value that applies in practice. The Scottish Government’s January 2025 domestic technical handbook notes that roof-element U-values are often quoted in the vertical plane and need adjustment for roof angle. Ask whether the quoted value has already been adjusted for your actual pitch and whether it covers the complete roof element, including framing.
Compatibility is equally practical. Confirm the proposed sealed units’ thickness, weight, edge details, drainage and fixing method against the existing roof system or the new frame design. A technically impressive pane is unsuitable if it cannot be supported, sealed or drained correctly. Request a drawing or schedule identifying every roof and wall unit, rather than accepting a single generic “double-glazed” description.
Make safety decisions pane by pane
Roof panes, low-level glazing, doors and glazed screens can have different safety requirements. Ask the installer to list whether each location uses toughened or laminated safety glass and to identify the standard or building-control basis for that choice. The GGF consumer guide says association members must use safety glazing throughout, but that statement should not be generalized into a legal rule for every project or country.
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Do not substitute a verbal assurance for a written schedule. Keep the safety-glass specification with the quotation, drawings and completion documents so a future repairer can identify the correct replacement. If your project involves a change of use, a structural alteration or a new extension, ask the local building-control authority which safety provisions apply before ordering units.
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Design ventilation and shading with the glazing
The GGF good-practice guide warns that insulation without solar control can leave a glazed extension overheating and identifies adequate ventilation as essential. Treat ventilation and shade as part of the glazing decision, not as optional remedies after installation.
Ventilation questions
- Which roof vents, opening lights or doors provide purge ventilation?
- Can occupants reach and operate them safely, and will insect screens or security hardware limit the opening?
- What is the plan for still, hot weather when the room is occupied?
Shading questions
- Is shade external, internal or a combination, and when should it be deployed?
- Will blinds fit the roof geometry, openers and frame without obstructing maintenance?
- Will the chosen shade preserve the daylight and view you want?
Blinds can help with sunlight and privacy, but a generic “conservatory roof blind” is not automatically compatible with every roof dimension or design. Obtain a measured compatibility check and a maintenance plan for the actual system.
Plan for daylight, view and privacy
Ask for a sample or mock-up that lets you judge colour, reflectivity and visible-light transmission from inside the room. A coating that appears neutral on a small sample may look different across a large roof or beside existing windows. Decide whether uninterrupted view, glare reduction, privacy or plant growth is the priority in each elevation.
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Consider cleaning and long-term maintenance
Self-cleaning glass is an option identified by the GGF consumer guide, not a promise of maintenance-free glazing. Ask what conditions activate the coating, how often the supplier still recommends washing, which detergents are permitted and how the coating is protected during construction.
For any roof, establish safe access for inspection and cleaning before work starts. Include gutters, drainage channels, seals, opening hardware and blinds in the maintenance schedule. Keep the product data sheets and installation instructions; they matter if a pane, coating or seal needs replacement later.
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Check the rules for your location and project type
Scotland
The Scottish Government’s 2025 domestic technical handbook, shown as updated in July 2025, gives conservatory wall-and-roof guidance of a maximum area-weighted average U-value of 1.8 W/m²K and a maximum individual element U-value of 3.3 W/m²K for the glazing and framing covered by that guidance. These are Scotland-specific handbook figures, not universal UK or global limits. The same handbook notes the need to adjust roof values for roof angle.
England and Wales
The UK government circular “Conservatories and porches,” dated 30 July 2013, describes conditions under which a conservatory or porch may benefit from specified Building Regulations exemptions and applies to England and Wales (with the circular’s stated treatment of excepted energy buildings in Wales). It is not a worldwide rule and should be checked against current local guidance.
England’s overheating provision
England’s Approved Document O FAQ, published 15 June 2022, says Part O applies to new residential buildings and does not apply to extensions or conservatories added after those buildings are built. It also says conservatories on new residential buildings must meet Part O requirements. Confirm the scope with building control because the answer can change with the project classification and current guidance.
Elsewhere, use the local authority or code applicable to the actual address. Do not copy a Scottish U-value or an English exemption into another jurisdiction without confirmation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Follow a supplier-comparison process
- Describe the room. Provide orientation, roof pitch, dimensions, surrounding shade, intended occupancy, connection to the house and any existing frame or roof-system details.
- Request a glass schedule. Require every roof, wall, door and low-level pane to be identified by build-up, coating, thickness and safety construction.
- Request performance data. Obtain whole-element U-value, g-value and visible-light transmission, with the calculation basis and roof-angle assumption stated.
- Review comfort measures. Ask where ventilation openings and shading are located, how they are operated and how they interact with blinds, security and cleaning access.
- Check compliance. Give the schedule to the designer or building-control authority and confirm the rules for your location and project type before manufacture.
- Compare like with like. Put quotations in a table with the same headings: glass build-up, frame, U-value, g-value, visible light, safety, roof angle, ventilation, shading, warranty and maintenance.
- Approve samples and drawings. Check appearance in daylight and sign off the pane layout, opening positions, drainage and access for maintenance.
- Inspect at handover. Verify markings or documentation for safety glass, confirm vents and shades operate, and retain the final product data.
Use this comparison table when evaluating quotations
| Item to record | Supplier A | Supplier B | What a complete answer includes |
|---|---|---|---|
| Glass build-up | Fill in | Fill in | Pane count, cavity or laminate description, coating and pane positions. |
| Whole-element U-value | Fill in | Fill in | Units, frame included, roof angle and calculation basis. |
| g-value | Fill in | Fill in | Solar energy transmittance for the specified unit. |
| Visible-light transmission | Fill in | Fill in | Percentage or declared value plus a daylight sample. |
| Safety construction | Fill in | Fill in | Toughened or laminated designation for every relevant location. |
| Ventilation and shading | Fill in | Fill in | Opening locations, controls, shade type and compatibility. |
| Maintenance | Fill in | Fill in | Cleaning method, access, self-cleaning limitations and replacement information. |
| Compliance confirmation | Fill in | Fill in | Named authority or designer confirming the applicable local requirements. |
Replace “Fill in” with the supplier’s written answer; do not accept a dash or an unexplained marketing term where a performance value is needed.
Common mistakes and how to correct them
Choosing only for winter
Symptom: The specification focuses on low U-value but says nothing about g-value, shade or ventilation. Correction: Add solar-control and overheating questions before approving the design.
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Choosing only for summer
Symptom: A heavily tinted product is selected without checking daylight, view or winter comfort. Correction: Compare visible-light transmission and appearance samples alongside the g-value.
Comparing centre-glass numbers
Symptom: Two quotations show different U-values calculated on different bases. Correction: Request whole-element values with frame and roof-angle assumptions stated.
Assuming safety is automatic
Symptom: The quote says “safety glazing” but does not identify panes. Correction: Require a pane-by-pane schedule and confirm it with building control where necessary.
Believing self-cleaning means no maintenance
Symptom: Access and cleaning are omitted because a coating is specified. Correction: Obtain the coating’s maintenance instructions and provide safe access.
Copying a rule from another project
Symptom: A supplier cites a figure from a different country, date or project type. Correction: Check the current authority for the actual location and classification.
What a sound final specification looks like
A defensible specification names the glass build-up for each location, gives whole-element U-value and g-value, states visible-light transmission and appearance, identifies toughened or laminated safety construction, and records the roof-angle basis. It also shows how ventilation and shading will be used, confirms compatibility with the roof system, and explains cleaning and replacement.
Use the GGF figures as guidance rather than independent product testing. The GGF consumer guide says modern solar-control glass units can “easily reflect over 70% of the sun’s heat”; no independent comparative test is established here, so treat that statement as the association’s claim, not a guaranteed result for every product. Select the unit whose documented performance and installation details fit your orientation, occupancy, roof and local requirements.
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