Glazing Performance Reference
U-value and Solar Heat Gain Coefficient describe two different aspects of window performance: how readily heat passes through the complete window and how much solar heat it admits.
U-value and SHGC are commonly shown in BASIX and NatHERS documentation because windows can transfer heat through both conduction and solar radiation. The two values describe different physical behaviours and should not be interpreted as interchangeable measures of general window quality.
A lower whole-window U-value generally indicates better resistance to heat flow. The preferred SHGC is more contextual: a lower value reduces solar heat gain, while a higher value allows more solar warmth to enter. Either may be useful depending on climate, orientation, shading and the heating or cooling issue being addressed.
This guide focuses on understanding and reading these two technical values. For the wider design relationship between window size, orientation, frames and shading, see Window Performance and BASIX.
In Brief
U-Value
Measures heat transfer through the complete window system. A lower value indicates greater resistance to conducted heat flow.
SHGC
Describes the proportion of incident solar radiation admitted through the window system as heat.
Whole-Window Values
BASIX and NatHERS window values relate to the combined performance of the glazing and frame—not the glass pane alone.
The two values should be read together. A window can have a relatively low U-value but still admit substantial solar heat, or it can restrict solar gain while transferring more heat through its glass and frame.
Window Heat Transfer
Windows affect thermal performance through more than one mechanism. Heat can move through the window because indoor and outdoor temperatures are different, and solar radiation can also enter through the glazing and become heat inside the dwelling.
U-value describes the first relationship: heat transfer caused by a temperature difference across the complete window. SHGC describes the second relationship: solar energy admitted through the window system.
Temperature-driven heat flow. U-value indicates how readily heat moves through the window when one side is warmer than the other.
Solar heat gain. SHGC indicates how much heat from incident solar radiation is admitted through the window.
Combined modelled effect. Thermal software considers both values together with window area, orientation, shading, climate and room conditions.
Conducted Heat Flow
U-value is a measure of the rate at which heat passes through a building element. For windows, it is expressed in watts per square metre per degree Kelvin, written as W/m²K.
The value indicates how much heat is transferred through one square metre of the window for each degree of temperature difference between indoors and outdoors.
A lower U-value indicates that the window provides greater resistance to this form of heat flow. This can help:
U-value does not describe how much sunlight enters the room. That solar relationship is represented separately by SHGC.
Reading the Number
For resistance to conducted heat flow, a lower whole-window U-value represents better insulating performance.
For example, a window with a U-value of 2.5 transfers less heat under the same temperature conditions than a window with a U-value of 5.5. This does not mean the first window is automatically the best overall selection for every opening, because its SHGC, frame, operating type, cost and architectural suitability also remain relevant.
Common factors affecting whole-window U-value include:
The U-value on the assessment documentation should therefore be treated as a performance limit for the complete nominated window system—not simply as a description such as single or double glazing.
Solar Heat Transmission
Solar Heat Gain Coefficient, or SHGC, describes the proportion of incident solar radiation admitted through the complete window system as heat.
SHGC is expressed as a number between 0 and 1:
For example, an SHGC of 0.30 admits less solar heat than an SHGC of 0.60 under equivalent exposure conditions.
SHGC does not indicate how effectively the window resists heat flow caused by indoor and outdoor temperature differences. That behaviour is represented by U-value.
Solar Trade-Off
No. A lower SHGC reduces solar heat gain, which can be useful where exposed glazing contributes to summer overheating or excessive cooling demand. The same reduction can also limit useful winter solar warmth.
A higher SHGC may be useful where well-oriented and appropriately shaded glazing is intended to admit winter sun. It may be unsuitable where large windows receive difficult east- or west-facing summer exposure.
The appropriate SHGC depends on the relationship between:
Climate. Heating-dominated and cooling-dominated conditions do not necessarily benefit from the same solar-gain characteristics.
Orientation. Northern, eastern, western and southern windows receive different patterns of direct solar exposure.
Shading. Eaves, balconies, screens, recesses and surrounding obstructions affect how much sun actually reaches the glazing.
Window area. The effect of a particular SHGC increases as the exposed area of glazing becomes larger.
Heating and cooling balance. Reducing one load can sometimes increase the other, so the complete modelled result should be reviewed.
Reading Product Information
The values used in BASIX and NatHERS documentation relate to the total window product, including both glazing and frame.
A centre-of-glass U-value describes only part of the glazed unit. It does not include the thermal effect of the frame, edge spacer, window proportions or complete assembled product. It should not be compared directly with a whole-window U-value shown on a BASIX or NatHERS document.
Similarly, glass manufacturers may publish solar data for an individual pane or insulated glass unit. The project team should confirm that the submitted SHGC is the applicable total-system value for the complete nominated window.
This distinction is important because the same glazing can produce different whole-window values when used with different frames, opening types or product dimensions.
Complete Window System
Glazing Layers
The number of panes and the space between them can materially affect resistance to conducted heat flow.
Glass Treatment
Low-emissivity coatings, reflective layers and tints can alter U-value, SHGC and visible-light transmission in different combinations.
Frame Construction
Aluminium, thermally broken aluminium, timber, uPVC and composite frames can produce different total-system results.
Operating Type
Different operating types use different frame profiles, seals and frame-to-glass proportions, which can affect the total values.
Product Dimensions
Smaller windows or products with heavier framing may contain a larger proportion of frame relative to vision glass.
Edge Construction
Spacer materials, cavity width and gas fill can affect the performance of an insulated glass unit and the complete window.
Glazing Description
No. Double glazing describes a general construction arrangement, not one fixed thermal specification.
Two double-glazed windows can have different whole-window U-values and SHGCs because of differences in:
Double glazing will commonly provide a lower U-value than a basic single-glazed system, but its SHGC may be high, moderate or low depending on the glass configuration.
The project documentation should therefore nominate performance values or an identifiable compliant product rather than relying on the words double glazed alone.
Separate Glazing Properties
No. SHGC describes admitted solar heat. Visible-light transmission, often abbreviated as VLT, describes the proportion of visible light transmitted through the glazing.
Although some darker tints reduce both solar gain and visible light, the relationship is not universal. Different coatings can produce different combinations of SHGC and VLT.
A low-SHGC product should therefore not automatically be assumed to provide poor daylight, and a clear-looking product should not automatically be assumed to have a high SHGC. The applicable product data should be checked.
Assessment Documentation
Where NatHERS thermal modelling supports the BASIX assessment, each modelled window or window group is assigned a recognised default or custom window system. The software uses its U-value and SHGC together with the building geometry and climate data to calculate heating and cooling loads.
The resulting NatHERS documentation identifies the window performance relied upon by the rating. These values then become part of the coordinated compliance information for the project.
In the BASIX thermal comfort DIY method, the Certificate may identify a maximum allowable U-value and an acceptable SHGC range associated with the selected frame and glass type.
The applicable document and pathway should therefore be checked before windows are ordered. A general product description is not enough where the Certificate or NatHERS schedule relies on defined total-system values.
Procurement Check
The project team should compare the proposed window product with the complete requirement shown in the applicable BASIX, NatHERS and architectural documentation.
The review should confirm:
A proposed window with a lower U-value may be acceptable from the perspective of conducted heat flow, but SHGC should not be changed indiscriminately. A materially different SHGC can alter the modelled heating and cooling balance.
Where the product differs from the assessed window, the applicable substitution rules should be checked before procurement or installation.
Common Misunderstandings
This is broadly correct for U-value, but not for SHGC. Solar gain can be useful or unwanted depending on the project conditions.
Glass-only figures do not necessarily represent the total system performance of the glazing, frame and complete assembled product.
Double-glazed products can vary significantly because of their frames, coatings, cavities, spacers and operating types.
Different elevations or window groups may require different performance because their solar exposure and thermal influence are not identical.
Descriptions such as low-E or double glazed do not confirm the whole-window U-value and SHGC required by the assessment.
Visible appearance and solar heat transmission are separate properties and should be verified through applicable product data.
Project Coordination
The final window performance should be consistent across the assessment, NatHERS Certificate where applicable, BASIX commitments, architectural window schedule and supplier documentation.
Common coordination problems include:
These issues are best resolved before windows are ordered. Once product procurement or installation has begun, changing the specification may affect cost, programme and approval documentation.
Technical Boundary
U-value and SHGC are important thermal properties, but they do not describe every aspect of window suitability.
A complete window selection may also need to consider:
This page defines the thermal values used in assessment documentation. The broader architectural and room-by-room window strategy remains a separate design exercise.
Frequently Asked Questions
U-value measures the rate of heat transfer through the complete window system. A lower whole-window U-value indicates greater resistance to conducted heat flow.
What does SHGC mean?Solar Heat Gain Coefficient describes the proportion of incident solar radiation admitted through the complete window system as heat.
Are lower U-values better?A lower U-value provides better resistance to heat flow through the window. The complete selection should still consider SHGC, window area, frame, operating type and other project requirements.
Are lower SHGC values better?Not in every situation. A lower SHGC can reduce unwanted summer solar gain but may also reduce useful winter warmth. The appropriate value depends on climate, orientation, shading and the modelled heating and cooling balance.
Do BASIX and NatHERS use glass-only values?No. The relevant values describe the total window system, including both glazing and frame. Glass-only or centre-of-glass data should not be substituted directly for whole-window values.
Does all double glazing have the same U-value?No. Whole-window performance varies with the frame, glass configuration, coatings, cavity, spacer, gas fill, opening type and product dimensions.
Can a window with a lower U-value replace the assessed window?A lower U-value may satisfy the conducted-heat requirement, but the SHGC, opening type and applicable substitution rules must also be checked against the BASIX or NatHERS documentation before the product is changed.
Is SHGC the same as visible-light transmission?No. SHGC describes admitted solar heat, while visible-light transmission describes the amount of visible light passing through the glazing.
Related Guidance
BASIX Project Review
Certified Energy can review the available plans, NatHERS documentation and supplier window data to help confirm whether the proposed whole-window U-values and SHGCs align with the BASIX pathway.
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