Understanding SHGC and U-Values | BASIX and Window Performance

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

What Do U-Value and SHGC Tell Us?

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

Why Are Two Different Values Needed?

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.

Conducted Heat Flow

What Is a Window U-Value?

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:

• reduce outward heat loss during colder conditions;
• reduce inward conducted heat gain when it is hotter outside;
• moderate internal surface temperatures near the window; and
• support more stable indoor thermal conditions.

U-value does not describe how much sunlight enters the room. That solar relationship is represented separately by SHGC.

Reading the Number

Is a Lower U-Value Better?

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:

• single, double or other glazing configurations;
• the gas or space between glazing panes;
• low-emissivity coatings;
• frame material and thermal breaks;
• spacers and edge-of-glass construction;
• the proportion of frame to glass within the complete product.

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

What Is Solar Heat Gain Coefficient?

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:

• a lower SHGC admits a smaller proportion of solar heat; and
• a higher SHGC admits a larger proportion of solar heat.

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

Is a Lower SHGC Always Better?

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:

Reading Product Information

Whole-Window Values Are Not Glass-Only Values

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

What Influences the Published Window Values?

Glazing Description

Does Double Glazing Have One Standard U-Value and SHGC?

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:

• frame material and frame design;
• glass thickness and pane configuration;
• cavity width and gas fill;
• low-emissivity coating type and surface location;
• tint or solar-control treatment;
• window operating type and dimensions.

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

Is SHGC the Same as Visible-Light Transmission?

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

How Are U-Value and SHGC Used in BASIX and NatHERS?

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

How Should a Window Schedule Be Checked?

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:

• the total-system U-value;
• the total-system SHGC;
• the frame material or nominated product family;
• the opening or operating type;
• whether different elevations use different performance requirements;
• whether skylights and roof windows have separate requirements; and
• whether the supplier’s evidence relates to the complete window rather than glass alone.

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

What Commonly Causes Confusion?

Project Coordination

The Window Values Must Remain Consistent

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:

• generic window notes that do not identify required performance;
• supplier quotes based on glass-only values;
• one blanket specification replacing several modelled window groups;
• a change in frame or operating type without assessment review;
• different values appearing in the BASIX and architectural documents; and
• window substitutions made after approval without checking the thermal consequences.

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

These Values Do Not Select the Window by Themselves

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:

• window size and location;
• structural and wind-load requirements;
• bushfire, acoustic and safety-glazing requirements;
• water penetration and air infiltration performance;
• daylight, glare, privacy and views;
• opening area and natural ventilation; and
• product availability, cost, finishes and installation details.

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 and SHGC FAQs

Related Guidance

Continue Through the Window Performance Pathway

BASIX Project Review

Do the Proposed Window Values Match the Assessment?

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.