G-Values

Natural light pouring through your windows feels fantastic, but not all glass works the same when it comes to controlling the heat. G-value is a key metric that tells us how much solar energy can pass through a material, directly impacting how comfortable your home stays year-round.

Solar Control Glass - What G-Values Are and Why They Matter

So, what exactly are G-values? At Solarlux by Reveal, we specialise in glass structures including wintergardens / glass extensions, glass rooms, sliding and bi-fold doors, and windows. We're here to explain the science behind G-values, why they matter, and how they can guide your choices when selecting glazing for your home. We’ll explore the technical details, practical benefits, and why so many UK homeowners are able to make smarter decisions with solar control glass.

The Science Behind G-Values Explained

How much sunlight your windows allow in can make a huge difference to your home's comfort, especially when balancing natural light and indoor temperatures. That's where G-values come into play. G-values are measured on a scale from 0 to 1 and indicate how much solar energy passes through your glass. The higher the G-value, the more sunlight - and heat - enters your home (i.e. 1 means there is no solar control), while a lower G-value means your windows and door glass are effectively blocking more solar energy.

In the UK, particularly here in Yorkshire where we operate, south- and southwest-facing glazing is recommended as requiring solar control as we have the sun rising in the east through to the south and setting in the west. Higher G-values help you take advantage of solar heat to warm your home naturally, ultimately reducing your reliance on your heating system. Lower G-values are ideal for blocking excessive heat, keeping your rooms cooler and comfortable without relying as much on air conditioning.

G-value = total solar gain + incident solar radiation

This formula involves adding the direct energy passing through the glass with the amount of heat absorbed by the materials that then radiates into your home.

What Is Solar Gain?

Solar gain refers to the increase in internal temperature caused by sunlight entering through glass surfaces. When sunlight reaches the glass in your doors, windows and roofs, its shortwave infrared energy penetrates the glass, warming objects and surfaces inside. This energy is absorbed and then re-emitted as longwave infrared radiation. Unlike shortwaves, longwaves cannot easily pass back through the glass, effectively trapping the heat within the space. This process creates a noticeable rise in your home's internal temperature, especially in sunny conditions (which are predicted to increase in frequency in the UK).

How Solar Gain & G-Values Influence Your Choice of Windows

Solar gain can either work for you or against you, depending on factors such as which direction your doors, windows and roofs face, the amount of glazing, the season, and the location you live in. For instance, south or southwest-facing glazing often benefits from increased warmth during colder months, leveraging solar gain to provide free heating. During warmer months, solar gain may end up overheating your home, particularly in spaces like sunrooms or areas with large glass panes.

By selecting glazing with a suitable G-value, you can optimise the thermal effects of natural sunlight. For better control in balancing these effects, our team at Solarlux by Reveal recommends choosing solar control glass with glazing that aligns with the orientation and amount of glazing in each room in your home. Our specialists can advise you on this.

G-Values & Building Regulations - Approved Document O

An important update to building regulations was introduced in 2022 to address overheating concerns in buildings that feature large glazed areas. This change aims to limit unwanted solar gains during summer months and support the reduction of overall emissions.

Approved Document O specifically outlines these new regulations, which at present only apply to new build homes. Part O defines guidelines on limiting glazed areas within rooms and details measures for effective heat management. While these updates cover new build properties only, they serve as a helpful consideration for anyone planning a construction project. Key factors influencing the size of windows and glazed areas include:

• The floor area of the room
• The orientation of the glazing (for example, south-facing or west-facing windows)
• Whether the building is situated in a high-risk location
• The provision of cross-ventilation through openings on opposite walls
• The requirement for external shading in high-risk areas. (Internal blinds, curtains, and foliage do not qualify as shading)
• Suitable G-Value to minimise overheating at certain points of the year

G-Values and Solarlux

At Solarlux by Reveal, our glazing systems demonstrate excellent G-values and solar control options (when required). Whether you're looking at our wintergardens, glass rooms, sliding doors, bi-fold doors, or windows, we ensure each solution is optimised for your home’s specific orientation and personal requirements. Our technical expertise allows us to recommend the perfect balance of natural light and thermal control for your project, ensuring compliance with building regulations whilst maximising comfort and energy efficiency throughout the year.