Homeowners often ask whether double glazing works in both directions. Does it keep heat out during summer, or is it only useful for retaining warmth in winter? The short answer is yes, double glazing does reduce the amount of heat entering your home on hot days, but this is a secondary benefit compared to its primary role of insulating against heat loss.
Yes, double glazing keeps heat out in summer, reducing solar heat gain by up to 30% compared to single glazing. The same insulating layers that retain warmth in winter also reflect solar radiation, keeping rooms 3–5°C cooler on hot days.
- Double glazing cuts summer heat gain by up to 30% vs single glazing.
- Insulating air gap and low-e coating reflect solar radiation outside.
- A double-glazed room stays 3–5°C cooler on hot days (BRE, 2026).
- Costs £4,500–£7,500 for a three-bed semi in 2026.
- Replacing single glazing can reduce heat loss by 50–60% (GOV.UK).
- Double glazing costs roughly £4,500–£7,500 for a three-bed semi but can cut heat loss by more than half compared to single glazing
- Double glazing reduces heat entering your home in summer by up to 30% compared to single glazing
- Does double glazing keep heat out? Yes, but the effect is secondary to its winter insulation function
- Quick numbers how double glazing performs on heat rejection vs. single glazing
- The energy savings from double glazing are greatest in winter, but summer cooling costs also drop
- To get the best heat-rejection performance, choose double glazing with a low solar heat gain coefficient (SHGC)
- Installers must be MCS-certified or FENSA-registered for double glazing to be eligible for any grants or warranties
Double glazing costs roughly £4,500–£7,500 for a three-bed semi but can cut heat loss by more than half compared to single glazing. The same insulating layers that keep warmth inside during winter also reflect some solar radiation back outside, meaning rooms stay measurably cooler in summer.
Double glazing costs roughly £4,500–£7,500 for a three-bed semi but can cut heat loss by more than half compared to single glazing
The average cost for double glazing in a typical three-bedroom semi-detached home in the UK in 2026 is between £4,500 and £7,500, depending on frame material (uPVC, timber, aluminium) and number of windows. A standard uPVC double-glazed window costs approximately £350–£500 per window installed, while timber or aluminium options are typically 30–50% more expensive (Energy Saving Trust, 2026).
The payback period for double glazing on energy savings alone is estimated at 15–25 years, but the increase in property value and comfort can make it worthwhile sooner. Government data from the Department for Energy Security and Net Zero shows that replacing single glazing with double glazing can reduce heat loss through windows by around 50–60% (GOV.UK, DESNZ, 2026).
Double glazing reduces heat entering your home in summer by up to 30% compared to single glazing
Double glazing works both ways. The insulating air gap and low-emissivity coating reflect solar radiation back outside, reducing solar heat gain. On a hot day, a double-glazed window can keep an internal room temperature 3–5°C cooler than a single-glazed room, according to lab tests by the Building Research Establishment (BRE, 2026).
The solar heat gain coefficient (SHGC) of a typical double-glazed unit is 0.5–0.6, meaning it transmits 50–60% of solar heat, versus 0.7–0.8 for single glazing. This means a double-glazed window lets in roughly 25–30% less solar heat than a single-glazed window on a sunny day (Energy Saving Trust, 2026).
Does double glazing keep heat out? Yes, but the effect is secondary to its winter insulation function
The primary purpose of double glazing is to reduce heat loss in winter. A typical double-glazed unit has a U-value of 1.2–1.6 W/m²K, compared to single glazing at 4.5–5.0 W/m²K. The same insulation layer that slows heat escaping in winter also blocks heat entering in summer (GOV.UK, DESNZ, 2026).
In practice, double glazing is less effective at blocking summer heat than specialist solar-control glass (for example, tinted or reflective coatings). However, it is still a meaningful improvement over single glazing. The heat-rejection benefit is most noticeable in south- and west-facing rooms, where solar gain is highest (Energy Saving Trust, 2026).
Quick numbers how double glazing performs on heat rejection vs. single glazing
The table below compares the thermal performance and cost of different glazing types. U-value measures how much heat passes through the window (lower is better). The solar heat gain coefficient (SHGC) measures how much solar radiation the window transmits (lower means less heat enters).
| Window type | U-value (W/m²K) | Solar heat gain coefficient (SHGC) | Estimated summer heat reduction (%) | Approximate cost per window (GBP) |
|---|---|---|---|---|
| Single glazing | ~5.0 | ~0.75 | 0% | £100–£150 |
| Standard double glazing | ~1.5 | ~0.55 | ~25% | £350–£500 |
| Low-E double glazing | ~1.2 | ~0.50 | ~30% | £400–£550 |
| Solar-control double glazing | ~1.0 | ~0.30 | ~50% | £500–£700 |
Source: Energy Saving Trust, 2026; BRE, 2026.
The energy savings from double glazing are greatest in winter, but summer cooling costs also drop
In winter, double glazing reduces heat loss by 50–60%, saving a typical home £100–£135 per year on heating bills (based on gas central heating). In summer, reduced solar heat gain can lower air-conditioning or fan use, saving an estimated £20–£40 annually (Energy Saving Trust, 2026).
The combined year-round energy saving is approximately £120–£175 per year for a three-bed semi. Ofgem data shows that the typical UK household uses around 12,000 kWh of gas and 2,900 kWh of electricity per year, so these savings are meaningful but modest compared to other measures like loft insulation (Ofgem, 2026).
To get the best heat-rejection performance, choose double glazing with a low solar heat gain coefficient (SHGC)
A low SHGC (0.3–0.5) means the window transmits less solar radiation, keeping rooms cooler. Standard double glazing has an SHGC of 0.5–0.6. Low-emissivity (low-E) coatings and argon gas filling improve insulation but do not drastically lower SHGC. For stronger heat rejection, you need to specify solar-control glass (Energy Saving Trust, 2026).
Homeowners in south-facing or west-facing rooms should prioritise solar-control double glazing for maximum summer comfort. guide to choosing window energy ratings can help you compare options. The Microgeneration Certification Scheme (MCS) sets product standards for glazing that affect energy performance (MCS, 2026).
Installers must be MCS-certified or FENSA-registered for double glazing to be eligible for any grants or warranties
For any government or energy-company grant (for example, the Great British Insulation Scheme or ECO4), double glazing must be installed by an MCS-certified or FENSA-registered contractor. FENSA (Fenestration Self-Assessment Scheme) covers windows, doors, and roof lights. MCS covers renewable technologies but is also accepted for glazing in some grant schemes (GOV.UK, 2026).
Always check the installer’s certification on the MCS or FENSA register before hiring. Unregistered work may invalidate building regulations compliance and warranties. how to check an installer’s certification explains the process step by step. You can verify FENSA certificates online (FENSA, 2026) and find MCS-approved installers through the MCS website (MCS, 2026).
Frequently Asked Questions
Yes. Double glazing reduces solar heat gain by up to 30% compared to single glazing. The Building Research Establishment found it can keep rooms 3–5°C cooler on hot days.
In 2026, double glazing for a typical three-bed semi costs £4,500–£7,500. The Energy Saving Trust says uPVC windows average £350–£500 per window installed.
Yes, it cuts heat loss through windows by 50–60% versus single glazing, according to GOV.UK data from the Department for Energy Security and Net Zero.
Payback on energy savings alone is 15–25 years. However, increased property value and comfort can make it worthwhile sooner, as noted by the Energy Saving Trust.
uPVC is the most cost-effective at £350–£500 per window. Timber or aluminium frames cost 30–50% more but offer different aesthetics and longevity (Energy Saving Trust, 2026).
