Understanding U-Values
The U-value measures how well a material conducts heat. It's expressed in Watts per square metre Kelvin (W/m²K). The lower the U-value, the better the insulation.
| Window Type | Typical U-Value | Heat Loss |
|---|---|---|
| Single glazing | 5.4-5.8 | Very high |
| Single + secondary glazing | 1.8-2.0 | Low |
| Standard double glazing | 2.8-3.0 | Moderate |
| Modern double glazing (Low-E) | 1.4-1.8 | Low |
| Triple glazing | 0.8-1.0 | Very low |
Key Point: Secondary glazing can improve a single-glazed window from 5.4 W/m²K to 1.9 W/m²K - a 65% improvement that often exceeds standard double glazing.
How Secondary Glazing Provides Thermal Insulation
Secondary glazing works by creating a substantial air gap between your existing window and the new glazed panel. This trapped air acts as an insulating barrier.
The Air Gap Effect
Air is a poor conductor of heat. The cavity between panes traps air, reducing convection and conduction heat transfer.
- • Optimal gap: 20-100mm
- • Larger gaps don't improve thermal performance much
- • Smaller gaps still provide good insulation
Reduced Draughts
Secondary glazing seals gaps around old windows that allow cold air infiltration - often the biggest source of heat loss.
- • Brush seals eliminate draughts
- • Sealed perimeter reduces air leakage
- • Works with original window for double barrier
Calculating Your Energy Savings
The amount you'll save depends on several factors:
Factors Affecting Savings
- Number of windows: More windows = more savings potential
- Current window condition: Draughty old windows show biggest improvement
- Property age and insulation: Older properties typically save more
- Heating system and fuel type: Gas, electric, or oil affects £ savings
Typical Annual Savings
1-2 Bed Flat
£100-200
per year
3 Bed Semi
£200-350
per year
Large Detached
£350-500
per year
Impact on EPC Ratings
Installing secondary glazing can improve your property's Energy Performance Certificate (EPC) rating, which is increasingly important for both selling and renting properties.
EPC Benefits
- Can improve EPC by 1-2 bands (e.g., E to D, or D to C)
- Helps landlords meet Minimum Energy Efficiency Standards (MEES)
- Increases property value and rental potential
- Makes properties more attractive to energy-conscious buyers
Reducing Condensation
One often-overlooked benefit of secondary glazing is its ability to reduce condensation on your original windows.
Why Condensation Forms
Warm, moist indoor air hits cold window glass and water vapour condenses. Single glazing gets very cold in winter, making this worse.
How Secondary Glazing Helps
The inner secondary glazing panel stays warmer (as it's insulated from the cold outside), so condensation doesn't form on the room-facing surface.
Optimising Thermal Performance
To maximise energy savings from your secondary glazing:
- 1
Ensure good sealing
Properly sealed frames are essential - any air gaps will reduce performance
- 2
Consider Low-E glass
Low-emissivity coated glass can improve U-values by a further 20-30%
- 3
Maintain original windows
Repair any draughts in your original windows for best combined performance
- 4
Prioritise north-facing windows
Windows that don't receive direct sunlight lose the most heat
Related Resources
EPC & Thermal Efficiency Guide
How secondary glazing cuts heating bills by 30% and improves your EPC rating for MEES compliance.
Energy Savings Estimator
Calculate your potential annual energy savings with secondary glazing.
Cost & Pricing Guide
Understand costs and payback periods for thermal upgrades.
Thermal Insulation Services
Our professional thermal secondary glazing installation service.
Get a Thermal Assessment
We can assess your property and provide personalised energy saving projections.