Double glazing is flagged as “poor” on an EPC because the rating system penalises windows that fall below a U-value of approximately 1.2 W/m²K, which many older double-glazed units fail to meet. The Standard Assessment Procedure (SAP), used to calculate EPC ratings, directly reduces the overall energy-efficiency score when windows have a high rate of heat loss (GOV.UK, 2026).
The EPC rating is a snapshot of a home’s fabric efficiency, not a measure of window quality per se. The key variable is the U-value, how quickly heat passes through the glass. Modern double glazing with low-E coatings and argon gas fill can achieve U-values around 1.0–1.2 W/m²K. But older double glazing (installed before the mid-2000s) often has U-values of 2.5–3.0 W/m²K or higher, which the SAP model treats as “poor” alongside single glazing in the worst cases. The “poor” flag appears when the window’s contribution to the total heat loss is significant relative to other elements such as walls and loft insulation.
Older double glazing has poor U-values
Double glazing installed before 2002 typically uses standard float glass with a 4–6mm air gap, yielding a U-value around 2.7–3.0 W/m²K. The Energy Saving Trust states that upgrading from such windows to A-rated double glazing (U-value 1.2 W/m²K) can reduce heat loss by up to 50% (Energy Saving Trust, 2026). The EPC software compares the actual window U-value against a benchmark; if your windows are performing at twice the heat loss of the reference, they are marked “poor”.
Poor frames and installation worsen the score
The EPC assessment also accounts for frame type and installation quality. Uninsulated aluminium or steel frames can have U-values exceeding 5.0 W/m²K, dragging down the window’s overall performance. Even if the glazing unit itself is decent, poorly sealed frames or gaps around the window allow draughts, which the SAP model counts as additional heat loss. The British Research Establishment (BRE) notes that air leakage through window frames can reduce the effective U-value by 10–20% (BRE, 2026).
EPC penalties for non-compliant glazing
Under current building regulations (Part L), replacement windows must achieve a U-value of 1.2 W/m²K or lower. The EPC software applies a penalty if the window U-value exceeds this threshold, the higher the U-value, the steeper the penalty to the overall rating. For example, a home with U-value 2.5 W/m²K windows may see its EPC score drop by 5–10 points compared to compliant glazing. The TrustMark scheme requires certified installers to meet these standards (TrustMark, 2026). If your EPC shows “poor” double glazing, the solution is to upgrade to windows with a U-value of 1.2 W/m²K or less, typically labelled as A-rated or B-rated under the BFRC scheme.
A worked example
For a typical 1930s semi-detached house in Manchester with 15 m² of old double glazing (U-value 2.8 W/m²K), replacing it with modern A-rated argon-filled units (U-value 1.2 W/m²K) costs around £4,500 after the 0% VAT saving that runs until March 2027. The Energy Saving Trust estimates this upgrade cuts heat loss through the windows by roughly 50%, saving about £195 per year on heating bills. Without any additional grant support, the payback period sits at around 23 years. Over a 25-year lifespan the total saving reaches approximately £4,875, though the EPC score typically jumps by 5 to 10 points, moving the property from an E or D band up to a C. If the household qualifies for ECO4 funding, the upfront cost could drop to zero, making the payback immediate and the lifetime saving much larger.
| Item | Figure |
|---|---|
| Upfront cost after grants | £4,500 |
| Yearly savings | £195 |
| Payback period | 23 years |
| 25-year lifetime savings | £4,875 |
What homeowners often get wrong
The most common mistake is assuming that any double glazing automatically earns a ‘good’ rating on an EPC. Homeowners often overlook the U-value threshold that determines the score. Here are the three biggest errors to avoid.
- Thinking all double glazing is equal Many believe that if the windows have two panes, they perform well. In reality, old double glazing from the 1990s can have a U-value above 3.0 W/m²K, which the SAP calculation treats as equivalent to single glazing, dragging the EPC score down by up to 10 points.
- Ignoring the frame material People focus only on the glass and forget that aluminium or steel frames without a thermal break lose heat rapidly. A window with good glass but poor frames can still be flagged as ‘poor’, wasting the £500–£1,000 spent on new panes if the frame is not upgraded too.
- Believing replacement is always the answer Some homeowners rush to replace all windows without checking if secondary glazing or draught-proofing would suffice. For a typical Victorian terrace, fitting low-E secondary glazing for £2,000 can lift the EPC by 2–3 bands at half the cost of full replacement, and it does not require planning permission in conservation areas.
Quick reference
- An EPC flags double glazing as ‘poor’ when its U-value exceeds 1.2 W/m²K, which applies to nearly all units installed before 2002.
- Upgrading from old double glazing (U-value 2.8) to modern A-rated units (U-value 1.2) cuts heat loss by roughly 50% according to the Energy Saving Trust.
- To qualify for the 0% VAT rate on window installations, the work must be carried out by a registered installer and invoiced before 31 March 2027.
- A typical 1930s semi-detached house saves around £195 per year on heating after replacing 15 m² of poor-rated double glazing with modern units.
- Fitting secondary glazing over existing poor double glazing can improve the EPC score without full replacement, but it may still leave the original windows flagged as ‘poor’ on the certificate.
Frequently Asked Questions
A U-value above 1.2 W/m²K is considered poor on an EPC. The Standard Assessment Procedure treats windows with higher heat loss as 'poor' regardless of their physical condition.
Yes, replacing old double glazing with modern A-rated units (U-value 1.2 W/m²K) can significantly improve your EPC rating. The Energy Saving Trust confirms this upgrade reduces heat loss by up to 50%.
New double glazing may show as 'poor' if it lacks low-E coatings or argon gas fill, resulting in a U-value above 1.2 W/m²K. Check your window's certified U-value against the EPC benchmark.
