What Lowers an EPC Rating the Most?

An Energy Performance Certificate (EPC) is influenced by a range of factors, some more impactful than others. While many homeowners focus on visible upgrades, the biggest contributors to a low EPC rating are often less obvious.

1. Inefficient Heating Systems

Heating systems have one of the largest impacts on EPC scores. Older boilers, limited controls, or outdated systems can significantly reduce a property's rating.

Even if a system is functioning properly, it may still perform poorly within the EPC model compared to modern alternatives.

2. Poor Insulation Levels

Insulation is a key driver of energy efficiency. Properties with limited loft insulation, uninsulated walls, or older glazing often lose heat quickly, which lowers the EPC score.

This is particularly common in older housing stock found in areas such as Reading and Belfast.

3. Lack of Heating Controls

Basic heating systems without thermostats, timers, or thermostatic radiator valves (TRVs) can negatively impact EPC results.

Simple upgrades here can often improve a rating more than expected.

4. Older Property Construction

Construction type plays a major role. Solid walls, older materials, and traditional layouts can result in lower efficiency compared to modern builds.

This is often seen in established markets like Oxford and regional areas such as Stockport.

5. Limited Low-Energy Lighting

Lighting contributes to EPC calculations. Properties with fewer energy-efficient bulbs may see a slight reduction in score.

6. Missing or Unverified Improvements

If improvements such as insulation or glazing cannot be verified during the assessment, standard assumptions are used — often resulting in a lower rating.

Why EPC Ratings Matter More in 2026

EPC ratings are increasingly used by buyers, tenants and lenders when comparing properties. A lower rating may not stop a sale, but it can influence perception and demand.

This is explored further in our guide on how EPC ratings affect house prices and buyer demand.

Estimating EPC Costs Before Booking

If you’re planning to improve or update your EPC, you can estimate pricing using our EPC cost calculator, which provides a quick guide based on property type and location.

Booking an EPC

If your EPC is outdated or you’re preparing to sell or let, it’s best to arrange a new assessment in advance.

You can book directly via the online booking page.

Conclusion

Low EPC ratings are usually driven by a combination of heating, insulation and property construction factors. Understanding what impacts your score allows you to make targeted improvements and avoid surprises during a sale or letting process.