Why Your EPC Could Be Wrong for an Old Stone House
If you own an old stone house, listed building or traditionally constructed property, you may have looked at its Energy Performance Certificate (EPC) and wondered how accurately it reflects the building you actually live in.
Perhaps the house feels warmer than the rating suggests. Perhaps the EPC recommends measures that seem unsuitable for a historic property. Or perhaps you've been advised to insulate walls that have successfully managed moisture for well over a century.
There may be good reason to question some of those recommendations.
Research into traditionally constructed buildings has found that standard EPC calculations can underestimate their actual thermal performance.
Historic England's updated 2026 guidance specifically warns that this can result in poorer EPC ratings and, potentially, recommendations that are inappropriate for historic buildings.
So why does this happen, and what should owners of older properties do about it?
What Is an EPC?
An Energy Performance Certificate provides a standardised assessment of a property's energy efficiency.
Properties are currently given a rating from A to G, with A representing the highest energy efficiency and G the lowest.
An EPC also provides recommendations for measures that could potentially improve the property's rating.
An EPC is normally required when a property is built, sold or rented, and a certificate is generally valid for 10 years.
For modern properties constructed using relatively standard materials and methods, this standardised approach can provide a useful comparison.
The difficulty comes when that same methodology is applied to buildings constructed in a completely different way.
Why Are Old Buildings Different?
A traditional stone, cob or solid-brick building doesn't behave in the same way as a modern cavity-wall house.
Many older buildings were designed around materials capable of absorbing and releasing moisture.
These can include:
- Solid stone or brick walls
- Cob
- Lime mortar
- Lime plaster
- Lime render
- Solid timber floors and structures
- Traditional timber windows
- Natural ventilation
Rather than relying upon modern cavities, membranes and impermeable barriers, these buildings often manage moisture through the building fabric itself.
Understanding this difference is fundamental when considering energy improvements.
How Can an EPC Get an Old Building Wrong?
An EPC isn't normally based on somebody measuring exactly how much heat passes through every wall, roof, floor and window in your house.
The assessment uses standardised methodology and assumptions.
For existing homes, RdSAP – Reduced Data Standard Assessment Procedure – is currently used in England to estimate energy performance and generate EPCs.
Historic England says this assessment methodology tends to inaccurately represent the performance of older buildings.
Where the precise construction or thermal performance isn't known, assumptions and default values can be used.
And that's where problems can arise.
A 300-year-old stone wall isn't a standard manufactured product.
Two walls that look similar could be constructed differently, contain different materials, have different thicknesses and behave differently depending upon moisture, exposure and previous alterations.
Applying a generic assumption doesn't necessarily tell you how that particular wall actually performs.
What Has Recent Research Found?
This is where things become particularly interesting.
Historic England published its Historic Building Energy Performance Potential: Demonstrator Project in January 2026.
Rather than relying entirely on assumptions, researchers investigated six traditionally constructed properties using techniques including:
- Air-tightness testing
- In-situ U-value measurements
- Whole-house heat-loss testing
- Occupancy surveys
- RdSAP 10 modelling
The results demonstrated a difference between modelled assumptions and what was actually happening in the buildings.
Historic England reports that most of the properties performed better than the standard assumptions suggested, particularly in relation to air tightness and fabric heat loss.
The research concluded that EPCs remain a useful baseline, but can frequently underestimate the energy performance of traditionally constructed buildings.
That's an important distinction.
It doesn't mean every EPC on an old property is wrong.
It means the standardised assessment may not always tell the complete story.
Your Stone Walls May Perform Better Than the EPC Assumes
This is particularly relevant to solid walls.
The thermal performance of a wall is commonly represented by its U-value.
The lower the U-value, the less heat passes through the construction.
Where the actual U-value isn't known, an EPC assessment may rely upon an assumed value based on the age and construction of the property.
But research has repeatedly found examples where traditional walls perform differently from those default assumptions.
That matters because an incorrect assumption at the beginning of the calculation can influence both the eventual EPC rating and the improvements recommended afterwards.
The Danger of Chasing an EPC Rating
This is where owners of historic buildings need to be careful.
If an EPC says a property would benefit from wall insulation, it's tempting to assume that installing it must be the correct thing to do.
But improving an EPC score and improving a historic building aren't necessarily the same thing.
Historic England's current guidance warns that EPC recommendations can sometimes suggest interventions that could harm the character of a historic building or risk deterioration of its fabric.
For example, installing an inappropriate insulation system on a solid stone wall could alter how moisture moves through the structure.
If the wall can no longer dry effectively, moisture can potentially accumulate within the construction.
That can contribute to:
- Damp
- Mould
- Timber decay
- Deterioration of masonry
- Failed plaster or finishes
- Reduced indoor air quality
- Condensation
The solution to an inefficient historic building therefore isn't simply to chase the highest possible EPC rating.
A Whole-Building Approach
Historic buildings generally benefit from being considered as a complete system.
Before specifying major insulation measures, it can make sense to investigate simpler issues.
For example:
Is the roof performing properly?
Are gutters and downpipes working?
Is the building suffering from penetrating moisture?
Has cement pointing or render been applied over traditional masonry?
Are there uncontrolled draughts?
Is the heating system operating efficiently?
Are the windows actually beyond repair?
Is the loft adequately insulated?
Is ventilation appropriate?
Resolving those issues can sometimes make a significant difference without radically altering the historic fabric.
What About Windows?
Windows are another good example of why an EPC recommendation needs context.
A certificate may recommend replacing single-glazed windows with double glazing.
In a modern house, that may be straightforward.
In a listed building containing original historic windows, it could be entirely inappropriate to simply remove them.
There are now several alternatives worth investigating.
These can include:
- Repairing existing timber windows
- Draught-proofing
- Secondary glazing
- Slim-profile double glazing
- Vacuum glazing
- Bespoke high-performance timber windows where replacement is appropriate
Modern vacuum glass is particularly interesting because it can provide substantially improved thermal performance while remaining much thinner than conventional double glazing.
This can make it far easier to retain traditional glazing-bar proportions and historic window appearance.
We are covering this subject in more detail in our guide to modern timber windows and vacuum glazing in historic and listed buildings coming out this week.
What Should You Do If You Think Your EPC Is Inaccurate?
Don't simply ignore it.
Instead, understand its limitations.
For a significant historic renovation or energy-efficiency project, it may be worthwhile obtaining more detailed information about the building.
Depending upon the project, this could involve:
- Detailed building surveys
- Thermal imaging
- U-value testing
- Air-tightness testing
- Heat-loss calculations
- Moisture assessment
- Investigation of existing construction
- Review of previous alterations
- Specialist heritage energy assessment
The objective isn't to prove the EPC wrong.
It's to understand how the actual building performs before deciding how to alter it.
That can lead to a much better retrofit strategy.
EPCs Are Changing
Interestingly, the Government itself is reforming the EPC system.
The proposed replacement methodology is based around the Home Energy Model (HEM) and is intended to provide consumers with more useful information about areas including fabric performance, heating systems, smart readiness and energy cost.
The Government originally considered introducing the reformed domestic EPCs sooner, but their launch has now been moved to the second half of 2027 while the methodology and implementation are developed.
So this is an area that is actively changing.
Does This Mean EPCs Are Pointless for Historic Buildings?
No.
An EPC still provides useful information and a standardised benchmark.
The important thing is understanding what it is – and what it isn't.
An EPC is a model.
It isn't necessarily a detailed building pathology investigation, structural survey or measured thermal assessment of every element of your property.
For a modern standardised house, many of the assumptions may be relatively straightforward.
For a listed farmhouse constructed from thick local stone, altered repeatedly over 250 years and containing a mixture of traditional and modern materials, things can become considerably more complicated.
Measure First. Then Improve.
At South West Heritage & Development Ltd, we believe successful historic-building renovation begins with understanding the building.
Before introducing major changes, it's important to understand:
How was it constructed?
What materials were used?
How does it currently manage moisture?
Where is heat actually being lost?
What elements are historically significant?
And what improvements can be made without creating unintended problems?
Sometimes the solution may involve insulation.
Sometimes it may involve repairing windows, improving draught-proofing, upgrading heating controls, installing secondary or vacuum glazing, improving roof insulation or dealing with defects elsewhere in the building.
And sometimes the best intervention is much smaller than initially expected.
Renovating an Old Stone or Listed Property?
South West Heritage & Development Ltd specialises in the restoration, renovation and redevelopment of historic and traditionally constructed buildings throughout Devon, Somerset and the wider South West.
Our work includes:
- Listed-building renovation
- Historic roof restoration
- Structural timber repairs
- Traditional carpentry and joinery
- Lime mortar, pointing and rendering
- Historic and bespoke timber windows
- Energy-efficiency improvements
- Drainage and external works
- Complete refurbishment and redevelopment
We work alongside clients, surveyors, architects, structural engineers, conservation professionals and specialist local trades to develop solutions appropriate to each individual building.
If you're considering improving the energy efficiency of an old stone, listed or traditionally constructed property, contact South West Heritage & Development Ltd to discuss your project.