Is my house thermally efficient? How to tell and what to do if it’s not
A house’s thermal efficiency describes how well it retains heat in cold weather and limits heat gain in warmer months. The more thermally efficient a house is, the less energy is required to maintain a comfortable indoor temperature.
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David Milner
By improving a property’s efficiency you can enhance the thermal comfort for occupants and reduce energy use, which, in turn, can lead to cost savings on energy bills and reduced carbon emissions. There are several ways to improve the thermal efficiency of a home, such as installing thermal insulation, improving airtightness and upgrading windows and doors.
Signs a property may not be thermally efficient
If you’re concerned about a property's thermal efficiency, start by considering its age. Analysis carried out by the Office for National Statistics (ONS) in 2021 found that the age of a property is “the most significant factor associated with energy efficiency”. It found that almost all homes built since 2012 in England and Wales had an EPC rating of C or better. Of the assessed homes built before 1900, only 12% in England, and 8% in Wales, had a high level of energy efficiency.
Aside from a property’s age, other indicators that a house may not be as thermally efficient as it could be include.
- Draughts
- Cold spots
- Uneven heating
- Condensation
- Higher than expected energy bills
How to assess a property’s thermal efficiency
One way to better understand a property’s thermal efficiency is to check its EPC rating. EPCs, or Energy Performance Certificates, have been mandatory for homes being sold or rented in the UK since 2008, so many homes already have one.
EPCs are produced by accredited energy assessors – they visit the property and gather data about its construction, home insulation levels, heating systems and energy use. The resulting certificate includes:
- An energy efficiency rating (A is the highest, G is the lowest).
- Estimated energy costs for heating, lighting and hot water.
- Recommendations for improvement, including potential savings and new ratings after upgrades.
EPCs are valid for ten years. If changes have been made to the property since the EPC assessment, it may no longer be accurate. Getting a new EPC can help to better understand a house’s thermal performance. Alternatively, an assessor who offers home energy audits may be sought. Depending on the assessor, these audits may include thermal imaging and other tests, and the report will include a list of recommendations for energy-saving upgrades.
Checklist: assessing a home’s thermal performance
If you’re curious about whether you live in an energy-efficient home, a quick walk-around assessment can help you understand its thermal performance and identify areas that may require improvements. The following checklist can help identify areas where improvements may be needed:
| Check for draughts | Feel for moving air around window frames, door seals, floorboards, loft hatches, and service penetrations (such as pipes or cables). Also, listen for whistling noises on windy days and look for visible light under or around doors. |
| Check for cold spots | Cold areas on walls, ceilings, or floors can indicate thermal bridging (areas where heat transfers more easily through structural elements), or insufficient insulation. |
| Compare room temperatures | Note if any rooms feel significantly cooler or warmer than others, if they take longer to heat up or cool down quickly. |
| Do a visual inspection | Look for areas of condensation, damp or mould, as well as gaps around doors, cracks in walls or ceilings and signs of deterioration around seals or joints that could allow heat to escape. |
| Assess insulation | Where safe and accessible, check whether lofts, roofs and floors are insulated, and whether it appears continuous and well-fitted. |
| Review your energy bills | Comparison websites can help you determine if your household energy bills are higher than those of other similar properties. If your bills are significantly higher, it could be a sign that your home needs thermal efficiency upgrades. |
Once you have finished carrying out these checks, if there are any areas of concern, a professional survey or further investigation may be beneficial.
Key areas where homes lose heat
Homes can lose heat in a variety of ways, but for properties that are poorly insulated, most heat loss can occur through the following elements:
| Roof and loft | Since warm air rises, up to 25% of an uninsulated home’s heat loss can be through the roof, according to the Energy Saving Trust. Adding roof or loft insulation can be a cost-effective way to upgrade a home’s energy efficiency. |
| Walls | The Energy Saving Trust reports that 33% of an uninsulated home’s heat can be lost through the walls. The method for insulating a home’s walls will depend on its construction. Most homes in the UK have masonry walls. These may be cavity walls, which comprise two layers of masonry with a gap between, or solid walls, which have a single layer of masonry. Other wall constructions may include timber frame, steel frame or concrete. The right method of insulation will depend on the wall construction. |
| Floors | Around 10 to 20% of an uninsulated home’s heat can be lost through the floor, reports the Energy Savings Trust. Installing floor insulation can help to reduce this heat loss by limiting heat transfer into the ground or unheated spaces below. In existing homes, insulation is typically installed between suspended timber floor joists or over the concrete slab, depending on the construction type. |
| Windows and doors | Single-glazing and older double-glazing can allow heat transfer, while poor detailing around windows and doors can also cause heat loss. Often, windows and doors will need to be upgraded for improved thermal performance. It’s also important to ensure adequate insulation and air tightness around windows and doors. This can help to minimise thermal bridging, draughts and the risk of condensation. |
Other ways to improve a home’s thermal efficiency
Adding insulation can be an effective way to improve a home’s thermal efficiency. However, it’s also important to consider other aspects of the property to ensure that the home isn’t leaking heat. These include:
|
Draught-proofing |
Sealing gaps around windows, doors and floorboards can help to prevent heat loss. |
| Upgrading windows | Installing windows with thermally efficient glazing and well-sealed frames can help to reduce air leakage. |
| Ventilation | While the point of thermal efficiency is to keep heat in, there’s also a need for fresh air. Ventilation systems with heat recovery can help to maintain indoor air quality while minimising heat loss. |
| Smart heating controls | Options such as zoned heating and smart thermostats can help to prevent unnecessary energy use. |
Improving thermal efficiency with Celotex insulation
Adding or upgrading insulation can help to improve a home’s thermal performance by limiting heat transfer through roofs, walls and floors. This contributes to more stable indoor temperatures and can help to reduce the need for heating and cooling systems. The actual thermal performance achieved will depend on the full construction build-up, installation quality and other design factors.
David Milner, technical team lead at Celotex, explains that Celotex manufactures a range of insulation products suitable for retrofit and refurbishment projects. “Our polyisocyanurate (PIR) insulation boards are designed for use across various parts of the building envelope, including roofs and lofts, walls and floors. We also offer extruded polystyrene (XPS) insulation for use in floors,” he says.
He also notes that both PIR and XPS are available as rigid boards in a variety of thicknesses to suit different build-ups: “Some PIR products are available in thicknesses as low as 20mm, helping to provide effective thermal insulation in areas where space is limited.”
Celotex PIR and XPS products offer low thermal conductivities (lambda values) – from 0.021 W/m·K for PIR and 0.032 W/m·K for XPS – which can contribute to achieving low U-values when used as part of a well-designed construction. Celotex XPS also provides compressive strengths of 300, 500 or 700 kPa, depending on the product.
To support accurate specification and installation, Celotex provides a range of technical resources and tools, including an online U-value calculator and detailed product data sheets. Our team is also available to offer project-specific guidance and advice, whether you’re improving the thermal performance of a loft, upgrading solid walls, or insulating a suspended floor as part of a retrofit project. Explore our dedicated retrofit application pages for more detailed information on each solution. For more information, to request a sample or to discuss your project requirements, please contact the Celotex technical team or visit the product pages on our website.