Solid Floor Insulation

At the time of writing, the methodology for EPC assessments of existing dwellings is RDSAP 10. A significant methodology overhaul from RDSAP to the Home Energy Model (HEM) is proposed for 2027. In addition, government consultations indicate future changes to the format of EPCs and minimum rating levels required for private rental properties. Methods for improving EPC ratings will therefore change in the future.

In this guide, we'll look at:

• How solid floors are identified and recorded during an EPC assessment.
• How insulation in a solid floor is recorded.
• What evidence may be required for retrofitted insulation.
• How the U-value of a solid floor is calculated.
• Whether solid floor insulation can improve an EPC rating.
• A real example showing the effect of different insulation levels.
• How a solid floor is described in the EPC features table.
• The practical considerations involved in insulating an existing solid floor.
• When solid floor insulation will appear as an EPC recommendation.
• How documentary evidence can affect the way a floor is represented in an EPC.
• What Building Regulations and Building Control may mean when renovating an existing solid floor

The floor forms part of a property's thermal envelope, so heat can be lost through it. However, the amount of heat lost through a ground floor is generally lower than through walls or roofs because the ground beneath the floor remains considerably more stable in temperature than the outside air.

Many properties built from around the 1950s onwards have solid ground floors. Older properties may have suspended timber floors, although it is also common to find solid floors in individual rooms such as kitchens and bathrooms.

Insulating a solid floor can reduce heat loss, but retrofitting insulation to an existing property can be a significant undertaking. Unlike loft or cavity wall insulation, it isn't usually a straightforward improvement.

There are also some important points to consider if your main objective is to improve your EPC rating. The potential EPC improvement from solid floor insulation can be relatively small compared with some other improvements.

On this page we'll look at how solid floors are assessed for an EPC, how insulation is recorded, how the floor U-value is calculated, and how much difference floor insulation might actually make to an EPC rating.

There are two stages to identifying the floor.

First, the assessor records its location. The available options include:

• Ground Floor
• Above Partially Heated Space
• Above Unheated Space
• To External Air
• Another Dwelling Below
• Same Dwelling Below

For example, a floor above an integrated garage could be recorded as Above Unheated Space, while the floor of a flat above another flat would be Another Dwelling Below.

Once a floor has been identified as Ground Floor, its construction type can be recorded as:

• Solid
• Suspended Timber
• Suspended, Not Timber
• Unknown

A block and beam floor, for example, would fall into the Suspended, Not Timber category.

A suspended timber floor will often have ventilation openings at low level in the external walls. These allow air to circulate beneath the floor and help prevent moisture from accumulating in the sub-floor void.

Modern properties may use periscope-style vents where the external grille is positioned above the internal floor level.

It is important that these vents remain unobstructed.

Another indication of a suspended timber floor can be the way the floor responds to movement.

The "drop-heel" or stamping test is one possible indication: a suspended floor may have some movement or produce a different sound compared with a solid floor.

However, the assessor will consider the available evidence rather than relying on a single indication.

This page focuses on solid floors.

Once the floor has been identified as a solid ground floor, the insulation can be recorded as:

• As Built
• Retro-fitted
• Unknown

This distinction is important because floor insulation is normally concealed within the floor and therefore cannot simply be seen during an EPC inspection.

As Built

As Built is the normal default where there is no evidence that insulation has subsequently been added.

It assumes certain levels of insulation are present depending on the original date of construction. RdSAP calculates the floor's U-value using information including the building part, floor area and the relevant construction date information.

Retro-fitted

If insulation has been added to the floor after the property was originally constructed, the assessor can record it as Retro-fitted.

Because the insulation is normally hidden within the floor, documentary evidence is required to support this entry. The evidence needs to be sufficient to withstand an audit of the EPC. This is an important point if you have had your solid floor insulated in the past.

Don't assume that an assessor can simply record the insulation because you tell them it is there.

If insulation has been added to the floor since the property was originally constructed, then the 'retro-fitted' option can be selected by the assessor. However, as floor insulation is not ordinarily visible, the property owner needs to provide documentary evidence to support any declaration. This evidence needs to be sufficient to satisfy any potential audit of the EPC. See our Documentary Evidence page for more information.

So if the 'retrofitted' option is chosen, the insulation levels available are:

• 25 mm, 50 mm, 75 mm, 100 mm, 125 mm, 150 mm, 175 mm, 200 mm, Unknown

'Unknown' uses an assumed minimum of 50 mm insulation but should only be used in exceptional circumstances where contradictions in evidence occur.

Convention 3.07 advises: Any insulation can be doubled in thickness [for entry in the assessment] if there is documentary evidence of the type of insulation and manufacturer's information that the lambda value (thermal conductivity) is equal or less than 0.025 W/m.K. 

This is one of those areas where good documentary evidence can make a significant difference to how accurately the property is represented in an EPC.

A U-value is a measure of how readily heat passes through a building element. Lower U-values represent better thermal performance.

For an EPC, the floor U-value is used alongside the U-values of other elements of the building fabric when calculating heat loss from the dwelling.

This is then used in the final calculation of the EPC rating.

There is an important difference between a solid floor and some other building elements such as walls, however.

The U-value of a solid ground floor is not simply selected from a single table according to its construction date-band.

Calculation of the U-value occurs 'on the fly' and takes account of the geometry of the floor, including factors such as the floor area and external perimeter length.

A floor U-value is calculated for each building part - ie the main building and any extensions (either real or virtual).

The calculation for floor U-value is shown in section 5.12 of the RdSAP 10 Specification document, available on the BRE website:

As you can see, the calculation is not straightforward.

Notice how the methodology assumes floor insulation is already installed in more recent construction datebands.

For reference, dateband I in England & Wales is 1996-2002.

The U-value for any building fabric element in an RdSAP assessment can be manually entered.

This includes solid floors, but it needs to be supported by sufficient documentary evidence.

Owners of properties that were built with energy performance significantly better than required by Building Regulations at the time of construction will be particularly interested in this path.

The most well-known construction type that comes to mind in this scenario is Passivhaus.

Regarding documentary evidence, the U-value calculation itself needs to be produced by a suitably qualified person and clearly attributable to the specific property.

Guidance on this is presented in RdSAP Conventions 3.08 and 9.02.

Note that the manual entry of any building fabric U-value (apart from windows and doors) results in an automatic Smart Audit trigger for the final EPC.

Your energy assessor will therefore want to make absolutely sure the documentary evidence is sufficient to support any potential audit.

The answer is yes, but the improvement can be relatively modest considering the effort and cost of installation.

The best way to demonstrate this is with a real example.

Here I'll model solid floor insulation on my own house, a 1950s mid-terraced property that we cover in our Case Study 2 page.

This property does have solid floors currently, and this measure is one that we therefore could apply:

EPC ratings are calculated behind the scenes to 4 decimal places and then rounded to the nearest integer.

You can see that for my property I could achieve a gain of 1 SAP point by installing 50mm of solid floor insulation. Even if I increased this to 200mm, the gain would still only be 1 SAP point as a single improvement measure.

Just as we observed on our Loft Insulation page, the benefits diminish as you continue to add further insulation in any one building element as a single improvement measure, and the rating approaches a plateau.

That doesn't mean that solid floor insulation is ineffective, it's just that as the floor becomes less significant in the heat loss of the dwelling, further improvements to it make less difference, and the other building elements become a more significant target for attention.

It also means that the EPC benefit needs to be considered alongside the amount of work required to install it, and the cost.

Always contact a local energy assessor near you and confirm what gain you might experience with your property for any measures you have in mind before proceeding. It's always better to work from a position of certainty rather than make changes and try and guess what improvement they might make.

Here, I've taken my property with a solid floor in England & Wales, and changed the construction dateband to see how it's description changes in the EPC Features table, and also noted whether a recommendation is presented for solid floor insulation:

When a solid floor is declared 'As Built' (required unless there is sufficient evidence to the contrary), the 'assumed' insulation status depends on the defaults in Table 19 discussed earlier on this page.

From 1996 (in E&W) the methodology assumes a level of insulation is installed at the time of construction. Prior to that, the methodology assumes there is no insulation present.

If you discover there is in fact a layer of insulation present in such a property then your energy assessor is likely to be able to take this into account in the assessment to get a better EPC rating. Sufficient documentary evidence is required.

We have a ground floor rental flat constructed in the mid 1970s. I know that there is a layer of polystyrene insulation present in the floor of that flat, but previously we have not included that in the EPC for that property. When we next assess that property, we will measure the thickness of that insulation and if it is at least 25mm in thickness, and is suitable as a material to qualify, we'll record it as present. This will provide a small uplift in EPC rating, but also extinguish the recommendation on the EPC for solid floor insulation which could be helpful towards MEES compliance in the future.

This is where solid floor insulation differs from some of the easier EPC improvements.

One option is to install insulation above the existing floor and then install the finished floor covering over it. The disadvantage is that this raises the finished floor level, which can create knock-on problems with:

• Door thresholds and frames
• Internal doors
• Initial staircase steps
• Kitchen worktops and units
• Other floor levels and transitions

The alternative is considerably more disruptive: the existing floor can be excavated and reconstructed with insulation incorporated into the new floor.

That can make much more sense where the floor is already being removed as part of a major renovation, but it is a much bigger undertaking simply to improve an EPC rating.

This example of excavation and reconstruction makes interesting viewing: https://www.youtube.com/watch?v=8HxxjJ5bvxI

This is why modelling is important

In my example property, 150 mm of insulation would produce only a two-point improvement.

I therefore wouldn't regard this as a particularly attractive measure for that property when considered purely from an EPC improvement versus cost and disruption perspective.

Your property could produce a very different result.

The calculation is affected by the floor area, geometry and building date information, among other factors.

Always have the proposed improvement modelled for your own property before committing to the work.

If you are substantially renovating an existing solid floor, Building Regulations may apply, so it is worth checking with Building Control before starting the work.

Regulation 12 of the Building Regulations 2010 requires the relevant building control procedure to be followed when a thermal element is replaced or renovated.

Under the current Approved Document L, the limiting U-value for an existing floor is 0.70 W/m²K, with an improved U-value of 0.25 W/m²K where the full improvement is technically, functionally and economically feasible. A lesser standard may be appropriate where achieving 0.25 W/m²K would cause significant problems, such as with adjoining floor levels.

This is particularly relevant where an existing floor is taken up and rebuilt, rather than simply having insulation added as a minor improvement.

If you are planning to substantially alter an existing floor, check with your local Building Control body before starting work. They can confirm whether an application or notification is required and what standard the completed floor needs to achieve.

Your Building Control completion certificate can provide useful supporting evidence of the work carried out, and may help support the existence of the insulation for your next EPC assessment.

Note that the requirements in Scotland can differ from those applying in England and Wales.

Because floor insulation is normally hidden within the construction, an energy assessor cannot usually verify its presence simply by looking at the floor during an EPC assessment.

Where insulation has been retrofitted, or where a specific insulation thickness or U-value is being claimed, suitable documentary evidence may be needed to support the information entered into the assessment. The evidence must be sufficient to support the EPC if it is subsequently audited.

Useful evidence can include documents relating to the installation, construction or refurbishment of the property. Where available, Building Control documentation can also provide useful supporting evidence for work carried out during a renovation.

The precise evidence required depends on what is being claimed and the circumstances of the assessment.

For a more detailed explanation of the types of evidence that can be used, see our Documentary Evidence for EPCs page.

A recommendation will appear on the EPC for solid floor insulation if the following criteria are met:

• Below the building part there is: Ground, and the floor is Solid
• The existing floor:
  • has no insulation or
  • U-value >0.7 W/m2K

The improved EPC rating displayed on the certificate with the recommendation included is calculated based on an Insulated floor with U-value as follows:

• Insulated floor with U-value U=0.25 W/m2K

• Solid floor insulation can improve an EPC rating, but the improvement can be relatively modest.
• Floor insulation is normally hidden within the construction, so documentary evidence can be important if you want retrofitted insulation recorded on an EPC.
• The U-value of a solid ground floor is calculated using the characteristics and geometry of the floor, rather than simply selecting a single value from a table.
• In our example of a 1950s mid-terraced house, 150 mm of solid floor insulation increased the EPC rating from 68D to 70C.
• Retrofitting insulation to an existing solid floor can involve considerable disruption, particularly where the existing floor needs to be excavated.
• Raising the existing floor level can create additional work around doors, stairs, kitchen units and other floor levels.
• If you're considering solid floor insulation primarily to improve your EPC rating, have the proposed improvement modelled for your property before starting any work.

Click here to return to our 'Improve Your EPC' main page, and see if there is another way you could improve your EPC rating.