Suspended Timber Floor Insulation

The current methodology for EPC assessments of existing dwellings in England and Wales is RdSAP 10. The Government plans to introduce reformed EPCs based on the Home Energy Model (HEM) in the second half of 2027.

Changes to EPC metrics will also affect how property energy performance is reported and how future Minimum Energy Efficiency Standards (MEES) are assessed. For privately rented homes in England and Wales, the Government has set 1 October 2030 as the compliance date for the new MEES standard.

These changes mean that the measures needed to improve a property's current EPC rating may not necessarily be the same as those required to meet future standards. The guidance on this page relates primarily to the current RdSAP methodology.

The floor of a property forms part of it's thermal envelope, meaning 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.

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

On this page we'll look at how suspedned 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.

On this page, we're interested in the Ground Floor option. Once selected, the construction type can be recorded as:

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

On this page we are interested in suspended floors.

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

A Suspended timber floor of course would be recorded as Suspended Timber.

Typical forms of suspended ground floors include:

• Block and beam
• Suspended timber

Block and beam floors use concrete beams with a cross sectional shape that supports lines of parallel blocks traversing the gap to neighbouring beams. This forms quite a rigid floor.

Suspended timber floors in contrast use a series of parallel wood joists with perpendicular floorboards above. This forms a floor with noticeably more 'bounce'.

Both types create a void area under the floor which is normally ventilated to the outside using 'air bricks' which are visible low down on external elevations, usually just below the damp proof course, and usually below the level of the internal ground floor.

Particularly in the case of suspended timber floors, this ventilation ensures any air-bourne moisture permeating down from the room above cannot linger in the void and it reduces the potential for condensation, mould and degradation of the timber structure.

An assessor will look out for these ventilation bricks during an assessment, and modern properties may even use periscope-style vents where the external grille is positioned above the internal floor level.

It is of course important that these vents remain unobstructed.

Another indication specifically 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.

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, or the level of insulation present has been positively confirmed, 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 suspended floor insulated in the past.

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

Unknown

This option should only be used in exceptional circumstances, such as when there is conflicting evidence about whether insulation has been installed. It should not be used simply because the assessor cannot confirm whether insulation is present. Use of this option suppresses any recommendation on the EPC for floor insulation.

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

If the insulation material is mineral wool then the thickness is recorded as the actual depth measured.

For other material types, if documentary evidence is available of the type of insulation and the manufacturer's data that the lambda value (thermal conductivity) is equal to or less than 0.025 W/mK then the thickness can be recorded as double that as measured (RdSAP Convention 3.07).

'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.

Of course, if no evidence of retrofitted insulation is available, the floor will be declared 'As Built'.

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 suspended floor and some other building elements such as walls, however.

The U-value of a suspended floor is not simply selected from a single table according to its construction type and 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, and uses default information (if necessary) from the following table:

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, so a level of floor insulation is assumed for properties with a construction dateband of 1996 onwards.

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 government have produced a comprehensive 'best practice' guide for installing insulation in suspended timber floors.

This is available here:

https://www.gov.uk/government/publications/insulating-suspended-timber-floors-best-practice

My house actually has solid floors but if I were to simulate a suspended timber floor in an assessment, and add some insulation, here are the ratings I would achieve:

This is a 1950s terraced house that we cover in our Case Study 2 page. The ratings here were calculated using the previous release of RDSAP (V9.94):

So the difference for my property is small considering the amount of physical work required and the cost of materials.

The increase from 50mm to 100mm and then 150mm does not result in an integer increase in the EPC rating for my property but it will be making a smaller fractional difference behind the scenes, just unfortunately not enough to be rounded up to the next integer value of 70.

It would be possible to do this type of work as a DIY project but you would need to submit a formal Building Control application with exact details of the materials used so that the documentary evidence could be used by an energy assessor to support the declaration of the insulation in the EPC assessment.

Always contact a local energy assessor and get them to discuss what EPC rating improvement you would experience with your own property before proceeding with any works, and what documentary evidence of the works would be required.

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.

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 for Suspended Floor Insulation will be included on an EPC certificate under the following conditions:

• Below the building part there is:
  • Ground or External Air or Unheated Space, and
  • The floor is Suspended

And:

• The Floor has:
  • No insulation or
  • A U value of > 0.7 W/m²K

The EPC rating indicated on the certificate with this recommendation implemented will have been calculated using:

• Insulated Floor with
  • U value = 0.25 W/m²K
  • For a park home, u value = 0.30 W/m²K

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

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