Solid Floor Insulation

At the time of writing the methodology for EPC assessments of existing dwellings is RDSAP 9.94. The much anticipated RDSAP 10 update is due in Q1 2025 and a significant methodology overhaul from RDSAP to the Home Energy Model (HEM) is proposed for 2026.

Floors are part of a building’s ‘thermal envelope’ and therefore, heat inside a dwelling will be lost through them to some degree.

Many floors from the 1950s onwards were solid and even when suspended timber floors were used in previous years, a kitchen or entrance hall may often have been laid with a solid floor.

The heat loss through a solid floor is often not as great as that potentially lost through walls and roofs because the temperature of the ground underneath in winter isn’t as cold as the outdoor air circulating around the rest of the property’s thermal envelope. The ground retains a certain amount of warmth with the temperature remaining comparatively stable all year round.

Floor insulation however helps reduce the heat loss, and modern building regulations significantly limit what is allowed for new builds.

Solid floor insulation can be retro-fitted to existing dwellings but the effort required to complete this work is usually significant, and EPC rating gains are relatively small compared to that available from other measures.

One method is to simply add a layer of insulation on top of the existing solid floor, with required floor coverings on top of that. This however will raise the level of the floor and can create some significnat remedial work around: door frames/heights, the bottom step of a flight of stairs, kitchen worktop levels etc.

Another method is to dig up a solid floor and reinstate with the presence of insulation. This may only be viable if you are already digging a floor up due to failure or are carrying out a significant renovation of a property. From a quick search of The Internet I found this example which makes interesting viewing: https://www.youtube.com/watch?v=8HxxjJ5bvxI   

During an EPC assessment, your energy assessor will determine the type of floor present in your property.

The assessment can contain a main building and up to four 'extensions.' These extensions could represent real physical extensions that were added after the main building was constructed, or they can be used to virtually sub-divide a building into parts where a mixture of different floor or roof types is present. Each of these building parts will be assigned a floor 'Location'.

The choice available is:

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

Ground Floor is where the ground floor of the property is located directly over ground. This could be with a solid floor or a suspended floor. This is the option we will be looking at on this page regarding solid floor insulation and the following page regarding suspended timber floors.

Above Partially Heated Space is used for example where a flat is located above a commercial premises below. The heating pattern of such a premises is likely to be different to that of a dwelling - such as not being heated during overnight.

Above Unheated Space is used for example for the part of a house located over an integrated garage.

To External Air is used where a part of the property is perhaps above a walkway. This might be the case in some terraced properties where a walkway exists on the ground floor between two houses to gain access to the rear gardens of the terrace.

Another Dwelling Below is used for example in the case of flats where there is another flat located below.

Same Dwelling Below is used for vertical 'extensions' located above another part of the same property.   

So on this page we're interested in the Location type: Ground Floor.

For floors with a Location of 'Ground Floor',  the following 'Type' options are available:

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

Suspended floors are often indicated by the presence of ventilation bricks in external walls at low level, often below the damp-proof course (DPC) in older propertues. In modern buildings these can be implemented using periscopic vents where the grilles on the exterior face of the wall are higher than the internal floor level.

In all cases these grilles should be kept free of obstrution to maintain ventillation under the floor and prevent the build up of moisture. Also the external ground height should be a couple of bricks below the grille. 

There are some similar looking grilles in a few older properties used for ventillation in cavity walls but these are rare. The property in our Case Study 1 page is an example. 

Another indicator used to determine the presence of a suspended timber floor is the drop-heel test. This is a simple jump or stamp on the floor. If you can hear any give to the floor or reverberation of the furniture then you probably have a suspended floor. If however the floor feels hard and that movement jars your body, then it's likely to be a solid floor.

A block & beam floor would be a case of 'Suspended, not timber'.

We're interested on this page with Solid Floors.

For Solid Ground Floors, an energy assessor can select from the following insulation type of the floor. 

• As Built
• Retro-fitted
• Unknown

Floor insulation is not normally visible to the naked eye, so the default option that an energy assessor has to choose is 'As Built'.

This is also the standard option for a floor that had insulation included in it when the property was originally constructed. 

In the case of 'As Built', the U value used for the EPC calculation will be worked out by the RDSAP methodology using algorithms that take into account the floor area of the building part and the assigned dateband or worst case requirements of building regulations at the time of construction.

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 insultion is not ordinarily visible, the property owner needs to provide documentary evidence to support any declaration. This evidence needs to be sufficient to satify 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:

• 50mm
• 100mm
• 150mm
• Unknown

When the RDSAP methodology is eventually updated to version 10, this list will be expanded to include all values at 25mm steps from 25mm up to 200mm.

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

The next lowest value in the list is always selected. So for example if 80mm of insulation is present then currently 50mm must be declared in the assessment.

Convention 3.07 advises: Any insulation can be doubled in thickess [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. 

Alternatively, if a calculated U value is available for the floor, produced by a suitably qualified person and clearly attributable to the specific property then this can be used to overide the value in the assessment.

This however will result in an automatic Smart Audit trigger of the EPC so the energy asessor will want to make absolutely sure the documentary evidence is sufficient.

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:

Here I use 'None' to mean 'As Built' for this 1950s solid ground floor.  

So for my property the gain would only be 2 SAP points for 150mm solid floor insulation.

I won't consider this as a viable option for my property because the physical effort and cost of conducting the works wouldn't be worth the comparatively small gain in EPC rating.

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.

Any rating increase for your property is likely to be different to the ones for mine. This is particularly so because the calculation of a solid floor's U value is complicated and involves the floor area and the building dateband.

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 Floor is:
  • As Built, age band <=J [ie prior to 2007 in Eng&Wales] or
  • has retrofitted insulation <= 50mm or
  • U value >0.5

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

• Eng & Wales and NI: u = 0.25
• Scotland: u = 0.18
• For a Park Home refer to Appendi S1.1.2

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