Retrofitting solid floor insulation

18 June 2019 Kingspan Insulation UK

Retrofitting solid floor insulation

Image of a living room, focusing on the floor

If you live in the UK, there’s a good chance your floor isn’t insulated. Around three quarters of British homes[1] were constructed before the first thermal performance requirements for floors were introduced in the 1976 Building Regulations. This means your floor could, quite literally, be draining the heat out of your home.

To help tackle this, the Building Regulations and Standards now ask that insulation is retrofitted on ground floors affected by refurbishment, extension or change of use. This can present a challenge, particularly on solid floors where available construction space can be highly limited.

What are the requirements?
Where technically and functionally feasible, ground floors should be upgraded to the minimum standards within the following documents:

More details about how these apply to floor insulation can be found on our Building Regulations page.

Above and beyond these regulatory requirements, homeowners and developers may also wish to insulate floors to improve comfort levels and to reduce energy consumption.

For homes with a suspended floor, the solution is relatively straightforward. Rigid insulation, such as Kooltherm K103 Floorboard, can be cut with a fine toothed saw and slotted between the floor joists (for best practice tips take a look at our suspended floor insulation guide).

Solid floors, however, present an altogether different challenge. These constructions typically feature a 75-100 mm screed layer above a thick concrete floor slab. The three retrofitting options are:

  • Digging down and lowering the floor slab level – This requires an extensive and costly manual labour programme. Workers must dig-out, remove and then re-lay several inches of concrete without the use of heavy machinery. 
  • Raising the floor level – This may not be possible in properties with lower ceilings, and the reduction in floor to ceiling height may make other rooms feel cramped.  It also requires fixtures and services such as plug sockets, door lintels and radiators to be raised in line with the floor level, creating yet more remedial work and again increasing the time and cost involved.
  • Installing a slimmer screed layer – Under this approach, the original screed layer is replaced with a thinner, modern alternative. This creates a slight space for insulation to be installed above the floor slab. It also avoids the added cost and complexity of raising floor levels. The challenge is finding an insulation product which can deliver the required level of thermal performance, within the strict confines available.

To better understand the cost efficiency of the different approaches, and how they can impact floor to ceiling height, we commissioned a report from industry experts, Currie and Brown.  The results of this are discussed in the white paper, ‘Maintaining floor to ceiling height in residential floor refurbishment’. 

It looks at several options for a solid floor retrofit, all designed to achieve a U-value of 0.25 W/m2.K, which is the minimum standard as set out in Approved Document Part L1b.

The specification is based on a domestic property refurbishment scenario, with a floor area of 84 m2, The scenario identified was the refurbishment of a typical 1950s semi-detached home. The ceiling height within the ground floor rooms is a little low, standing at 2475 mm, so any loss of floor to ceiling height would be best avoided. It assumes an existing 100 mm concrete slab with a 75 mm screed layer and an intact damp-proof membrane. For details of the cost modelling, please see the white paper.

The specifications feature Kingspan Kooltherm K103 premium performance phenolic insulation or expanded polystyrene insulation (EPS) with either liquid or sand and cement screed.


The results show that overlaying insulation on the existing floor slab is far more cost-effective than breaking out and replacing it but it will affect the floor to ceiling height, unless using Kingspan Kooltherm K103 with a high-performance screed. This is cheaper than the alternate solution of replacing the floor slab.

For full details of the research, read our white paper, linked below.
“Kooltherm K103 with the high performance screed would be the cost effective solution to maintain ceiling heights and meet the floor thermal performance requirements.”

Maintaining floor to ceiling height in residential refurbishment