Polished concrete floor by Architecture for London. Photo by Lorenzo Zandri.
Homes in the UK lose around 15% of their heat through the floor, compared to about 35% and 25% through walls and the roof, respectively. Despite this seemingly smaller percentage, insulating floors can significantly improve thermal comfort and reduce energy bills.
There are two main types of floor construction in UK homes: solid floors and suspended floors. The type of construction will determine the appropriate insulation method.
In this guide, we’ll explore the insulating solid concrete floors, including the different insulation methods, integrating underfloor heating, building regulations requirements, and costs.
Rigid foam materials like PIR, XPS and phenolic boards are commonly used to insulate new concrete floors, as they possess high compressive strength and excellent thermal performance.
Low carbon insulation materials such as wood fibre are largely incompatible with new concrete construction. However, they can be used in the retrofit of old concrete floors and new solid floor buildups which utilise sustainable aggregates and dry screed systems. Continue reading for more information.
Early concrete floor construction dating from the 1950s typically consists of a hardcore layer, a 100mm concrete slab, and a timber or tiled floor finish laid on bitumen adhesive, with some having a liquid-based damp proof membrane. From the mid-1960s to the mid-1990s, mesh reinformcent, polythene damp proof membranes (DPM) and sand cement screed were introduced, though insulation was rarely present.
Insulating old concrete floors can be extremely challenging as it requires raising the floor level, impacting door and ceiling heights and necessitating additional remedial work throughout the building.
If adding insulation is feasible, first install a proper damp proof membrane (DPM).
Then, install a timber floor over the top, as follows: 50 millimetre timber battens at 600 millimetre centres, PIR or wood fibre insulation between battens, vapour control layer (VCL)/ airtightness membrane, 18 millimetre tongue and groove chipboard, and then the floor finish.
If raising the floor is impractical, options include installing super-thin, high-performance insulation like phenolic boards, or an insulating floor finish like cork for improved thermal comfort. Alternatively, you may want to consider digging up the existing floor and installing a new concrete buildup, though this can be highly disruptive and costly.
A typical construction buildup for insulating an existing concrete floor: 1- Existing slab; 2- DPM; 3- Timber battens; 4- insulation; 5- VCL/ airtightness membrane; 6- Tongue & groove chipboard; 7- Floor finish; 8- Skirting.
Since the mid 1990s building regulations required insulation to be added to all new concrete floors.
A typical build-up consists of 150 millimetres of well-compacted hardcore, plus a sand blinding layer, to provide a firm, level base and then a reinforced concrete slab. A DPM can be installed above or below the slab, depending on site conditions and detailing. Rigid foam insulation is then laid over, followed by a VCL which also acts as a separation layer. Then it’s a floating screed, usually about 75 millimetres thick, which forms a smooth, level sub base for the floor finish.
This ground floor construction is widely used in new build houses and extensions in the UK.
A typical construction buildup for insulating a new concrete floor: 1- Slab; 2- DPM; 3- Rigid insulation; 4- VCL; 5- Screed; 6- Floor finish; 8- Skirting.
Sometimes a ground bearing slab is unsuitable, for example if the building is on sloping site. In these situations suspended concrete floors are widely specified.
Floors are usually made from 150 to 200 millimetre upside down T-beams with concrete blocks filled between. The buildup then follows the ground-bearing concrete slab noted above, insulation is then laid on top and then a sand cement screed. The void below is vented to the outside walls via telescopic vents and air bricks.
Suspended concrete floors are typically more expensive than ground-bearing slabs. However, in larger homes, economies of scale can offset these costs by speeding up construction through the use of dry trades, making this method more cost-effective.
Precast concrete beam and block construction. To insulate, install a DPM, rigid insulation, VCL, then floating screed above the structure. Image by Hanson.
Concrete floor construction is carbon-intensive due to the use of concrete and rigid foam insulation. In recent years, more sustainable solid floor build-ups have been installed, combining foam glass aggregates and dry screed systems.
The construction involves laying two layers of well-compacted foam glass aggregate (such as Geocell)to about 200-250 millimetres with a geotextile membrane above and below, followed by a 10-30 millimetre thick dry mix levelling layer (such as Cemwood CW2000). Next, a combined DPM and VLC membrane (such as Ampatex Solero) is used for waterproofing, vapour control, and airtightness. Wood fibre sarking boards (such as Beltermo Top) are then applied for a stable base, followed by a dry screed system (such as Lithotherm underfloor heating tiles) and the final floor finish.
As the build-up uses only dry trades, the floor can be finished and used immediately, making it a good choice when time is a key factor.
Low carbon solid floor construction: 1- Geotextile membrane; 2- Foam glass aggregate; 3- Levelling layer; 4- DPM/ airtightness layer; 5- Rigid wood fibre insulating sarking board; 6- Dry screed underfloor heating tiles; 7- Floor finish; 8- Skirting.
Underfloor heating (UFH) can be integrated within solid or suspended floors, although it is more effective in new solid floor construction where the thermal mass of the screed provides a more effective thermal store.
There is wet and electric underfloor heating. Wet underfloor heating, although more expensive, provides the best performance and is cheaper to run.
In a classic solid floor construction buildup, for a wet underfloor heating system, pipes are clipped above the VCL and the screed is poured on top. The screed is heated, acting like an enormous radiator.
Due to the higher surface area of the heating, UFH operates at a lower flow temperature than radiators (about 40C), making it an ideal companion for heat pumps.
Underfloor heating pipes clipped to a VCL laid over rigid insulation. Stock Adobe image.
Approved Document L sets out different insulation requirements for upgrading existing floors and new floors in extensions and new dwellings.
Calculating ground floor U-values also depends on the size and shape of the floor (exposed perimeter/area ratio).
Existing floors
The 2022 update of the Approved Document L acknowledges the challenges of retrofitting insulation into older buildings and includes provisions to ensure that installations do not have a detrimental impact on the building.
It sets target U-values, referred to as ‘Improved U-values,’ and minimum, or ‘Threshold U-values,’ to allow for design flexibility.
For example, the Improved U-value for upgrading an existing floor is 0.25 W/m²K, while the Threshold U-value is 0.70 W/m²K (Table4.3).
In a 1950s semi-detached house with a perimeter/area ratio of about 0.4, 50-70 millimetres of high-performance phenolic insulation could achieve 0.25 W/m²K, while 25 millimetres is likely to achieve about 0.35 W/m²K, which is still a significant improvement.
New and extension floors
For new floor elements the regulations are more stringent and set a U-value requirement of 0.18 W/m²K (Tables 4.1 and 4.2).
100-150 millimetres of PIR or XPS insulation, or 70-90 millimetres of high performance phenolic insulation could achieve 0.25W/m²K for a ground-bearing slab with floating screed floor buildup if the exposed perimeter to area ratio is 1.
The cost of insulating a concrete floor varies based on size, type and complexity.
As a rough guide, the material costs for multi-purpose PIR, XPS, and flexible wood fibre insulation are around £10 per square metre.
High-performance insulation can be considerably more expensive. For example, Kingspan K103 phenolic boards, costs approximately £22 per square metre at the time of writing.
Labour costs to install the insulation are likely to be between £250 and £350 per day, depending on your location.
For new floors, the cost of additional materials for the buildup will be extra. To achieve the best overall value and save on labour costs, install insulation as part of a wider programme of works whenever possible.
The information provided in this article is intended for general guidance and educational purposes only. For specific expert advice on your project, appoint a skilled residential architect.
About the author
Aron Coates, an architect with over two decades of experience, draws upon a wealth of residential architectural expertise, encompassing projects ranging from the restoration of historic structures to the design of contemporary homes.
