Edinburgh apartment, by Luke McClelland Design. Photo by zac and zac.
Prior to the Georgian era in the UK, floors were typically created by laying tiles or stone slabs on compacted earth. In the early 18th century, timber joists resting directly on the ground were introduced, though they were prone to decay without separation from damp earth. To address this, the floor structure was raised up off the ground, creating a ventilated air space below to ensure effective airflow and reduce dampness.
The ventilated air space or subfloor void also brought draughts into the rooms which aided the combustion coal fires – the main source of heating.
Floor insulation was never a consideration.
In this guide we’ll look at the best practices for insulating existing suspended timber floors and explore the benefits of insulating a floor, timber floor construction, insulation materials, considerations before insulating, insulation methods, integrating underfloor heating, costs and building regulations requirements.
UK homes lose around 10% of their heat through the floor, compared to about 35% and 25% through walls and the roof, respectively.
Even though more heat is lost through the walls and roof, insulating floors remains crucial for improving the energy efficiency and comfort of your home. While wall and roof insulation address significant heat loss, floor insulation prevents heat from escaping through the floor, maintaining a more stable indoor temperature and reducing the need for constant heating. This not only leads to lower energy bills but also helps to reduce your carbon footprint.
Floor insulation also minimises draughts and cold spots, making it a practical investment that enhances both value and thermal comfort of your property.
A suspended timber floor in a property built before the 1920s typically comprises a series of joists supported by external and intermediate sleeper walls and covered with floorboards. The joists, usually 100 by 50 millimetre sections at approximately 400 millimetre centres, span over compacted earth and form a sub-void.
To prevent the timbers from rotting, the sub-void is ventilated by air bricks (normally cast iron) built into the external walls. Additionally, the sleeper walls often have gaps between the bricks (known as honeycomb construction) to aid cross ventilation.
From the late Victorian period, slate damp proof courses were introduced to protect against rising damp. By the 1920s, more effective bitumen DPCs were introduced, while oversite concrete covered the ground within the sub-void to prevent the buildup of water and the growth of vegetation.
To successfully insulate a suspended timber floor, the insulation needs to be positioned between the joists while keeping the subfloor void clear for ventilation.
Early suspended timber floor construction: 1- External masonry wall; 2- Timber joists; 3- Sleeper wall; 4- Floorboards; 5- Air bricks; 6- Sub-void.
Insulating older buildings carries risks, particularly the development of interstitial condensation - moisture build-up within the building fabric - which can lead to dampness, mould, and structural damage.
To mitigate these risks, it is advisable to choose vapour-open and hygroscopic materials, such as sheep’s wool, hemp, or wood fibre. These materials can safely absorb and release moisture, which reduces the risk of condensation.
When insulating between timbers, opt for flexible insulation. This type of insulation can be easily squeezed into place, minimising air gaps and speeding up construction. For insulating over a timber structure, a solid insulation, such as a wood fibre rigid board, provides a more robust substrate.
Mineral wool insulation is a flexible, vapour-open, and economical alternative to natural flexible insulation. However, it is hydrophobic, meaning it cannot absorb moisture and is less effective at handling moisture.
Plastic foam insulation, including polyisocyanurate (PIR), polyurethane, and polystyrene, is vapour-closed and hydrophobic, posing greater condensation risks for older buildings. Additionally, its rigidity makes it difficult to fit between joists, often requiring foam sealants to fill gaps, which can fail over time.
Wood fibre insulation between floor joists with a breather membrane (blue) and VCL/ airtightness membrane (green). Image by Eco Building Systems.
Before insulating, consider the impact on the building's heritage, the condition of the existing structure and ventilation, and building services.
Evaluate the disturbance to floorboards, skirtings, and joinery. If original boards are present, determine if they can be reused. Removing boards without damage is challenging, especially in older properties, and listed properties may require retaining original boards. Ideally, lift, mark, and resecure original boards with a skilled joiner. If the boards are in poor condition or not historically valuable, consider replacing them with tongue and groove chipboard flooring for a new finish.
After removing the boards, ensure the floor structure supports the new insulation. Clear debris, repair damaged timbers and assess if damp proofing is needed to protect against rot. It’s also important to maintain or improve subfloor ventilation to prevent moisture buildup and ensure timber longevity – an increased risk when adding insulation to old floors.
Additionally, many floor voids contain water, heating pipes, and electrical cables notched or clipped to joists. These may need relocation to accommodate the insulation.
Consider original details such as floorboards before insulating timber floors. Image by Vincent Reed.
In buildings with a cellar or floor void deep enough for crawl space access, insulation can be added between the joists from below, making the process a lot less disruptive.
First, install a continuous vapour control layer (VCL)/ airtightness membrane along the sides of the joists, ensuring it laps up around the perimeter walls to keep the insulation from direct contact with the masonry. Cut flexible wood fibre insulation batts (such as SteicoFlex) to fit snugly between the joists, minimising gaps to prevent air movement, heat loss, and condensation. Avoid compressing the batts too tightly, as the air between the fibres provides the insulation.
For better thermal performance, if space and budget allow, add rigid tongue-and-groove wood fibre boards (such as Beltermo Top) beneath the floor structure if the joists are flat and true.
Next, install a continuous breather membrane below the joists and insulation, securing it with battens. This prevents the insulation from slipping out and enhances its effectiveness by reducing air movement through the floor.
This buildup ensures a well-insulated, airtight, and breathable construction.
Insulating from below (in order of installation): 1- VCL/ airtightness membrane; 2- Flexible insulation; 3- Breather membrane; 4- Timber battens. If space permits, a layer of rigid insulation can be added underneath the joists before installing the breather membrane.
In many cases, the only option is to insulate from above, which requires lifting the existing floorboards.
The general build-up is similar to the method used when working from below, but the materials are fitted in reverse with a few variations.
Once the floorboards are removed, first install the breathable membrane. This will support the insulation and act as a damp proof layer - a much better detail than the common method of using netting. Similarly to the previous method, the membrane needs to be lapped up around the perimeter to prevent the insulation making direct contact with the external walls.
Next, fit the flexible insulation batts, followed by the VCL/ airtightness membrane lapped up against the breather membrane.
Finally, reinstall the original floorboards, or use tongue-and-groove chipboard subflooring and floor finish, and skirtings to complete the floor buildup. The skirtings hide the membranes around the edge of the floor.
Insulating from above (in order of installation): 1 -Breather membrane; 2- Timber battens; 3- Flexible insulation; 4- VCL/ airtightness membrane; ; 5- Floorboards; 6- Skirting board.
The Energy Saving Trust estimates that insulating a floor could save a typical household about £100 per year on energy bills. A relatively modest sum.
If access to the subfloor is possible, installing insulation can be relatively simple and cost-effective, offering better returns on investment.
However, if the insulation needs to be installed from above, the process is more invasive, resulting in higher build costs and making payback harder to justify as a standalone project. Financially, this type of installation becomes more viable when included in a larger refurbishment project, allowing for savings through economies of scale.
If the floorboards are particularly valuable and cannot be lifted, there’s no access to the void beneath the floor, or the payback is insufficient to justify the works, then the best option is to focus on draught-proofing measures. This will not reduce heat loss, but can considerably improve thermal comfort by reducing the cold air entering the building from the unheated subfloor void.
Fitted carpet laid over floorboards has the advantage of reducing draughts and so improving comfort. Choosing a natural fibre underlay (such as a sheep’s wool underlay) and carpet (such as sisal, seagrass, jute etc.) will maintain the permeability of the floor for moisture movement.
Alternatively a VCL/airtightness membrane can befitted over the floor boards, then the underlay and flooring of your choice.
If you want to expose the floorboards various materials can be used to seal gaps between the boards and below the skirtings. These include timber fillets to match the existing floor boards and proprietary compressible foam rods or beads.
Floorboard gap filler by DraughtEx.
Underfloor heating (UFH) can be integrated into suspended timber floors, though it is more effective in solid floors where the thermal mass provides better heat retention.
There are two types of UFH: wet and electric. Wet UFH, while more expensive to install, offers superior performance and lower running costs.
Installing a wet UFH system with heat plates (such as the Tectora System by Warmup) between the joists keeps the floor depth to a minimum, though performance is slightly reduced due to the joist interruptions. A boarded UFH system placed over the joists (such as VLo-Econna-12 by Warmup) offers continuous coverage and improved performance, with a slight increase in floor depth (22mm thick systems are common).
UFH operates at a lower flow temperature than radiators (around 40C), making it an ideal pairing with a heat pump.
Insulated suspended timber floor with slim line underfloor heating plates over joists. Image by Eco Building Systems.
The 2022 update of the Approved Document L acknowledges the susceptibility of older buildings, constructed with traditional materials and methods, to moisture issues from thermal upgrades. Consequently, the guidelines include provisions to mitigate potential defects when retrofitting insulation, with installations limited to a maximum 15-year payback (Section 4.13).
The target U-value, or the Improved U-value, for upgrading an existing floor is 0.25 W/m²K, while the minimum Threshold U-value is 0.70 W/m²K (Table 4.3).
Floor U-values depend on the size and shape of the floor (perimeter/area ratio), and insulating an existing suspended timber floor is limited by the depth of the joists, which can range from 100 to over 250 millimetres.
Generally, 0.70 W/m²K can be achieved with even the thinnest joists, while 150 millimetres of sheep’s wool or 120 millimetres of flexible wood fibre insulation is likely meet 0.25 W/m²K.
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.
