Compare insulation methods for suspended timber floors
Which underfloor insulation is right for your home?
If you have a suspended timber floor with floorboards and a ventilated void underneath, there are several ways to insulate it. The four main options are sprayed foam, rigid insulation board, mineral wool and wood fibre.
The same materials go by several names. Sprayed foam is also called spray foam or PUR foam. Rigid board is usually PIR, sold under brand names such as Celotex and Kingspan. Mineral wool covers rock wool, stone wool and glass wool. Wood fibre is sometimes sold as a natural fibre batt.
Each has different requirements for void depth, installation, thermal performance and the condition of the floor. This guide compares the four side by side, so you can understand the differences before choosing an installer or getting a quote. For more detail on how each material performs, read our guide to the best insulation for suspended floors.
Which is best for a suspended timber floor?
There is no single best material. Sprayed foam can be installed in shallow voids using a robot as well as higher voids, it always encases the joists to reduce joist thermal bridging. Rigid board, mineral wool and wood fibre need enough depth for an installer to work inside the void. A survey will determine the best solution for your floor structure and condition, and the services already in place.
How the four options compare
On a small screen the table is grouped by criterion. Scroll down to compare.
| Sprayed foamspray foam · PUR foam | Rigid boardPIR · rigid insulation board | Mineral woolrock wool · stone wool · glass wool | Wood fibrenatural fibre batt | |
|---|---|---|---|---|
| Installation method | Polyurethane foam is sprayed in place onto the underside of the floor, where it expands and sets. | Rigid PIR boards are cut to fit each joist bay and fixed in place. | Mineral wool quilt is draped between the joists and held up on netting. | Wood fibre batts are cut and friction-fitted between the joists. |
| Thermal performance | High thermal performance can be achieved with 120mm foam thickness, as low as 0.18 W/m²K in some cases (standard for a brand new floor). | High thermal performance can be achieved, depending on the board specification, thickness, detailing and how well the boards are fitted and sealed. | Depends on the product, thickness, joist depth and installation quality. | Depends on the product, thickness, joist depth and installation quality. Wood fibre typically performs similarly to mineral wool at the same thickness. |
| Thermal bridging | The insulation encases the joists, reducing thermal bridging through exposed timber. | Rigid board is normally fitted between the joists, leaving the joists as a thermal bridge unless the design includes additional insulation or vapour barrier. | Mineral wool is normally fitted between the joists, leaving the joists as a thermal bridge unless the design includes additional insulation or vapour barrier. | Wood fibre is normally fitted between the joists, leaving the joists as a thermal bridge unless the design includes additional insulation or vapour barrier. |
| Access to the underfloor space | A small access point is enough, because the foam is applied by a robot rather than by a person working inside the void. | Larger access is needed to fit the boards and for the installer to enter the void. | Larger access is needed for the installer to enter the void. | Larger access is needed for the installer to enter the void. |
| Best suited to | Suspended floors, from 200mm and deeper, when a continuous high-performance insulation layer is required or desired. | Suspended floors with sufficient depth and good access where rigid-board installation is practical. | Suspended timber floors with sufficient depth and installer access, where a flexible insulation system is appropriate. | Suspended timber floors where a fibre-based, vapour-open insulation approach is appropriate and the floor construction has been assessed as suitable. |
Note that suitability, access requirements and cost depend on the floor construction and installation conditions, and will be determined with a survey.
Underfloor insulation questions answered
What is underfloor insulation?
Underfloor insulation is an insulation layer added to a suspended timber floor, which is a floor made of floorboards over a ventilated void. It can be installed from below, without lifting the floor, or from above with the floorboards lifted. The four materials commonly used are spray foam, rigid insulation board, mineral wool and wood fibre.
What are the different names for underfloor insulation materials?
Sprayed foam is also called spray foam or PUR foam, and is a polyurethane applied as a liquid that expands and sets. Rigid board is usually PIR, a rigid polyisocyanurate board sold under brand names such as Celotex and Kingspan. Mineral wool covers rock wool and stone wool, spun from molten rock, and glass wool, spun from glass. Wood fibre is a batt made from compressed wood fibres, sometimes described as natural fibre insulation.
How much does underfloor insulation cost?
Q-Bot spray foam installation starts at £3,250 for a floor up to 25m², then £126.50 per additional m². Rigid board, mineral wool and wood fibre are priced by the installer and vary widely with the product specified, the floor construction and the access available. Costs are confirmed after a survey.
How deep does the space under my floor need to be?
Q-Bot spray foam can be installed from 200mm. Rigid board, mineral wool and wood fibre are fitted by hand by an installer working inside the void, which typically needs a lot more space. A void is rarely the same height throughout, and sleeper walls, pipework, ducting and debris can slow down or prevent access to some areas.
Can underfloor insulation be installed without lifting the floorboards?
Yes, where there is enough depth and access. All four materials can be installed from below. Sprayed foam needs only a small access point because it is applied by a robot. Where the void is too shallow or too obstructed to work in, insulating from above with the floorboards lifted is the alternative.
What U-value should my floor reach?
Building Regulations set 0.25 W/m²K as the minimum for an existing floor being upgraded, and 0.18 W/m²K for a new floor in an existing home. Both are in Approved Document L, Volume 1. With 120mm of sprayed foam, a floor can reach 0.18 W/m²K in some cases. What your floor can achieve depends on several factors, and is confirmed by a survey.
What is thermal bridging, and does it matter?
Rigid board, mineral wool and wood fibre are normally fitted between the joists, which leaves the joists themselves uninsulated. Those timbers conduct heat, so they remain a thermal bridge across the floor unless the design includes additional insulation or vapour barrier. Sprayed foam encases the joists, so no timber is left exposed within the insulated layer.
Which underfloor insulation is best for an older or damp-prone home?
There is no single best insulation for an older or damp-prone home. Existing moisture, timber decay, poor ventilation or water ingress should be investigated and addressed before any underfloor insulation is installed.
More questions are answered in our underfloor insulation FAQs, and in our guide to the best insulation for suspended floors.
Not sure which insulation is right for your floor?
Our survey measures the void, joists and floor build-up, so we can inform you which underfloor insulation measure(s) could be fitted under your floor.