How to insulate a suspended floor

17 November 2020 EcoTherm Insulation

With the weather turning colder and many of us spending more time at home than ever before, homeowners are looking at ways to make their homes more comfortable and cost-efficient. Insulating suspended timber floors is a fairly simple yet highly effective way to do this.

In this blog, we look at how homeowners can benefit from insulating floors. We then provide some tips for checking the condition of the floor and how to fit rigid underfloor insulation such as EcoTherm Eco-Versal

How much can you save by insulating floors?

Most people know about the benefits of fitting loft insulation but have never thought about fitting insulation under the floor. U-Switch estimates that about 10% of heat loss from an average home is through the ground floor. This is especially true for older properties that typically feature cellars and suspended timber floors, but can also be true of any floors that are above unheated spaces such as garages or car ports. 

Insulating these areas can not only help to keep homes warm but, as research from the Energy Savings Trust shows, it can result in significant savings on energy bills, depending on the house type:
House Type Estimated annual energy bill savings (£/year)
Detached £85
Semi-detached £50
Mid-terrace £35
Bungalow £75

How much insulation should you use to insulate under a floor?

Any floor insulation retrofit needs to comply with the relevant Building Regulations for that area. For example, if you are installing underfloor insulation in an existing home in England or Wales, you will need to ensure the floor achieves a U-value of 0.25 W/m2K or less to meet the minimum requirements. Where possible, it makes sense to target an even lower U-value as this will provide improved heat savings over time. 

The thickness of insulation you will need to do this depends on what type of insulation you use and its thermal conductivity. If a material has a lower thermal conductivity value, it is more effective at stopping heat escaping. This means you get hit your U-value target with a thinner layer of insulation compared with a less efficient insulation. Our Eco-Versal insulation is suitable for a range of applications, including suspended floors and has a thermal conductivity of 0.022 W/mK which is much lower than other options such as mineral fibre rolls. 

To help you decide what thickness of Eco-Versal boards to use, take a look at our U-Value calculator. This no-jargon tool will help you to quickly determine the build-up you need to achieve the project’s targeted U-value. 


3 things to check before fitting underfloor insulation

Before you start your installation, there are a few things you need to check to make sure the timbers are dry and in good condition before any insulation is fitted. 


1. Check for damp

Damp and rot can cause serious issues, so when you are removing the floorboards, check them and the joists for any wet patches or areas where the wood is soft or crumbling. If you have one, it is a good idea to use a moisture meter to quickly check the moisture content of the wood, which should be 14% or below. Also be on the lookout for any signs of insect damage such as tiny holes in the timbers.

These issues will all need to be fixed before insulation is installed. 

Even if the timbers are not showing signs of damp or rot, it is important to consider if there are any issues that are causing condensation now, or that that may cause it in the future. For instance:
•    check adjoining walls around joists for any signs of damp;
•    look for any rust on pipework;
•    ask the owner if the property has suffered flooding in recent years which might saturate walls and timbers; and
•    carefully seal any gaps or voids.


2. Assess ventilation

One of the added benefits of fitting insulation is that it can close off gaps between the floor joist, cutting down on draughts in the home. At the same time, it is important to make sure the home is still well ventilated to let in fresh air and get rid of moist indoor air before it condenses. 

To make sure there is adequate ventilation, you should:
•    check all of the existing ventilation channels, such as air bricks, to make sure they aren’t blocked up with leaves and other debris and are providing enough ventilation to keep the area dry.
•    remove any rubble or materials which have been left under the floors that could potentially block the airflow;
•    if the house has sleeper walls, ensure that these also allow ventilation either through a honeycombed construction or a vent; and
•    check that the room has adequate ventilation strategies in place; for example, trickle vents on the windows or, if installing in a damp environment such as a kitchen or a bathroom, that there is a fan vent. If you need to, you can also get air-leakage tests carried out. 


3. Address any practical issues

If pipework and cabling are running between the floor joists then you will typically need to relocate them to make room for the insulation. If the existing floorboards have a straight edge, it is also a good idea to replace them with tongue and groove boards to avoid gaps from shrinkage.


How to install suspended timber floor insulation from below the joists

If the home has a cellar, the insulation can be fitted from below without having to remove the floorboards. To do this, you should:
1.    Measure the gap between the floor joists and cut the Eco-Versal boards to the right dimensions using a fine-toothed saw. It is important to measure each gap as these can often vary slightly and the insulation needs to fit snugly. Any small gaps can be filled with expanding urethane sealant.
2.    The boards should then be pushed in until they are flush with the underside of the floorboard. The ends of the boards should also be tightly butted with no gaps.
3.    When you are installing two layers of insulation, offset the joints between the two layers.
4.    Side-nail 25 mm x 25 mm timber battens to the joists or partially drive galvanised nails into the side of the joists in the appropriate position to keep the insulation boards in place. 
5.    Narrow strips of insulation should be cut to fill gaps of 25 mm or more between joists and perimeter wall and support on blocks nailed to the underside of the joists. Again, any gaps of less than 25 mm can be filled with expanding urethane sealant.


How to install suspended timber floor insulation from above the joists

If there is no way to get access from the joists below, then you will need to clear the room, lift up the carpets/flooring and remove the floorboards before you can start. Once this is done, the installation process is very similar to the above, but in reverse order:
1.    Measure and mark the joist at the required depth to install the insulation so it will sit flush with the floorboards once it has been fitted.
2.    Side-nail the timber battens or partially drive the galvanised nails into the joist as described above.
3.    You can then cut and fit the boards, as detailed above.

(Note that Eco-Versal should not be installed directly above the joists.)

Still have questions?

If you have any other questions about how to fit EcoTherm insulation in any application then take a look at our Educate section, where you can find everything from how to videos to FAQs, or get in touch with our Technical Services team for some fuss-free advice.

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EcoTherm Insulation Ltd

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