21st Nov 2022 -

The ground floor is a vital part of the thermal envelope within a property, and installing thermal insulation can help to reduce the level of heat escaping through it. Installing insulation under your floorboards can be accomplished in both modern homes with solid floors and older buildings with suspended timber floors. There are some drawbacks to underfloor insulation, which is often an expensive and destructive process.

Is Installing Insulation Under Floorboards Right For You?

Compared to insulating the other areas of the thermal envelope (the roof and outer walls), insulating your floors is by far the most disruptive process, while offering the lowest reductions in heat loss. Installing underfloor insulation will involve completely removing all flooring, including any carpeting, laminate, or tiles, and the floorboards underneath. In some cases, these can be relayed afterwards but for others, they may need to be replaced. This process will be time-consuming and render the ground floor of your house unusable during work. When building a new house, underfloor insulation is a necessity, but if looking to add insulation to an existing property, you will probably get a better increase in energy efficiency by adding insulation to the walls or loft.

How to Install Insulation Under Floorboards in a Solid Floor

There are two options for insulating a house with a solid concrete floor, excavating the floor to install insulation, or installing it over the concrete. Houses built from the 1950s onwards are likely to have a solid concrete floor, as the design became standard due to the decline in timber imports following WW2.

Digging Out the Concrete

In new-build properties, underfloor insulation will normally be laid beneath the concrete floor during construction. For an existing house, it is possible to retrofit insulation under concrete floors, though this is an extremely expensive method. The process to do this will involve removing all flooring, excavating the concrete subfloor below, installing insulation, and then relaying concrete.

Beneath the excavation, you would first install a damp-proof membrane, topped with a solid insulation board, and a new layer of concrete. This is normally only completed in events where the foundation is already damaged, as it costs several thousand pounds and causes a great deal of disruption to your home. It’s also worth noting that most modern homes will already have a layer of insulation installed beneath the concrete, so it may not be redundant to try and add more. This is a job for a professional, and not one that can be undertaken as a DIY project.

Installing Insulation Over the Concrete

A less disruptive process is to install a rigid insulation board over the concrete subfloor. This will still require you to remove all flooring for access. There can be some issues with adding underfloor insulation this way, the primary one being that it will effectively raise the height of your floors. This can cause issues throughout the ground floor, requiring doors to be trimmed, radiators and plug sockets to be refitted, and skirting boards to be raised.

If opting for this method you’ll first have to remove all flooring from your ground floor. You should check the quality of your concrete floor, potentially using a concrete floor sealer if it has issues with cracking. Then, a damp-proof membrane (sometimes called a vapour barrier) will need to be laid over the solid floor, at a size slightly larger than the floor to allow it to cover the bottom of the walls. This can then be topped with the suggested 50mm of solid insulation, which will then be covered with a chipboard. This chipboard can serve as the base for whatever flooring you opt for, be that carpeting, tiles, or laminate.

How to Install Insulation Under Floorboards in a Suspended Floor

Suspended timber floors are present in most houses built between the early 18th century and the 1950s. Houses built prior to this were usually built with wooden lattices or bricks laid on the compacted earth. These floors, particularly when made from timber, were susceptible to decay. Suspended timber floors, built over a void, solve the issues of decay but can lose a considerable amount of heat.

Insulation can be installed between the joists of a suspended timber floor. Like insulating a solid floor, this will likely involve removing all flooring to access the joists, unless your home possesses a crawl space large enough to insulate through. Reinstalling your floorboards should be possible, providing you carefully remove them, and mark them so you know which order to reinstall them in. Alongside insulation, properly draught-proofing your floorboards can improve heat retention, especially in older properties where floorboards are worn or otherwise damaged. When looking to insulate suspended timber floors your floor void must be properly ventilated to allow air movement, with air bricks on the external wall of the property. This will allow water vapour from the damp soil to escape, reducing the risks of damp and interstitial condensation on your insulated floor.

insulation boards

Insulating Between the Joists

The method you use to insulate between the joists of your suspended floor can differ based on sizing and your willingness to enter small spaces. In certain older houses, there will be a crawl space running under the entirety of the ground floor. Provided you or your contractor feel comfortable navigating this space you can insulate the underside of a suspended timber floor this way, negating the need for removing flooring. There are also modern options to use spray foam insulation, which can be installed by a remote-controlled robot that navigates your crawl space. If your house is not suitable for this method you will have to remove all flooring including lifting the floorboards within your property.

You’ll be looking to insulate between each joist in a floor, using insulation batts or boards. Rather than affixing your insulation to the joists, the popular method is to use netting between the floor joists, stapling it in place. Your insulation can then be snugly fit within this netting, ensuring that it stays in place. For insulation boards, you can use about 70mm of a quality brand, and for mineral wool or sheep wool batts, you’ll need a greater distance to offer the recommended level of thermal support, roughly 150mm.

How to Draught Proof a Suspended Floor

Reducing or outright eliminating the draughts passing through a wooden floor can further contribute to energy savings when done in combination with underfloor insulation. There are a few options in place for easily plugging draughts in your floor:

  • Silicone-Based Filler: These fillers come in a variety of wood styles and can be inserted into the gaps between floorboards using a mastic gun. This can be a little messy, and won’t offer a permanent solution.
  • Pine Slivers: Pine Slivers are thin pieces of wood that can be hammered into the gaps between floorboards using a rubber mallet. You can then sand them, and stain them to match the look of your floor. This is a long-lasting solution, though it will be far more expensive.
  • Branded Floorboard Filling Products: There are options available like Draughtex, which is a long rubber line in a range of sizes that can be pressed into the gaps between floorboards using a specialised tool. It will mimic the look of shadows between floorboards whilst offering thermal insulation. If you are happy to leave the impression of gaps then these are fine, and they will also be great if you are covering your floorboards.

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