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Suspended Timber Floor Insulation: A Complete DIY Guide

Insulating suspended timber floors has become an essential strategy for owners of older homes to combat common issues. It effectively prevents draughts, enhances the warmth of the house, reduces energy wastage, and preserves the structural integrity of suspended wooden floors.

Though insulating floors can sometimes be challenging, it’s a crucial measure for not only draught-proofing your home but also minimizing the risk of rot or dampness in the flooring. Neglecting these issues could lead to significant damage.

Our guide highlights the significance of suspended timber floor insulation and outlines practical steps to maintain the health and longevity of your timber floors. Continue reading.

The Reason Behind Suspended Timber Floors

Suspended timber floor insulation was developed to solve the problems caused by laying floor timbers directly on the ground, historically leading to dampness and rot.

This innovation involved elevating the timbers above the ground, creating a ventilated space beneath the floor. This design prevented the wood from becoming damp due to direct ground contact and allowed air circulation through sub-floor vents, reducing condensation risks.

However, a drawback of suspended timber floors is their susceptibility to drafts. An uninsulated base of this type can lose more heat than the walls or loft, making it a critical yet often neglected aspect in insulating older homes.

Benefits Of Adding Floor Insulation

Adding underfloor insulation to suspended timber floors is essential due to their inherent draftiness.

  • These floors usually consist of floorboards laid over timber joists. While liners beneath timber or laminate finishes can reduce drafts, they don’t completely eliminate them as their edges are not typically sealed for airtightness.
  • Properly installed underfloor insulation improves a house’s heating efficiency. Assuming the insulation is installed correctly, the house can warm up faster and retain heat longer.
  • Moreover, hygroscopic insulation is beneficial in older homes with suspended timber floors. This type of insulation can absorb water vapour from the air during high humidity periods, reducing the likelihood of condensation and mould on internal surfaces like walls and ceilings.
  • It then releases this moisture gradually when humidity levels are lower, helping to maintain a balanced indoor environment.

How To Do DIY Suspended Timber Floor Insulation?

There are practically two ways of insulating your suspended timber floors. They are as follows:

Suspended Timber Floor Insulation from Above the Floor Joists

  • Begin by installing the insulation boards before boarding the floor.
  • Ensure the insulation boards are cut to snugly fit between the floor joists. Measure the distance between joists before cutting, as spacings may vary.
  • To align the insulation boards flush with the top surface of the joists, support them on at least 25 mm x 25 mm treated softwood timber battens, proprietary galvanized steel saddle clips, or partially driven galvanized nails into the side of the joists.
  • If using battens or nails, adjust their height to match the board’s thickness. Nails should protrude 40 mm from the joist.
  • Place the insulation boards between the joists, ensuring they are supported by the battens, saddle clips, or nails.

Suspended Timber Floor Insulation from Below the Floor Joists

  • Start by fixing the floorboards over the joists before installing the insulation boards from below.
  • Cut the insulation boards to fit snugly between the floor joists. Measure the distance between joists before cutting, as spacings can differ.
  • Insert the cut insulation boards between the joists, ensuring they are flush with the underside of the floorboards.
  • Secure the insulation boards by side-nailing 25 mm x 25 mm timber battens to the joists or by partially driving galvanized nails into the joists at the correct height to hold the panels in place.

Things You Should Be Careful About

  • When installing two layers of insulation, ensure they are tilted oppositely. This means the joints of the boards in the two layers should not align directly with each other.
  • All joints between insulation boards must be closely butted against each other.
  • Insulation boards need to be tightly fitted between the joists. Fill any remaining gaps with expanding urethane sealant for a secure fit.
  • Use specially cut insulation board pieces to insulate narrow spaces between a joist and the perimeter wall. These pieces should be supported by blocks nailed to the underside of the joists.
  • Fill gaps narrower than 25 mm with expanding urethane sealant. Ensure there are no gaps wider than 50 mm for optimal insulation.

Is Suspended Timber Floor Insulation Worth It?

On average, a UK household requires approximately 9,000kWh of energy annually for space heating. In a home with a 150m2 floor area that has undergone improvements, including adding 100mm quilt insulation, energy savings can reach about 1,500kWh each year.

While this saving might not seem substantial, it’s certainly significant. Financially, this could translate to savings of around £100 annually, a notable amount in the context of current energy costs.

Projecting the savings from insulating suspended floors over five to ten years is uncertain due to fluctuating energy prices. However, the rationale is that if insulating walls is beneficial, floor insulation should also be considered a worthwhile investment.

How Much Does A Suspended Timber Floor Insulation Cost?

The cost of insulating a suspended timber floor can vary widely based on multiple factors, including the size of the area to be insulated, the type of insulation material used, and regional labour costs. Here’s a general breakdown:

  1. Material Costs: Insulation materials, such as mineral wool, foam boards, or natural fibres, vary in price. As a rough estimate, materials might cost between £5 to £20 per square meter, depending on the type and quality of insulation.
  2. Labour Costs: If you’re hiring professionals, labour costs can significantly influence the overall price. This can range from £150 to £400 per day for a tradesperson, though this can vary based on location and the job’s complexity.
  3. Size of the Area: The larger the area, the higher the overall cost. For example, a small room will cost less to insulate than an entire house.
  4. Additional Costs: These include costs for removing and reinstating floorboards or any necessary repairs to the existing floor structure.

As an average figure, for an average-sized room, you might expect the total cost (including materials and labour) to be in the range of £500 to £1,500.

However, for a more accurate estimate, getting quotes from local contractors who can assess your specific needs and provide a tailored quote is recommended.


Installing underfloor insulation correctly is straightforward, with adequate planning and attention to detail. The key is to start by thoughtfully installing the breathable airtightness layer. Make sure to leave a flap at the edges, allowing it to be seamlessly taped to the walls. Next, lay the insulation carefully, ensuring no gaps for optimal efficiency.

Regarding the vapour control layer, turn it against the wall edges and secure it with tape. By following these steps, you can achieve a floor that is not only well-insulated but also breathable and airtight, enhancing the overall comfort and energy efficiency of your home.

Read more: What Is Underfloor Insulation? Best Under Floor Insulation Grant 2023

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