Does Underfloor Insulation Work?

Underfloor insulation can make a noticeable difference in homes with suspended timber floors. It helps reduce heat loss through the floor, limit cold draughts and make ground-floor rooms feel more comfortable.

It is particularly suitable for older properties where timber floorboards are positioned above a ventilated underfloor void.

However, good results depend on more than simply placing insulation between the joists. The insulation must be fitted carefully, supported correctly and installed without restricting the ventilation beneath the floor.

PMR Eco Insulation installs 150mm Knauf OmniFit mineral wool underfloor insulation, supported securely with netting. Where the existing underfloor ventilation is inadequate, we can also install additional air bricks to help maintain cross-flow ventilation beneath the suspended floor.

How does underfloor insulation work?

Suspended timber floors normally have an open void beneath the floorboards. Outside air enters this space through air bricks to help control moisture and protect the timber floor structure.

Although this airflow is important, it also means that the underside of the floor is exposed to cold air.

Underfloor insulation is fitted between the timber floor joists, directly beneath the floorboards. It creates a thermal layer that slows the movement of heat from the room above into the colder space below.

A correctly installed system should:

• Fit closely between the floor joists

• Cover the underside of the floor consistently

• Avoid large gaps between sections

• Remain securely supported

• Preserve ventilation beneath the floor

• Avoid trapping moisture against the timber

• Be fitted carefully around pipes, cables and obstructions

Our underfloor insulation installation uses Knauf OmniFit mineral wool fitted between the joists and held securely in place with a suitable netting system.

What underfloor insulation system does PMR use?

PMR Eco Insulation normally installs 150mm Knauf OmniFit mineral wool, where the available joist depth and floor construction allow it.

The insulation is installed between the timber joists and supported from below using securely fixed netting.

Knauf OmniFit is well suited to suspended timber floors because it can be fitted closely around:

• Uneven timber joists

• Water pipes

• Electrical cables

• Small obstructions

• Irregular spaces

• Older floor structures

The netting supports the insulation across the full floor area and helps prevent it from sagging or falling out of place.

The exact installation depth and method will depend on the property, the joist depth and the access available beneath the floor.

Which properties are suitable for underfloor insulation?

Underfloor insulation is normally suitable for properties with suspended timber ground floors.

These commonly include:

• Victorian terraced houses

• Edwardian properties

• Older semi-detached homes

• Traditional cottages

• Properties with accessible cellars

• Homes with crawl spaces

• Houses with timber floorboards above a ventilated void

Solid concrete floors require a different insulation method and cannot normally be insulated from underneath in the same way.

A professional underfloor insulation survey is needed to confirm the floor construction, available access and condition of the area beneath the property.

What are the benefits of underfloor insulation?

The main benefits can include:

• Reduced heat loss through the ground floor

• Warmer-feeling floor surfaces

• Fewer cold draughts through floorboards

• Improved comfort in ground-floor rooms

• More consistent room temperatures

• Reduced demand on the heating system

• Improved energy efficiency

The improvement may be especially noticeable in living rooms, dining rooms, hallways and other areas with exposed timber floorboards.

Underfloor insulation does not generate heat. It helps retain more of the warmth already being produced inside the home.

Can underfloor insulation reduce draughts?

Yes. Underfloor insulation can significantly reduce cold air moving through gaps in suspended timber floors.

Draughts may enter through:

• Gaps between floorboards

• Openings around pipes

• Gaps beneath skirting boards

• Damaged or missing floorboards

• Service penetrations

• Open edges around rooms

Mineral wool fitted closely beneath the floorboards helps reduce this unwanted air movement.

Larger openings, damaged boards or gaps around services may also need separate draught-sealing work.

Will underfloor insulation make the floor warmer?

It can help the surface of the floor feel less cold because less heat is being lost into the ventilated space beneath the property.

The amount of improvement will depend on:

• The insulation depth

• How accurately it is fitted

• The condition of the floorboards

• Existing draughts

• The floor covering

• The heating system

• The temperature beneath the property

• The condition of the underfloor void

Homes with exposed timber floors may notice a particularly clear improvement in comfort.

Why is netting used beneath the insulation?

The insulation must stay securely positioned against the underside of the floor.

Without proper support, insulation may:

• Sag between the joists

• Fall out of place

• Leave gaps beneath the floorboards

• Become damaged

• Lose effectiveness

• Obstruct access beneath the floor

Our netting system is fixed beneath the floor joists to support the full depth of the Knauf OmniFit insulation.

The insulation should be held securely without being unnecessarily compressed, because compression can reduce the amount of air trapped within the material and affect its thermal performance.

Why are air bricks important?

Air bricks allow outside air to move through the space beneath a suspended timber floor.

This ventilation helps manage moisture and reduces the risk of damp conditions developing around the timber joists and floorboards.

For cross-flow ventilation to work effectively, air needs to enter from one side of the property and leave through another.

Problems may occur when air bricks are:

• Blocked by soil or paving

• Covered by extensions

• Damaged

• Too few in number

• Blocked internally by debris

• Positioned below the outside ground level

• Unable to ventilate all sections of the floor void

Where the existing ventilation is inadequate, PMR Eco Insulation can install additional air bricks to help improve cross-flow ventilation beneath the suspended floor.

Should air bricks remain open after insulation is installed?

Yes. Air bricks should remain open and unobstructed.

Underfloor insulation must not block the ventilation paths beneath the property. Maintaining airflow is important because insulation reduces heat entering the underfloor void, which may make the space colder.

During installation, we take care to ensure that:

• Existing air bricks remain clear

• Cross-flow ventilation is maintained

• Insulation does not block airflow routes

• Netting does not hang excessively low

• Debris is not left obstructing ventilation

• Additional air bricks are considered where required

Insulating the floor and maintaining ventilation should be treated as part of the same system.

Can underfloor insulation cause damp?

Correctly installed underfloor insulation should not cause damp. Problems are more likely where moisture defects already exist or where ventilation has been restricted.

Before work begins, the accessible underfloor area should be checked for signs such as:

• Wet or decaying floor joists

• Standing water

• Leaking pipes

• Blocked air bricks

• Damp masonry

• Mould growth

• A strong musty smell

• Fungal growth

• Previous timber repairs

Insulation should not be used to cover an unresolved water leak, damp problem or structural defect.

A proper underfloor insulation survey should assess the visible condition of the floor timbers, pipework, ventilation and access before insulation is recommended.

Why should ventilation inside the home also be considered?

Insulation makes a home more energy efficient by reducing uncontrolled heat loss. However, moisture produced inside the property still needs a controlled route to escape.

Everyday activities add moisture to the indoor air, including:

• Cooking

• Showering

• Bathing

• Drying clothes indoors

• Using an unvented tumble dryer

• Breathing

• Boiling water

For this reason, PMR Eco Insulation may also advise customers to consider trickle vents for background ventilation and suitable mechanical extraction in kitchens and bathrooms.

These measures help manage moisture inside the occupied rooms and reduce the likelihood of condensation developing elsewhere in the property.

What are trickle vents?

Trickle vents are small controllable vents normally fitted within or above windows.

They provide a steady level of background ventilation without requiring the windows to remain fully open.

Trickle vents can help:

• Replace stale indoor air

• Reduce excessive humidity

• Support bathroom and kitchen extraction

• Improve background airflow

• Reduce condensation risk

• Provide controlled ventilation throughout the home

Where a property has very limited background ventilation, we may recommend trickle vent installation alongside insulation improvements.

They should form part of a considered ventilation approach rather than being treated as a replacement for proper kitchen and bathroom extraction.

Why are mechanical extractor fans important?

Bathrooms and kitchens generate large amounts of moisture.

A suitable mechanical extractor fan removes humid air directly from the room and discharges it outside before that moisture can spread throughout the home.

Mechanical extraction may be required in:

• Bathrooms

• Shower rooms

• Kitchens

• Utility rooms

• Rooms containing tumble dryers

• Other areas producing high moisture levels

The extractor should discharge to the outside of the property and should not terminate inside a loft, wall cavity or underfloor void.

We may advise customers to consider suitable bathroom mechanical extraction and kitchen mechanical extraction where existing ventilation is inadequate.

Can underfloor insulation be installed through the floorboards?

In some properties, access may be created by carefully lifting sections of flooring. However, this depends on the construction and condition of the floor.

Possible access routes include:

• An existing cellar

• A crawl-space hatch

• An access panel

• An opening beneath the stairs

• Carefully lifted floorboards

• Access from an adjoining underfloor area

Installing the insulation from beneath is often preferable because it can avoid disturbing large areas of finished flooring.

The survey should establish whether safe and practical access is available before work is agreed.

Can pipes and cables remain beneath the floor?

Pipes and cables are common within suspended-floor voids and can usually remain in place.

The insulation must be fitted carefully around them without:

• Pulling or placing pressure on cables

• Damaging pipework

• Hiding leaking joints

• Preventing access to valves

• Forcing insulation tightly around services

• Covering visible defects without reporting them

Water pipes may also benefit from separate pipe insulation, particularly because the underfloor void will remain cold and ventilated.

Electrical faults, plumbing leaks or damaged services should be repaired by an appropriately qualified tradesperson.

How long does underfloor insulation last?

Mineral wool underfloor insulation can remain effective for many years when it remains dry, securely supported and undisturbed.

Its performance may be affected if:

• Supporting netting becomes damaged

• Water enters the floor void

• Pipework begins leaking

• Rodents disturb the material

• Building work damages the insulation

• Large gaps were left during installation

• Ventilation defects lead to excessive moisture

A properly installed system should not normally require frequent replacement.

Is underfloor insulation worth installing?

Underfloor insulation can be worthwhile for suitable homes with cold suspended timber floors.

It is most likely to provide a noticeable benefit where:

• The floor feels persistently cold

• Draughts rise through the floorboards

• Ground-floor rooms lose heat quickly

• The property has an accessible floor void

• No insulation is currently present

• The heating struggles to maintain comfort

• The underfloor timber and ventilation are in suitable condition

The likely cost and benefit depend on the floor area, access, joist depth, existing ventilation and condition beneath the property.

Arrange a free underfloor insulation survey

PMR Eco Insulation provides underfloor insulation installation across Liverpool, the North West and North Wales.

Our system uses 150mm Knauf OmniFit mineral wool fitted between the timber floor joists and supported securely with netting.

We inspect the available access, visible floor timbers, air bricks and underfloor ventilation before recommending work. Where necessary, we can install additional air bricks to help maintain cross-flow ventilation beneath the floor.

We may also recommend trickle vents, bathroom mechanical extraction and kitchen mechanical extraction as part of improving ventilation and controlling moisture throughout the property.

Use our underfloor insulation calculator for an instant estimated price or contact us to arrange a free survey.