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Floor insulation, a brief guide to planning and installing


Floor insulation is often overlooked as an upgrade or improvement to a home. It can be very disruptive and it can present many challenges. Finding the right installer who has knowledge and experience is often the first hurdle. Making sure that the correct materials na processes are chosen can also present problems.

This is aimed to help overcome these challenges.

There are two main types of floor and the majority of UK homes will fall into one or the other category. It is also true that most homes will probably have a combination of both types.

Solid Floors

Until the 18th century solid floors were the predominant type of construction. Often just compacted earth with straw spread over the top, or sometimes stone flags.

More recently decorative tiles and flags were used with a mortar bed. From the 19th onwards concrete and gypsum were more commonly used.

The majority of houses built in the 19th and 20th century that have solid floors will have concrete floors.

Suspended Floors

The first type of suspend floor was simply a timber structure laid directly on compared earth or soil. These were prone to decay and in order to deal with damp and decay issues suspend floors were lifted off the ground. Often sleeper walls were constructed underneath the floors as support.

To further aid damp control the areas under floors were ventilated with air bricks. But suspended floors were still prone to damp and rot problems until the advent of damp proof courses and damp proof membranes.

Many floors had extra structure timbers in place until joists were built to support the whole weight of the floor from wall to wall.


modern floors will have significantly larger floor joists and in very recently built homes the suspended floor might even be concrete joists with concrete panels infilled between. This is called Beam and Block.

Planning to insulate a floor

Surveying the floor properly is a first step in the planning process. Finding out whether it’s a suspended floor or solid and how its constructed. Finding out the size of joist or type of material used in construction.

Once the type of floor has been found and how it’s constructed and from what, it is is possible to choose a route to insulate. But first the risks that insulating poses must be dealt with.

Assessing the risks of floor insulation


Controlling moisture and water movement through a floor is the first consideration. Solid and suspended floors can both be .

Allowing the floor to “breathe”, whether its suspended or solid is of paramount importance. Our floors have stood the test of time due to their ability to regulate moisture. Well ventilated suspended floors should never have problems with damp or rot. But this can become a problem if the physical attributes are changed by adding insulation.

Solid floors are also well designed to allow excess moisture to escape either into the living space or out through the walls to outside. If we alter the capacity of a floor to seal with moisture by adding impermeable materials we ca nee increasing the likelihood of future damage and problems.


Is only really a problem when considering suspended timber floors. The choice of material can have a large effect on how effective a floor is if fire becomes an issue.


Adding materials and extra weight to a floor can have an effect on its overall structure. This should be taken into account when deciding on which approach to take.

Choose a material for floor insulation

Materials choice is normally dictated by the risks that the specific floor present. controlling the flow of moisture through the floor is aided or inhibited by the material that you choose. If moisture is not an issue, then it may be possible to use a rigid insulation board. Because of the high performance of these boards, they can be used in areas where joists are slimmer and space is at a premium. Rigid insulation boards are made from fossil fuels, and have the highest embed energy of most insulation materials.

These boards are impermeable and will trap moisture if they are not correctly installed.

If moisture is a problem then it would be better to choose a water permeable or semi permeable insulation like mineral wool or rock wool. They will also need some sort of fire protection as they can be a risk in fire situations.

These types of insulations are made from minerals, glass or rock. It is crushed and spin or blown into fibres and binders are added to help keep its shape.

They come in a variety of types, like loft roll, more rigid batts for cavities in brick walls, or more dense boards which can be used for internal and external insulation. These boards are good for floor insulation as they are water permeable, and fire resistant. they are rigid enough to hold there own weight, although if more pliable boards are used, the ycan be held up with net or wire strung underneath.

This is the way that natural materials like sheets wool or hemp should be installed. This type of material is a great sustainable material but it needs extra support to keep it in place.

Wood fibre is similar to these materials in that it is vapour open and is good at controlling moisture.

It is often advisable to add a vapour control layer to the cold side of the insulation and if possible add an airtightness layer to the warm side. If the installation method does not allow this, a combined layer of vapour permeable and air tight membrane can be added to the cold side.

Solid floors are more difficult to insulate and the material choices are more limited. If the existing floor can be cut out then it is possible to insulate with rigid boards, polystyrene or polyurethane are good examples of this type of insulation. But a good depth is required. It is possible to insulate with aerogel, which is a very high performing insulation and can be used in very thin layers.

there are proprietary boards which come with floor boards attached to the upper side and manufacturers have developed systems for retrofitting floor insulation onto solid floors.

Pre-installation considerations for post installation

Some aspects of the installation are unique to the project. Some final questions to ask, before you decide upon the specific route to installation.

How will the insulation or floor be repaired?

is access required after installation?

What will the floor finish be?

is it carpet or bare floor boards?

Will underfloor heating be installed as well?

For further reading go to:

Historic England

The Alliance for sustainable building products