How do I insulate a floor

How do I insulate a floor

Resources

How do I insulate a floor?

Got cold feet about cold floors?  We look at how to insulate a floor, exploring options for both a wooden floor and concrete floor, showing you how to insulate under the floor or above a concrete slab, whilst minimising thermal bridging.

Target U-Value

Generally, ground floors are either solid, usually concrete slabs, or suspended, i.e. timber floorboards on joists. When uninsulated, they will both have a U‑value of between 0.45W/m2K and 0.70W/m2K, but this depends on the geometry of the floor and the amount of thermal bridging. In each case the target U-value after installing insulation is between 0.20W/m2K and 0.25W/m2K (depending on floor geometry).

Insulating a solid floor is a lot more trouble and expensive than insulating a timber floor. Typically, the former has a payback of between 8 or 10 years or more, but the payback period for suspended timber floor insulation is between 3 and 5 years.

Avoiding thermal bridging

Thermal bridging occurs where a solid building elements continues from the inside to the outside of the structure, conducting heat out or cold in. It severely compromises the value of the work and can lead to cold spots and condensation.

Techniques to minimise thermal bridging with insulation around floors

The above diagrams show how to apply insulation around the edges of building elements to prevent thermal bridging.

Also consider in each case where the airtightness layer beneath the floor next to the one in the walls around the edges. The airtightness layer prevents unwanted draught infiltration. It can be pictured as a continuous envelope around the building.

For example, in the diagram above for underfloor heating, which shows how the membrane laps up the side of the insulation behind the skirting board, it should then go behind, or be taped to, the plasterboard or plaster on the walls.

In the diagram above illustrating insulation beneath a solid floor, where it goes underneath the wall to the outside, it should then connect to any render covering the wall.

Best insulation material to use to insulate a floor

Comparing the k-values of some insulants used in underfloor insulation

The graph above shows the relative thermal conductivity (k-value) of some insulants which might be considered for underfloor insulation. The standard choice is phenolic foam, XPS or EPS, but these are not ideal in an eco-home, as it is not a natural material but non-breathable and a product of the fossil fuel industry.

Other materials are discussed below and compared in tables for each type of floor. Figures for k-value vary according to product so you should do your own research. These tables are guides only.

Phenolic foam, EPS, polyisocyanurate and XPS should only be used if space is a absolute premium. As can be seen, batts of woodfibre or mineral wool are almost as good as EPS. Cellulose is also ideal if the insulant does not have to be load-bearing. This is made of recycled newsprint. Granulated cork has also been used.

We will now look at how to deal with each type of floor in turn.

How to insulate a wooden floor

If there is access from below, is easy to stuff the insulation between the joists. Press the insulation against the floorboards, but do not over-compress.

Sheeps wool insulation between rafters in a suspended floor, secured with netting

Its possible to add more insulation than the depth of the joists permits in two stages, by, after stuffing the spaces between the joists, adding battens to the underneath of the joists and fastening the second layer of retaining boards, fabric or netting to them.

Holes in the floorboards where pipes and other wires go through, plus the gaps around the edges and between floorboards, all need to be plugged with mastic and tapes to prevent draughts.

If there is no access from beneath, there is nothing for it but to take up the floorboards.

If there is a possibility that the room could flood, a damp proof layer should not be put beneath the insulation; instead, draught resistant fabric should be used, through which the water can drain away.

Three stages of installing insulation beneath a suspended wooden floor

Here, insulation in the form of sheep’s wool has been installed between the joists and kept in place with 60mm thick wood-fibre boards. The thermal conductivity of the wood-fibre boards is 0.04W/mK. This is fully breathable. The detailing for this installation is below. The U-value for a typical semi-detached house with this configuration is 0.16W/m2K.

Resources

How do I insulate a floor?

Got cold feet about cold floors?  We look at how to insulate a floor, exploring options for both a wooden floor and concrete floor, showing you how to insulate under the floor or above a concrete slab, whilst minimising thermal bridging.

Target U-Value

Generally, ground floors are either solid, usually concrete slabs, or suspended, i.e. timber floorboards on joists. When uninsulated, they will both have a U‑value of between 0.45W/m2K and 0.70W/m2K, but this depends on the geometry of the floor and the amount of thermal bridging. In each case the target U-value after installing insulation is between 0.20W/m2K and 0.25W/m2K (depending on floor geometry).

Insulating a solid floor is a lot more trouble and expensive than insulating a timber floor. Typically, the former has a payback of between 8 or 10 years or more, but the payback period for suspended timber floor insulation is between 3 and 5 years.

Avoiding thermal bridging

Thermal bridging occurs where a solid building elements continues from the inside to the outside of the structure, conducting heat out or cold in. It severely compromises the value of the work and can lead to cold spots and condensation.

Techniques to minimise thermal bridging with insulation around floors

How do I insulate a floor

The above diagrams show how to apply insulation around the edges of building elements to prevent thermal bridging.

Also consider in each case where the airtightness layer beneath the floor next to the one in the walls around the edges. The airtightness layer prevents unwanted draught infiltration. It can be pictured as a continuous envelope around the building.

For example, in the diagram above for underfloor heating, which shows how the membrane laps up the side of the insulation behind the skirting board, it should then go behind, or be taped to, the plasterboard or plaster on the walls.

In the diagram above illustrating insulation beneath a solid floor, where it goes underneath the wall to the outside, it should then connect to any render covering the wall.

Best insulation material to use to insulate a floor

Comparing the k-values of some insulants used in underfloor insulation

The graph above shows the relative thermal conductivity (k-value) of some insulants which might be considered for underfloor insulation. The standard choice is phenolic foam, XPS or EPS, but these are not ideal in an eco-home, as it is not a natural material but non-breathable and a product of the fossil fuel industry.

Other materials are discussed below and compared in tables for each type of floor. Figures for k-value vary according to product so you should do your own research. These tables are guides only.

Phenolic foam, EPS, polyisocyanurate and XPS should only be used if space is a absolute premium. As can be seen, batts of woodfibre or mineral wool are almost as good as EPS. Cellulose is also ideal if the insulant does not have to be load-bearing. This is made of recycled newsprint. Granulated cork has also been used.

We will now look at how to deal with each type of floor in turn.

How to insulate a wooden floor

If there is access from below, is easy to stuff the insulation between the joists. Press the insulation against the floorboards, but do not over-compress.

Sheeps wool insulation between rafters in a suspended floor, secured with netting

Its possible to add more insulation than the depth of the joists permits in two stages, by, after stuffing the spaces between the joists, adding battens to the underneath of the joists and fastening the second layer of retaining boards, fabric or netting to them.

Holes in the floorboards where pipes and other wires go through, plus the gaps around the edges and between floorboards, all need to be plugged with mastic and tapes to prevent draughts.

If there is no access from beneath, there is nothing for it but to take up the floorboards.

If there is a possibility that the room could flood, a damp proof layer should not be put beneath the insulation; instead, draught resistant fabric should be used, through which the water can drain away.

Three stages of installing insulation beneath a suspended wooden floor

Here, insulation in the form of sheep’s wool has been installed between the joists and kept in place with 60mm thick wood-fibre boards. The thermal conductivity of the wood-fibre boards is 0.04W/mK. This is fully breathable. The detailing for this installation is below. The U-value for a typical semi-detached house with this configuration is 0.16W/m2K.


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