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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.

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

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.

Sheep's wool insulation between rafters in a suspended floor, secured with netting.

Sheep’s wool insulation between rafters in a suspended floor, secured with netting

It’s 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

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.

Detail for insulation beneath suspended wooden floor

Detail for insulation beneath suspended wooden floor

How much insulation do I need under a wooden floor?

Insulation thicknesses for a suspended wooden floor

Insulation thicknesses for a suspended wooden floor

How to insulate a concrete floor

If a floor is being excavated and replaced, this is a perfect opportunity to improve the standard of insulation. A U-value of 0.2W/m2K can be achieved for a typical semi-detached dwelling using around 100mm of air-based insulation and careful detailing. Better U-values could be achieved with the same thickness of higher performance materials, or a deeper thickness of the same material.

Insulation can be added over the existing concrete slab or under a new one.

Above the concrete slab

If the insulation is placed above the slab, the room will warm up more quickly
when the heating is switched on, but cool down more quickly too – this applies to all internal insulation.

It will also affect the floor level, which has an impact on door openings, stairs and other fixtures. This is why phenolic foam is the most common solution in this instance, because it can achieve greater insulation for less depth.

In this case, the damp proof membrane goes above the concrete. Moisture resistant chipboard goes on top, beneath the flooring surface. Leave room for expansion around the edges.

Below the concrete slab

Insulation below the slab is generally better, especially in a warm south-facing room, since the concrete helps absorb heat and limits overheating.

The damp proof membrane is placed over a layer of sand to prevent it being punctured by sharp stones. As it also serves as an airtightness membrane, it laps up the sides of the walls to link to the plaster or plasterboard.

Insulation goes on top of this and around the edges, and concrete is poured on top of that. If underfloor heating is being used, which is fixed to the top of the concrete mix, and then a small amount of concrete poured on top of that. The floorcovering can be either tiles or floorboards, but any timber should be left in the room for some weeks with the heating on before being laid to prevent it warping.

How much insulation do I need for a concrete floor?

Insulation thicknesses for a solid concrete floor

Insulation thicknesses for a solid concrete floor

Why not install underfloor heating along with solid floor insulation?

It is highly recommended to install underfloor heating at the same time as tackling underfloor insulation, especially with solid floors. You get better value for money.

Underfloor heating is far more efficient than conventional central heating and gives a greater feeling of comfort. It will pay for the cost of installing it with fuel savings, as less energy is required to achieve the same feeling of warmth.

Section through solid floor insulation with optional underfloor heating

Section through solid floor insulation with optional underfloor heating

If it is a suspended floor, always leave the floorboards in the room for a few weeks with the heating on before laying them to avoid the wood warping when in place.

© David Thorpe, Manager of Green Deal Advice and author of Sustainable Home Refurbishment: The Earthscan Expert Guide to Retrofitting Homes for Efficiency.

Also see:
Floor insulation at free green open house events in Mar/Sep
Boarding a loft over insulation
Retrofit underfloor heating

eco renovation - talk to homeowners

Notes

A U‑value is the overall rate of energy transfer (in watts) through 1m2 of a particular building element when the air temperatures on either side differ by 1ºC. The lower the U-value the better the material is at preventing either the loss of heat or penetration of cold.

R‑values are a measure of thermal resistance, and depend on the depth (d) of the insulation material divided by its thermal conductivity (λ or k-value): R = d/ λ. It is measured in m2K/W. The thermal conductivity (λ or k-value) of an insulating material is given manufacturers. The thermal resistance of each component (as well as the resistances of cavities and surfaces) is used to calculate the overall thermal resistance or R‑value for the entire structural element, in this case the floor. The U‑value is the reciprocal of this R-value.

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