Flooring is the forgotten surface when it comes to meeting thermal performance targets. But Scott Brownrigg and Barbour Index explain that a little insulation can go a long way

The thermal performance of a ground floor is an increasingly difficult issue for specifiers. The push towards better insulated buildings means they must consider it in more detail.

If floors are left uninsulated, they may be the weak spot that lets down a design when scrutinised for compliance with the newly revised Part L of the Building Regulations. If a BREEAM or EcoHomes "excellent" rating is required, however, the floor insulation can contribute to the overall performance of the building for little extra capital cost.

However, it is not easy to understand the benefits of the specification. Furthermore, the increasing popularity of underfloor heating makes floor insulation more critical. Below, we take a look at how specifiers can meet the regulatory requirements:

1. Principles

An uninsulated ground-bearing floor will have a U-value of 0.7 to 2.1 W/m2/K depending on the moisture content of the ground underneath. A value of 1.4 W/m2/K is usually used for calculation purposes. However, this is complicated by the fact that heat losses through the floor will increase towards the perimeter because of the relatively short path to the outside atmosphere. This means that the overall losses are related to the size and shape of the building footprint.

As the size of the building increases, the ratio between the perimeter and the footprint also increases, and the effect of losses at the edges decreases. Therefore, the relationship between the perimeter length and plan area

is important when considering the amount of underfloor insulation to specify. Bear in mind that increasing the levels of floor insulation in buildings with a relatively small perimeter area has become a useful way of meeting the requirements of the revised Part L.

A suspended floor will have similar losses as a slab in contact with the ground, although losses may be greater if air is circulating in the sub-floor void or if this is particularly large. If the suspended floor is made from timber, it is more susceptible to condensation-induced damage so consideration of vapour control and airflow is critical. Increased airflow will increase heat losses through the floor.

2. Regulations

Part L requires a minimum standard of insulation for all floors to an average weighted value of 0.25 W/m2/K over the whole floor, or 0.7 W/m2/K if it is considered on its own as an individual element. Domestic buildings must now comply with the new standard assessment procedure. BS EN 13370:1998 gives the calculation methodology for all types of floor. This is based on keeping the losses to a controlled level, especially at the perimeter. For small buildings, the regulations require a significant degree of insulation across the whole floorplate.

Larger buildings may only require insulation at the perimeter, but in practice this can be difficult to detail. It is worth considering adding a layer of insulation over the complete floor. Although this may not have a significant effect on energy losses, it will improve the response time of the building to heating and cooling cycles.

The regulations also require cold bridges to be minimised. The specifier should always consider perimeter edge insulation integrated with the cladding or wall construction to eliminate any cold bridges.

3. Practical details

Consideration should be given to making the insulation level proportional to that in the remainder of the building. If higher levels of insulation are used elsewhere, this should be reflected in the floor.

Additionally, when underfloor heating is specified, U-values of less than 0.2 W/m2/K, or preferably 0.15 W/m2/K should be considered.

The type of insulation should be matched to the loading for the floor. Most practical is the use of close-cell expanded polystyrene. Care should be taken to specify the correct density in terms of compressive strength, usually not less than 45 kPa for light commercial uses and domestic, and at least 70 kPa for commercial.

Ensure that you have checked compatibility with the dampproof course. This should be located outside the insulation, keeping it dry and improving its performance. Some closed-cell insulation can tolerate being waterlogged but its performance will drop.

4. Types of insulation

For suspended floors, mineral rock fibre or plastic-based insulation can be used. Check the anticipated water vapour generation in the building, as a vapour control layer may be required, particularly if the insulation is not closed cell.

Ensure that the insulation specified is actually used on site; less dense materials may cause problems with the finish, making the surface too flexible under percussive impact.

Sustainability of materials is an important issue. Plastic insulation that is effectively sealed within the floor construction will last the life of the building if specified correctly. Assuming no defects, it will continue to perform year after year and is therefore effective.

5. Key points

  • Consider the regulatory requirements and the practicalities of installation at the same time
  • Ensure the insulation matches the crushing strength of the floor requirements
  • Some types of insulation can collapse in time and leave a cavity
  • Vertical-edge insulation is important as this is the area of greatest heat loss
  • Check that cold bridging around penetrations and columns is kept to a minimum
  • Ensure that the same standards are maintained at service trenches and lift pits.