Beam and aircrete block floors

Beam and aircrete block floors – thermal insulation performance

Bright, Norman

The Building Regulations for England and Wales Conservation of fuel and power in dwellings introduced more stringent requirements from 1 April 2002. In the case of ground floors, the guidance in Approved Document L1 states that floors with a U-value of 0.25W/m2K meet the requirements when the elemental method is used. The corresponding value under the previous regulation was 0.45W/m2K. (The lower the value, the more stringent it is.)

Appendix C in Approved Document L1 gives a simple method of calculating the U-values although more rigorous procedures are available(1,2).

The U-value of a ground floor is dependent both on the inherent properties of the floor construction and the ratio of the perimeter length to the floor area. The dimensions should be measured between the finished surfaces of the external walls. Excessive thermal bridging should be avoided at the floor edges to control condensation and mould growth risk.

Suspended ground floors

Precast suspended ground floors have increased in popularity over the years as they offer many advantages over in-situ construction. The operation of filling to the underside of the floor slab and the associated requirement to compact the fill is avoided. No blinding is necessary. All forms of precast suspended ground floor avoid the problems of heave or subsidence under the floor slab as there is a sufficient space under the suspended floor. The floor remains clear from any poor, wet or contaminated ground at all times. Bad weather delays can also be avoided by using this dry form of construction.

The calculated U-value of suspended ground floors depends on the ventilation opening area per unit perimeter of underfloor space. A value of 0.0015m^sup 2^/m is assumed in this article.

Beam and block floors

Beam and block floors represent a large proportion of precast suspended ground floor construction. This form of construction comprises inverted T-beams (usually prestressed) and standard 215 X 440 X 100mm thick blocks laid flat, supported on the beam flanges. Grout is placed between the ends of the blocks and the precast beams. An immediate safe working platform is produced. Added insulation in the form of panels can be laid over the floor area before the finish is applied.

Beam and aircrete block floors

Beam and aircrete block floors have lower self-weight compared with other types of beam and block floors using heavier blocks. Longer spans from a given beam cross-section can be obtained. Sometimes this enables a reduction in the need for sleeper walls.

Beam and aircrete block floors provide an open system of construction. The beams are produced by most precast concrete manufacturers, and suitable aircrete blocks can be supplied by any of the aircrete block manufacturers. Double size aircrete blocks (440 x 440 improve the infill block placing productivity (see Figure 1). Aircrete T-shaped end blocks are also available (see Figure 2).

Using aircrete blocks in floors reduces heat loss. The floor U-value is not only affected by the length of the floor perimeter but by the edge insulation. Aircrete blocks used below the damp-proof course to form the perimeter wall are a good method of improving the edge insulation.

Design

Elemental method

The Guidance in Approved Document L1 Clause C1 indicates that a U-value of 0.25 W/m^sup 2^K can normally be achieved without added insulation for solid ground floors if the perimeter length to the area ratio (P/A) is less than 0.12m/m^sup 2^ or 0.09m/m^sup 2^ for suspended floors. For most practical situations, all types of ground floor in dwellings require additional insulation to meet the AD L1 levels for the elemental method. It can be seen from Figure 3 that when the P/A is larger, there is considerable benefit from using the beam and aircrete block construction particularly with an aircrete perimeter wall extending below the damp-proof course.

The example of how to obtain a U-value of dwelling floors used in approved document L1 has a PIA of 0.517. Figure 1 shows that there is not much difference between the different types of floor up to a P/A of 0.2. But for higher P/A ratios, the beam and aircrete block floor with a solid aircrete perimeter wall below dpc demonstrates considerably better performance than uninsulated solid or suspended floors. When using the elemental method, beam and aircrete block floors require less added insulation.

Target U-value method

The Target U-value method gives greater design flexibility than the Elemental method. It allows a combination of some elements with better and others with worse U-values than the Elemental method. The same calculated total heat loss from the dwelling can thereby be provided as would be obtained from a calculation where all elements had U-values set at the Elemental method minima. Taking a P/A of 0.517 as used for the example in Approved Document L1, the beam and aircrete block floor with an aircrete block perimeter wall below the dampproof course gives a U-value of approximately 0.48W/m^sup 2^K without additional floor insulation. Under the target U-value method this floor construction can then be used, with widows, roof, boiler etc which have better performance than assumed for the elemental method minima. This produces a satisfactory over all design to meet the regulations. The other forms of floor included in Figure 3 would require added insulation to achieve a U-value comparable with that of the beam and aircrete block floor or would otherwise require the other elements of the dwelling to provide much higher levels of insulation.

Acknowledgements: Information on floor U-values provided by Durox Building Products Ltd.

References:

1. BRITISH STANDARDS INSTITUTION. BS EN ISO 6946: 1997 Building components and building elements: thermal resistance and thermal transmittance – calculation method, London.

2. THE CHARTERED INSTITUTION OF BUILDING SERVICES ENGINEERS. CIBSE Guide A: Environmental design, Section A3: Thermal properties of building structures, London, 1999.

Norman Bright, consulting engineer

Copyright The Concrete Society Jun 2003

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