A. Simpson

The effect of moisture on the thermal conductivity of aerated concrete

The thermal conductivity (Λ) of five specimens of aerated concrete varying in density from 390 to 900 kg/m3 has been measured at several moisture contents between 0.2 and 7 per cent volume. Although the Λ-value increased with increasing moisture content, the change was less than that predicted by the Jakob factors. In general, the moisture factors were found to be density dependent and to increase with decreasing conductivity. A general expression has been derived relating the moisture factor to both density and moisture content by volume over the range 0 to 5 per cent.

The effect of moisture on the thermal conductivity of lightweight aggregate concrete

The thermal conductivity (Λ) of 13 specimens of lightweight concrete, incorporating either Lytag, Pellite or Aglite aggregates, varying in density from 1144–1424 kg/m3 have been measured at several moisture contents between 0.03 and 19.75 per cent by volume. Although the Λ-value increased with increasing moisture content, the change was much less than that predicted by the Jakob factors. In general, the moisture factors were found to be density dependent and to increase with decreasing conductivity. A general expression has been derived relating the moisture factor to both density and moisture content by volume over the range 0–4 per cent. A comparison has been made between Λ obtained up to 6.5 per cent moisture content, corrected to 3 per cent using this expression, and experimental results at 3 per cent moisture content. Close agreement was obtained. This expression has also been used to correct the Λ-values to 5 per cent moisture content by volume and reasonable agreement has been obtained with the limited experimental data obtained at this level.

Thermal conductivity of porous materials: I Theoretical treatment of conduction processes

The theoretical background relating to thermal conduction in porous materials is discussed and extended to interpret these processes more rigorously. It has been shown that most conventional models can be derived from a set of general equations adapted from electrical theory. The general model takes into account the shape and orientation of pores (or inclusions) as well as the thermal conductivities and volume fractions of the component phases. The effect of shape and orientation has been examined and shown to be a major influence on the conductivity of composite materials. Improvement of the performance of thermal insulants by entraining suitably shaped inclusions is discussed.

Condensation, heat transfer and ventilation processes in flat timber-frame cold roofs

An experimental study of the heat and moisture transfer processes in an insulated flat timber cold roof, 4.4 m long x 2 m wide, has been carried out under controlled steady state winter conditions and wind speeds up to 5 m s-1. The experiments were performed with intact, perforated and absent vapour barriers on the ceiling. The cavity between the insulation and plywood roof deck was ventilated via openings of various sizes at the ends of the roof. Cavity air speed, relative humidity and vapour pressure were measured, as well as moisture content in the timbers and the heat flow, temperatures and thermal properties of the roof. In accordance with current standards the roof was designed to have a 50 mm clear cavity, but in practice this was effectively 10 mm, thus restricting ventilation. The results show that vapour barriers and ventilation are of paramount importance if the airspace is to be prevented from reaching saturation conditions, resulting in dangerous levels of moisture in the timber deck and support elements. The minimum requirement for ventilation slot area was found to be an insufficient safeguard at low wind speeds, particularly in roofs without vapour barrier protection. At high wind speeds substantial heat losses from the cavity, accompanied by consequent increases in the thermal transmittance of the roof, were measured. Predictive models are given for the vapour pressure distribution and heat losses from the cavity.

Thermal conductivity of vermiculite concrete: Effect of inclusion shape

The thermal conductivity of three normal granular expanded vermiculite aggregate concretes, and six plate-like (approximately disc shaped) expanded vermiculite aggregate concretes, in the density range 500–1100 kg m−3, together with a sample of the cement paste matrix material of density 1428 kg m−3 has been measured. The plate-like vermiculite concrete was found to have conductivities some 20 to 30% less, for any given density, than the granular type. The large reduction in conductivity has been explained in terms of the shape of the inclusions, based on equations derived in an earlier paper.

Moisture factors and thermal conductivity of concrete

The results of thermal conductivity (Λ) measurements at several moisture contents on two types of aerated concrete in the density range 391 to 892 kg/m3, and four types of lightweight aggregate concrete in the density range 1144 to 1424 kg/m3, have been analysed to find a simple mathematical relationship between moisture content and Λ. The only measurements which have been taken into account were made since the British Standard was amended and since the British Calibration Accreditation scheme came into operation. It is shown that experimental results can be represented with reasonable accuracy by a linear relationship between Λ and moisture content by weight.