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and involves very long times in sample conditioning. BS EN ISO 10456 (BSI,
1999b) provides guidance on conversion factors for a limited range of materials
where the dry thermal conductivity value is known and the current moisture
content is known.
Dryness, according to BS EN 12664, is when the mass of the material heated in
an oven at 1050C, vented to air at 230C with 50% relative humidity, does not
change by more than 0.1 kg. M-3 over a 24 hour period. Once dried, the sample
normally needs to be contained in a vapour tight envelope to prevent
hygroscopic moisture take up. In both BS EN 12667 and BS EN 12664,
consideration has to be given to the contact resistance between the
equipment's plates and the sample, and the effects of the vapour tight envelope
on this resistance.
The thermal properties of samples dried or conditioned in this way, even where
accurate values are obtained, are unlikely to be representative of the materials
as they would be in real buildings, exposed to variable external and internal
environmental conditions. However, in a market economy, manufacturers may
have a preference for measured values that show their products' performance is
as proficient as possible, and dry materials invariably have lower thermal
conductivities than materials containing moisture. Dry values, obtained through
established and accredited means, are then used in design specifications and,
as Doran (2000) observed when reporting Building Research Establishment
work for the Department for Environment, Transport and the Regions (DETR),
this leads to buildings not performing as expected.
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