BUILDING PERFORMANCE INSULATION
Figure 3: Cold air can bypass the insulation, substantially reducing its performance. Note the level of insulation here – 50mm would be insufficient for most modern buildings
Figure 4: A fully-filled cavity wall using a compressible insulation. There are no gaps at all in the insulation. The inner face of the blockwork will be plastered to provide an airtight barrier, while the outer face will be rendered to prevent moisture penetration
Passivhaus design principles have been developed with a much more stringent interrogation of these factors
The inclusion of regularly-repeating thermal bridges – particularly the extent of timber frame – within U-values also needs care. The default figures given in the standards for Building Regulations, BR497, are too generous: 15% is taken as the timber fraction of timber-framed structures; but various studies have found typical values in excess of 25% (www.jrf.
org.uk/sites/files/jrf/low-carbon-housing- full.pdf). Ideally, the structure should be removed from the insulation, so the U-value is not dominated by the extent of bridging. Some structures use cavities in the construction that are ventilated to the outside. It is wishful thinking to hope that any insulation between these cavities and the outside will do anything thermally, given the vagaries of onsite installation. It is generally best to exclude from the U-value calculation any material on the outside of ventilated cavities.
Bypass and blowthrough Air movement through or around insulation significantly decreases the performance. There are two primary ways this can happen: blowthrough and convective bypass. Convective bypass occurs where there
are gaps between and around insulation. Because of the temperature difference between the two sides of the insulation, convective air currents will move heat
36 CIBSE Journal November 2011
from the warm side of the insulation to the cold side, bypassing it and reducing its performance. The impact is significant; a 7.5 mm gap between wall insulation boards can cause a 200% increase in heat loss. Figure 3 shows a classic case of a partially
filled cavity that is ventilated to the outside. The picture demonstrates particularly poor installation; the architect’s drawing would have shown the insulation flush against the inner course of blocks on the right- hand side. Because of the tolerances of rigid insulation, together with the uneven surface of the blockwork, it is almost impossible to construct this detail well; as it is covered up so quickly, it is even harder to check. Where rigid insulation is used, it should be installed with tongue-and-grove edges, or with layers overlapping each other. Expanding foam can then be used in the odd case where gaps are present and then all joints should be taped to further mitigate air movement. A more thermally robust detail would
be to replace this insulation with a mineral wool batt, which could closely follow the contours of the wall, and could butt up together with a tolerance fit. However, this causes another problem: blowthough, which occurs in open-cell insulation, such as a mineral wool in contact with a ventilated cavity. As the cold air moves through the cavity (primarily due to external pressure from the wind) there
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WARM: Low Energy Building Practice
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