Heat transfer through a well insulated external wooden frame wall : an investigation of the effects of normal defects in the insulation resulting from incident wind and air flow through the wall

University dissertation from Byggnadsfysik LTH, Lunds Tekniska Högskola

Abstract: The heat requirement of a building can turn out to be greater than was calculated at the design stage. The reason for this may be that heat transport though the building envelope is greater than expected. This in turn can be due to the structure not fulfilling the design requirements in respect of windtightness and airtightness. In addition, there may be defects in the quality of the workmanship of the insulation that significantly reduce the thermal resistance of the external wall. The objective of this investigation has been to ascertain how normal variations in the insulation can affect heat transport through a well insulated wooden frame external wall under the influences of incident wind or wind flow through the wall. These 'normal variations' have been taken to be the presence of electrical conduits, breaks in the insulation, airgaps and nogging pieces, either singly or in combination. For incident wind, measurements in a hotbox and theoretical simulations have shown that the presence of electrical conduits, breaks in the insulation, airgaps or nogging pieces in a well insulated wooden frame external wall, whether singly or in combination, have negligible effect on thermal transport when subjected to incident wind. Heat transport is affected, however, by the presence of a break in the insulation: the combination of electrical conduits, airgaps, a nogging piece and a 16 mm gap in the insulation increased the U value of the wall by 0.028 W/(m²·K) at the most. Measuring the effects of a flow of air through the insulation involved simulating a break in the air seal. A pressure difference of between 10 Pa and 20 Pa was established, causing air to flow from the cold side to the warm side. Measurements and calculations have shown that normal variations in the insulation have a negligible effect on the transmission losses on the cold side, due to the heat exchange effect of the insulation, which raises the temperature of the air flowing through it. This has meant that as the air flow increases, the transmission losses on the cold side have decreased while those on the warm side have increased. Air flow through a wall does not necessarily mean that the heat demand of the building will increase.

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