Airtightnes Design Guide
Design Guide

Air Leakage

Effective Air tightness involves securing air leakage paths and therefore reducing the risk of condensation through warm air from the inside of the building coming into contact with cold surfaces in roofs and walls. The amount of moisture created in a typical domestic dwelling is approximately 15 litres of water per 24 hours. If not ventilated away, the moist air will enter the building fabric and take with it heat that will eventually condense. In winter time this will often accumulate on the roofing underlay including roofs designed with vapour permeable membranes where the amount of condensation is higher than the membrane capacity. In severe cold weather and if a building is newly built or even up to 12 months after construction, condensation will be a major concern. The AirSmart system controls the amount of vapour that can escape into the building fabric making it possible for vapour permeable membranes to perform as designed even in winter conditions.

Given a winter situation where outside air temperature is 5°C at 90% relative humidity (RH) then it would take approximately 10 m3 of air change to remove 1 m3 of the moist indoor air at 20°C at 50% relative humidity. This is not always possible as periods of cold weather often provide very little air movement resulting in the formation of condensation. The moment vapour condensates it will change from being airborne to becoming liquid and can therefore no longer pass through the vapour permeable membrane. This in turn means that the condensate water will drip down onto the insulation and affect the insulation’s U-value and may subsequently form mould, mildew and rot within the construction.

Due to these factors there is a much higher focus on building airtight, and ensuring a comfortable indoor climate. Building airtight also means it is essential to have adequate air changes within the building as stated within Part F of the Building Regulations. In essence it is about the combination of building airtight and ventilating correctly.

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U-value Performance

Building Regulations


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