Abstract
A recently Danish study reported that no vapour barrier is needed in ceilings, if the attic is well ventilated and the ceiling towards the dwelling is airtight. Based on that study, new investigations were initiated with focus on the hygrothermal behaviour in ventilated attics with different air change rates. A test house with three sets of four different ceiling constructions – all airtight – was used in this study. The ventilation rate was reduced in two of the sets with approx. 35 % and 50 %, respectively. Air change rates were measured with tracer gas. Furthermore, temperature and relative humidity was measured every hour. Measurements in similar ceilings with mineral wool or cellulose-based insulation material show that hygroscopic properties of the insulation have very limited effect on relative humidity. Furthermore, only at low ventilation rate the effect of a vapour barrier could be measured with minor impact. Based on the short-measured period the calculations of the risk of mould growth showed no risk. The results indicate that even when the ventilation is reduced by 50 %, the ventilated attic still performs well if the ceiling is highly airtight. However, the importance of vapour barriers becomes more important at lower air change rates.
Highlights
A study on the need for vapour barriers below ventilated attics was finished [1, 2]
Manufactures of cellulose based insulation material have stated that due to the hygroscopic properties of cellulose, their product can handle more moisture compared to e.g. mineral wool, and no hygrothermal problem will occur at thicknesses above 150 mm, even without vapour barrier
The outcome of that study [1, 2] was that if the ceiling is airtight and the ventilation follow normal Danish guidelines, there is no need for a vapour barrier neither when using mineral wool nor cellulose based insulation material
Summary
A study on the need for vapour barriers below ventilated attics was finished [1, 2]. The outcome of that study [1, 2] was that if the ceiling is airtight and the ventilation follow normal Danish guidelines, there is no need for a vapour barrier neither when using mineral wool nor cellulose based insulation material. This has led to changes in the common understanding of whether to install a vapour barrier when renovating cold attics.
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