Hygrothermal conditions in ventilated attics with different air change rates and ceiling constructions

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.

Exterior Wood-Frame Walls—Wind–Vapour Barrier Ratio in Denmark

“…For cold, ventilated attics, the use of vapour barrier in ceilings was investigated by Hansen and Møller [2,3]. The conclusion was that if the ceiling was airtight, the vapour barrier could be omitted; however, several other assumptions should be met [4]. For ventilated attics, the exterior side of the thermal insulation is not protected by a material layer and an exterior vapour diffusion resistance does not exist compared with exterior walls, where a wind barrier is used to protect the thermal insulation from being wind washed, but at the same time contributes water vapour diffusion resistance.…”

Openings in ventilated Attics

“…This study is a continuation of the study described in [4], as the experiment was continued for two more years. The mould measurements after two years are new.…”

“…The test setup has been described thoroughly in [2] and [4], therefore, only the main configurations are described here.…”

“…When the study [2] was based on was finished, the test facility was free for other experiments.. This idea was in [4] followed up by a small study where ventilation openings in the test facilities from [2] were modified and reduced by 0 %, 35 %, and 52 %, respectively. The results showed very little differences in temperature and relative humidity in the attic.…”