Hygrothermal performance of cold ventilated attics above different horizontal ceiling constructions: Full-scale test building

Abstract

It is often assumed that reduced heat flux to a ventilated attic reduces the temperature and thereby increases the relative humidity. Consequently, the importance of having a tight air and vapour barrier in the ceiling becomes more important with increased insulation. Therefore, in Denmark, the recommendation is to have a tight vapour barrier when re-insulating ceilings against cold ventilated attics to a total of 150-mm insulation material. The recommendation is independent of the insulation material’s hygroscopic properties or the indoor moisture level. The aim of this project was to test the relevance of the recommendation through testing in a full-scale test building (7 × 22 m) with a series of six different ceilings with or without a vapour barrier and variation in the insulation material comprising insulation thickness and its hygroscopic properties. The examination was performed with a controlled indoor climate after an European humidity classes 1–3 and a natural outdoor climate. The study showed very little differences in temperature and relative humidity in the cold ventilated attics, while the humidity class of the indoor climate significantly affects the absolute moisture content in the attic. However, the climate in the attic did not cause mould growth in the test sections, even for humidity class 3. Consequently, in the test building the thickness or hygroscopic properties of the insulation material did not have a significant effect on the moisture level in attics and did not determine whether a vapour barrier should be installed. In this study with the given climate, a vapour barrier is unnecessary in well-ventilated attics if the ceiling is airtight.

Towards moisture safe ventilated cold attics – Monitored conditions in a full-scale test building

“…Although results according to Fig. 5 seem to prove the benefit of vapour barriers, the picture was not equally clear in other humidity classes, and as described in [6], there was no statistical evidence that vapour barriers reduced the relative humidity in airtight attics.…”

“…Furthermore, many of the curves are so close together, it is difficult to distinguish them from each other. Statistical analyses made in [6] showed no statistical significant difference between spaces with or without vapour barrier or between the different insulation materials. Fig.…”

“…Mineral wool and cellulose based insulation material were used as materials with low and high moisture buffering capacity, respectively. A detailed description of the experimental set-up as well hypotheses are found in [6]. This paper presents a selected part of the monitoring results.…”

“…Full-scale test house with 6 different attic designs with varying insulation type and thickness, presence of vapour barrier or not, and 3 different humidity classes of the indoor environment.Figure from[6].…”

Danish Dwellings with Cold Attics—Ventilation Rates and Air Exchange between Attic and Dwelling

“…According to [18], cellulose-based insulation can absorb 10-40 times more moisture than mineral wool, due to the hygroscopic properties; hence having a moisture-buffering effect. However, full-scale studies as well as field studies have not been able to establish this effect [16,19]. Some measurements do however indicate marginally lower moisture conditions in cold attics with hygroscopic insulation materials [20].…”