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To save energy ceilings towards a ventilated cold attic space are normally insulated. In Denmark the insulation material thickness and type varies but the common used insulation material type is mineral wool or cellulose-based insulation. Furthermore, some attics have vapour barriers, whereas others do not or only have poorly sealed vapour barriers. The effect of these variations on the humidity conditions in cold ventilated attics was investigated in preparation for evaluating the existing recommendations on when and how to add additional insulation to ventilated attics to reduce energy consumption. The investigation has been done by examining and measuring the relative humidity and temperature for at least one year in 34 attics in inhabited single-family houses. In addition, outdoor and indoor temperatures and relative humidity were measured. The results were divided into different groups based on the vapour barrier type, amount of insulation, and insulation material type. Most of the attics were ventilated according to the Danish recommendations. Contrary to the expectations, the thickness of insulation material had no effect on the temperature or relative humidity in the attic. The type of insulation material or the presence of a vapour barrier also did not have any noticeable effect. Apparently, the air change rate (ACH) is more important than the investigated parameters. Combined with previously published results from controlled full-scale experiments, the results from this study are valuable input to new recommendations on how to apply additional insulation in existing ventilated attics. • The ventilation of a cold attic is an important factor to the moisture level. • Ceilings under sufficiently ventilated cold attic does not need a vapour barrier. • Hygroscopic insulation properties have no significant impact on the moisture level. • Insulation thickness has no significant impact on the moisture level in attics. • Additional insulation on ceiling against attics do not influence the moisture level. [ABSTRACT FROM AUTHOR]