Hygrothermal assessment of north-facing, cold attic spaces under the eaves with varying structural roof scenarios

Abstract

The objective of this study was to test whether compliance with the current Danish best practice recommendations concerning design of the cold attic space will prevent damaging moisture levels. The project was performed as a full-scale experimental setup in the cool temperate climate of Denmark. The setup comprised 18 north-facing attic spaces with varying ventilation principles and varying infiltration scenarios. The relative humidity and temperature were measured in attic spaces, indoor and outdoor, for almost 3 years. The hygrothermal performance of the attics was evaluated by post-processing and comparing the data with predicted mould growth risk and with visual observations of mould growth. The results showed that following the recommended passive ventilation strategies made the hygrothermal performance in attics with diffusion-open roofing underlay worse. In addition, increasing vapour diffusion tightness of the roofing underlay made the hygrothermal performance of the cold attic spaces under the eaves worse, except for attics with passive ventilation but without infiltration. The hygrothermal performance of the attics with diffusion-tight roofing underlay was poor when combining infiltration and the assessed ventilation strategy. The performance of the same attic without infiltration showed that some degree of ventilation was needed. External roof insulation did not significantly improve the hygrothermal performance of the attic.

Factors affecting the performance of ventilation cavities in highly insulated assemblies

“…During the phase with the 70-mm XPS on the roof, the cavity temperature remained 0°C-1°C above outdoor air. After closing of the ventilation inlet, the cavity temperatures rose to 1°C-2°C above outdoor air on average, which corresponds to the temperature increase obtained by Jensen et al (2020) in an unventilated roof cavity with 50-mm-thick exterior polystyrene insulation. The outdoor temperature increased 4 days later, and the cavity temperature excesses decreased to zero.…”

“…In the study, the lowest attic RH during the cold season was achieved with the additional insulation. The experiments by Jensen et al (2020) showed that exterior insulation increased temperatures in an unventilated roof cavity, but the mold risk remained similar to the case without external insulation. It seems that increase of the exterior thermal resistance (R value) may improve cavity conditions in HI structures, but finding an optimal ventilation rate is challenging.…”

“…It seems that increase of the exterior thermal resistance (R value) may improve cavity conditions in HI structures, but finding an optimal ventilation rate is challenging. The results of Macias-Melo et al (2018), Langmans et al (2016, Viljanen et al (2020) and Jensen et al (2020) suggest that exterior insulation may reduce the ventilation rate of cavity in summer, because it partially prevents the warming of the cavity by solar irradiation. Earlier studies aimed at improving the hygrothermal conditions in the cavities of HI walls have not been found.…”

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

“…The airtightness of the ceiling prevents both convection of moist air from the dwelling, but also reduces the possibility of downward airflow of possibly contaminated air. A recent Danish study of small cold attic spaces under the eaves stressed the importance of an airtight ceiling, as ventilation is not always sufficient for removal of moist air [21]. A Swedish study [1] also determined the side effect of mould odour in the dwelling due to mould growth in the attic.…”

“…Note that some measurements from house 22 exhibit excess moisture up to 16 g/m 3 in single measurements. Top: houses with vapour barriers(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19), bottom: houses without vapour barriers(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30).…”