• Preben Holst Bang
4. term, Master of Building Physics (Continuing Education Programme (Master))
The last ten years of tightening of the Building Code with the purpose of reducing energy consumption
in buildings have included requirements for the reduction of heat loss through the
thermal envelope. The higher demands have led to increased thermal insulation of buildings. It is
most often seen as favorable to apply very thick layers of insulation to roofs as compared to other
parts of the building envelope. In highly insulated, ventilated roofs the thermal loss is reduced
and attics and ventilated cavities are consequently colder than before. This reduces the amount of
humidity the ventilation air can thus absorb, leading to increased risk of moisture build-up in the
roof structure.
This master project aims at investigating the moisture safety in highly insulated, ventilated roofs
with wooden structure that comply with requirements in the Building Code and technical instructions
from generally recognized organizations and institutions.
Current Danish regulations and related studies regarding moisture and moisture safety in ventilated
roofs are summed up. The investigation of moisture conditions in highly insulated, ventilated
roofs is done as a parameter study with a reference construction for both a cold roof and a cold
attic. The parameter variations regard different forms of construction and different materials as
well as other aspects of the structure, and also the edge conditions concerning indoor climate and
outdoor climate. For the calculations the hygrothermal calculation tools WUFI Pro 5.3 and BSim
is used.
There will always be a risk of mould in outdoor-air ventilated roofs subject to Danish climate
conditions. However, this risk can be limited through correct project design and execution of a
ventilated roof. A range of factors influence the moisture conditions and these must therefore
enter into the work on moisture risk. The most important factors are listed below:
• Increased insulation thickness increase the risk of mould in the roof constructions, but so
long as no other factors influence the moisture conditions in the roof construction in a
negative way the increased risk will be limited. On this background alone this does not
lead to alterations of ventilated roofs.
• The air change in the ventilated cavity must be sufficient to eliminate the humidity which
will inevitably be fed to the roof construction. An insufficient air change will not do this
and the risk of moisture problems will increase. However, the air change must not be so
high that it decreases the temperature in the roof construction since this would increase
the relative humidity and thus also the risk of mould.
• Wooden roof constructions are sensitive to any type of leakage, leaks in the roof underlay
leading to water ingress, leaks in the vapor barrier and humidity in the structure stemming
from the building process.
The results are based on 75 hygrothermal calculations on ventilated cold roofs and cold attics.
Publication date28 Nov 2016
Number of pages56
ID: 244487348