Lisø PhD Dissertation Manuscript - NTNU

with large amounts of precipitation and high wind
speeds. Normally there is a clear need for air gaps
and openings, for reasons discussed here. For reasons
of penetration by precipitation and preventing
the spreading of fires, air gaps and openings should
however be no larger than absolutely necessary. In
particularly affected areas, snow penetration problems
are often solved by closing all air gap openings,
trusting that necessary ventilation will be ensured
by air leakiness through small unintentional air
gaps in the roof structure. Such proofing can however
lead to insufficient ventilation, poor indoor climate
and the formation of dampness and mould. The
protection of gap openings against snow penetration
should be based on local experience from particularly
weather-beaten districts in Norway.
Figure 3. The present design criteria for air gaps have been
found to be the risk of ice formation on roofs caused by melting
snow (photo: Y.O. Sæbbe, Nordlys).
The amount of building damage on roofs is
closely connected with a complicated roof design,
insufficient venting (narrow ventilation ducts under
the roofing) and other inadequate details. Roofs are
not only being exposed to external climatic loads
and mechanical stresses. It is also important to understand
the impacts of the indoor environment on
the design of constructions, especially the air tightness
of roofs and outer walls. One of the most common
causes of failure in the design of roofs is air
leakage from the inside - with subsequent condensation
problems. This is especially true when it comes
to roofs covering rooms with severe moisture loads.
Buildings with large roofs (more than 7 meters from
eaves to ridge) and high headroom often experience
moisture problems because the same roof design
principles are being used as for smaller buildings
(e.g. single-family houses). The problems are primarily
related to insufficient ventilation of larger
roofs, and the consequences are likely to be more
severe for these roofs than for smaller roofs.
Changes in amounts of precipitation could influence
the whole-life performance of roofs. An increase
might change the performance requirements
of the roofing, the ventilation air gaps and the underlay
for roofing. The present design criteria for air
gaps have been found to be the risk of ice formation
on roofs caused by melting snow (Blom 1990, 2001)
(Fig. 3). The design criteria are valid provided the
airtight layers (the vapour retarder and the wind barrier)
are performing sufficiently. Increasing the
thickness of the insulation will decrease the transfer
of heat through the roof surface and the likelihood of
melting snow. The thermal insulation and continuous
wind and vapour barriers in the roof are also deciding
factors regarding the prevention of ice formation.
Figure 4. A modern pitched insulated roof construction with
separate wind barrier and sheathing.
The amount of moisture damage in larger pitched
roofs calls for a more reliable moisture-proof roof
design. Pitched roofs should be insulated continuously
from ridges to eaves, and not have partly insulated
attics which in practice does not get cold
enough to avoid ice formation on the roof. Ideally,
roofs should have all thermal insulation placed on
the cold side of the roof’s load-bearing structures.