Lisø PhD Dissertation Manuscript - NTNU

Organic materials within the insulation layer that
could be damaged by moisture ought to be avoided.
The traditional way of ventilating an insulated
pitched roof is the use of an air gap underneath the
underlay for roofing (to ensure sufficient venting of
excessive moisture) and one air gap above the underlay
for roofing (to prevent melting snow), illustrated
in Fig. 4. New materials, like the combined
underlay for roofing and wind barrier (watertight
breather membrane), have led to a design of roof
structures where all the ventilation of the roof surface
takes place through an air gap between the roofing
and the underlay for roofing. This is still a rather
new roof design in Norway, introduced back in the
1980’s. The Norwegian construction industry has
not fully adapted this solution, mainly due to a profound
scepticism concerning water tightness under
severe weather conditions. Larger amounts of precipitation
in parts of the country could entail more
scepticism.
This expresses the need for more comprehensive
investigations on issues concerning ventilation and
drying of pitched insulated roofs.
5.1.1.2 Compact roofs
Compact roofs are roofs where the different layers
of materials are placed close to each other without
ventilated layers. These roofs normally have a water-
and vapour-tight roofing membrane on top of
the insulation and a vapour retarder on the inside,
and thus limited drying properties. Compact roofs
are built as both flat roofs (angle of pitch of roof <
6°) and pitched roofs (angle of pitch of roof > 6°).
As far as larger buildings are concerned, this is the
dominant type of roof structure. Compact roofs are
used on all sorts of buildings, and properly constructed
compact roofs rarely sustain damage.
Built-in moisture in compact roofs is a topic that
once again became of great interest following the
mentioned extreme rainfall in the autumn of 2000 in
eastern Norway. Heavy precipitation during the construction
period increases the risk of built-in moisture
during roof construction. The development of
weather-protective measures (e.g. tent solutions) and
guidelines for the construction phase will be of great
importance.
5.1.2 Façades
5.1.2.1 Definitions
External walls should be designed to form a climatic
envelope against the surrounding environment, in
order to ensure the desired indoor climate. Façade
materials and systems, as well as the correct design
of construction details, is therefore of crucial impor-
tance as far as the functionality and lifetime of buildings
are concerned. Driving rain represents the
greatest challenges concerning the design and construction
of outer wall structures. Façades or outer
walls are also exposed to moisture impact from the
indoor environment.
External wall structures can be divided into systems
with one stage tightening (massive façades/outer
walls) and systems with a two stage
tightening (ventilated façades/outer walls). The design
requirements depend on many factors, but the
local climatic conditions at the building site are of
crucial importance.
5.1.2.2 One stage tightening
Field and laboratory tests have been carried out to
analyse whether the current design and construction
practice for rendered masonry façades (one stage
tightening) in Norway provides sufficient protection
against moisture-related problems. The impacts of
structure and composition of rendering layers have
been studied through laboratory tests (rain tests in
field testing) (Kvande & Waldum 2002a, b). This
experimental programme emphasised the importance
of the type of binder to obtain an optimal water
tightness of a rendering system. A cement-rich mortar
should always be used as spatter dash to resist
water penetrating the wall. A two-coat render is not
sufficient to withstand heavy driving rain, and so a
three-coat system has to be used. The final coat in a
three-coat render may be a suitable inorganic coat
like silicate paint. The results are of special interest
for massive masonry walls, and for masonry walls
without sufficient ventilation.
5.1.2.3 Two stage tightening
The principle of façade systems with separate wind
and rain barriers was thoroughly studied in Norway
in the 1960’s. The principle was introduced in order
to achieve better weatherproofing of façades and façade
elements. A two stage waterproof façade has
the principle design of an outer rain protection layer,
a ventilated and drained space and an airtight layer.
The outer rain protection layer, i.e. the cladding,
could be different kinds of wood panelling, metal
sheeting or board claddings. The rain protective
properties are dependent on the type of material, the
number of joints and the performance of the joints.
Detailed experimental studies were conducted in
Norway in the mid 1960’s (Birkeland 1963, Isaksen
1966), and the results are still applicable.
More precipitation in the form of rain (or all
forms of water originating from the atmosphere) will
to a greater extent challenge the performance of ventilated
claddings. The surrounding drying conditions