Installation Guide

VENTILATION DETAILS
The importance of good attic ventilation beneath the
roof cannot be over-emphasized. Such movement of
air will prevent or inhibit condensation of moisture
on the undersurface of the shingles or shakes, or on
the roof decks. Vents should be provided at the
soffits (eaves) as well as at gable ends (screened to
prevent ingress of insects), with cross-ventilation
desirable. A rule of thumb for adequate ventilation
is that the ratio of total net free ventilation area to
the area of the attic should be not less than 1:150,
with compensation made for screens over vent
apertures. Attic fans may be beneficial, these
supplying additional movement of air in attic spaces.
Several examples of construction techniques which
provide roof ventilation are shown in Figure 14.
The decision on whether to use a separate vapour
barrier must be made by the designer, based on the
type of building, its end use, and its geographic
location. A separate vapour barrier is sometimes
omitted on a sandwich-type roof deck when the
weather-shedding skin is not a membrane-type
impervious to the transmission of water vapour.
Although some types of rigid insulation have the
properties of a vapour barrier, a layer of roofing felt
is often placed between the deck and the insulation
as an air check. Many specifiers still prefer to use a
separate vapour barrier because it prevents vapour
from condensing in the insulation, which reduces
the overall efficiency.
Where a vapour barrier is used, care must be taken
to ensure that the dew point is well to the outside of
the vapour barrier in order to prevent condensation
on the deck. Ideally, the vapour barrier should be as
close as possible to the warm side of the roof, and
the thickness of the insulation should be increased as
the deck thickness increases to maintain the correct
location of the dew point. In unevenly heated
buildings such as churches and halls, or buildings
such as swimming pools where an unusually high
level of moisture is generated, the excess humidity
may have to be removed by mechanical means to
prevent condensation on the deck. In air-
conditioned buildings, use of the cold weather roof
system allows a constant flow of air between the
insulation and the roofing, helping to reduce the
energy required for cooling.
Full details on the cold weather roof system are given
on page 14
.
Roof rafter or truss
No. 1 Red Cedar shingles
or shakes
Air
flow
Insulation
Air flow
Continuous
screened vent
Gable Roof with attic
Insulation
Insulation
Roof rafter
No. 1 Red Cedar
shingles or shakes
Air flow
Air flow
25mm-50mm
diameter screened
vents each
roof rafter space
Shed Roof
Insulation
Roof rafter
No. 1 Red Cedar
shingles or shakes
Continuous
screened vent
Air flow
Louvered vent at each end
Continuous ridge vent
Gambrel Roof with ridge vent
Roof rafter
Louvered vent at each
end of attic
Continuous
screened
vent
No. 1 Red Cedar
shingles or shakes
Air flow
Air flow
Cathedral Ceiling
with partial attic
Louvered
vent at each
end of attic
Fig. 14: Ventilation Details
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VAPOUR BARRIER GUIDELINES