Clinton News-Record, 1985-4-17, Page 66Insulation methods for roofs
A lot of energy conservation advice tells
you that insulating the attic will reduce your
heat loss. But what if you don't have an attic?
Manyhouses don't: flat roofs and cathedral
ceilings are quite common in Ontario.
As much as you might expect, it is more
difficult to re -insulate without an attic --
there's just not much room for the insulation.
Even so, it's still possible to effectively
re -insulate the roof.
Before you make any decisions about
materials or methods, you should inspect the
roof to discover what condition it's in.
Remove a section of ceiling in some
concealed place, like a closet. Then find out
how much insulation there is by measuring its
thickness; for example, 31 inches of
fibreglass batts means an RSI value of 2.1 (R
12). See how well the insulation has
weathered. Do this, if possible, in mid -winter
to discover whether your insulation is soggy,
which means that warm moist air is leaking in
from the house and condensing. In that case,
sealing against air leaks is an essential first
step. Finally, assess the adequacy of the
ventilation space and look for an air -vapour
barrier; these are two of the most important
things to keep in mind when you insulate a
flat roof or cathedral ceiling.
The air -vapour barrier keeps warm air from
penetrating the insulated space and condens-
ing. It also reduces heat loss by blocking the
flow of air into and out of the house. The
material that best performs this double
function is 6 mil polyethylene, attached on
the warm side of the insulation. Full
installation directions can be obtained from
the Ministry of Municipal Affairs and
Housing fact sheet on Air -Vapour Barriers. If
an air -vapour barrier is already present, you
should slash it if you're re -insulating from the
inside, to avoid "locking" moisture in
between the two vapour barriers. However,
an existing vapour barrier can be left intact if
you're insulating from the outside.
No air -vapour barrier gives complete
protection, so you need a ventilation space to
diffuse any moisture that does get past it.
Adequate ventilation requires a space of at
least 64 mm (21/2 inches) on the cold side of
the insulation. This prevents the possibility of
moisture ruining your insulation and rotting
the wood that supports the roof.
However, you don't need to provide
additional ventilation if you're insulating
with rigid foam boards on the outside of the
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roof. In that case, any existing ventilation
space should be blocked.
How do you decide whether to insulate
from inside or -outside the roof? Consider -your
c ircwnstances. Insulating cathedral zeeilings
and flat roofs on the inside makes good
economic sense if you're already renovating
your house. If you're planning to re -roof,
exterior insulation can conveniently be
included in the work.
If interior insulation is the most suitable
approach in your situation, you have three
options.
Insulating with Rigid Boards. This is an
especially satisfactory approach if your head
room is limited. Foam boards, such as
extruded or expanded polystyrene, are used
here, with the boards attached to the existing
ceiling joists or rafters. You don't have to
worry about additional space for ventilation,
since the roof cavity provides enough. But it
is essential to cover the new insulation with a
good air -vapour barrier before you put up
drywall.
Re -Insulating Inside the Rafter Cavity.
This approach necessitates removing the
existing drywall, but allows you to add a
higher level of insulation without losing head
room. The amount of insulation is restricted
by the size of the roof cavity and by the space
required for ventilation. If you nail cross
strapping across the rafters at 400 mm (t&
wch) distances, you can create enough room
for a second layer of insulation.
Extending the Existing Rafter Cavity. This
is an improvement on the preceding method,
because it allows for an even greater amount
of insulation. You expose the rafters or
ceiling joists and use new rafters to extend
the depth of the cavity by as much as 150 to
200 mm (6 to 8 inches). The new rafters
attached to the original ones with pieces.
plywood (called "gussets" ). Nail the gusse
3 pairs for every 8 feet, to the bottom rafters
before they go up. F511 the enlarged space
with fibreglass batts, leaving the top 64 mm
(21/2 inches) dear for ventilation.
The ministry fact sheet Flat Roofs and
Cathedral Ceilings describes these proce-
dures in detail; it also discusses the exterior
insulation options. You'll need professional
help with these, so if you tackle the job from
outside, consult an experienced roofer first.
The exterior method involves placing rigid
board insulation directly over the sheathing
(Continued on Page 33)
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