Unvented Attic and Cathedral ceiling construction
History of roof venting
Before the introduction of insulation, moisture was not
a problem in roof spaces. Roofs were exposed to warm, humid
interior air. This warm air raised the interior temperature
of the roof space and decking materials. The roof itself,
made of vapor-permeable, natural materials, allowed water
vapor to pass through it to the outside without condensing
on the interior surface.
With the introduction of insulated roof spaces (attic and
cathedral ceilings), the temperature in attics was reduced
and water vapor passing through the ceiling to the attic
encountered decking materials that were now colder than
before. Condensation resulted, causing moisture problems
and in winter, a build-up of ice.
The solution was to install a vapor diffusion retarder
(VDR) on the warm side of the insulation, and to ventilate
attics to remove any water vapor that succeeded in passing
through the VDR and the insulation.
Similarly with cathedral ceilings, designers and builders
faced similar moisture problems. Their solution was to leave
an air space between the roof deck and the insulation material.
Vents at the soffits and ridges allowed outside air through
the space. The function of the air space in a cathedral
ceiling is exactly the same as the function performed by
attic vents.
Modern attic and cathedral ceiling technology
Polyurethane Spray Foam (PSF) provides architects and builders
with a new tool. While this modern material may be used
with ventilation in the same configuration as glass fiber,
it also allows us to roll back the clock and build as our
forefathers did. There is no longer a need for ventilation
or air spaces. This is accomplished with spray-in-place,
air sealing foam insulation.
Purpose of present conventional roof space ventilation
practices
Conventional purposes for ventilation and the air spaces
in roof spaces are:
- remove moisture
- lower the temperature of the roof to impede the buckling
of roof shingles; and
- prevent temperature rise in the roof deck, which could
result in ice damming
Moisture problems and ice damming in roof spaces are caused
by air leakage from within the conditioned building and
vapor diffusion, which allow moisture to pass through fibrous
insulation materials and to condense on the nearest cold
surface. It is well documented that, in most situations,
diffusion accounts for only 1% of moisture transfer, while
movement of air accounts for 99% of the total moisture load
on roof spaces and materials. Thus, controlling air leakage
virtually eliminates moisture problems.
Scientific research (University of Illinois Small Homes
Council, and Florida Solar Energy Center) has determined
that the maximum exterior roof temperature for roofs without
ventilation or air spaces is virtually the same as those
with ventilation and air spaces. The tests, conducted over
six months, found that, in the critical high-temperature
range above 140† F (60† C), there was only a
3† to 5† F (1.6† to 2.8† C) difference
in the non-vented roof. They concluded that any effect of
the ventilation was far out-weighed by the solar gain- so
that no difference could be expected to occur to shingles,
with or without ventilation.
In Montana, a mountain state with heavy snowfall, insulating
cathedral ceilings with spray foam without ventilation or
air space has been standard practice for many years. There
is no evidence of an ice-dam problem in those homes. Similarly
foam-core, stress-skin panels (SIP panels), without ventilation,
have been used as roofing for decades, with no evidence
of problems.
Unvented roofs are now widely accepted in humid climates
like Florida.
Why PSF does not require ventilation: Air leakage control
Foam has a low air permeance – low enough to be classed
as an air barrier. Therefore moisture movement through polyurethane
foam by air transfer is virtually nil.
Why PSF does not require ventilation: Vapor diffusion
permeance
The 1% of moisture that is conveyed by diffusion is usually
not a problem, because the amount is so small that it is
measured in nanograms (one billionth of a gram). Its effect
is easily overcome by normal drying cycles inherent in wood
materials.
Five inches (25 mm) of polyicynene foam has a vapor permeance
of 10 perms (565 ng/m2/s). This property allows low rates
of moisture diffusion to occur, just enough to allow breathing
of adjacent framing and decking materials to prevent moisture
entrapment. The minimal diffusion that does occur through
polyicynene foam will pass through the material without
condensing, provided that the substrate to which it is attached
is equally (or more) vapor permeable.
Final Word
SPF has been applied in unvented roof spaces in all climatic
extremes for many years. There has not been a single reported
complaint about any lack of performance or failure with
the material. However, the building code in your community
may dictate that ventilation and air spaces are still required.
The final authority may not be the building code, but a
professional consultant. Many jurisdictions allow for alternative
installation techniques provided they have been reviewed
and approved by professional architects or engineers. Finally,
many building codes have been amended in recent years to
allow for unvented roof installations. This is a promising
sign that this approach to insulating attics and cathedral
ceilings may be universally adopted in the future.
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