Breathable Roof Underlayments
Roofingcontractor.com March 31, 2001
In North America, we read literature and test reports
providing evidence that we should incorporate this
principle of breathability into the design of our
buildings; but unfortunately, we are slow in
recognizing this issue.
In Europe, understanding the need for buildings to
breathe is a given. Products for cladding and
decorating the exterior of buildings are expected to be
breathable. In North America, we read literature and
test reports providing evidence that we should
incorporate this principle of breathability into the
design of our buildings; but unfortunately, we are slow
in recognizing this issue.
In North America, the most widely used underlayment
is 15- and 30-pound roofing felt. In the last two
decades, ice and waterproofing membranes have
become the norm in situations where one anticipates
ice-damming problems. Since the 1950s,
humidification in buildings has become increasingly
popular, which adds considerable humidity in
residential, commercial and institutional structures.
Older buildings simply are not equipped to handle this
additional humidity. As a result, water vapor drives
through the walls and ceilings and eventually
condenses on insulation or other building components.
Historic buildings, in particular, are at risk because
they generally lack vapor retarders. Yet with the goal
of providing a secondary roofing system, these
buildings are wrapped in roofing felt, or worse, ice and
waterproofing membranes.
Roofing felts are marginally breathable and certainly
suffer from condensation through the winter months.
Ice and waterproofing membranes that are self-sealing
at the joints and at nail penetrations are almost perfect
vapor retarders. The designer, with the intention of
providing a high-quality secondary roofing system,
inadvertently produces a situation where long term
performance of the building may be catastrophic.
The following are excerpts from the “Concrete and
Clay Tile Roof Design Criteria Manual for Cold
and Snow Regions,” published by the National Tile
Roofing Manufacturers Association Inc. and the
Western States Roofing Contractors Association
(produced by Leland E. Gillan and Terry Anderson).
They discuss various problems with existing
underlayments.
Chapter 4
Section IV. Criteria for Underlayment, Asphalt
Felts or an Ice and Waterproofing Membrane
A.1. Felt underlayments are intended to protect the
building from wind driven snow and rain. They are
required by code. Even a good tile may allow a dry
light snow to blow under it and onto the felt where
melting will eventually occur.
B.1. On a well designed tile roof, the underlayment
generally wears out before the tile. Two layers of
ASTM D226, Type II, No. 30 asphaltic felt overlaid in
shingle fashion is the recommended minimum.
C.1. Asphaltic felt deterioration is caused in various
ways.
C.1.1. Exposure to the sun’s ultraviolet rays deteriorates
the felt’s surface very quickly. This exposure often
takes place after the roof felt has been installed but not
yet covered with tiles. In some cases, months have
passed before the felts are covered with tile.
C.1.2 Water running over or, much worse, ponding on
the felts removes the asphalt’s oils. Dirt collecting on
the felts behind horizontal batten boards placed
directly on the felts will hold water. This will keep
felts moist for extended periods of time, causing rapid
deterioration. Placing vertical batten boards (i.e.
counter battens) below the horizontal batten boards
that hold the tile allows water to drain off the felts
quickly and helps prevent dirt accumulation.
C.1.3. As the asphalt oxidizes from exposure to air, it
becomes brittle. This is okay on a roof as long as it is
not exposed to sunlight or handled since the old brittle
asphalt felt will rip very easily.
D.1. While an adhered waterproofing membrane may not
be required in valleys and along eaves, it can be a costeffective valuable addition to prolong the life of the roof.
E.1. Caution: A roof completely covered in a
waterproofing membrane, sometimes with asphaltic
felts, and either an inadequate vapor retarder or no
vapor retarder with no venting, may prevent adequate
vapor removal from the attic or rafter joist space.
Apparent roof leaks that are actually the accumulation
of condensation may develop. See Figure 4-17, 4-18.
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Appendix B-1
Ventilation
Section II. Controlling Vapor Drive
A.1. Vapor drive must be controlled so that there is
less moisture allowed into a ceiling or wall than can be
removed to the outside. Often people refer to a vapor
barrier when they are really talking about a vapor
retarder; there are very few building systems that
create a true vapor barrier. Materials such as gypsum
board and most paints used on building interiors
provide almost no vapor retardation.
A.1.1 An example of a problem roof seen often in
mountain areas is where an ice and waterproofing
membrane is used on the roof deck directly above an
insulated roof system with no attic, a 4-mil thick
polyethylene sheet vapor retarder on the inside or
warm side, and gypsum board. No ventilation is
provided in the joist space. The vapor drive forces
moisture through the staple holes, nail holes and screw
holes and lap joints of the polyethylene and into the
interior ceiling. The moisture is not allowed to move
to the outside as fast as it comes in because of the ice
and waterproofing membrane placed on top of the roof
which seals itself when nails are driven through it.
Moisture content in wood of over 20 percent will
allow most woods to start rotting. In a few years a roof
joist can rot to the point of collapsing.
B.1. In all buildings in cold climates the vapor retarder
must be better on the inside of the building than on the
outside or have sufficient vents.
Solutions
Having established that there are problems with the
underlayments that are presently in use, it is
worthwhile to consider the performance of hard
roofing products in the marketplace. High quality slate
and clay tile roofs have produced life expectancies in
North America approaching, and in some cases
exceeding, 100 years. Modern conventional roofing
felts have nowhere near this life expectancy. Roofing
felts begin to deteriorate immediately when exposed to
sunlight or moisture (in the form of rain or
condensation) (ARMA/NRCA Research Report on the
Performance of Asphalt-Saturated Underlayment
Felts).
Superior coated polypropylene products are now
available that are strong as well as tear-, rot- and UVresistant. These membranes perform reasonably well
as temporary roofs and can be expected to exhibit
long-term performance once protected from UV by the
slate or tile. In terms of breathability, they are similar
to 30-pound roofing felt.
Most ice and waterproofing membranes have good life
expectancies once they are protected from UV. They
remain, by far, the best solution to dealing with ice
damming conditions. Installation of these products is
required at the eaves, valleys and other areas where
significant snow accumulations occur.
There now exists another category of underlayments
(breather membranes) that are extremely breathable
and durable. The best of these have moisture vapor
permeability in the range of 2,400 grams per square
meter per day, which is approximately 64 gallons per
10.76 square feet per day. This is a magnitude of more
than 200 times more breathable than 30-pound felt or
coated polypropylene membranes. These breather
membranes are waterproof, tear-, rot- and UVresistant. They are not self-sealing underlayments and
are installed in a similar fashion as 30-pound roofing
felt.
A superior version of the breather membranes is an
uncoated spun-bonded polypropylene three-ply
membrane. This product can be exposed to UV
radiation for months without deterioration and
therefore can perform as a temporary roof. Fewer nails
are required for securement because the product is
tear-resistant and very strong. Cap nails further reduce
securement requirements. Roll sizes are available in 59
inches by 164 feet or 118 inches by 164 feet, which
allows for reduced labor time in installation.
Considering the life cycle of slate and tile roofs, it is
essential to employ an underlayment that will be as
durable.
In the UK, “Daltex Roofshield” has been awarded a
British Board of Agrément Certificate for cold roof
design. The benefits are: no additional risk of
condensation, no requirements for eave and ridge
venting, a warmer roof space, increased efficiency of
insulation, easy installation, easier site supervision and
simpler specifications.
There is also an argument for breather membranes for
cedar shingle roofs and cedar shake roofs. These
membranes can be used as underlayment and as
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interlayment. It has long been believed that cedar roofs
perform better if they are able to dry out.
Both traditional and modern metal roofing systems are
well suited for breather membranes. Traditional metal
roofs suffer from condensation generally at the level of
the felt underlayment or the slip sheet. A breather
membrane can act as both a waterproof secondary roof
and a slip sheet. Modern composite metal roofs suffer
from internal condensation that can eventually
compromise the insulation and even possibly the
securement. Breather membranes allow water vapor to
escape by condensing on the underside of the metal
roof, dripping back onto the underlayment and
draining to the eave.
In North America, we struggle to provide adequate
ventilation in roof spaces. Often it is difficult, because
of building design, to introduce enough vents and
place them where they will be effective. Wherever
breather membranes are installed, they will reduce the
possibility of condensation forming and help protect
structures.
Breather membranes have already found their way
onto numerous historical buildings of significance in
both Canada and the United States. Modern composite
metal roofing systems such as the Detroit Metro
Airport benefit by using a breather membrane to
protect the integrity of the rigid insulation.
The principle of breather membranes will not only
impact the roofing industry in North America, but also
will quite possibly play a growing roll in building
envelope design.
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