The Underlayment Craze
It started in the mid-80's. On the
rooftop, roofers were required to use a new type of underlayment
called “Ice & Water” underlay. The roofers of old would
sometimes use 90 lb. or a heavier underlayment at the eaves and up to
a point 24” inside the eave wall to protect for ice damming events.
This sticky underlayment is applied to the plywood and it
eventually melts itself to the roofing shingles, sealing around nail
shanks to provide a seal at the eaves that protects against ice
damming. These products represented a boost to the manufacturers'
bottom lines as profits increased because of the increased sales of
these underlayments.
The real cause of ice damming on
rooftops is the “hot-spots” associated with poor ventilation.
The spotty areas of interior heat combined with the sun would cause
uneven melting of a snow covered roof. Once the runoff reached the
eaves it would freeze similar to a bridge roadway freezing before the
connecting roads. The overhangs are cold and suspended in air while
the attic areas are warmer. This is caused by a lack of proper
ventilation which tends to even out temperatures on rooftop planes.
The use of these products increased in
popularity. The tradesmen of the rooftop are generally young and
with the influx of foreigners into the workforce, the expert to
journeymen passing of proper roofing disciplines was lost in the same
era. Manufacturer specifications were looked upon for direction as
building codes are vague and the makers of the products specified the
use of the “Ice & Water” underlayments at every critical
juncture of the roof. Even on chimneys and rakes and where the
heavier underlayment would conceivable transfer its runoff to the
lesser felts (or tar-paper). The underlayments became more important
than the primary shingles in the specifications and in the minds of
the installers!
There had been a lack of proper
discipline in the applications of metal flashings for years. The
loss of transfer of information came with the many workers in the
trades being from other countries. Soon these underlayments were
looked upon as roofing systems, themselves. They were specified even
by manufacturers for roof slopes down to 2” rise per foot (low
slope) under asphalt roofing shingles. By trade standards and for
years, for roofing shingles it was always anything under a 4” rise
per foot was considered “low slope” and a sealed roofing system
was used. The crowd went along with the new specifications only to
find that problems emerged. Bob Wewer - Low Slope Shingle Failure The underlayment did not work as
specified. The fingers were pointed and the leaky roofs were blamed
on the installers that installed them per spec. See Bob Wewer - Mold from leaks
Soon it was realized that proper
flashings were compromised because the “Ice & Water”
underlayments were used and roofers would just haphazardly install
sub-par flashings. The underlayment became more important than the
proper and fitting old world specifications.
Now the siding arena was suffering a
similar fate with the cladding of side walls that leak. The products
made with channels that collect and allow water behind them had to be
underlayed with significant products to prevent water intrusion.
Housewraps with perm ratings were invented because of the moisture
vapors that push outward in the winter. Condensation was a problem
with the necessity to install a formidable underlayment, so a
permeable underlayment had to be used. See Bob Wewer - Siding Leaks
This idea of a permeable underlayment
was the answer to provide protection for liquid moisture while
allowing moisture vapor to escape. The problem with this scenario is
that when the temperature variables are added into the equation, the
idea does not work well. The comparison has been made that if you
were to open your window on a cold winter's day and place a pot of
boiling water on the window sill with the screen closed, the exiting
moisture would be stopped by the screen, beading up and running down
onto the sill. The screen is many, many times more permeable than
any housewrap. So the perm theory does not work in the real world.
Making sure your home is air-sealed and a proper vapor retarder is on
the inside of the wall, on the warm side of the insulation is the
answer. Bob Wewer - Mold In Wall
In the above image, moisture from inside the home was slowed down by the underlayment and condensation occurred. The "perm" rating has little to do with reality.
The argument will be made, as it has
been, that the wall must be allowed to dry out and trapping moisture
is wrong. The wall can dry from the outside, in the northern
climates. Never in 35 years have I torn open a wall to find, what is
referred to as “reverse condensation” in the Philadelphia
climate. The condensation always shows on the outside of the wall.
Flashing tapes similar to “Ice &
Water” were developed to “flash” windows from the onslaught of
water that is channeled under the siding. Such emphasis was put on
underlayment that again, the proper installation of wall cladding and
head flashing was lost to the new age. The main problem with these
window flashings is that when an opening is not sealed (with foam
insulation or batt insulation and a proper vapor retarder) it will
emit interior moisture vapor that condenses on the “flashing”
tape.
A story of interest: Moisture Inside Walls by Bob Wewer discusses how interior moisture migrates through walls to condense and cause confusing issues.
by Bob Wewer
So are you advocating using no underlayment at all?
ReplyDeleteNo, I am not saying that no underlayment should be used. What I am saying is that a little more than blindly following manufacturers instruction on the job-site is required. Underlayment should not be looked upon as a means to stop leaks - ever! Perm ratings are the result of testing in a laboratory. Real world findings differ immensely.
ReplyDeleteThe craftsmen of old looked upon underlayment as a way of protecting a structure while under construction. This is the view that should be taken instead of looking to underlayment as part of a system. I know there will be folks that go crazy when they read this but to rely upon underlayment is to mean that you expect your primary cladding to leak. That means that there must be something wrong with your spec or your product or your installation.
ReplyDeleteThe craftsmen of old looked upon underlayment as a way of protecting a structure while under construction. This is the view that should be taken instead of looking to underlayment as part of a system. I know there will be folks that go crazy when they read this but to rely upon underlayment is to mean that you expect your primary cladding to leak. That means that there must be something wrong with your spec or your product or your installation.
ReplyDelete