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