Asphalt shingles are a little bit permeable in windy conditions but not very.
Ice and water shield entire roof.
It s typically only installed on the first leslie.
Gcp invented fully adhered roofing underlayments more than 35 years ago.
Both products have a separate purpose.
Ice and water shield the whole roof.
However because the modified bitumen back surface is continuous these membranes can result in a vapor barrier.
Ice and water shield is a thin rubberized asphalt membrane that goes beneath the shingles of your roof typically only along the overhangs and in the valleys.
Many types of roofing including epdm and standing seam steel roofing are virtually impermeable to water vapor.
You should seriously consider using ice water shield if you live in ice dam country.
Ice and water shield is a great product that can be used in spots to protect the most vulnerable areas of your roof or as a whole roof underlayment to protect from ice dams water dams storms and leaking in any area of the country and any weather.
It can protect your home from water leaks caused by ice dams and wind driven rain.
Now ice and water shield is designed to stop ice damming or water that backs up say from your gutters and getting into the roof surface.
Other types of roofing including cedar shingles and concrete tile roofing are fairly permeable.