Because water runs downhill, they should be installed “shingle style” by starting at the bottom of the building and overlapping lower courses with subsequent courses. Any holes should be repaired, and the WRB should be lapped over (not behind) window head flashing and step flashing on abutting roods.
To prevent the water that gets behind the siding from causing problems, a house needs a water-shedding layer behind the siding. In the old days, builders used felt paper to prevent water intrusion. More recently, plastic house wraps have been introduced that shed water. Described collectively as water-resistive barriers (WRB), these materials are only as good as their installation. A properly detailed water-resistive barrier such as asphalt felt paper or plastic housewrap prevents water that gets behind the siding from doing any damage. It’s important to tape housewrap seams and overlap them.
It is a common misconception that siding is waterproof. Water can be forced behind siding through wind-driven rain. It can also leak in around windows and doors and around pipes, ducts, and wires that penetrate the exterior walls. If water routinely gets behind siding, it can lead to rot and mold.
As we all know, water runs downhill. This means that any lower level areas that have holes or cracks are susceptible to having leaks. This can be very dangerous to the home and the health of those living in the house. It is very important, therefore, to make sure your home is sealed from these water leaks to prevent hazardous risks. By following these building science recommendations, you can keep your home and health safe.
For more information on building sciences, visit Fine Homebuilding.