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Extension > Environment > Housing Technology > Moisture Management > Protecting walls against wind and water intrusion

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Protecting walls against wind and water intrusion

By Richard Stone, Extension Educator in Housing Technology
Reviewed 2010


Photo by Richard Stone, University of Minnesota

Someone went to a lot of work to tape this opening but it is not flashed "shingle fashion", not sealed, and holes are still visible

When completing flashing details for mechanical penetrations, walls should be treated similarly to roofs. A number of millwork manufacturers are providing installation instructions with windows and doors. The instructions must be followed to receive the manufacturer's warranty protection. As a result, most windows and doors are now installed in walls using flashing details that compare to those for installing skylights on roofs.


Photo by Richard Stone, University of Minnesota

When openings are cut in like this, they create the need for unnecessary repair work by someone else

In addition to window and door openings, a house typically has 20 or more mechanical penetrations on the outside walls. If roofs and walls are exposed to the same rain, wind, and snow, shouldn't they be detailed in the same way? It would be pretty surprising to see a PVC plumbing stack on a roof with contractor's tape pasted all around it to protect against water intrusion yet this is a commonly seen practice on walls. A list of the mechanical penetrations through walls, including electrical and gas service entrances, heating and gas fireplace air intakes and exhausts, air conditioning lines, outdoor water faucets, light boxes, outdoor outlets, phone and cable service entrances, and ventilation system intake and exhaust hoods, quickly adds up to equal the potential for a lot of holes. If they are not all flashed properly, they can become a lot of wind and water leaks into wall cavities or even across the entire building envelope and into the home. These leaks create the potential for energy loss, comfort complaints, and especially for durability losses. We know that all exterior finish systems leak to some degree. Some finishes, like metal or vinyl sidings allow a significant percentage of wind and water to get past them to the drainage plane. It is commonly accepted that the drainage plane on the exterior of the home is the primary defense against wind and water intrusion. As with windows and doors, each mechanical penetration should be integrated with the drainage plane so it effectively seals against water and wind intrusion while directing water outward and downward in a shingle fashion. It is important that every trade working on the job understands the importance of keeping wind and water out of the house and completes their work in a way that either assures effective weather resistance or makes it easy for the trades who follow them to finish the next steps in the process until the assembly is completed.


Photo by Richard Stone, University of Minnesota

These wall blocks should have been installed shingle fashion to direct water to the outside and stop wind intrusion

Wall blocks are most often installed against the outside of the Weather Resistant Barrier (WRB). Sometimes the perimeter is taped to the WRB, but usually not. This may look good and prepare the wall for siding installation, but it will probably not keep air and water from getting into the wall assembly or behind the WRB. The opening for a wall block should be prepared in the same way as the opening for a window. The wall penetration should be sealed behind the wall block so it directs water down and over the outside of the WRB below the opening. The wall block is then installed as part of the drainage plane, lapping over the WRB below the penetration. The sides and the head of the block are then taped and sealed shingle fashion, similar to a window, to complete the detail.


Photo by Richard Stone, University of Minnesota

Every opening is sealed and flashed to keep wind and water out and direct water to the outside of the Water Resistant Barrier

Many builders now use manufactured flashings for exterior wall penetrations. Instead of tapes and sealants, a neoprene gasket creates the seal against the pipe or duct and the top of the flashing is tucked into a slit in the Weather Resistant Barrier. Then the top flap and the sides are secured in place with durable tape. This is very similar to the process used to install a neoprene flashing over a plumbing stack on a roof. On roofs, flashings are integrated with the shingles, roofing paper, and other roofing components. On walls, flashings are integrated, shingle fashion, with the Weather Resistant Barrier and other wall components to create a continuous drainage plane from top to bottom.

Before manufactured wall flashings were available, some builders used neoprene roof flashings for wall penetrations like air conditioning lines or PVC heating exhaust and intake pipes. Other builders shaped flexible window flashing products to create flashings for mechanical penetrations of all shapes and sizes on exterior walls. Whether it is a new product developed to save time or a creative site-built solution adapting versatile products for use in a new application, all of these options can effectively protect the exterior of a home from wind and water intrusion. The important thing to remember when completing the exterior drainage plane of the home is that everything should be applied in a shingle fashion to direct the water downward and outward. When under full assault by wind and rain, the drainage plane should protect the sides of the building as effectively as the roof protects it from the top.

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