Controlling Moisture - Keep it Dry

Keeping a house dry begins by identifying the sources of moisture that cause the trouble.  Moisture is generated both inside and outside a home. On the outside the sources include, 

  • Rain, sleet and snow

  • Ground water

  • Exterior humidity

An effective water management strategy starts with a well-designed roof incorporating a suitable pitch (steepness), generous overhangs, with minimal penetrations, covered with a weather-tight membrane. Avoid chopped up, complicated roof designs. Complex roofs may be architecturally interesting, but they are difficult to build and more susceptible to leaks. Don’t expect a leak free roof unless it includes proper flashing. Protect valleys and other places where water gathers by using flashing and ice and water shield. Never rely on caulks, tar or roof cement to stop water leaks. Use a premium 15 - 20 mil thick underlayment instead of felt paper. Install a drip edge at the gutter line where practical. Keep roof shingles in good repair. Frequent inspections, looking for damage and missing shingles is always a good practice. Use amply sized gutters and down spouts. Manage the site by piping downspouts away from the foundation and sloping grade away from the house.

Start at the Top

Nice Pitch

No Overhangs

Complex Roof

A Roof Acts as An Umbrella Over Your House

Valley Flashing

Kick Out Flashing

Robust Underlayment

Caulk is Temporary


Poor Subsitute for Proper Flashing 


Pipe Gutter Down Spouts Away From the Foundation


Drain the Site - Slope the ground away from the foundation 6 inches within the first 10 ft. Use swales to direct water away from the house. 

Water Managed Walls

One of the biggest myths in building construction is that siding, brick, vinyl, stucco and stone doesn't leak. While exterior finishes may appear watertight, in fact, all residential “cladding” systems leak water.  When rainwater penetrates these claddings, water usually ends up inside the wall, causing problems. (see photo to the right) To avoid failure, a wall should incorporate  a pathway that directs it down and away.


Pathways are called drainage planes or pressure equalized rain screens. Drainage planes are created with water repellent materials such as building paper, house wraps, sprayed on membranes or foam insulation. The material is called a weather resistant barrier or WRB. It’s essential that drainage planes be properly shingled with suitable overlaps. Include an air space sufficient for the water to drain freely. 

Some exterior siding choices can store large amounts of rainwater. In other words, they act like sponges when they get wet. These materials are called reservoir claddings.  Common reservoirs include brick veneers, stuccos and fiber cement cladding (Hardie & Nichiha). Once they get wet, the stored water can migrate elsewhere and cause problems

Notice the water being absorbed by this siding. The wettest sections are characterized by the darker color. The pattern reflects water penetrating into the vertical joints and being absorbed laterally. 


The close up to the left shows edges of another type of reservoir cladding. This material is manufactured with wood chips glued together much like press board or OSB (oriented strand board).


Reservoir claddings on the exterior of a home can be a huge problem, unless they are de-coupled from the wall. There are several techniques that can limit the moisture storage of these materials. 

Seeking Forgiveness

There is no such thing as a perfect house or a perfect weather resistant barrier. Chances are best efforts will still fail to keep 100% of water out of the walls. Admitting this upfront provides an opportunity for a second line of defense or a belt and suspenders approach. Sadly, you seldom see this principle in action, even in fine quality homes. 

In the case of walls, simply designing a system that allows them to dry solves the problem. For example, drainage plane material should always be vapor permeable. Vapor permeable drainage material allows trapped water to pass through the material and move out of the wall.


The drawing to the right show a 1-inch air space between the brick veneer and the weather resistant barrier. This allows water to fall to the base of the wall. Once at the bottom, a flashing directs the water out of the space using weep holes. Proper shingling of the weather resistant barrier and keeping the space free from mortar droppings is critical for this system to work as intended.



There is no right way to do the wrong thing. The photo to the left shows brick veneer with less than a 1/4 inch air space. How quickly this fails depends on the annual rainfall in this climate zone and how the bottom of the wall drains. 

Through the wall flashing detail

Consequences of improper flashing

The drawing to the left shows proper detailing of a through the wall flashing required when an adjoining roof structure abrubts an exterior wall. The photo to the right shows the consequences of leaving it out or not following best practices. This type flashing is frequently installed improperly on new homes. 

Drainage behind brick veneer is substanially reduced when mortar drops into the air space. 

Back venting stucco, stone, wood and cementious siding provides exceptional drying potential. This enhances the life of siding paint and reduces maintenance costs. 

The photo to the left shows a custom home under construction. There are four (4) different manufacturers used in the water control system for this home. This implies "systems approach thinking" was not a priority. 


Both brands of housewrap material have different vapor permeance ratings. The Barracade product used on the lower sections is 88% less vapor permeable than the Dupont Tyvek brand of house wrap used on the top. Most manufacturers test their products as a system. As such, they may not be compatible with a different manufacturer's products. This makes the performance of these walls extremely difficult to predict usually void warranties. 

Its not the Heat, its the Humidity

If you live in the South, the preceding phrase has special meaning. Our climate is consistently challenging with scorching summer temperatures along with high humidity. Buildings designed for better health and wellness must always be able to manage water AND humidity. In the South, the task is nearly overwhelming. 


When humid air sees something cold, the result is condensation. Its the same phenomenon as a glass of ice tea or a cold can of soda. When the humidity in the air sees the cold surface, it changes from moisture vapor to liquid water. 


Humid outside air sees an air conditioned (cooled) building as a cold glass of ice tea. As outside air moves into the building, there is always a risk that the moisture will condense on a cold surface. When humidity becomes trapped inside a wall, it will condense on any surface below its dew point. If the wall cannot dry, the condensation lingers, typically on the backside of drywall. This invites mold and fungal growth. Once again, if it is inside the wall cavity, it is hidden from view, 


It' okay for walls to get wet as long as they can dry out before bad things happen. Scientifically speaking, walls can get wet as long as their drying potential exceeds their wetting potential. Modeling the wall performance with a computer program called WUFI can reduce the risk of inadequate drying. WUFI is a hydrothermal modeling tool that models a building assembly with all of the layers to a specific climate. A multi-year model shows if there is a risk of condensation in the wall and assesses the potential for mold growth. 



In addition to the flange, an airtight seal between the jamb and rough opening reduces the pressure drop and the risk of leaks. 

Renowned building science engineer, Joe Lstiburek, PE PHD once said, “there are two kinds of windows, those that leak now and those that leak later.” In other words, all windows leak water.


Satisfactory window performance demands a good product and proper installation. Providing a drainage system in the framed opening and the window unit itself reduces the risk.


The photo to the left shows the first opportunity for water leaks into and around a window. If the flashing above a window is installed properly, all of the water coming down behind the siding will be diverted around the sides of the window. If the flashing is improperly installed, as shown below, the window is guaranteed to leak water.  

Water also leaks into the sides of windows because of the pressure (ΔP) exerted by wind. 

Water also leaks into the sides of windows because of the pressure (ΔP) exerted by wind. Sealant behind the flange creates a substanitally airtight seal. But any imperfections in this seal still allow water to enter.  

Proper flashing between the window unit and the framed opening greatly reduces the risk of water leaking into the wall. But rainwater may also enter because of deficiencies in the window components.

Ultimately most water drains down and collects at the sill (photo to the right) Water collecting at the sill must be drained out. Installing a sloped sill allows the water to flow out of the window opening.

This video demonstrates the proper flashing technique for a new construction window. It does not include the beveled sill detail, which is best practice.