Watertightness of windows and doors
Perhaps the most common defect on new and near-new buildings is water penetration – and doors and windows are prime offenders. Australian Window Association executive director Tracey Gramlick outlines how to avoid some common pitfalls.
As driving rain is the largest single source of moisture on roofs and walls, controlling rain penetration is one of the most important parts of a successful moisture control strategy. Although most precipitation will run down the external face of a window system, wind driven rain behaves as a fluid under pressure and may penetrate gaps if the joints are incorrectly designed or installed.
Failure to control rain is more than likely the oldest and most common building problem and despite thousands of years of experience, avoiding rain related building damage is still one of the most difficult tasks facing designers and builders. There are, however, both traditional and modern means of providing rain control.
There are usually three factors that are needed for water to penetrate windows and doors and enter a building: the presence of water on the outside, an opening for the water to move through and a driving force (such as wind or even gravity). Wind creates a pressure difference between indoors and outdoors.
If we can eliminate one of these three conditions we can prevent rain from penetrating but, while wide eaves may help to shelter the windows and doors of most single storey houses, it is important to note that even for windows that meet a specific water penetration rating, leakage can occur during periods of wind driven rain. Serviceability is the return period that allows for that.
Doors, with the exception of glazed sliding units, are not rated for water tightness as per the current Window Standard AS 2047. They can be expected to ‘leak’ at high exposure areas, therefore it is important to provide protection as much as possible including large overhangs and proper flashings at the perimeter.
Even if they are partially protected by eaves, leakage may occur through the door seal and threshold. The amount of actual leakage will depend upon the exposure, height of the sill and typical weather conditions and these in turn can vary greatly with local site conditions and any exceptional weather conditions.
Means to minimise water leakage include:
- Installing untested doors only at low exposure areas and avoiding elevations where the wind and rain levels are the highest due to local weather patterns
- Providing canopies or substantial eaves where possible
- Specify outward swinging windows and doors with good weather seals
- Use of window flashing which provides drainage to the exterior
- Inclusion of a sub-sill when recommended by the window designer or supplier
- The equalisation of internal and external air pressure
The most obvious method of rain penetration is the movement of moisture through cracks and openings due to the kinetic energy of falling rain (the open window scenario). This is not an important mechanism if the joints of the product interfaces are designed to prevent this and have been tested by an accredited laboratory to AS2047. Rain penetration due to kinetic energy requires a clear path through which the water can travel into the interior of the building.
The high surface tension of water increases its ability to wet surfaces and gives rise to the phenomenon of ‘capillary action’. Capillarity allows water to penetrate very small openings in the framing systems without an external driving force. It can only occur if the crack is sufficiently small (less than about 3mm) and the surface hydrophilic (wettable by water). As the size of the crack decreases, the capillary force increases and rainwater can be driven into very small cracks.
Gravity is the most common cause of serious problems, although it is the easiest to understand and avoid. Lack of flashing or proper seals around windows and doors can collect surface rain water and direct it into the enclosure assembly. This is the mechanism that causes serious problems quickly. Flashing must be of a waterproof material and installed in a continuous manner with an outward slope. Leaks often occur at the laps between lengths of flashing, so these should be sealed, not just lapped.
Window joints and corners often leak rain water into the wall. A sub-sill could be installed to deal with this likelihood. Because the joint between the wall and the window is sensitive to workmanship, the head of the window should also be protected by flashing.
The principle of pressure equalisation through design is to reduce the pressure difference across a window and any consequent air flows by creating a pressure on the inside of the opening that matches, as closely as possible, the pressure on the outer face. It offers advantages for weather tightness, structural requirements and ease of construction and mainly applies to commercial, especially curtain walling, applications.
Practically everyone involved in the window and door industry, whether they are a manufacturer, installer, or component supplier, has received that ominous phone call regarding non-performance of an installed product. Before the telephone receiver has hit the hook, the finger-pointing begins.
Unfortunately, this scenario occurs within our industry more times than any of us would like to admit. A great deal of this anxiety stems from a lack of understanding of the causes and processes necessary to successfully resolve the problems. Rather than immediately looking for someone to blame, the wise window manufacturer sees this situation as an opportunity to provide his client with assurance that the problem is a mere annoyance. Knowing that a quality manufacturer services what he sells will many times result in repeat business as well as valuable referrals.
Ultimately, the water management/barrier choice comes down to expectations. We work in buildings and house our families in homes designed and built by people who comply with standard practices to ensure our safety and protect our investment.