Choosing the correct vapour barrier results in a lifetime of energy saving!

Buildings with composite walls such as Light Steel Frame and Timber are engineered to last and have less impact on the environment over the lifetime of the building.   Light Steel Frame and timber buildings, need less cooling and heating than traditional structures, while also providing for clean building sites and time saving during the construction phase.

The energy saving of a properly constructed composite building has been proven to be at least 30% and this does not diminish over the lifetime of the building.  This cost saving is largely dependent on external walls having an effective weather resistant vapour barrier.

The purpose of a weather vapour barrier is to allow the walls to resist bulk water penetration and also to facilitate the drying out properly of the wall construction.  Drying is one of the critical components of moisture management which can be impacted by wall construction and climate.  The faster a wall is allowed to dry, the more durable and problem-free a wall system and a building will be.

In order to be effective, a good weather resistant vapour barrier has to function in four important ways:

  1. It MUST have a high level of bulk water resistance – to help protect the wall cavity from water that gets behind the cladding. Bulk water is water in liquid form such as rain, dew or snow.
  2. It MUST have moderate to high vapour permeability – to promote drying out of wall systems. Vapour permeability is commonly referred to as breathability.
  3. It MUST have a high level of air resistance – to help prevent drafts, reduce energy bills and resist the flow of moisture laden air though wall cavities.
  4. It MUST be durable to withstand the rigors of the construction site and continue to perform once construction is completed.

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BULK WATER HOLD OUT

Because wall cavities do get wet, roofs leak, plumbing leaks, condensation occurs and construction materials are installed wet, the internal moisture loads can be very high. However it happens, walls get wet and require a way to dry out. When a wall can’t dry out, it becomes vulnerable to moisture-induced damage including mould and rot.  In an ideal situation walls must be designed and engineered to allow maximum drying while still maintaining the other critical, tensile strength properties.

Many products that are commonly used in the building industry to act as a bulk water barrier, such as perforated wrap, builders paper and felt often result in wicking and walls that absorb rather than repel moisture.  These materials create the ideal breeding ground for mildew that is toxic to humans and destroys buildings.

Tyvek® Housewrap achieves the optimal balance of air resistance, water resistance and moisture vapour permeability.

MOISTURE VAPOUR TRANSMISSION AND PERM RATINGS

Moisture Vapour Transmission Rate (MVTR) is the measurement referenced in building codes and is measured in a lab using ASTM E96. The test method measures how much moisture vapour is allowed to pass in a 24-hour period (adjusted for vapour pressure) to get the moisture vapour permeance (MVP) or perm rating.  Although building standards allow for measures as low as 5 perms, this is often not sufficient to allow for proper drying of the wall cavity.  Perm values of up to 58 are possible and considered a high permeability.

Laboratory and practical tests have yielded some interesting results for perm rating of some of the materials used daily as vapour barriers.  These *tests are available on request.

Perforated branded wrap:  6.7 perms
Unbranded builders paper: 5 perms
Unbranded felt:  15 perms
Tyvek Housewrap high density polyethylene:  58 perms.

The higher the number of perms, the more moisture vapour the material will allow to pass and the better drying the material allows.  If drying of walls is negatively impacted, moisture issues are more likely to occur, increasing the potential for mould, mildew and rot.

AIR RESISTANCE AND PROTECTING THE BUILDINGS R-VALUE

The third function of a weather resistant vapour barrier is to help the insulation maintain its R-value by keeping air from infiltrating the wall system.  Air filtration into wall cavities is directly related to the energy costs of heating and cooling a home.  Many house wraps fail to meet the basic air barrier requirement of < 0.06 cfm/ft2.

Because weather resistant barriers have no inherent R-value, they are often neglected and under specified in the building planning phase.  Even if a vapour barrier has a high perm rating and is technically able to withstand air filtration, it is important that products are correctly installed and that overlaps are either shingled or taped as recommended by manufacturers.

Installation shortcuts and inferior vapour barrier materials are unfortunately only evident once buildings are occupied and it is too late to reconsider.

All builders and construction companies plan to build quality homes and buildings that last.  With sufficient knowledge and the correct product choices, a building can be a durable structure with wall systems that resist water and air while maintaining breathability and save costs for a lifetime of energy saving.

For more information* on vapour barriers that are engineered to provide the optimum balance of properties for superior performance contact Denise at Marshall Hinds on 021 701 1271. www.marshallhinds.co.za

Pictures courtesy of Rancor

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