Weathertight basics – ‘the 4 Ds’

By applying the correct procedures at the time of construction, builders can minimise the threat of decay within a building’s wall.

This article, based on weathertight remediation workshops run by MBIE – Building and Housing Group, goes back to the basics of avoiding leaky homes – ‘the 4 Ds’ of design.

NZ adopted these principles from Leaky building veterans Canada. They can be applied to all types of building projects but have particular relevance to a building that has already failed.

Deflection

If less of the cladding is exposed to moisture, there will be less of the building with potential to let moisture in. A house with eaves, for example, can shelter areas of the cladding from rain. This is supported by an analysis of Weathertight Homes Resolution Service assessment reports, which shows that houses with no leaves feature at a disproportionately higher percentage than the overall building stock.

Drainage and Drying

A majority of the costly damage to a leaky home is from trapped moisture, when any moisture that has breached the cladding can’t escape and insufficient ventilating air is present to permit drying.

Durability

This is arguably the most important of ‘the 4Ds’; making durability a primary concern when designing and constructing a building will often dictate the use of the other basic principles.

New Zealand has many standard detailing solutions, such as E2/AS1,which, if used, is enough to demonstrate design compliance in certain situations. But is using these ALWAYS the best way to enhance durability of a building in any situation? Is there sometimes a better way? In many cases, it may not make a material difference to the overall cost, but it’s important to remember that each scenario is different and will often require a different solution.

“Properly installed cladding cavities are excellent at addressing all of ‘the 4 Ds’, as well as providing redundancy in the system and simplifying the detailing solutions

Differential air pressures, their effect on a building and how they are dealt with can really affect the durability of a building, but they also vary greatly depending on building design and location.

High air pressure always wants to go to an area of lower pressure, as does positive pressure to negative. Positive pressure on a building (eg wind pushing on an elevation) will create negative pressure (uplift) on both sides of a 25 degree pitch roof. Similarly, on the other three elevations of the building, air is effectively sucked off the building, creating negative pressure.

As positive pressure travels to negative pressure, it’s important pay attention to its effect on the building wrap at external wall corners, or wall to roof junctions, as it could lead to moisture bridging to the cavity’s dry side, and/or paper rattle, which could affect the paper’s effectiveness, durability and, in many cases, be audible.

To avoid this, should you choke them to prevent moist air being potentially sucked through the corners of a cavity system, under parapet tops or into roof spaces via barges? Every situation is different and builders need to consider their options. Similarly, air seals around windows effectively choke the high pressure outside, possibly allowing moist air to be sucked through to the low pressure interior.

Properly installed cladding cavities are excellent at addressing all of ‘the 4 Ds’, as well as providing redundancy in the system and simplifying detailing solutions. With the 2011 changes to E2/AS1, there are now very few situations where an Acceptable Solution design will not require a cavity.

The ‘dry’ side of a cavity is the framing side, with the ‘wet’ side directly behind the cladding. Ideally, all moisture should be kept on the outside. However, if a design detail does not make this possible, then moisture should be contained on the wet side. If moisture is allowed to easily and regularly get to the dry side, where the structural framing is, it potentially compromises durability.

BRANZ research has found that the size of the vents in a wall cavity is the single biggest factor in the drying rate. If building wrap is baggy (especially if pushed out by insulation) or the cavity batten installation hinders the airflow, it’s important to consider how that will affect the drying capabilities of the cladding system.

Changes to the original design of a building, such as a different cladding type, installation of eaves or a pitched roof, could also be considered by a client for better adherence to ‘the 4 Ds’, an improved resale value or simply for aesthetic reasons.

In consideration of all of this, builders need to ask the question: ‘Is there a better way of achieving a durable weathertight building?’

If you believe the specified detail may fail or could be improved, talk to the designer and request a change to the specifications. In serious cases, it may even be advisable to flag and document your concerns to the homeowner and consenting authority. By doing so, it may assist in showing you have done your best to fulfil your obligations in case the detail fails later on.

Suggested follow up areas for more information:

Building and Housing website: http://www.dbh.govt.nz/ws-info-for-building-professionals

Building and Housing publications:

• Guide to Remediation Design
• Guide to the Diagnosis of Leaky Buildings
• Dealing with Timber in Leaky Buildings
• Code Watch Issue 1: October 2011