Weathertightness of new buildings

weather

Recently I wrote an article on weathertightness basics as part of feature in the Oct/Nov issue of BRANZ Build magazine. There was an excellent range of articles from local and central government , a lawyer, BRANZ scientists and from technical writers. Usually such articles disappear into the ether, however this time I received some rare and encouraging feedback;

“I thought you might like to know that a reader – an architect – contacted BRANZ to say how much he liked the latest issue of Build, your article in particular.”

As any writer would appreciate, someone going to the trouble of showing their appreciation is really valued. So I’d like to share with you some pieces from my article. You may want to read it in full and some of the other articles, so go to: www.branz.co.nz/welcome_to_build

Weathertightness basics

Following the widespread use of cavities, introduced with the revision of Acceptable Solution E2/AS1 in 2005, and the return to boron-treated timber, our understanding of weathertightness is far ahead of what it was in the 1990s and early 2000s.

The 4Ds

The 4Ds concept of deflection, drainage, drying and durability, developed by Canadians Paul Morris and Don Hazleden in 1999, is the overarching principle underpinning E2/AS1. Here I only consider the first 3Ds in relation to wall assemblies.

Cavities provide drainage and drying

The primary purpose of a cavity is for drainage and drying, but it offers so much more. It separates less durable wall framing, insulation and internal linings from more durable wall cladding elements – cladding, external joinery and flashings. I call this the dry side, wet side principle.

Accordingly, certain flashings in E2/AS1 (such as saddle flashings) have been kept on the wet side rather than bridging to the dry side. That idea still seems to be novel to some. This E2/AS1 solution contrasts with the additional complexity of saddle flashings used in British Columbia that cross to the dry side (see Figure 1).

Be sparing with battens

The ideal cavity is empty with no battens except those necessary to support the cladding. So less is best with battens.

My preference is vertical battens only, as is the practice in British Columbia, Canada. If intermediate support is needed, short vertical battens can be used. It’s also much easier to inspect vertical-only battens, accidental blockage is reduced.

Better drying with top venting

BRANZ testing has confirmed that, while bottom venting provides enough drying, top venting allows greater drying potential than bottom-only venting. Top venting is common sense. Vapour – water as a gas – is buoyant in air. Invisible vapour rises until it gets cold and condenses into tiny water droplets and becomes visible as clouds.

Therefore top venting allows buoyant vapour to escape to the outside rather than becoming trapped at the top of cavities. Air in the cavity also warms and rises – the stack effect – as the cladding and joinery are heated with sunlight or internal warmth. Both these effects produce a conveyor belt of drying.

Differential wind pressures top and bottom of cavities also create air movement both up and down – it is important to keep the openings on the same face to avoid ventilating between positive and negative pressure zones and creating too much air movement.

Bottom vents can get blocked

Not only are well detailed top vents considered beneficial, they also provide redundancy. Bottom vents can be readily blocked – more often than most realise. For example, landscaping levels can rise as mulch is added.

Brick veneer requires venting to avoid drawing air from subfloors or venting into roof spaces. The same care of where to draw and discharge air applies to all wall cavities.

Top venting allows more rapid and reliable pressure moderation. I use this principle with windows set into concrete and concrete masonry walls. Sill details can be problematic as they are the opposite of a good rainscreen, so I vent at the head as well. This gives more reliable pressure moderation and better drying of the cavity between the fame and wall.

At the junctions

All cladding junctions are best formed by providing an effective rainscreen, a drained cavity and an air-barrier – that’s all! A rainscreen separates water from air, which then enters and balances cavity and external air pressures. Once equalised, there is no driving force to propel rainwater across the cavity.

Compartmenting cavities

Any building subject to wind has a wide variance of positive and negative wind pressures over its various faces. The face exposed directly to the wind typically has positive pressure, whereas the side and rear faces typically have negative pressure. Wind speed, and thus pressure, also increases with height, due to less obstructions and ground friction.

So, to improve pressure moderation on larger buildings, we divide the cavity into compartments, both vertically and horizontally, and especially at external corners where there are adjacent positive and negative pressure.

Car door excellent example

If in doubt, consider your car door, as this works well. It has an interior air seal that keeps out the road noise and dust and a drained cavity between the door and car body that also provides the rainscreen. The gaps that are large enough so water can’t block the air flow needed to achieve pressure equalisation. It’s a good reminder of how we should design and construct weathertight building envelopes.

Written by: Philip O’Sullivan

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