Weathertightness Basics

December 17, 2013

No one really knows the weathertight state of new buildings, which is ironic given our performance-based Building Code. To be serious about the Code, we need to monitor and understand the outcomes.

By Philip O’Sullivan, Director, Prendos New Zealand, and President, NZ Institute of Building Surveyors

Weathertightness Basics in New Zealand

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.

However, poor understanding of weathertightness principles and concepts remains. When I was involved in the E2/AS1Workgroup in 2003-04, I wanted explanations so people could understand what they were doing, rather than just applying a new set of rules.  This was not permitted, so let’s go over some of them.

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.

A cavity is like a moat around a castle, and anything, like flashings, that bridges a cavity can create a weakness.  

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.

I often see three battens jammed together below window corners clogging the cavity where it is needed most. Battens are seen as beneficial, but this is just very poor practice (see Figure 2).

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. While top venting has not been adopted in E2/AS1, it has been by the Ministry of Education for new school buildings.

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. Such movement can cause flexible wall underlays to vibrate and could carry some moisture into the cavity./span>

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.

Things like drainage grates or insufficient ground separation mean damp air can enter the cavity and elevate moisture levels. The practice of terminating cladding below ground-level timber decks is worse – damp air below the deck may enter the cavity and provide sufficient moisture to support certain moulds and decay.

When sheet claddings are used, vertical battens form a series of chimneys. Damp air may cause timber battens to swell and clamp, reducing secondary lateral movement of air. Top venting reduces this risk.

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.

Pressure moderation

So how much air needs to enter the cavity to achieve pressure balance? A 100 km/hr wind gust is equivalent to 500 Pa or 0.5% of atmospheric pressure. Boyle’s Law (pressure x volume = constant) tells us 0.5% of the air volume of the cavity needs to enter to balance pressures.

An individual wall cavity 4 m high with 20 mm battens at 600 mm centres over a rigid air barrier has an air volume of 44 litres, so only 0.2 litres of air – a cupful – is needed for pressure balance to occur.

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. Effective compartmentalisation means that each cavity has a reasonably low pressure gradient across its face. This is more important with larger buildings – if air is rushing through cavities, pressure moderation can’t be achieved and air may carry water with it. 

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.

We would like to acknowledge BRANZ ‘Build’ Magazine for allowing us to republish this article, which was originally featured in Build 138 – October/November 2013

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