Leaky Buildings — Part 2 — What we now know

By Ken Collins 08/06/2010

Following on from my blog on Leaky Buildings – Part 1 -and how we got to where we are, this blog covers some of the science and research that has gone into the building industry as aponding water lounge roof result.

At this stage I must point out that there are other people with specialist areas of knowledge and research, in what is now quite a wide topic.  So, as blogs tend to be, this is more of an overview from my experience, rather than a detailed technical paper.

With all buildings that have ’leaking’ issues, the problem is that water gets into an area it shouldn’t be (most commonly the structural timber frame), the water stays there because it can’t drain or evaporate away. When the timber remains wet (typically above 30% moisture) and relatively warm, these conditions allow fungi to grow, which rots the timber.

The ways that water gets into a building falls into 4 broad categories, with many iterations in between where a combination of these forces are at work.

Gravity: generally a hole that water drips into, or where water is flowing down a cladding (or a flashing) that doesn’t adequately direct the water away, out of the building fabric.

Capillary Action: where water in the ground is soaked up by building materials (including DSC01305concrete) and transferred along to structural elements over time. This is commonly what is referred to as rising damp. It also happens where water is allowed to pond and hydroscopic materials are soaking in it (or close enough for rain splash to soak the material).

Condensation: the interior of your house is full of water vapour. From cooking, showering, laundering, un-vented gas heaters, and your own hot breath. If this vapour isn’t extracted or vented out of the house then it can condense on cold surfaces. Such as you see on your windows in winter. This also can happen inside your wall if the conditions are right.

Air Pressure: or more specifically a pressure differential. When it is windy there is a higher air pressure on the outside of the building than the inside. This in effect sucks air through any holes, cracks or openings. If it is raining then the water is taken in along with the air flow.

If you think about all the things that can happen in and around our buildings, the number of ways water can get into our buildings are too numerous to mention. It also follows that just because water has got into a building doesn’t mean it is a ’leaky building’ as such, which commonly implies a cladding failure.

The action (or in-action) of owners has always been an issue. All buildings require regular maintenance, and sometimes a bit of good old fashioned TLC is all that is needed to keep the building water tight. A recent article on the Beehive roof leaking is a perfect example of this.

Another classic is for gardens to be built up around the house. If the sub-floor vents are covered this significantly reduces the sub-floor ventilation and the water coming out of the ground under your floor isn’t removed, allowing sub-floor framing to remain wet. If the bottom of the cladding is buried in the soil (or even too close to the ground) then this will allow water to easily wick up into the framing.

There are a number of variables and reasons for condensation to form inside a wall cavity. Relative humidity, air pressures, vapour pressures, and temperature differentials all contribute to where the Dew Point is. This means that in certain circumstances water vapour could be condensing on the timber framing, inside the insulation, on the back of the cladding, or even on the building paper. This is a known cause of some so-called leak problems and rotten timbers.

When people talk about ’Leaky Buildings’ the most common image that comes to mind is of water getting into the timber framing, through a hole in the exterior cladding, and that timber remaining wet. In the early days of the current leaky buildings problem, existing brick veneer and cavity stucco designs were simply adapted to a wider range of claddings. It was recognised that if (when) water gets through a cladding, a cavity between the cladding and the building paper which is attached to the timber framing allows it to either evaporate or to drain away.

Further research has also shown that the reason for this is that cavity helps to equalise the Window-detailsair pressure behind the cladding, and the lack of air flow allows water to drop out and drain away. But of equal importance, it has shown just how effective a cavity is at allowing any moisture to dry out. BRANZ released initial results of it’s research in Build Magazine in June/July 2007.

They found that water dries 100 times faster from the back of the cladding than from inside timber framing, mainly due to how fast water diffuses through timber. When you add in that we are demanding higher levels of insulation and air tightness in our buildings, the ability for wall framing to dry out is further reduced. This unwanted water then tends to evaporate and condense repeatedly until it soaks into the wall materials or migrates inside the building.

The dilemma we now face is now how to allow for air movement and moisture drainage in a wall while still maintaining a high level of insulation. A cavity behind the cladding allows for ventilation and a drainage path, but it also decreases the insulation value of the wall. So more insulation is shoved in the wall, reducing the ability of the wall to breath even further.

The BRANZ research also highlighted what a significant part air pressure has to play in leaks. The Acceptable Solutions to the Building Code requires that all window and door frames be fully sealed to the structural timber frame to eliminate the air leakage path around these openings. Testing showed that even a small gap in the sealant had a big impact on air flow, and the water it carries. So the important thing is for the cavity behind the cladding to remain at an equal air pressure to the outside. In effect the cladding is now acting as a rain screen, rather than trying to achieve a waterproof membrane.

The truth is that we have ‘thought-built’ buildings. We always have had this, and it is even more so now. To build houses the way we do (in New Zealand) requires knowledge, skill and understanding. Construction clearly requires the designer, inspector, and builder to work from the neck up. They need to think as they draw, observe, and install the building components, like flashings, building paper and claddings. Thinking about where water will be coming from, and where it’s going to go. Miss something and the whole stack of cards can come down.

This is even more so with the rise of ’cladding systems’ where the one manufacturer provides all of the flashings, fixings, and finishings. Even good old fashioned things like weatherboards and bricks are starting to fall into this category. You now need detailed knowledge of how to install a particular system to make it work. Specialist installers, trained by the manufacturers are growing in number. On a recent project there was even a company who specialised in installing just the sealant between the windows and the timber framing. Almost gone are the days of generic claddings where you could use what ever individual components you liked and the whole thing still worked.

This obviously isn’t the be all and end all of this problem. There is a lot more to be learnt about how our buildings work in our environment. We are already seeing that some supposedly remediated buildings, aren’t, and they are leaking again. The story still has some way to go — unfortunately.

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