What are the four Ds of weather-tightness?
At its most basic, a house should be warm, dry and sturdy, and a healthy place to live. While that sounds easy on paper, but in reality even the smallest mistakes during a build can let in unwanted moisture or even small creatures.
A good rule of thumb is to remember the four Ds to prevent moisture damage in any build: Deflection (check claddings and flashings are keeping water out); Drainage (arrange for drainage paths to outside, should water get in); Diffusion (have ventilation and vapour diffusion to dry out remaining water) and Durability (choose components that will withstand moisture while drying out).
Deflection means including capillary breaks to prevent water from being “sucked” inside a building through capillary action. If you watch water running down a vertical surface, sometimes it will cling to the material and start flowing under the bottom of it as well. Changes in air pressure can also make water start travelling upwards, meaning it can get inside wherever there’s a gap. Capillary gaps, rebates, kick-outs or drip edges at critical points help prevent water from getting all the way through.
As for drainage, wall cavities behind cladding will enable water to dry even if it gets inside. The Department of Building and Housing has some great tips for determining when a cavity is needed, and on choosing and installing the right wall underlay products and systems. Don’t forget to fit a cavity closer made of aluminium, stainless steel or uPVC at the bottom of every cavity, allowing drainage and ventilation but preventing vermin from getting in.
Diffusion relies on having good ventilation design and systems, but you can improve performance by increasing thermal insulation to help prevent condensation inside, as it often leads to timber damage. It’s also important to ensure drainage and subfloor cavities are sealed off from framing and roof cavities to stop humidity migrating from warmer areas into cooler ones.
To maximise durability, you’ll need to go beyond choosing the right timber. Choosing the right kind of joint for your cladding will give sealants a longer life and ensure they can stand up to weather and movement. As the minimum life of a building is normally at least 50 years and the life of a sealant is between 15 and 25 years, this will have to be replaced at some stage.
BRANZ has a useful guide on how to design and seal joints for maximum weathertightness and durability. This includes avoiding complex intersections between surfaces, or joins that are too wide, and designing joints that make it easy for property owners to access and reseal them while minimising their exposure to the elements.
The worst thing that can happen is for elements of a well-built home to fail because the homeowner isn’t aware of how to maintain it. Communication is key, before the keys are handed over, to ensure everything remains in good condition for the lifetime of the structure. For more information on preventing moisture damaging your timber home, visit the Department of Building and Housing or the BRANZ website.
