Roger, one of Pacific Environments architectural designers, shares his thoughts on the importance of 'Breathable Homes.'
Much like wearing a raincoat on a warm, humid rainy day, (a home’s exterior envelope should stop the rain from getting inside, but allow the building to breathe sufficiently to release moisture created inside from bathing, cooking, and even from people breathing and to remove condensation which can build up on the inside of the windows in winter. Double-glazing is becoming standard even in Auckland and Northland to keep houses warmer in winter, as well as cooler during summer, however thermally-broken glazing (particularly with aluminium frames which conduct heat well) is just as important, and where it sits in relationship to the insulation of the surrounding wall.
NZ houses in the past were often poorly insulated, and draughty. The draughts actually helped to dry out any moisture in the structure, but as homes become better insulated and wrapped, other problems can occur.
Wall Insulation and thermal bridging
Because insulation is typically installed within the structural timber wall framing voids, the point at which moisture falls out of the air often happens within the depth of the structure, potentially causing rot and decay over time. Compounding this problem, a recent preliminary BRANZ study2 has found that the percentage of timber framing in the walls studied in new houses was an average of 34% of the net wall area. This means that the overall performance of the wall for thermal insulation purposes is lower than would be expected by the Building Code, particularly when the minimum level/cheapest insulation product is specified, which is more common when projects are driven by maximisation of profit. Junctions in walls at corners and where the internal partitioning meets the exterior walls is commonly not insulated at all due to sequencing of trades within the project, further decreasing the thermal performance.
Typically, in the UK and other European countries, the insulation is pushed outside the structural frame, and then a drained cavity is used on the outside of this to deal with any water getting behind the cladding. The advantage with this approach is that the insulation is able to provide a continuous thermal break, without any short-cuts from thermal bridges through the structural frame.
Alternatively the use of additional cross-battens either on the inside or outside of the main timber structural frame allows for a “service cavity” for running waste pipes or cables etc, and can be insulated to create a thermal break, while keeping the construction style closer to traditional NZ methods, and can even save costs on framing because there is no need to cut timber between the studs for nogs, as the battens are simply fixed across the face of the studs.
I believe that these methods of construction will become more and more common in NZ as home-owners become more informed, and through legislative change, similar to the
demand for double-glazing in new homes.
As new NZ homes become more air-tight, with air seals around the windows and doors part of the code since 2004, and the use of rigid air barriers becoming more common, the risk increases that we do not allow homes to breathe. Opening a window is often the solution in NZ, but during winter we are less likely to want to let out all the expensive heat used to warm our homes. This leads us onto the intelligent use of ventilation, alongside the need to continuously insulate, to be looked at in the following part of this blog!
2. Measuring the Extent of Thermal Bridging in External TimberFramed Walls in New Zealand (External Research Report ER53 ) Verney Ryan, Guy Penny, Jane Cuming, Ian Mayes, Graeme Baker, Beacon Pathway Inc.