CLADDING
The Building Code Clause explaining the minimum requirement for cladding systems is E2 External Moisture. It is contained here.
https://www.building.govt.nz/building-code-compliance/e-moisture/e2-external-moisture/
Roofs
The principal purpose of your roof is to keep the weather out and collect rainwater for controlled discharge. Aesthetic or design requirements may call for complexity or a combination of materials. The roof slope can determine the material used. The complexity and size/scale of the roof can influence these decisions and geographical location and ground conditions may also determine the material choice (hazard zones). A roof also plays an important role in the seismic performance of the structure. It may act as a diaphragm and transfer lateral seismic forces to the vertical framing elements and down to the foundations.
For further information http://www.seismicresilience.org.nz/topics/building-envelope/residential-buildings/
Warm and Cool Roofs
New Zealander’s love our metal, tile and membrane roofs on trusses; however, most people who own these homes or even designers of these homes, don't fully understand the complications that arise with these systems. For many years we've been told that if we insulate the ceilings, we will keep the interior space warm. While this is correct, there are a wide array of other considerations. Adding ceiling insulation has an impact on moisture movement by diffusion and transport via air leakage.
For those of you unfamiliar with the concepts of warm and cool roofs, it has to do with the placement of the insulation layer. In a cool roof assembly (traditional NZ roof) the insulation is in the ceiling cavity or between the joists. As the insulation is below the structural deck (plywood, steel or concrete) the deck is on the cold side of the insulation and thus “cool” (see below for comments on how cool). In a warm roof assembly, you place an insulation layer above the structural deck and then place the membrane over that. This keeps the structural deck “warm”. This assembly is also called a conventional or built up roof. A similar concept can also be applied to steep slope roofs, just think about where the insulation layer resides in relation to the sheathing layer and the same concepts apply.
Ideally, in a Superhome, traditional cool roofs should be eliminated and instead a warm roof assembly should be used. Alternatively, it is possible to design a cold roof to manage condensation and mould growth with a vapour control layer and checking this before building using hygrothermal analysis. WUFI is a software package that can do this type of analysis. A study by BRANZ published in Canada showed that WUFI replicated real-world findings very accurately. https://pdfs.semanticscholar.org/d535/4752146be86515e2e484bcf5f75cb73c67a4.pdf
For further information https://www.buildmagazine.org.nz/index.php/articles/show/dont-be-cool-about-warm-roofs
https://www.buildmagazine.org.nz/index.php/articles/show/cold-roofs-warm-roofs.
The designer needs to fully understand building science to be able to design the roof assembly correctly to mitigate the potential for interstitial condensation. One simple way of mitigating risk is to design a warm roof which keeps the interior surface temperature of the roof assembly above the dew point temperature. In other words, condensation is not possible.
The cold roof system has "worked" in New Zealand for decades, mostly by accident. Newly constructed or retrofitted attics are typically better insulated and more airtight than older lofts. As a result, these attics experience less leakage of warm and humid air into the attic. By keeping this space warm, it reduces the condensation risk of interior moisture within the attic. Modern and retrofitted lofts experience significantly less conductive heat transfer from the rooms below and create cooler attic spaces. However, without a vapour control layer, the moisture from the habitable spaces below finds its way into the roof space. If an attic space has similar conditions to outdoors with increased ambient moisture, there is an increased likelihood that night sky radiation can lead to condensation.
Research shows that mould growth associated with vented attic spaces in New Zealand is commonplace. https://www.metalroofing.org.nz/sites/default/files/legacy/pdfs/scope/NZMRM_Scope_Issue_35.pdf (p20) Wetting of the underside of the roof sheathing or purlins due to exposure to moist outdoor ventilation air can cause visible surface mould growth. Ventilated attics can work in summer when there is potential for the radiant heating of the sun. However, the scales tip in the winter months when relative humidity is generally higher, far less solar drying potential and night sky radiation causing cold surfaces in the roof.
Sometime in the past few decades, people decided that being warm in their house was a novel concept and they started adding insulation in the attic. While keeping people warmer on the inside, this also reduces the amount of heat that gets into the attic, creating a cooler roof (still not that cool). As insulation increases, sheathing temperatures continues to decrease to almost the same as the outside temperature (lets say -10C in Queenstown). Now that air migrating into the attic space has a lovely cold surface to condense and causes problems (mould, rot etc.) Bear in mind that NZ does not have any exterior passive attic ventilation requirements like North America does (because condensation hasn’t been recognised as an official problem in NZ yet).
So with the continuing trend of higher insulated roofs (unlikely that is going to go the other way), and cooler and cooler sheathing temperatures, how do you build an assembly to get to R4-R6 without causing massive systemic condensation problems?
One way is to first minimise air entry into the attic with an airtight ceiling. If air does get into the attic, let it escape through ridge vents and allow outside air to enter at the soffits. This is the defacto method in much of North America for a cool roof assembly, but this method may also not work that well in some climates. In addition, all the challenges that creating an air barrier in a ceiling presents, alternative methods are required.
Another option for highly insulated roofs is a warm roof assembly. In this assembly, the air barrier is created from above rather than at the ceiling level. There are naturally fewer penetrations through the roof sheathing than through the finished ceiling in a modern home, meaning an effective warm roof air barrier may actually be feasible. This assembly also has less thermal bridging meaning more insulating value per metre. The air barrier can be either be sealing a plywood sheathing layer, a taped roof underlay or taped rigid insulation. These could all work well depending on the circumstances and be a super assembly if done correctly.
Above Sheathing Ventilation
ASV can be implemented by adding counter battens (running vertically) directly above the weathertightness membrane. Regular battens (running horizontally) are then attached above the counter battens to form the fixing structure for the roof cladding. The counter batten construction provides an air space between the exterior face of the roof sheathing and the underside of the roof cladding so that a clear air pathway exists beneath the roof cladding from the soffit to the ridge.
Benefits of Above Sheathing Ventilation (ASV)
Various building science studies from around the world have found that no matter what the roof cladding type, there are significant benefits to providing ventilation between the cladding and a secondary weathertightness layer using battens and counter battens.
The three main benefits of ASV are:
● The counter battens enable free drainage of water on the membrane or the underlay boards.
● The air exchange reduces overheating of the attic, regardless of the colour of the roof cladding, thereby reducing the cooling demand of the building while creating more comfort internally.
● Air movement in the layer helps to remove any moisture that might accumulate either via water vapour permeating from the interior of the building, or from dew condensing from ambient air entering the cavity.
Low pitch (5 – 10 degrees)
High Pitch (>15 degrees)
Roofing Materials
Roof cladding systems tend to be considered as either light or heavy as the weight impacts the amount of bracing you need for earthquake and wind events. Clearly it also impacts on the gravity structure you need to hold it up. Roof claddings are measured in kg per m2.
Heavy roof claddings (over 20kg/m2 and up to 60kg/m2 of roof area), are products like concrete or clay tiles or slate and stone. These are generally fixed to a timber batten system and start at a minimum of 15deg pitch.
Lightweight systems (mass up to 20kg/m2), consist of a greater range dealing with roof slopes from 1.5deg and up (1deg possible beneath max 40sqm deck), for example:
● Steel, aluminium or copper sheet steel with various profiles and coatings.
● Metal tiles.
● Asphalt and fibreglass shingles (usually fixed to a backing sheet).
● Sheet membranes on plywood sheet.
Super low pitch “flat” roofs are generally a membrane material such as EPDM butyl, a torched on multi-layer bitumen impregnated product or a TPO. These systems are generally adhered to a 20mm treated plywood substrate, (except for a warm roof).
Other materials include shakes, shingles and bitumen (asphalt) impregnated tiles. The timber used has to be either suitably treated or naturally rot resistant, such as western red cedar. These suit steeper simple roofs.
Recommended Healthy Home guidelines for Roof design are set out in the table below:
Cladding
An external wall cladding’s basic job is to keep the weather out.
Types of Cladding
Wall cladding systems can be considered as either light, medium or heavy and the weight impacts the amount of bracing you need for earthquake events. Clearly it also impacts on the gravity structure you need to hold it up. The weight of the cladding is likely to impact on foundation size and on sites with poor ground conditions, heavy claddings may not be appropriate for resilient homes. Wall claddings are measured in kg per m2.
Heavy Weight Cladding
Heavy claddings have a weight over 80kg/m2 for example clay brick and concrete block and natural stone veneers, and up to 220kg/m2 for precast concrete panels. These “permanent” cladding systems generally are an external weather tightness and finish over a structure (eg timber frame) over a drained cavity. Brick ties are used to prevent the heavy veneers from parting company from the structure when subjected to horizontal acceleration from the likes of an earthquake.
The advantage of these materials is that they require little maintenance compared to other cladding systems.
Medium Weight Cladding
Medium cladding has a mass over 30kg/m2 and up to 80kg/m2. Medium-weight claddings include 25mm thick stucco and some thicker autoclaved aerated concrete (AAC) panels.
Light Weight Cladding
Light wall cladding has a mass up to 30 kg/m2. This includes a range of wall claddings commonly used in New Zealand, such as:
● Weatherboard claddings made of timber, fibre-cement, PVC or aluminium.
● Metal claddings with various profiles, colours and finishes.
● Sheet and panel materials made from plywood and fibre-cement, with a variety of treatments and facings.
● Exterior insulation and finishing systems (EIFS).
● Maximum 10 mm thick glass curtain walls.
An alternative approach to traditional light weight claddings is the rain screen. Strip timber, Corten steel panels, terracotta tiles or the like are installed over a cavity. Their purpose is to reduce the effect of wind driven moisture from the actual moisture resistant layer; generally a membrane layer located at the back of the cavity. Rain screens offer visual texture, colour and diversity.
Structural Cavity Battens
Either 40 or 45mm structural cavity battens are ideally recommended to be used for all vertical cladding systems to minimise the use of timber blocking within the thermal envelope. The cladding is fixed to the structural cavity battens, with the battens then fixed back to the wall framing.
Recommended Healthy Home guidelines for Wall Cladding design are set out in the table below:
