Page 38 - Build 161

the exterior side of the insulation, so
condensation is unlikely to occur.
Warm roofs have signi cant bene ts over cold roofs because:
● energy e ciency is improved
● heating and ventilation costs are reduced.
Statutory requirements
There are no speci c New Zealand standards for warm roofs. However, a warm roof must meet the requirements of the following New Zealand Building Code clauses:
● B1 Structure requires that the design must be able to support the dead and live loads imposed by the roof.
● B2 Durability requires that the materials must provide a minimum 15-year durability.
● C1 Protection from re is applicable where ‘foamed plastic’ insulation is used.
● E2 External moisture deals with the roof’s ability to shed precipitated moisture.
● H1 Energy e ciency sets out the energy e ciency requirements of the roof.
Installation facts
Warm roof construction requires a continuous substrate, typically 17.5 mm minimum plywood or pro led sheet metal, to support the rigid board insu- lation. The insulation is laid over the substrate, and the roof membrane is then laid over the insulation.
A vapour barrier may be installed between the insulation and the substrate if desired. It is recommended in climate zone 3 due to lower average annual ambient temperatures.
Insulation options
Insulation for warm roofs generally consists of either polyurethane (PUR) or polyisocyuranate
(PIR) rigid foam boards at least 75 mm thick. Advantages of PUR and PIR include that they: ● are excellent insulators
● have a closed-cell structure with high
resistance to moisture absorption
● are self-extinguishing in re and do not melt ● have excellent durability
● have low environmental impact
● have high acoustic absorption
● have high compressive strength
● can be faced with di erent materials to
provide compatibility with di erent membrane
types.
The insulation may be mechanically fixed or glued. Mechanically fixed systems must not create thermal bridges through the fixings.
Alternatively, expanded (EPS) or extruded (XPS) polystyrene board insulation may be used. However, they must be protected from fire and are unsuitable where hot bitumen, asphalt or solventborne adhesives – unless specifically formulated for the EPS or XPS – are to be applied.
Roof membranes
Typical roo ng membranes for use with warm roofs are:
● single-ply sheet membranes
● a multi-layer torch-on modi ed bitumen.
Mainly installed in roof refurbishment
At present, warm roofs are mainly installed in New Zealand in roof refurbishments such as when a low-slope roof needs replacement or the roof slope of an existing roof needs to be increased.
However, the bene ts of warm roofs are making them more popular for new low-slope
or skillion roofs. Although they are somewhat more expensive to construct, the bene ts of improved thermal e ciency and reduced heating and cooling costs are able to o set increased construction costs.
The major advantage of a warm roof in a roof refurbishment is that it can be installed directly over the existing roof. This eliminates demolition costs removing the existing roof, and building weathertightness is maintained throughout the refurbishment as the building interior is not exposed.
Several proprietary systems
There are several proprietary warm roof systems available in New Zealand, some of which are BRANZ Appraised.
Proprietary warm roof systems should be installed strictly according to the manufacturer’s instructions.
Inverted warm roof variation
An inverted roof system is a variation on the warm roof.
The insulation is installed over the waterproof membrane and then protected
by pavers, planting or loose gravel ballast – typically 20–40 mm diameter washed pebbles. These shield the insulation from the effects of UV light and wind uplift. Pavers also allow the roof to be used for recreation or for housing service units.
An inverted warm roof has additional load imposed by the ballast layer, which may increase the building structure costs.
Inverted warm roofs are not commonly used in New Zealand.
36 — August/September 2017 — Build 161