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DESIGN
Don’t be cool about warm roofs
While they may be more expensive to install, the popularity of warm roofs is growing as they are more thermally e cient, resulting in lower energy costs.
RIGHT
CODE REQUIREMENTS AND INSTALLATION FACTS FOR WARM ROOF CONSTRUCTION
BY ALIDE ELKINK,
FREELANCE TECHNICAL WRITER, WELLINGTON
THE TRADITIONAL METHOD of constructing a low-slope, membrane roof is to install insulation between the rafters, leaving an air space between the insulation and the roof deck above (see Figure 1). This roof construction, called a cold roof, does have some disadvantages:
● Ventilation of the ceiling space is required as
there is a risk of condensation forming on the underside of the roof deck, which may result in the insulation becoming wet and losing e ectiveness where moist air can migrate into the roof space.
● The rafters can provide a continuous path or thermal bridge for heat movement from the interior to the exterior of the building.
What is a warm roof?
Warm roof construction is where rigid board insulation is placed over the roof deck and the waterproof membrane is laid over the insulation (see Figure 2). This means that the insulation is on the outside of the building envelope and the roof structure and ceiling space are at a similar temperature to the building interior.
Advantages and bene ts
The advantages of warm roof construction:
● Thermal bridging is largely eliminated as the
insulation is continuous over the whole roof
structure.
● The need for roof space ventilation is
eliminated or reduced. The ceiling space is on the warm side of the insulation. The dew point temperature (the temperature at which water vapour in the air condenses as water) is on
cold surface and colder air within roofspace
rafters insulation
dew point occurs where warmer moist air meets colder
air resulting in condensation forming – cross-ventilation of roof space is required to remove moisture
waterproof membrane
substrate
airtight ceiling lining thermal (cold) bridge path
Figure 1
Cold roof construction.
mechanical xing must not create a thermal bridge
rafter
airtight ceiling lining
waterproof membrane insulation
vapour barrier (recommended in climate zone 3)
framed support to substrate (plywood, pro led metal) (substrate can also be a concrete slab)
Figure 2
Warm roof construction.
Build 161 — August/September 2017 — 35