The 'Cold Roof' Method

The mainstream method of insulating roofs in New Zealand has been what's commonly referred to as the 'cold roof' method, where the insulation (usually glass wool), is stuffed between rafters/joists in the ceiling cavity i.e. on the inside of the building. But there are a number of shortfalls with this system.

With internal moisture levels in modern buildings higher than ever before, excess moisture rises-up through gaps such as down-lights into the roof cavity. If the roof cavity is insufficiently vented, the moist air finds its dew point when it meets the underside of the roof substrate (the 'cold deck'). This condensation can lead to rotting roof substrates, damp ceilings and wet insulation (which eventually renders it ineffective).

Even if the roof cavity is sufficiently vented, a great deal of heat can be lost in the process. The maximum thickness of the wool insulation is restricted by the depth of the rafters/joists.

What is a 'Warm Roof'?

The most commonly accepted definition of a warm roof (for low-slope roofs) is 'a roof where the insulation is installed on top of the roof structure' i.e. insulated from the outside of the building. The Viking Warm Roof System includes rigid polyisocyanurate (closed-cell foam) sheets from Kingspan Insulation on top of which the waterproofing membrane is adhered.

With a correctly designed warm roof:

• The dew point is on the outside of the building, which means no damp or rotten materials inside
• Thermal bridging is avoided
• The roof cavity is clear for wiring (not stuffed with insulation)
• The savings in air conditioning and heating costs hugely shorten the amortisation period of the initial warm roof cost
• The rigid insulation sheets add further structural integrity to the roof
• Significantly higher R-Values are achieved (uniformly across the entire roof; not just in parts)
• Warm Roof insulation systems can easily be retrofitted on to an existing building

Viking Roofspec's Warm Roof System can achieve an R-Value of R-3.0 when 75mm-thick polyiso is used.