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Energy-Efficient and Easy to Install Low Pitched Roof Insulation

Homeowners want to improve the overall energy efficiency of their homes. NZ Foam applied to the underside of a roof makes homes drier, warmer, and more energy-efficient.

BEAL has stated that spray foam insulation is an effective way to provide an airtight envelope solution; useful for energy-efficient Passive Houses — and especially those with complex and difficult to access framing.

There will always be considerable moisture in the air due to:

  • Temperature
  • Proximity to corrosive atmospheres
  • Wind force

In housing, the most common surfaces affected by water vapour are metal roofs, and single glazed windows.

Metal Roofs

The drawings above illustrate how typical sloping metal roofs are installed, to meet New Zealand Building Code requirements (NZBC).

  1. Timber or metal framing comprising structural rafters and small timber or metal purlins that sit-on-top of the rafters.
  2. Roof underlay — a fabric-like sheet that provides the second line of defence against unwanted rain getting in but doesn’t protect against condensation.
  3. Metal roofing is usually fixed down over the underlay onto the purlins.


All new metal roofs need either natural (passive) or mechanical ventilation into the roof space to prevent a build-up of moisture-laden air in the roof space. This prevents unwanted condensation getting inside the house. However, most roof spaces either don’t have mechanical ventilation or rely on passive ventilation (typically a series of vents in the roof), which unfortunately can be poorly designed.

The incorporation of airtight, sprayed polyurethane foam, applied by registered applicators to walls and especially the underside of the roof is already a popular choice Europe and North America.

NZ Foam Application — Underside of the roof

Typically, 90mm to 110mm thickness or more is applied, depending on the client’s wishes. For example, 90mm thickness will produce a typical R-Value of 4.29 while 110mm thickness will produce a typical R-Value of 5.24 — both considerably more than the minimum requirement.

The advantages of NZ Foam spray foam insulation are:

  • Thermal resistance is high
  • Economical to apply — especially when wanting airtightness
  • Able to provide an airtight envelope solution — useful for energy-efficient Passive Houses — especially with complex and difficult to access framing
  • Water resistance is excellent
  • Durable product

Excerpts taken from the article was written by Colin Prouse, Director of BEAL in August 2020 and was updated in December 2020.

View more information on NZ Foam, including contact details.
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September 2021 EBOSSNOW Product News

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