This article was written by Colin Prouse, Director of BEAL in August 2020 and is focused on the use of NZ Foam's spray foam insulation applied to the underside of metal-clad roofing and what design features need to be incorporated into the design and construction to meet New Zealand Building Code requirements (NZBC).
All metal roofs are subject to three key environmental influences: first, the temperatures experienced, second, the proximity to corrosive atmospheres — especially salt-laden wind, and third, the wind forces applicable to the site. When considering the use of metal roofing, each of these needs to be accounted for, whether for commercial or residential use.
Apart from the air in the Arctic Circle or in the Antarctic, there will always be considerable moisture (water vapour) in the air. The amount of moisture will be dependent on the temperature of the air, since the warmer the air, the greater the ‘capacity’ of the air to contain moisture. For example, a parcel of air near saturation may contain 28 grams of water per cubic metre of air at 30°C, but only 8 grams of water per cubic metre of air at 8°C. Therefore, when there is a warm day, there will be a higher capacity of the air to contain water vapour and this phenomenon is called humidity. Generally, warmer climates have higher humidity levels.
However, when the same vapour saturated air, drops in temperature, there will become a point when the air’s capacity is reduced to the point that the water vapour will turn into a liquid form, i.e. water droplets. Usually, these water droplets will form on the warm moisture saturated side of a surface. In housing, the most common surfaces affected by the drop in temperature of water vapour saturated air, are metal roofs and of course, single glazed windows.
In most houses, metal roofs are installed on a sloping frame in the roof — called a “roof truss/rafter”. The drawing (2nd image above) illustrates how typical metal roofs are installed, and where spray foam insulation can be applied.
First, there is the timber or metal framing comprising structural “rafters” (sometimes called the top cord of the truss) and small timber or metal elements called “purlins” that sit-on-top of the rafters — onto which the metal roofing is fixed to. The purlins are also used to enable a roofing underlay to be installed over the top.
Second, there is a water-impermeable but vapour-permeable roof underlay — a fabric-like sheeting that provides the second line of defence against unwanted rain getting into the roof space and falling on the unprotected ceiling. It also prevents unwanted air movement from the outside getting into the house which can reduce energy efficiency.
Third, there is a metal roofing product, which is available in several different shapes or profiles. The metal roofing is usually fixed down over the underlay onto the purlins. Corrosion-resistant screws with rubber seals are used to allow for thermal expansion and contraction that occurs, as well as providing watertightness.
Currently, in many house roof spaces, there is no 'mechanical ventilation’ to remove excess moisture in this air. To prevent a build-up of moisture-laden air in the roof space, which can lead to unwanted condensation inside the house, outside air — containing less water vapour — must be introduced. This is usually done by way of passive ventilation or in some cases, by mechanical ventilation. Typically, there are a series of vents in the roof overhang (known as a soffit) to allow the outside air in. However, a ridge vent — to allow the moisture saturated air out, especially in metal-clad roofs — is often missing, indicating the need for better education in this area.
All new metal roofs need to have either natural or mechanical ventilation into the roof space.
Adding Insulation to the underside of the waterproof roof underlay
In recent times, homeowners have been seeking to improve the overall energy efficiency of an existing or new home. One popular solution used in Europe and North America, is the incorporation of air-tight, sprayed polyurethane foam, applied by registered applicators, to the basement walls, sometimes the walls and especially the underside of the roof, being the roof underlay.
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 air-tight envelope solution — useful for energy-efficient Passive Houses, especially with complex and difficult to access framing
- Water-resistance is excellent
- Durable product
Interestingly, since the foam adheres directly to the underside of the underlay and its expansion upwards is minimal, the underlay does not need any support. The first layer of foam sprayed stiffens underlay. This also changes the point at which any un-managed vapour-laden air might develop condensation inside the roof space in the event of a fall in air temperature.
The net effect of using NZ Foam spray-applied PU foam to the underside of a roof is a drier, warmer interior of the roof space and overall, more energy-efficient home.