Air films sit against the outer and inner most surfaces of the system. These air films still provide a small insulating effect ie provide a small R value.
Air-spaces are cavity spaces between bricks or blockwork, the air between stud walls if there is no bulk insulation, the air around battens between plasterboard and a pre-cast panel wall, a free space between the roof and the ceiling, a free space under a floor etc. Note air-spaces filled with bulk insulation are not air-spaces. Also reflective air-spaces are air-spaces where there is a reflective surface (usually a foil) facing towards this air-space. A reflective air-space achieves a higher R value than a non reflective air-space. Non ventilated (still air) air-spaces achieve higher R values than ventilated air-spaces.
Emissivity is the value given to materials based on the ratio of heat emitted compared to a blackbody, on a scale of 0 to 1. A blackbody would have an emissivity of 1 and a perfect reflector would have a value of 0. So, more reflective items have a lower Emittance value. An alternative definition: emissivity is the relative ability of a materials surface to (absorb and) emit energy by radiation. Shiny surfaces cannot (absorb and) emit radiation significantly.
So there are high emissivity polyweave faces, low emissivity antiglare faces (usually on one side only) and very low emissivity bright/reflective faces
Reflectivity is inversely related to emissivity and when added together their total should equal 1 for an opaque material. Therefore, if asphalt has a thermal emissivity value of 0.90 its thermal reflectance value would be 0.10. This means that it absorbs and emits 90% of radiant thermal energy and reflects only 10%. Conversely, a low-e material such as aluminum foil has a thermal emissivity value of 0.03 and a thermal reflectance value of 0.97, meaning it reflects 97% of radiant thermal energy and emits only 3%. Low-emissivity building materials include window glass manufactured with metal-oxide coatings (low e glass) as well as housewrap materials, reflective thermal insulations and other forms of radiant thermal barriers.”
- Emissivity of polyweave foil – bright face 0.03, polyweave face 0.87
- Emissivity of antiglare foil – bright face 0.03, antiglare face 0.1
- Emissivity of bubble or foam foils – bright face 0.03, antiglare face 0.1
|Solar Absorptance values – from BCA vol2 126.96.36.199 – light coloured roofs with lower solar absorptance reduce the flow of heat from solar radiation better than dark coloured roofs
|Slate (dark grey)
|Zinc aluminium – dull
|Galvanised steel – dull
Sarking or sisalation (reflective sheet) may change a tiled roof space from ventilated to unventilated. (1)
A thermal break (which prevents thermal bridging) may be provided by materials such as timber, expanded polystyrene strips, plywood or compressed bulk insulation. Reflective insulation alone is not suitable for use as a thermal break because it provides very little insulation effect in its own right – it requires an adjoining airspace to achieve its often quoted R values. (1)
The thermal performance of a roof may vary depending on the position of the insulation, the climatic conditions, the design of the house, and the way in which it is operated. Insulation installed under the roof rather than on the ceiling of a conditioned house with a large roof space will be less effective because of the aditional volume of roof airspace that would need to be heated or cooled.(1)
Insulation values are only accurate when insulation is installed correctly on site and all gap widths, materials, and reflective surfaces are facing airspaces etc per the descriptions below. Different temperatures, temperature differences between inside and outside, and ventilation (air velocities) affect airspace and airfilm R values. The age of materials and moisture levels in bulk insulation also affect R values. As such the total R values will be approximate and to be taken as a guide only. If in doubt we recommend additional insulation be installed to manufacturers recommendations.
Airspaces are spaces between two materials. Air films are a layer of air that sits against a surface plane.
The R value of an item, other than an airspace, air film or air cavity, may be increased in proportion to the increased thickness of the item.
Airspaces are either between parallel or non parallel surfaces ie where the ceiling is flat and the roof flat (or ceiling is cathedral and parallel to the roof), or when the ceiling is flat and the roof is pitched ie such as in an attic space.
The R value of a material (other than airspaces and airfilms) is not affected if it is horizontal or vertical. It is also not significantly affected by temperature.
The R value performance of bulk insulation batts is slightly affected by temperature, however the quoted average R value is used in this table for simplicity.
Where the R value of a material is known by us to age over time, the aged R value is provided in this table.
Where a cavity or airspace is filled for example by bulk insulation, the R value listed for the cavity does now not apply and cannot be included in the total R value for the wall. (1)
The values provided in these tables are for the material itself only – not the material and air spaces or total system value.
*manufacturers nominal R value is provided.
Note for comparison R values of windows are calculated as the reciprocal of the U value ie 1/U value = R value. Typical R values for single glazed aluminium windows are R0.2 – R0.3 and R0.3 – R0.4 for double glazed windows.
Tiled roofs without sarking/sisalation or blanket under the tiles are considered ventilated roofs.
Suppliers wishing to include details of their products on this site should email certified material R values to the email address below.
Your access to the Low Impact Development (LID) Consulting online calculator is conditional upon your acceptance and compliance with this Disclaimer. Your use of the website constitutes your acknowledgement that you have read and agreed to this Disclaimer.
These calculators are populated by data from the listed sources. Low Impact Development (LID) Consulting accepts no responsibility for the interpretation and/or use of this data (incl. correct and/or appropriate entry of data in the tables, or the figures in the tables). To the maximum extent allowable under the law of Australia, neither Low Impact Development (LID) Consulting, nor its employees, or agents shall be liable to you, or any third party for any loss, damage, claims or costs suffered by you or any third party whatsoever from your use of this website, or reliance on the data in these tables, calculators and software contained on it. The data is provided ‘as-is’ without any warranty, express or implied, and/or representations of any kind. Use of these calculators is entirely at your own risk.
The raw figures are taken from the BCA, AIRAH Handbook 2007, ICANZ manual, AS/NZS 4959.1:2002 with interpolation as appropriate, or suppliers figures where indicated. Figures from independent sources were preferred, but when unavailable, suppliers figures have been used. This calculator has been prepared in consultation with RMIT’s Centre For Design. Low Impact Development (LID) Consulting accepts no responsibility for the ultimate accuracy of these figures and recommends values to be read to the closest 1 decimal place only as NCC/BCA “required R values” are.
Furthermore, The Centre for Design at RMIT University makes no claim as to the accuracy or authenticity of the data, and does not accept liability to any person for the information provided in or incorporated on this website by reference.
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