Glare, Reflectivity And Reflectance
In Scope 32, Stuart Thomson wrote a technical article about Solar Reflectance and Thermal Emittance and the relationship between the apparent visual colour of a metal roof and its ability to absorb or not absorb heat. The way in these factors can be modified by the use of “cool pigments” to allow the use of darker visual colours while reducing heat gain was also covered. Recently I have been asked about the old favourite “Glare” from roofs, and its relationship to Cool Roofs. What follows is a less technical take on what is still seen as an issue in many locations in New Zealand (and elsewhere), and covers only domestic roofs, not larger commercial buildings. These are my views, and not necessarily the views of others.
Glare
Over the years there has been much talk of glare and “reflectivity” and “reflectance” with the two latter terms being used more or less interchangeably. Various dictionaries define glare as “To shine with or reflect a very harsh, bright, dazzling light”. Motorists who drive in the morning or evening (like nearly all of us) will have at one time or another been nearly blinded by incoming or reflected sunlight straight into their eyes. This can be very dangerous and recently a train/truck crash in the South Island was attributed to this. The TipTop icecream factory was notorious for creating blinding reflections on the Auckland Motorway at Mount Wellington, although this seems to have been mitigated by the use of different glazing.
The glare from house roofs has also long been an issue ranging from annoyance to serious eyesore and BCAs in New Zealand have been proactive in trying to prevent this, particularly in rural areas. Queenstown Lakes District Council specifies acceptable colours. In “Cool Roofs” Stuart Thomson shows a yellow hut and comments that DOC don’t like it and would prefer it to be of a low visibility colour. I suggest that this would be even more so if it were clad with unpainted Zincalume®.
Heat Gain
More recently the emphasis internationally has shifted from glare reduction (minimum reflection of visible light) to “cool roofs” where the cost of air conditioning for cooling can be reduced by roofs which have minimum thermal gain (maximum reflection of IR radiation) and maximum thermal emissivity.
Glare reduction and low heat gain can be seen as mutually incompatible. Things which reflect IR normally reflect visible light and so minimum heat gain/ maximum heat loss will probably result in higher glare.
The use of “cool pigments” can offset this to some extent by allowing reflection and emission at one level in the IR part of the light spectrum and to a lesser degree in the visible part of the spectrum. Many roof paints today claim to use “cool pigments”, although such pigments have been used for decades without being called “Cool” because they are also more cost effective and durable. In the New Zealand environment preventing thermal gain/maximising thermal emission is not seen as particularly important (at present). There is also a difference between a house with a space and then insulation separating the roof cladding from the inside and commercial buildings where typically the roof cladding (plus foil/ underlay etc) are directly exposed to the internal building space and can radiate heat into this space.
If you believe advertisements for ventilation systems claiming to extract heat from the roof space, it would seem that getting hot is useful, so domestic roofs more thermal gain may be beneficial in reducing heating costs, although the numbers derived from the thermodynamics don’t really bear this out. When MRM have looked at potential benefits from heat gain into buildings from the roof, there seems little real movement either way.
“Glare” however continues to be undesirable in many visual environments.
In NZ while there are some attempts to promote “cool roofing” or White Roofs as environmentally friendly this is debateable in the current heating/cooling building environment, and avoiding glare is more important in many locations.
What is “low glare”?
This is to some extent this is in the eye of the beholder, but there is a test method for reflectivity which is used by PCC – ASTM E903-96. The Queenstown guideline implies “reflectance” of 0-20% as best, and 21-35% as acceptable and over 35% as unacceptable. The figures quoted in “Cool Roofs” are RV (Reflectivity Value) which is not necessarily the same (but are presumably related in that more is more and less is less). The Queenstown guide also says that materials with non-shiny, textured or matt/powder finish are preferable.
Another measurement that is applicable to visual glare (but not to thermal reflection) is gloss and a Sheen Gloss meter can be used to determine the “glossiness” of roofing materials “Cool Roofs” considers only long run metal roofing, but in fact there are other roofing materials which need to be considered once we are looking at houses, not commercial buildings. Metal tiles are also part of NZMRM’s range and concrete tiles, clay tiles and asphalt shingles/built up roofing all have their own properties.
The following qualitative diagram shows the relationship between the three properties of Colour, Gloss, and Texture and Glare/Heat gain.
Gloss (60° Sheen Gloss) figures for various products that I am aware of are –
- Textured tiles – zero
- Satin tiles (post painted) – 4-8%
- Low gloss coilcoated – 10-15%
- New Zincalume® 20-25%
- Standard coilcoated – 25-30%
Clearly where glare is seen to be an issue standard gloss coil-coated product of light colours are likely to be worse. Unpainted Zincalume® with its very light colour may be worse. Dark coloured textured tiles are likely to be the best with no glare at all.
New concrete tiles normally have a high gloss acrylic coating, and asphalt shingles use flat granules, which would be slightly less effective than textured tiles. Unglazed clay tiles should have low glare, but a noticeable colour. It would be necessary to do some testing to ASTM E903-96 and using Sheen Gloss of all materials in various colours to get a definitive quantitative statement, but in general – post-painted metal tiles (textured or satin) are better in reducing glare than roofing made from any prepainted coil. This is unrelated to colour from a gloss point of view, although darker prepainted colours should have less visual impact.
Trade-off between glare and heat gain
The diagram shows clearly that for unmodified products the two properties are in opposition – highest glare has least heat gain, and vice versa. The heat gain factor for darker coloured finishes is claimed to be mitigated by the use of “cool pigments” which are said to be reflective in the IR (heat) more than in the visible part of the spectrum and so allow the use of darker colours (to reduce glare) without as much heat gain as using standard pigments. Stuart Thomson explains this in his technical Scope article “Cool Roofs”. However this article covers only coil coated metal, and all these products have a relatively high gloss and are quite smooth. So the use of cool pigments only deals with one of the three properties.
From the above the most obvious way to minimise glare in sensitive locations is to use dark coloured textured chip metal tiles. Use of cool pigments could allow use of lighter coloured matt post-painted metal tiles.
Mitigation of glare (and heat gain)
Quite a lot of work has gone into this and I have uploaded several related documents to the MRM website.
Glare
- Minimise by use of low-gloss, textured dark colours with lower “Reflectance”. The Queenstown Lakes document “Reflection Brochure” covers this.
- Reduction of visible glare by adjustment of the location/ screening. The Bluescope Technical Bulletin TB-28 deals with this method.
- Use of cool pigments allowing darker colours. Stuart Thomson’s article “Cool Roofs” deals with this.
- Use of glass covered solar panels (either thermal or pv) or skylights may create spots of high glare in an otherwise low-glare roof.
Heat
- Under roof (eave to ridge) ventilation.
a) Oak Ridge National Laboratory in the US has done work on under roof ventilation of concrete/clay tiles as reported in ASV NBNL report.
b) They have also done work on metal tiles as reported in the Decra Green Brochure. These two reports show that underroof ventilation for tiles is significantly better at reducing heat gain than any colour modification. This is logical.
c) As far as I can find, no work has been published on the effect of under roof ventilation with long run products.
2. Earlier research done by NZMRM was not able to show significant heat gain or loss from long-run metal roofs installed in the normal way.