Shed roof colour for keeping heat down

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Paul1977

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#280887 18-Jan-2021 11:40

We're ready to order our new garden shed, for aesthetic reasons we don't want to go any lighter than something like "Grey Friars" for the walls/door so are hoping they'll be some benefit in at least making the roof lighter.

The ColorSteel website has all the TSR and LVR ratings the for colours, but not for plain unpainted Zincalume. I'd like to know if a light colour for the roof would be better than unpainted, but haven't managed to find any comparisons.

~~I think~~ Titania (TSR 67, LSR 69) is the lightest the shed company offers, ~~but have enquired whether Cloud is available (TSR 72, LSV 76).~~

EDIT: Titania is the lightest they offer.

Hammerer

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#2637327 18-Jan-2021 12:43

Zincalume may be as high as 80% but oxidation happens very quickly so it will drop faster than paint and, I think, move to 60%. Clear anodised aluminium will maintain high TSR for longer.

https://www.metalroofing.org.nz/technical/cool-roofs-hot-topic

Unpainted Zincalume and unpainted aluminium cladding have a low emissivity of 0.1. but a relatively high total solar reflectance, 0.6 to 0.8 however the reflectance and emittance of bare metals are very sensitive to the smoothness of the surface and the presence or absence of surface oxides, oil film or dirt. Usually, bare metals are not very cool in the sun. Rough surfaces also cause the light to be deflected at different angles according to the surface profile resulting in a scattering of light away from the angle of reflection. This causes the object to appear dull or matte. The higher the degree of surface roughness, the greater the scattering of light occurs which results in a lower gloss level. Painted roof cladding is a different story as even a thin coil coated white coating “hides” the low emittance of the metal underneath, and the thermal emittance increases in most cases, to more than 0.8, regardless of the colour.

Local bylaws can be an issue if you use high-reflectance paint as, if anyone complains about glare, you may be required to paint unpainted zincalume or repaint in a darker colour.

https://www.resene.co.nz/homeown/painting-your-home/Painting_Roofs.pdf

Many Councils and Regional Authorities have specified limits on colours that may be used on roofs. Some even state that light colours, including unpainted Zincalume, must be painted to stop reflection and visual pollution of the environment - better to have a deep green roof on the house below you than a bright silver.

timmmay

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#2637333 18-Jan-2021 12:55

I can tell you my house has gray friars color steel, can't remember how hot it gets up there but I'd say into the 40s on a sunny day. Too hot to be up there really. Ventilation up there isn't great.

Kraven

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#2637348 18-Jan-2021 13:12

We have two sheds at home, one is Karaka green and the other is unpainted zinalume. Both get uncomfortably hot inside so I don't think the colour makes a lot of difference. I think ventilation would be much more effective.

Paul1977

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#2637359 18-Jan-2021 13:31

Kraven:

We have two sheds at home, one is Karaka green and the other is unpainted zinalume. Both get uncomfortably hot inside so I don't think the colour makes a lot of difference. I think ventilation would be much more effective.

Agreed, but a lighter colour roof can't hurt!

Paul1977

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#2637369 18-Jan-2021 13:58

Hammerer:

Zincalume may be as high as 80% but oxidation happens very quickly so it will drop faster than paint and, I think, move to 60%. Clear anodised aluminium will maintain high TSR for longer.

I'd read the same article, and thought "0.6 to .08 - not very specific". Titania might be the way to go then.

Hammerer:

Local bylaws can be an issue if you use high-reflectance paint as, if anyone complains about glare, you may be required to paint unpainted zincalume or repaint in a darker colour.

I think those usually only apply to roofs with a pitch of 10 degrees or more (but can't remember where I read that, so might be wrong). This will just be a standard low pitch monopitch type shed roof so hopefully won't be an issue, but if I had to repaint it I could.

Hammerer

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#2637640 19-Jan-2021 01:20

Paul1977:

I'd read the same article, and thought "0.6 to .08 - not very specific". Titania might be the way to go then.

I figured that 80% is plain and 60% is textured/matte where the surface structure reduces reflactance by another 20%.

Infrared and visible light, 91% of solar radiation, are largely unaffected by aluminium oxide (which forms on Zincalume) so reflectance remains >90%. See https://wp.optics.arizona.edu/optomech/wp-content/uploads/sites/53/2016/10/An-Overview-of-Aluminum-Protective-Coating-Properties-and-Treatments.pdf Figure 3 on page 5.

Also FYI, solar radiation at earth's surface (250-2,500nm) has the average proportion:
9% of ultraviolet light (250-400nm)
42% visible light (380-700nm)
49% (700-2,500nm)

So >90% of 91% of solar radiation is ~81% which is the basically that top figure quoted.

What will reduce it, and may be the cause of the lower 60% reflectance, is coastal/salt corrosion and the like.

Paul1977:

I think those usually only apply to roofs with a pitch of 10 degrees or more (but can't remember where I read that, so might be wrong). This will just be a standard low pitch monopitch type shed roof so hopefully won't be an issue, but if I had to repaint it I could.

That is of interest for me if that is true.

We have a 15 degree slope on the roof we want to paint with high-reflectance paint, maybe even silver. It will make for easier maintenance of our evacuated-tube solar hot-water system if we can get rid of the reflective strips which corroded quite quickly.

raytaylor

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#2639294 20-Jan-2021 23:58

Paint it gloss white I find to be the most effective.

Ray Taylor

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mattwnz

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#2639300 21-Jan-2021 01:07

I am seeing a lot of dark coloured roof and clad houses at the moment with their skylights fully open and the heat pumps on during the hot summer days. I went into the roof /storage space of a grey friars roofed house recently, and it was so hot.

timmmay

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#2639362 21-Jan-2021 07:21

From here: “Research shows that light coloured roof tiles can lower the air temperature in the roof space by up to nine degrees compared to dark tiles.”

Nine degrees is a lot if you have to go up there.

Paul1977

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#2639401 21-Jan-2021 08:56

I contacted ColorSteel and according to them plain Zincalume is TSR 67.4% LRV 65.1%, compared to Titania TSR 67% LRV 69%. Since there isn't much difference they suggested Titania would be the better choice as the painted steel will last longer.

neb

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#2639473 21-Jan-2021 11:46

Kraven:
We have two sheds at home, one is Karaka green and the other is unpainted zinalume. Both get uncomfortably hot inside

Karaka Green looks good but is a terrible colour for heat absorption, not quite as bad as black but it's getting up there. For a new build I'd definitely avoid that. We went with Gull Grey which isn't quite as good as Titania, but Titania looks very white while Gull Grey is still noticeably grey and fits in with other roofs better.

Hammerer

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#2639486 21-Jan-2021 12:22

neb:

Karaka Green looks good but is a terrible colour for heat absorption, not quite as bad as black but it's getting up there. For a new build I'd definitely avoid that. We went with Gull Grey which isn't quite as good as Titania, but Titania looks very white while Gull Grey is still noticeably grey and fits in with other roofs better.

We have Karaka and it may as well be black because the difference is negligible. It turns our ceiling space into a 70C oven on sunny summer days. Even with 30-50cm of insulation, I can still feel the heat radiate from our ceiling at the hottest time of the year. That's a major reason to go to the other extreme with a light roof paint.

neb

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#2639497 21-Jan-2021 12:53

Hammerer:
We have Karaka and it may as well be black because the difference is negligible. It turns our ceiling space into a 70C oven on sunny summer days. Even with 30-50cm of insulation, I can still feel the heat radiate from our ceiling at the hottest time of the year.

If there's that much heat up there, say a 40 degree difference between that and outside air, could you run a Stirling engine to push outside air through the ceiling space? It'd self-regulate, once the space was cool enough it'd stop running.

Hammerer

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#2639505 21-Jan-2021 13:06

neb:
Hammerer:

We have Karaka and it may as well be black because the difference is negligible. It turns our ceiling space into a 70C oven on sunny summer days. Even with 30-50cm of insulation, I can still feel the heat radiate from our ceiling at the hottest time of the year.

If there's that much heat up there, say a 40 degree difference between that and outside air, could you run a Stirling engine to push outside air through the ceiling space? It'd self-regulate, once the space was cool enough it'd stop running.

That would be fun but we've already decided against such options because we need to pain the roof anyway.

We've painted the exterior of the house mid blue so a light roof colour will look good and remove the heat problem reducing at least half and maybe two-thirds of the heat.