Energy Matters Forums

[sandystone]

I recently measured the temperature of a dark tile roof, and a white colourbond roof.
The dark was 70 degrees, and the white was 41 degrees.
While reflective foil does limit the heat getting inside the roof, it is rarely installed with ideal circumstances, with the required 25 mm air gap. Much heat still gets through via thermal bridging.

Reflecting heat, before it has a chance to enter the roof space is a far better option than trying to stop absorbed heat from entering a roof space...... which wont happen.

A white roof and well installed reflective foil will massively reduce air conditioning requirements.

[TerryB]

Just a thank-you note to everyone who replied to my original post. We ended up going with "Shale Grey" as the colour on the roof - not quite a white but a light grey that was both acceptable to my wifes colour taste (very important!!!) and also much lighter in colour than either the old roof or the colour she originally chose for the roof

Due to all the rain we have had up here the roof replacement was delayed several times so it wasn't done until the start of April so now the weather has cooled off and will have to wait for next summer for a definite conclusion (of course we also installed some better insulation whilst the roof was off so I will never know for sure how much of the difference is due to the roof colour and how much is the insulation). I did go up on the roof on a hot sunny day both before and after it was done and the temperature difference between the old roof (which was dark blue) and the new was substantial - basically the new one I could touch with bare skin without it feeling overly hot whilst the old one was too hot to touch comfortably after the sun had been on it for any length of time

[TsaTsa2CV]

I have used the color bond of the north facing roof of our studio/workshop to our advantage for winter heating, at least when the sun shines.

The horizontal 65mm steel purlins form the cavity between the Woodland Gray wall sheet and the mesh tensioned between the pulins to maintain the reflective foile 65mm from the sheet. A stud frame flush with the 200mm C-column on the inside so that wall lining continues on the studio side provides the second cavity for R-3.5 insulation. The outside, 2 above each other, (hot) air cavities are 9m long and fitted with 2 small air extraction fans drawing 30W combined. They are switched on and off by a simple thermostat ($20.00 EBay) as soon as air temperature exceeds 27oC.

My observation to date have exceeded all expectations. I have observed digital readout temperatures in excess of 60oC pumped into the studio. 40oC with outdoor temperatures around 2oC and as soon as a cloud hovers in front of the sun the fans turn off. We are off the grid up here in Buxton and with the 5.55kW try to make the most of every bit of sunshine. It would be interesting to calculate the number of kWh of heat produced on a winters day with an input of about (7hx30W) 210W.
The beauty is the setup works in winter with the sun low and little heat absorption in summer as the sun is high and only drawing power when the sun is shining.
I have no doubt that every roof could be designed to to contribute a great deal of energy in winter to reduce the heating bill and in summer to use natural convection to cool the hause during the day and use the cold generated by the roof by absorbing the night time cold for the house either by shutters or small fans.

[bpratt]

Went and painted a brown shipping container 'Thredbo White', and it has made a huge change. On hot days you can still work in it, without breaking in to a sweat as you once used to by simply walking in to it.

Colour makes a huge difference, and this just proves it again.

When we finally sell here and move, the shed at the new house will at least have a 'Thredbo White' colourbond roof, and the rest if of it will be a very light colour.

[caleb]

I performed some tests on a standard 3 bed house design in Melbourne, using FirstRate 5 house energy rating software to see how much effect the roof colour made. Here are the results:

Light colour roof
Energy required for heating: 98.3 MJ/m2
Energy required for cooling: 38.5 MJ/m2
Star rating: 6.0 Stars

Dark colour roof
Energy required for heating: 92.9 MJ/m2
Energy required for cooling: 45.2 MJ/m2
Star rating: 5.9 Stars

It can be seen that the light colour roof had a greater effect on reducing the overall heating/cooling energy required for the home (and thus a slightly better energy rating). Although the heating requirements were slightly higher (most likely due to the roof structure not absorbing as much solar energy as a dark roof), the reduction in cooling requirements more than made up for this.

This simulation was performed on a house located in Kinglake just outside of Melbourne - which is a cool climate zone. In a warmer climate zone such as Queensland, the effects of a lighter colour roof would be even more beneficial.

[Gordon-Loomberah]

It's interesting that only a relatively small difference (~ +/-3%) is made to the heating energy requirements, and about 3X that for cooling, no doubt due to the roof being well insulated. Can you run simulations with lower, more typical of older houses, insulation values? What insulation value was assumued for the above result? Tiles or Colourbond makes a difference too I think, I've been in dark tiled roof houses that just don't cool down much at night in summer, as the tiles keep radiating heat onto the ceiling.

What internal and external (I assume those are BOM averages) temperatures are assumed to arrive at those results?

[Tracker]

I performed some tests on a standard 3 bed house design in Melbourne, using FirstRate 5 house energy rating software to see how much effect the roof colour made..

Out of curiosity, is this a software analysis, and what actual "Measurements" were taken..

I recall when one of those "I'll do a free energy analysis" guys insisted on his share of tax-payer dollars, he simply looked and took my word about insulation etc.

I can't see how such a product would be useful in determining roof colour.. .. Sorry - just the cynic in me..!
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[caleb]

Tracker, this was a software analysis performed on a conceptual 6-Star house 'off the plan'. The house is yet to be built, however detailed working drawings were used for the assessment.

Gordon, the house has a colorbond roof with R3.5 insulation and 1 layer of reflective sisalation under the roofing sheets. This is fairly typical for a new house in Melbourne.

The FirstRate 5 software is built on the Accurate platform which has been developed by the CSIRO. It works off average external temperatures calculated for every hour of the year in 61 climate zones around Australia. I assume this data has been sourced from BOM, but cannot verify this.

To arrive at the energy requirements for heating and cooling of the house, the software has inbuilt parameters which cannot be altered. Depending on the climate zone, the software assumes occupants will use artificial heating or cooling to maintain a certain 'comfortable/acceptable' temperature range for each type of room and at certain hours when each room is likely to be occupied. A range of occupant behavior assumptions are also built in, such as opening a window (and calculating the internal temperature and air speed) to try and make the room reach an acceptable temperature before air-conditioning is used.

Off course all the occupant behavior assumptions and 'comfortable/acceptable' temperatures will not apply to everyone, however this has been used simply to set a benchmark to compare the relative thermal efficiency of the building under a common use scenario.

[Tracker]

The FirstRate 5 software is built on the Accurate platform which has been developed by the CSIRO. It works off average external temperatures calculated for every hour of the year in 61 climate zones around Australia. I assume this data has been sourced from BOM, but cannot verify this.

Given the sad history of modeling, one can only hope that this one does work...
Problem - How do you prove it? .. .. or, is it all relative anyway, and if we all use the same software, the relative benefits will be correct..

One silly curiosity...
In this "Nanny-State" where we are now required to use Blue-Painted building products to minimise glare for builders, I find it interesting that there is no LAW that says...
"..and the maximum reflectance of ANY surface finish shall be......" ... ie precluding silver or white paints..

We now have Church Leaders requiring parishioners to sign agreements that under OHnS, they will not step on anything higher than a toad-stool..etc. etc.
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[bpratt]

All I can say is that since I painted our plain zincalume roof "Thredbo White", there is a significant reduction in overall heat inside the house during summer time.

I've since then painted half the garage roof (ran out of paint at the time, and never got back to it), and that isn't quite as hot as it once used to be.

We've also got a shipping container that was reddy brown in colour, and whenever the sun came out, it became unbearably hot, and that's pretty normal for a shipping container. I went beserk and painted it in the same Dulux Weathershield as the house roof, and now I can go in to it at anytime and it remains comfortably cool. The only change is the colour of shipping container, no insulation whatsoever, no special coatings.


All I can say is that if you have the choice of roof colour for your house, then get a very light colour for it. Sure it won't be quite as warm in winter, but insulate properly, and what heat you generate inside the house will stay inside.