Painting a dark roof white - any experience?

Forums › Home Workshop DIY › Painting a dark roof white - any experience?

potscrubber

25 posts

Geek

#279642 29-Oct-2020 16:01

Hi there

This may seem a strange forum to post this question, but it's one where I am interested in & value the opinions of this particular community.

Our house - 1957 Rimu weatherboard / brick villa - has a very northern aspect. The living areas have huge windows for the house style, with a brick exterior on that side. The Colorcote zincalume roof is 10 years old, and a dark karaka colour. It is a low pitch, with vast areas of roof pointing directly at the summer sun. The roof cavity has a grab bag of different pink batts, probably about 15 years old.

The living areas have become unbearable in summer, despite blinds, great natural airflow, ceiling fans.

Obviously mitigating this requires a multi-pronged approach. We will probably end up with some form of aircon & solar (why not).

But I'm interested if anyone here has painted their dark roof white or light coloured, and if it made a real world difference, or if you think it's a myth, or over-stated.

Any input graciously received, thank-you!

Hammerer

2379 posts

Uber Geek

Lifetime subscriber

#2593478 29-Oct-2020 16:24

Same for us.

We have a 1960 rimu house with a Karaka steel roof which replaced an older asbestos roof. The asbestos was a good insulator; the steel is not. So we got a lot of heat in summer and a lot of cold in winter. We could feel the heat radiating from the ceiling on the sunny summer days.

We fixed most of the problem by adding more insulation - a thick polyester blanket - over all the joists and over the old pink batts insulation which was only placed between the joists. Since then our summers have been cooler and our winters a lot warmer.

We are planning to paint over the karaka with a silver colour some time in the next decade and expect a small drop in summer heat.

Sharesies

Affiliate link

Affiliate link: Trade NZ and US shares and funds with Sharesies.

frankv

5118 posts

Uber Geek

Lifetime subscriber

#2593482 29-Oct-2020 16:30

I don't know about roofs, but I can tell you that the colour of the walls is hugely significant to how hot they get.

Many years ago, my ski club (along with all the others) was compelled by the Tongariro National Park Board to paint our lodge a dark brown, so it would be less visible. (Which it did, at shortish distances. However at longer distances, reflections from the the planar surfaces stood out just as much as orange or white or silver or any other colour. But I digress.) Subsequently we had issues with doors and windows jamming and such-like, as the north-facing walls heated and expanded relative to the south-facing walls. We then used a data logger to record the temperatures of white, old light brown, and mandatory dark brown paint on the north-facing wall. The floppy disk with the data on is long gone, I'm sorry to say, so, from memory, we found that the dark paint got IIRC to 65C in the daytime, whereas the white topped out IIRC about 20C. The dark paint was also significantly colder at night than the white. This was in August, at about 4800ft, with top air temperatures about I guess 15C.

So I think that painting your roof white would probably help a lot. I wonder whether some ventilation into your ceiling space would help too? You might even be able to get some convective cooling... hot air escaping out the vent would draw cool air into the house.

potscrubber

25 posts

Geek

#2593483 29-Oct-2020 16:32

Thanks for your input Hammerer. Do you remember what product you used, it's R rating? Part of our issue is that the pitch is so low, especially towards the walls, and the roof such a mess of timber, that installation will be v. difficult, especially towards the walls.

Gordy7

1537 posts

Uber Geek

ID Verified

Lifetime subscriber

#2593484 29-Oct-2020 16:36

I repainted a west timber wall to our bedroom that copped sun from 1pm until 8pm.

The previous paint had a solar reflectance of 23% and the new paint about 80% reflectance.

Made a significant drop in wall temperature and meant we could get to sleep earlier.

So I would say worth a shot with your roof too.

Edit:

I used Wattyl Solagard Low Sheen...

Dark Greenish - Blixen Formula 59762 LRV23%

Off white - Lamb’s Fleece LRV77%

Gordy

My first ever network connection was a 1MHz AM crystal(OA91) radio receiver.

kotuku4

440 posts

Ultimate Geek

Lifetime subscriber

#2593489 29-Oct-2020 16:57

I have used NOVAtherm polyester insulation blanket, 200mm thick R3.2, over existing pink batts. The brands have similar ratings, just the price and availability to choose this brand, self installed.

Have permanent green Zincalume roof, needed repair/repaint within 10 year warranty and is coming up for another paint, will consider lighter colour or reflective roof paint. Was warned off Karaka as it had fading and peeling issues.

Have a double garage that had permanent green steel roller doors, repainted Titania. As garage baked in summer, heat radiating from the doors.

potscrubber

25 posts

Geek

#2593494 29-Oct-2020 17:08

kotuku4:

Hi

I have used NOVAtherm polyester insulation blanket, 200mm thick R3.2, over existing pink batts. The brands have similar ratings, just the price and availability to choose this brand, self installed.

Have permanent green Zincalume roof, needed repair/repaint within 10 year warranty and is coming up for another paint, will consider lighter colour or reflective roof paint. Was warned off Karaka as it had fading and peeling issues.

Have a double garage that had permanent green steel roller doors, repainted Titania. As garage baked in summer, heat radiating from the doors.

Thanks for the input. Yeah the karaka has faded a little bit. But not as much as our first re-roof! True story, we were quoted and sold on the Colorcote product. Less than 2 years later, the karaka colour was a powdery, shedding tan brown. Turned out, what the (local, small town) roofers had installed was "Kiwisteel" (i.e. NOT "kiwi" product). Product substitution. We made them replace it.... but I digress.

neb

6599 posts

Uber Geek

Trusted

Lifetime subscriber

#2593501 29-Oct-2020 17:42

frankv:
I wonder whether some ventilation into your ceiling space would help too? You might even be able to get some convective cooling... hot air escaping out the vent would draw cool air into the house.

Or use the heat difference between the ceiling space and the outside air to run a Stirling engine that ventilates the space. When it reaches equilibrium, the engine stops by itself.

Bung

4633 posts

Uber Geek

#2593506 29-Oct-2020 17:56

While you are still at the thinking about it stage check that your local Council doesn't have any restrictions on roof colour.

neb

6599 posts

Uber Geek

Trusted

Lifetime subscriber

#2593508 29-Oct-2020 18:02

Our new roof will be Gull Grey, TSR 56%, LRV 50%, to match the existing grey roof. There seem to be different ways of measuring this, the highest LRV for the Colorcote scale I've got is Off White at 67% so Lamb's Fleece at 77% sounds very high. Karaka, for comparison, is 8%, just above Black at 5%.

Hammerer

2379 posts

Uber Geek

Lifetime subscriber

#2593513 29-Oct-2020 18:33

kotuku4:

I have used NOVAtherm polyester insulation blanket, 200mm thick R3.2, over existing pink batts.

We also used this, from memory.

Our roof space is full of cross bracing and it was a hindrance to laying the blankets. But the polyester is easy to cut and easy to conform to the braces and irregular surfaces. The braces mainly divide the roof space into ~1m wide lanes parallel to the roof peaks so it is easy to cut the blankets a bit wider than the "lane" width (so they can spread around the bracing) and drag them through.

The problem with a low roof is that there is meant to be, from memory, a 30cm ventilation clearance between the insulation and the roof covering. We have a 15 degree pitch with 0.8m eaves so we could extend the insulation to the edge of the living zone but not into the eaves.

The blankets work much better than laying another layer of pink batts over the joists. I've relaid pink batts in a few houses - both between the joists and over the joists - but I wouldn't recommend this. The glass wool compresses too easily and doesn't recover its original loft. Also smaller rectangles of insulation are too easily moved. Whereas the polyester springs back quite quickly and larger blankets don't move around easily.

neb

6599 posts

Uber Geek

Trusted

Lifetime subscriber

#2593514 29-Oct-2020 18:38

neb: There seem to be different ways of measuring this, the highest LRV for the Colorcote scale I've got is Off White at 67% so Lamb's Fleece at 77% sounds very high.

It's not different ways of measuring, it's magic! Colorcote Bulletin #4, September 2015:

ColorCote® Colour LRV Value %

Pacific Cloud 51.78

Pacific White 49.90

Colorcote Bulletin #4, July 2020:

ColorCote® Colour LRV Value %

Pacific White 77

Pacific Cloud 69

Others have changed as well, but those are the most drastic.

Hammerer

2379 posts

Uber Geek

Lifetime subscriber

#2593520 29-Oct-2020 18:54

We don't plan to paint the light silver directly over the karaka (dark olive). We'll use a primer to make sure the karaka doesn't show through.

We'll probably use a mid-gray primer because that will also reduce any possible impacts of the different paint layers expanding/contracting at different rates. We probably don't have to worry about delamination with light over dark, I just prefer to have a stepped transition between different colours. It is a definite problem to paint dark over light on a roof because the dark outer coat will heat up faster and will expand more than the light colour underneath.

mattwnz

18770 posts

Uber Geek

#2593530 29-Oct-2020 19:37

So many new houses get almost black roofs installed. Third forms science people should have learnt that black roofs will make the inside very hot. We installed sandstone grey and it doesn't have the problem. Yet many are probably only having the minimum insulation installed, rather than having a high R value. So they then need heat pumps to keep the house cooled in summer.

Also I found that out that our electrician bunched up the ceiling insulation when they install downlight electrical wiring, so you end up with big thermal holes in the ceiling insulation envelope. We had this problem, even though we had IC downlights installed. Above this, there were still holes in the insulation where the electrician had pulled the wire across to the light bunching up the insulation, and you can feel the heat / draft coming through that gap. Many home owners wouldn't even know this had occurred, especially if they have a skillion roof.

mdooher

Hmm, what to write...
1329 posts

Uber Geek

ID Verified

Trusted

Lifetime subscriber

#2594035 30-Oct-2020 15:34

My neighbour painted over his Karaka Green 5 rib roof with white roof paint from a basic Wagna spray gun... you know the little ones you get at bunnings. I thought he was mad.... anyway the paint job has outlasted him and looks a good as it did the day he did it 5 years ago.

He swore it made the place cooler in summer.. so not scientific, but interesting none the less

Matthew