Roof Colour Choice

[The Black Jack]

Has anyone ever tested whether light coloured roofs really keep boat interiors cooler on hot days?

With a properly insulated boat, I wonder if it makes much difference. A dark roof certainly produces less glare for comfortable cruising.

[nicknorman]

3 minutes ago, Mike A R Powell said:

Has anyone ever tested whether light coloured roofs really keep boat interiors cooler on hot days?

With a properly insulated boat, I wonder if it makes much difference. A dark roof certainly produces less glare for comfortable cruising.

I would say yes. We have a compromise light grey, but the ceiling is warm to the touch in hot sunny weather. "properly insulated" is the thing - narrowboats are a compromise due to the maximum distance between the bottom of the canal and the underside of the bridge/tunnel and so there isn't as much room for ceiling insulation as one would like, unless you are an oompa loompa.

 

And the dark coloured sides of the boat do get a lot hotter than the ligher roof in direct sun.

Edited November 8, 2023 by nicknorman

[Mike Tee]

Also textured paint or non slip will not glare as much - probably why many use raddle red on the roof

[robtheplod]

When i repainted our roof i noticed a difference. it was a matt green and painted it with a green 'gloss' finish - the boat is now much cooler.

[GUMPY]

Light colours are cooler to the touch.

Barbie Pink is particularly cool😎

[The Black Jack]

9 hours ago, nicknorman said:

I would say yes. We have a compromise light grey, but the ceiling is warm to the touch in hot sunny weather. "properly insulated" is the thing - narrowboats are a compromise due to the maximum distance between the bottom of the canal and the underside of the bridge/tunnel and so there isn't as much room for ceiling insulation as one would like, unless you are an oompa loompa.

 

And the dark coloured sides of the boat do get a lot hotter than the ligher roof in direct sun.

Interesting. The reason I thought I'd ask what boaters thought was that the physics involved is likely more complicated than it appears on the surface. Some desert nomads traditionally wear black robes.

[Mike Tee]

19 minutes ago, robtheplod said:

When i repainted our roof i noticed a difference. it was a matt green and painted it with a green 'gloss' finish - the boat is now much cooler.

That’s interesting - choice of reduced glare or reduced temperature!

[David Mack]

23 minutes ago, GUMPY said:

 

Barbie Pink is particularly cool😎

This one probably is!

 https://www.instagram.com/p/CmBsrbXD56_/?utm_source=ig_web_button_native_share

 

[Jen-in-Wellies]

6 minutes ago, David Mack said:

This one probably is!

 https://www.instagram.com/p/CmBsrbXD56_/?utm_source=ig_web_button_native_share

 

It also has a white roof and gunwales that day, so will be particularly cool inside. Which is why the stove is lit.

[rusty69]

2 minutes ago, Jen-in-Wellies said:

It also has a white roof and gunwales that day, so will be particularly cool inside. Which is why the stove is lit.

I always find with a thick layer of snow on the roof the boat is noticeably warmer than without. I am thinking of trying it in the summer months to see if it has the opposite effect.

[Jen-in-Wellies]

3 minutes ago, rusty69 said:

I always find with a thick layer of snow on the roof the boat is noticeably warmer than without. I am thinking of trying it in the summer months to see if it has the opposite effect.

Me too. A quick on-line trawl suggests fresh snow has a lot of air trapped between the flakes, so works very well as insulation. Also, reflects a lot of light from the surface too.

[robtheplod]

1 hour ago, Mike Tee said:

That’s interesting - choice of reduced glare or reduced temperature!

Never had an issue with glare... but i have lots of solar panels on the roof!

[PeterF]

Our tiller is chrome, when the boat surfaces are very hot on a summer day the reflective tiller is cool. Therefore you need to mirror coat your roof to keep it cool.

[GUMPY]

1 hour ago, PeterF said:

Our tiller is chrome, when the boat surfaces are very hot on a summer day the reflective tiller is cool. Therefore you need to mirror coat your roof to keep it cool.

Think of the uses for a mirrored roof 🥴

[Jen-in-Wellies]

1 hour ago, PeterF said:

Our tiller is chrome, when the boat surfaces are very hot on a summer day the reflective tiller is cool. Therefore you need to mirror coat your roof to keep it cool.

Have a new boat specified with a polished stainless steel roof.

[Clanky]

I repainted ours using SML deck paint in deep cream, it was dark green previously, the difference was quite noticeable and the glare was minimal as it was a matt finish still looks good 2 years later.

[ditchcrawler]

1 hour ago, Clanky said:

I repainted ours using SML deck paint in deep cream, it was dark green previously, the difference was quite noticeable and the glare was minimal as it was a matt finish still looks good 2 years later.

My roof is painted with Epifane's grey and the if you wash the roof the water always turns grey 

[Slim]

My roof was originally Midnight Blue with aggressive anti slip grit in it. Some years later I overpainted it in Oxford Blue. It was noticeably cooler on sunny days following the re-paint. Make from this what you will. 🖌️🖌️ 

[Grassman]

Go through a lock on a hot day and feel the difference between the black and the white paint on a lock gate beam and you'll see how much cooler the white painted parts are.

[blackrose]

On 09/11/2023 at 06:29, Mike A R Powell said:

With a properly insulated boat, I wonder if it makes much difference. 

 

There's some confusion here about how insulation works. With the sun beating down on a steel roof on the underside of which a layer of foam insulation is bonded, all the foam can do is retard the time taken for the heat to get through. Everything tends towards equilibrium so once the heat gets through the foam isn't doing very much other than retarding the "next" lot of heat trying to get through but by that time it's already hot in the boat. 

 

In winter it's the same, however sprayfoam works much more efficiently in retarding the time taken for heat to escape from a boat than it does in summer trying to stop heat getting in. That's simply because on the inside the foam doesn't have a sheet of hot steel bonded to it and is only dealing with warm air.

Edited November 10, 2023 by blackrose

[IanD]

11 minutes ago, blackrose said:

 

There's some confusion here about how insulation works. With the sun beating down on a steel roof on the underside of which a layer of foam insulation is bonded, all the foam can do is retard the time taken for the heat to get through. Everything tends towards equilibrium so once the heat gets through the foam isn't doing very much other than retarding the "next" lot of heat trying to get through but by that time it's already hot in the boat. 

 

In winter it's the same, however sprayfoam works much more efficiently in retarding the time taken for heat to escape from a boat than it does in summer trying to stop heat getting in. That's simply because on the inside the foam doesn't have a sheet of hot steel bonded to it and is only dealing with warm air.

 

And I'm afraid you're adding to the confusion...

 

Your statement assumes there's no way to get hot air out of the boat, and also ignores thermal mass and thermal time constants -- if the insulation slows down the rate of heat flow and therefore temperature rise by enough, the inside of the boat won't get as hot during the day and will cool down at night. It's true that the *average* temperature doesn't change, but the maximum temperature in the daytime is lower -- and the minimum in the night is higher, that's how lowpass filters (which is what this is) work.

 

For this to be effective you need enough insulation to slow down the rate of temperature change so inside never gets as hot as outside during the day -- and never gets as cool as outside during the night as a result.

 

But you can change this by keeping windows/vents closed during the day when it's hot, and opening them at night when it's cool.

 

Your last statement is just plain wrong, at least according to the laws of thermodynamics which AFAIK still apply to boats... 😉

 

(heat flow in both directions only depends on the temperature difference between inside and outside -- if this is bigger during daytime (hot outside, cool inside) than at night (cold outside, warm inside) then the heat flow will be faster, but this is nothing to do with materials or direction)

Edited November 10, 2023 by IanD

[blackrose]

25 minutes ago, IanD said:

 

And I'm afraid you're adding to the confusion...

 

Your statement assumes there's no way to get hot air out of the boat, and also ignores thermal mass and thermal time constants -- if the insulation slows down the rate of heat flow and therefore temperature rise by enough, the inside of the boat won't get as hot during the day and will cool down at night. It's true that the *average* temperature doesn't change, but the maximum temperature in the daytime is lower -- and the minimum in the night is higher, that's how lowpass filters (which is what this is) work.

 

For this to be effective you need enough insulation to slow down the rate of temperature change so inside never gets as hot as outside during the day -- and never gets as cool as outside during the night as a result.

 

But you can change this by keeping windows/vents closed during the day when it's hot, and opening them at night when it's cool.

 

Your last statement is just plain wrong, at least according to the laws of thermodynamics which AFAIK still apply to boats... 😉

 

(heat flow in both directions only depends on the temperature difference between inside and outside -- if this is bigger during daytime (hot outside, cool inside) than at night (cold outside, warm inside) then the heat flow will be faster, but this is nothing to do with materials or direction)

 

No I didn't assume there's no way to get air out of the boat, please don't put words in my mouth. That was obviously your assumption, not mine. Yes insulation slows down the rate at which heat can pass though it but on a boat in direct sunlight (which is what the OP was talking about)  on a hot day there's often no way to add enough thickness of insulation in order to slow that rate adequately. 

 

Have you actually tried keeping windows and vents closed on a hot summer day in direct sunlight? It will stay cool until about midday and then it will be roasting. Also you seem to be contradicting yourself here because if you close all the windows and vents your won't be able to get heat out of the boat as you previously said.

 

My last statement wasn't wrong at all. It's you who's wrong because materials and direction definitely do come into it and I have a PhD in materials science to back that up. The entire cross section of a boat roof isn't symmetrical so the insulation has to work much harder in summer than it does in winter. Plus there's much more heat coming from one direction than the other.

 

It's actually you who's adding to the confusion here. By all means feel free to think whatever you want even though it's wrong. I really can't be bothered to argue with you about it because having seen how you get on with people here I don't think you're worth talking to let alone arguing with.

Edited November 10, 2023 by blackrose

[IanD]

30 minutes ago, blackrose said:

 

No I didn't assume there's no way to get air out of the boat, please don't put words in my mouth. That was obviously your assumption, not mine. Yes insulation slows down the rate at which heat can pass though it but on a boat in direct sunlight (which is what the OP was talking about)  on a hot day there's often no way to add enough thickness of insulation in order to slow that rate adequately. 

 

Have you actually tried keeping windows and vents closed on a hot summer day in direct sunlight? It will stay cool until about midday and then it will be roasting. Also you seem to be contradicting yourself here because if you close all the windows and vents your won't be able to get heat out of the boat as you previously said.

 

My last statement wasn't wrong at all. It's you who's wrong because materials and direction definitely do come into it and I have a PhD in materials science to back that up. The entire cross section of a boat roof isn't symmetrical so the insulation has to work much harder in summer than it does in winter. 

 

It's actually you who's adding to the confusion here. By all means feel free to think whatever you want when though it's wrong. I really can't be bothered to argue with you about it because having seen how you get on with people here I don't think you're worth talking to let alone arguing with.

 

You may have a PhD in materials science but you've obviously never worked on thermal design of systems and heat flow analysis, or you wouldn't keep making claims like these.

 

The insulation on a boat has to work hard in summer and winter, but this is nothing to do with direction, just the temperature difference between outside (meaning, the steel roof) and inside (meaning, the roof lining). If the roof outside gets very hot in the summer sun (e.g. 45C) and you want to keep the inside at 20C, that's a 25C difference, and heat will flow in through the roof at a rate proportional to this. If the roof outside is very cold in winter e.g. -5C) and you want to keep the inside at 20C that's also a 25C difference, and the heat will flow out through the roof at exactly the same rate as it flows in in summer if the temperature difference is the same.

 

The better the insulation is the lower the heat flux, and the slower the rate at which the temperature will change inside the boat. Direction doesn't matter, only temperature difference -- including any solar heating of the roof. A nice thick covering of snow in the winter will help, snow is a good insulator because it's mostly air.

 

If you don't believe me, get hold of a thermal analysis package (plenty available, some even free) and try this out.

Edited November 10, 2023 by IanD

[The Black Jack]

One thing we seem to all agree on is that steel narrowboats can get uncomfortably hot in full summer sunshine. With all that canal water around, I'm surprised no one has come up with an idea for cooling the roof by evaporation of that water. We just need an efficient way of keeping the roof wet.

[IanD]

16 minutes ago, Mike A R Powell said:

One thing we seem to all agree on is that steel narrowboats can get uncomfortably hot in full summer sunshine. With all that canal water around, I'm surprised no one has come up with an idea for cooling the roof by evaporation of that water. We just need an efficient way of keeping the roof wet.

IIRC someone on the forum tried exactly that in the heatwave last year using wet sheets or similar, and said that it helped.