Roof Colour Choice

[Stroudwater1]

A boater by us at the marina used a set of hoses and pumps during the heatwave in 2022. This shifted a decent volume of water onto the roof. His roof was white. 
 

He reported it was fairly successful but he did offer us some of the hoses to do some engine work so he didn’t obviously plan a repeat last year. 
 

 

[PeterF]

10 hours ago, Stroudwater1 said:

A boater by us at the marina used a set of hoses and pumps during the heatwave in 2022. This shifted a decent volume of water onto the roof. His roof was white. 
 

He reported it was fairly successful but he did offer us some of the hoses to do some engine work so he didn’t obviously plan a repeat last year. 
 

 

Just don't do this on your white boat when moored near Harecastle or Worsley.

[DandV]

22 hours ago, Stroudwater1 said:

A boater by us at the marina used a set of hoses and pumps during the heatwave in 2022. This shifted a decent volume of water onto the roof. His roof was white. 
 

He reported it was fairly successful but he did offer us some of the hoses to do some engine work so he didn’t obviously plan a repeat last year. 
 

 

During the hot 2013 summer I did consider buying a plastic water butt to put on the roof as far forward as possible and connecting a cabin top length of irrigation soaker hose via a valve, to dribble water onto the crown of the roof to shed heat by evaporation and flow off the sides.

Perhaps aided by laying towels on the roof. A variation on the Australian Coolgardie safe. 

Would though have needed constantly refilling by a bucket from the canal.

Decided instead, like every one else, just to seek out moorings under trees.

But I did try gently bucketing water over the roof at times. It was surprising how hot the water that flowed off got. But the discernable  heat gain by heaving water over the roof more then offset any discernable change in cabin temperature.

[Francis Herne]

I think the two earlier are talking past each other a bit. Not sure if blackrose's understanding is wrong or just imprecisely described.

• The roof usually transfers much more heat in summer than winter for the same differential in air temperature. This might be unexpected.
• The asymmetry is that in summer much of the heat arrives by radiation, heating the warm side far above the warm air temperature. In winter there's not much radiated heat inside the boat (except very near the stove) so the warm side is roughly equal to the air temperature. The cold side is close to cold air temperature in both cases.
• The asymmetry in material layers (steel-foam-plywood, say) has little effect. An 'inside-out' plywood-foam-steel roof shouldn't behave differently in a steady state if the outside is painted with the same colour/finish.

"on the inside the foam doesn't have a sheet of hot steel bonded to it and is only dealing with warm air" can be read as either of those, so it's either wrong or it's not 😛

 

I stretched a tarpaulin over the south-facing side, and part of the roof not covered by solar panels, to keep direct sunlight off which helped quite a bit. My panels are spaced off the roof and shade much it quite well. The sticky-backed kind seem like the worst possible case for gaining heat into the roof.

 

I've vaguely dreamed of having a cooling loop with tubes welded or glued to the walls, roof and baseplate. Water would flow in a helical pattern down the boat. Perhaps it could incorporate or replace some of the usual bracing. Could never be retrofitted and does have some obvious potential drawbacks.

Edited November 14, 2023 by Francis Herne

[IanD]

12 hours ago, Francis Herne said:

I think the two earlier are talking past each other a bit. Not sure if blackrose's understanding is wrong or just imprecisely described.

• The roof usually transfers much more heat in summer than winter for the same differential in air temperature. This might be unexpected.
• The asymmetry is that in summer much of the heat arrives by radiation, heating the warm side far above the warm air temperature. In winter there's not much radiated heat inside the boat (except very near the stove) so the warm side is roughly equal to the air temperature. The cold side is close to cold air temperature in both cases.
• The asymmetry in material layers (steel-foam-plywood, say) has little effect. An 'inside-out' plywood-foam-steel roof shouldn't behave differently in a steady state if the outside is painted with the same colour/finish.

"on the inside the foam doesn't have a sheet of hot steel bonded to it and is only dealing with warm air" can be read as either of those, so it's either wrong or it's not 😛

 

I stretched a tarpaulin over the south-facing side, and part of the roof not covered by solar panels, to keep direct sunlight off which helped quite a bit. My panels are spaced off the roof and shade much it quite well. The sticky-backed kind seem like the worst possible case for gaining heat into the roof.

 

I've vaguely dreamed of having a cooling loop with tubes welded or glued to the walls, roof and baseplate. Water would flow in a helical pattern down the boat. Perhaps it could incorporate or replace some of the usual bracing. Could never be retrofitted and does have some obvious potential drawbacks.

 

Which was exactly the case I used as an example, a roof at 45C in summer (heated by the sun) and -5C in midwinter (air temperature) with the inside of the boat (air temperature) at 20C in both cases -- since the temperature difference across the roof is the same in both cases, so is the heat flow regardless of how the "roof sandwich" is built and which side is hot or cold.

 

If the roof gets even hotter than 45C in summer the heat flow will obviously be bigger, and if it's less cold in winter than -5C the heat flow will be smaller, but the principle remains. Adding an external sun shield (like the tarp, or solar panels) spaced away from the roof will keep the roof cooler, the sunlight gets the shield *really* hot but the roof stays cooler so long as there's a big air gap in between.

 

Either way, it can get far too hot inside a narrowboat in an extended summer heatwave even with thick sprayfoam insulation, and ventilation doesn't help if the air outside is also hot (e.g. above 30C)... 😞

 

The only real solution is some kind of added cooling, for example using cool water from the canal (as in the post above) or evaporative cooling as posted earlier, or an air conditioner -- but to be effective these take significant electrical power, which is also in short supply on many boats without having a *lot* of solar panels.

Edited November 15, 2023 by IanD

[Tacet]

2 hours ago, IanD said:

 

Which was exactly the case I used as an example, a roof at 45C in summer (heated by the sun) and -5C in midwinter (air temperature) with the inside of the boat (air temperature) at 20C in both cases -- since the temperature difference across the roof is the same in both cases, so is the heat flow regardless of how the "roof sandwich" is built and which side is hot or cold.

 

If the roof gets even hotter than 45C in summer the heat flow will obviously be bigger, and if it's less cold in winter than -5C the heat flow will be smaller, but the principle remains. Adding an external sun shield (like the tarp, or solar panels) spaced away from the roof will keep the roof cooler, the sunlight gets the shield *really* hot but the roof stays cooler so long as there's a big air gap in between.

 

Either way, it can get far too hot inside a narrowboat in an extended summer heatwave even with thick sprayfoam insulation, and ventilation doesn't help if the air outside is also hot (e.g. above 30C)... 😞

 

The only real solution is some kind of added cooling, for example using cool water from the canal (as in the post above) or evaporative cooling as posted earlier, or an air conditioner -- but to be effective these take significant electrical power, which is also in short supply on many boats without having a *lot* of solar panels.

The point you omitted earlier was that the arrangement of the materials has a significant impact on the roof temperature for a given air temperature.  

[Francis Herne]

1 hour ago, Tacet said:

The point you omitted earlier was that the arrangement of the materials has a significant impact on the roof temperature for a given air temperature.  

It doesn't. A roof made solely of (symmetrical) Celotex panels would show the same effect.

 

The usual steel/plywood/insulation layers shuffled in any order (but keeping the same thickness of each) will behave identically in constant sun.

The only thing to make a difference is surface colour/finish, which isn't closely tied to the material.

 

One with steel on the inside, rather than the outside, would heat up slightly slower in the mornings and cool slightly slower in the evenings because the steel's heat capacity is then inside the insulation. Compared to the heat capacity of other things in the boat I don't think the difference would be very noticeable.

 

[Tacet]

4 minutes ago, Francis Herne said:

It doesn't. A roof made solely of (symmetrical) Celotex panels would show the same effect.

 

The usual steel/plywood/insulation layers shuffled in any order (but keeping the same thickness of each) will behave identically in constant sun.

The only thing to make a difference is surface colour/finish, which isn't closely tied to the material.

 

It does.  For the reason you mention - the various materials have different surface colours and finishes.

[Francis Herne]

19 minutes ago, Tacet said:

It does.  For the reason you mention - the various materials have different surface colours and finishes.

 

The difference in surface finish of paint over steel/wood/fibreglass/cheese won't be significant compared to the colour and choice of gloss/matt/anti-slip paint, which is totally unrelated to the material underneath.

Edited November 15, 2023 by Francis Herne

[Ronaldo47]

The apparent  paradox of  black robes being  preferred by desert nomads was discussed in the letters section of the  "New Scientist" several decades ago (circa 1980 I think).   From memory, given  their loose fitting robes, one view was that the heat promoted convection currents in the outer layers of their garments, thereby removing sweat more effectively, but it was a long time ago that I read it, and the discussion stretched over several.issues. 

Edited November 15, 2023 by Ronaldo47
typos

[Tacet]

1 hour ago, Francis Herne said:

 

The difference in surface finish of paint over steel/wood/fibreglass/cheese won't be significant compared to the colour and choice of gloss/matt/anti-slip paint, which is totally unrelated to the material underneath.

We can agree on that - my point is that the order of the materials will have an effect, not that this is the only or main factor.

 

In any event, fibreglass (quilt) , cheese and sprayfoam are typically self-coloured and finished.  It is rather unlikely all the materials making up a narrowboat superstructure will have similar colours and finishes, so there will be appreciable differences in absorbed radiation depending on the order in which they are arranged.

[IanD]

34 minutes ago, Tacet said:

We can agree on that - my point is that the order of the materials will have an effect, not that this is the only or main factor.

 

In any event, fibreglass (quilt) , cheese and sprayfoam are typically self-coloured and finished.  It is rather unlikely all the materials making up a narrowboat superstructure will have similar colours and finishes, so there will be appreciable differences in absorbed radiation depending on the order in which they are arranged.

 

Still wrong -- the amount of solar radiation absorbed (and the temperature the surface gets to) only depends on the top surface colour and finish, because these determine how much light/infra-red is reflected and how much is absorbed and converted to heat -- the sun can't "see" what's under the surface. If this surface is a given colour and roughness of paint it doesn't matter what's underneath.

 

Once the solar radiation has been absorbed and converted to heat, how it travels through the roof only depends on the thermal resistance, which depends on the materials and their thicknesses and how they're joined together (good or bad thermal contact), but *not* the order that they're in. This is also true for heat absorbed from the air by conduction.

 

This is basic thermodynamics, it's how radiation and conduction work respectively.

Edited November 15, 2023 by IanD

[Francis Herne]

10 minutes ago, IanD said:

How it travels through the roof only depends on the thermal resistance, which depends on the materials and their thicknesses and how they're joined together (good or bad thermal contact), but *not* the order that they're in.

 

Well, only if they're in direct contact. Any air gap between layers and the reflectivity/emissivity of the surfaces will matter (hence shiny multi-layer foil insulation). Not very applicable to a sprayfoamed boat but with a curved roof lined with flat panels of foil-surfaced insulation board there should be some difference.

 

A couple of earlier posts imply (as I read them, however intended) that the sequence of materials in the roof is the primary difference between heat gain in summer and loss in winter, which is what I was trying to refute. If there is a difference for this reason I don't think it has a noticeable effect compared to other factors.

Edited November 15, 2023 by Francis Herne

[IanD]

20 minutes ago, Francis Herne said:

 

Well, only if they're in direct contact. Any air gap between layers and the reflectivity/emissivity of the surfaces will matter (hence shiny multi-layer foil insulation). Not very applicable to a sprayfoamed boat but with a curved roof lined with flat panels of foil-surfaced insulation board there should be some difference.

 

A couple of earlier posts imply (as I read them, however intended) that the sequence of materials in the roof is the primary difference between heat gain in summer and loss in winter, which is what I was trying to refute. If there is a difference for this reason I don't think it has a noticeable effect compared to other factors.

 

We're agreeing here on both points, hence my comment about "how they're joined together (good or bad thermal contact) and the order as such not mattering, only the materials/thicknesses/interfaces... 🙂

[Tacet]

35 minutes ago, IanD said:

 

Still wrong -- the amount of solar radiation absorbed (and the temperature the surface gets to) only depends on the top surface colour and finish, because these determine how much light/infra-red is reflected and how much is absorbed and converted to heat -- the sun can't "see" what's under the surface. If this surface is a given colour and roughness of paint it doesn't matter what's underneath.

 

Once the solar radiation has been absorbed and converted to heat, how it travels through the roof only depends on the thermal resistance, which depends on the materials and their thicknesses and how they're joined together (good or bad thermal contact), but *not* the order that they're in. This is also true for heat absorbed from the air by conduction.

 

This is basic thermodynamics, it's how radiation and conduction work respectively.

Which bit is wrong?

 

If you have a matt black material on one face of a sheet of anything and a silver surface material on the opposing face, the order in which they face the sun will have a bearing on the solar radiation absorbed and therefore the amount of heat transferred.  It's basic thermodynamics.

 

Paint is no more or less than another material in the sandwich.

 

[IanD]

12 minutes ago, Tacet said:

Which bit is wrong?

 

If you have a matt black material on one face of a sheet of anything and a silver surface material on the opposing face, the order in which they face the sun will have a bearing on the solar radiation absorbed and therefore the amount of heat transferred.  It's basic thermodynamics.

 

Paint is no more or less than another material in the sandwich.

 

The bit that you keep saying which is that order of the materials matters.

 

If the paint finish is the same it doesn't matter what's underneath, it will absorb the same amount of heat from the sun -- because anything that's not reflected turns to heat, and reflection only depends on the surface (colour/roughness) not what's underneath. White paint reflects more than black so it absorbs less so it stays cooler.

 

Once this has happened, it doesn't matter what order the sandwich of materials underneath are in, the rate of heat conduction will be the same, and the temperature of top and bottom surfaces will be the same.

 

This isn't the same as saying they'll *feel* as hot if you touch them, this depends on the conductivity of the bit you touch -- for example steel under the paint will feel hotter to the touch than wood even though it's at the same temperature. This is how materials like aerogels and ceramic thermal tiles work (and charcoal for firewalkers), even if they're hot the conductivity is low so they don't burn you.

 

I can't see how to explain this more clearly, but it's definitely the way that heat propagation works... 🙂

Edited November 15, 2023 by IanD

[Francis Herne]

36 minutes ago, IanD said:

We're agreeing here on both points, hence my comment about "how they're joined together (good or bad thermal contact) and the order as such not mattering, only the materials/thicknesses/interfaces... 🙂

If the interfaces contain air gaps, then radiation comes into play internally rather than just conduction, and the order (in terms of which faces are adjacent to the air gap(s), and the temperature and emissivity of those faces) matters.

 

It all comes down to quibbling about what exactly is meant by "the materials" and "the order".

Edited November 15, 2023 by Francis Herne

[Tacet]

27 minutes ago, IanD said:

The bit that you keep saying which is that order of the materials matters.

 

If the paint finish is the same it doesn't matter what's underneath, it will absorb the same amount of heat from the sun -- because anything that's not reflected turns to heat, and reflection only depends on the surface (colour/roughness) not what's underneath. White paint reflects more than black so it absorbs less so it stays cooler.

 

Once this has happened, it doesn't matter what order the sandwich of materials underneath are in, the rate of heat conduction will be the same, and the temperature of top and bottom surfaces will be the same.

 

This isn't the same as saying they'll *feel* as hot if you touch them, this depends on the conductivity of the bit you touch -- for example steel under the paint will feel hotter to the touch than wood even though it's at the same temperature. This is how materials like aerogels and ceramic thermal tiles work (and charcoal for firewalkers), even if they're hot the conductivity is low so they don't burn you.

 

I can't see how to explain this more clearly, but it's definitely the way that heat propagation works... 🙂

OK.  We agree if we include the fresh condition that the paint finish remains the same. 

 

But that was neither my assumption nor stated to be yours in earlier posts - and I am unsure why it is a relevant condition in a discussion on thermodynamic principles

 

To respond to your final paragraph, stating critical assumptions would help you explain yourself more clearly.

 

 

[IanD]

56 minutes ago, Tacet said:

OK.  We agree if we include the fresh condition that the paint finish remains the same. 

 

But that was neither my assumption nor stated to be yours in earlier posts - and I am unsure why it is a relevant condition in a discussion on thermodynamic principles

 

To respond to your final paragraph, stating critical assumptions would help you explain yourself more clearly.

 

 

I thought I did -- and also that you didn't... 😉

 

It's very easy to misinterpret what people write on t'internet... 🙂

Edited November 15, 2023 by IanD

[Tacet]

1 hour ago, IanD said:

 

I thought I did -- and also that you didn't... 😉

 

It's very easy to misinterpret what people write on t'internet... 🙂

True enough!

3 hours ago, Francis Herne said:

If the interfaces contain air gaps, then radiation comes into play internally rather than just conduction, and the order (in terms of which faces are adjacent to the air gap(s), and the temperature and emissivity of those faces) matters.

That did occur to me - but I won't pretend it came into my earlier posts.  Something to do with T^4, I think.

 

A slightly (oh so slightly) reduced heat transfer will occur if the materials are arranged so that the air gap is towards the warmer side of the wall.

[Snakey]

A boater nearby (CC/Liveaboard) has a roof covered in turf, complete with wild flowers etc. Leaving aside concerns as to corrosion and changes to the boat's centre of gravity, this would seem to be the ultimate in insulation. I don't know what provision, if any, for keeping it watered is provided but it all looked very verdant during the hot weather 18 months ago.

[nb Innisfree]

Once moored our yacht green nb next to a white one, the difference in temp was amazing, you could fry an egg on our roof, unable to keep my hand on it for more than a second whereas the white roof felt cool in comparison. 

[Ronaldo47]

One morning a couple of decades ago at the freshly-painted wooden balance beam of a lock on the Southern Oxford,  I was able to give my children an impromptu physics lesson in heat transfer.   The black part was bone dry and warm to the  touch. The white-painted end was icy-cold and still covered with drops of water from the previous night's rain.

[manxmike]

Is Hell Exothermic or Endothermic?

The following is an actual question given on a University of Washington chemistry mid-term:

"Is Hell exothermic (gives off heat) or endothermic (absorbs heat)? Support your answer with a proof."

Most of the students wrote proofs of their beliefs using Boyle’s Law (gas cools off when it expands and heats up when it is compressed) or some variant.  One student, however, wrote the following:

First, we need to know how the mass of Hell is changing in time.  So, we need to know the rate that souls are moving into Hell and the rate they are leaving.  I think that we can safely assume that once a soul gets to Hell, it will not leave.  Therefore, no souls are leaving. As for how many souls are entering Hell, let’s look at the different religions that exist in the world today.  Some of these religions state that if you are not a member of their religion, you will go to Hell. Since there are more than one of these religions and since people do not belong to more than one religion, we can project that all people and all souls go to Hell. With birth and death rates as they are, we can expect the number of souls in Hell to increase exponentially.  Now, we look at the rate of change of the volume in Hell because Boyle’s Law states that in order for the temperature and pressure in Hell to stay the same, the volume of Hell has to expand as souls are added.  This gives two possibilities.

1)  If Hell is expanding at a slower rate than the rate at which souls enter Hell, then the temperature and pressure in Hell will increase until all Hell breaks loose.

2)  Of course, if Hell is expanding at a rate faster than the increase of souls in Hell, then the temperature and pressure will drop until Hell freezes over.

So which is it ?  If we accept the postulate given to me by Ms. Therese Banyan during my Freshman year that "It will be a cold night in Hell before I sleep with you,"  and take into account the fact that I still have not succeeded in having sexual relations with her, then (2) cannot be true, and thus I am sure that Hell is exothermic.

The student got the only A.

 

Just thought this was appropriate . . . . . . .

[wakey_wake]

 

On 09/11/2023 at 09:02, 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.

On 09/11/2023 at 09:07, Jen-in-Wellies said:

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.

 

I like that idea. ☃️

Planning ahead for summer 2024, should I order my snow from Midland Chandlers (one off delivery in late May, hope that an early order will freeze the price) or will I need top-ups from the fuel boat? 🤣

Can I get a preservative to stop it going off?

 

On 13/11/2023 at 22:14, The Black Jack 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.

When I was last far too hot in mine (2021), the air temperature difference inside-to-out was five or ten °C. Then I was away for the hot parts of summers, sorry I forgot the details.

I concluded that extra air flow was needed, and a chimney wasn't practical, so I did get as far as obtaining a car radiator cooling fan.

Without covering mesh, it's a bit scary-spinny-choppy and will need some kind of box.

 

However I think the noise and power draw shouldn't be a problem: put it near the batteries, rely on plenty of solar, and sit up the other end.

 

 

On 15/11/2023 at 17:20, IanD said:

Once the solar radiation has been absorbed and converted to heat, how it travels through the roof only depends on the thermal resistance, which depends on the materials and their thicknesses and how they're joined together (good or bad thermal contact), but *not* the order that they're in. This is also true for heat absorbed from the air by conduction.

 

It's kinda simple and complicated.  The heat arrives and the various thermal conductivities decide where it ends up. "In theory, theory and practice are the same; in practice they aren't" ( https://quoteinvestigator.com/2018/04/14/theory/ ).

 

What seems weirdly magical is the self-cooling paint.

...work in progress, but many people are keen on the idea.  It's likely to glare somewhat, across a lot of the spectrum.