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Old 11-03-2019, 04:58 AM   #1
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External Spray Insulation?

Hey Skoolie Fanatics,
I was watching more YT build videos and I came across a lovely couple's build video, they are not raising the roof for added height. For their new floor they are just using ply and the composite flooring to minimize the added depth to the interior. Their plan is to spray foam the undercarriage and then top that with a sort of sealant to protect the foam.

Has anyone else seen, tried or competed a job like this?
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Old 11-03-2019, 05:37 AM   #2
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There's too much stuff under a bus to insulate very effectively from undeneath. Folks do it, but its not really ideal at all. The floor is still thermally bridging.
External foam would probably work but man would it ever be ugly. Much uglier than a roof raise.
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Old 11-03-2019, 06:33 AM   #3
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Hey Skoolie Fanatics,
I was watching more YT build videos and I came across a lovely couple's build video, they are not raising the roof for added height. For their new floor they are just using ply and the composite flooring to minimize the added depth to the interior. Their plan is to spray foam the undercarriage and then top that with a sort of sealant to protect the foam.

Has anyone else seen, tried or competed a job like this?
The metal part of your bus is like a giant radiator that has vastly greater capacity to shed heat (or absorb it in summer) into the surrounding air than you have capacity to generate it inside. Spray-foaming a part of the exterior (e.g. the floor and/or the roof) will accomplish effectively nothing for this reason - it would slightly impair the ability of your bus' metal to shed heat, but it would still have more than enough capacity (through all the external parts that aren't covered) to near-instantly get rid of the heat you're generating inside.

The way to keep warm inside a metal structure is to have insulation between the living space and the metal of the structure (i.e. inside). Then, the fuel consumption of your heater will be determined by the rate at which heat flows through the insulation before it reaches the metal. The rate of heat flow through metal is a couple of orders of magnitude (i.e. 100 times or more) greater than the rate through insulation, which is why having a thermal break between your living space and the metal is so important. Insulating outside the bus is basically the exact opposite of achieving a thermal break.
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Old 11-03-2019, 02:56 PM   #4
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The metal part of your bus is like a giant radiator that has vastly greater capacity to shed heat (or absorb it in summer) into the surrounding air than you have capacity to generate it inside. Spray-foaming a part of the exterior (e.g. the floor and/or the roof) will accomplish effectively nothing for this reason - it would slightly impair the ability of your bus' metal to shed heat, but it would still have more than enough capacity (through all the external parts that aren't covered) to near-instantly get rid of the heat you're generating inside.

The way to keep warm inside a metal structure is to have insulation between the living space and the metal of the structure (i.e. inside). Then, the fuel consumption of your heater will be determined by the rate at which heat flows through the insulation before it reaches the metal. The rate of heat flow through metal is a couple of orders of magnitude (i.e. 100 times or more) greater than the rate through insulation, which is why having a thermal break between your living space and the metal is so important. Insulating outside the bus is basically the exact opposite of achieving a thermal break.
I'm working with a bus that has about 6'4" of headroom. I was thinking about 2 inches of foam board on the floor and 2 inches of spray foam, or whatever thickness I can get and between the roof and the ceiling panel, and then four or more inches of foam on the outside of the roof. I was thinking of the building ribs that I would attach to the existing roof that would mimic the arch of that roof. Then I would spray foam on the exterior of the roof and shape it to the new roof line as determined by the ribs. I would then install a new exterior skin over the those ribs. I think I could finish it out nicely at the front and back ends with custom fiberglass pieces, such that most people could not tell that it was not a factory design. With the original metal skin of the roof sandwiched as I have described it would pretty much be neutralized as a heatsink or radiator.
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Old 11-03-2019, 06:04 PM   #5
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speaking of thermal bridging I measured the ceiling temp this morning in my bus. 35 degrees outside, inside about 65. The ceiling temp measured with an infrared thermometer was about 70 give or take a few degrees everywhere but where the roof beams are. They were 10 degrees cooler. My ceiling is 2 " fiberglass, original to the bus.

I have spray foamed under railroad cars. What a mess that can be, and it made very little difference in the heat needed.
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Old 11-03-2019, 10:50 PM   #6
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speaking of thermal bridging I measured the ceiling temp this morning in my bus. 35 degrees outside, inside about 65. The ceiling temp measured with an infrared thermometer was about 70 give or take a few degrees everywhere but where the roof beams are. They were 10 degrees cooler. My ceiling is 2 " fiberglass, original to the bus.

I have spray foamed under railroad cars. What a mess that can be, and it made very little difference in the heat needed.
The two buses I have experience with both had what looks to me like polyester batting in the walls and in the ceiling. This includes my current bus. I would like to see some authoritative source where I could find the insulative value of that material.

As a person with some good experience insulating homes, I am aware how important the details are in an insulation job. For instance gaps in the coverage can really compromise the effectiveness of the insulation, as can the failure to have a proper vapor barrier when that feature is a necessary component of the total insulating package. In conventional installations a vapor barrier is necessary on the inside face of the insulation to prevent warm moist air from condensing on the inside of the exterior wall and in the cooler layers of insulation. All of the factory insulation jobs I have seen so far are poorly executed and have no vapor barrier. The way buses are constructed they also have a metal structures with metal skins on the interior and exterior, which are very hard to insulate, as was noted earlier in this thread. Undoubtedly the best answer for insulating buses, is to use spray foam in the cavities, plus enough foam or a layer of rigid foam insulating board to separate the structure from the interior wall. Closed cell spray foam has the additional benefit of acting as the perfect vapor barrier.

The factory insulation on a bus is about as close to nothing as you can get.
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Old 11-04-2019, 09:29 AM   #7
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The metal part of your bus is like a giant radiator that has vastly greater capacity to shed heat (or absorb it in summer) into the surrounding air than you have capacity to generate it inside. Spray-foaming a part of the exterior (e.g. the floor and/or the roof) will accomplish effectively nothing for this reason - it would slightly impair the ability of your bus' metal to shed heat, but it would still have more than enough capacity (through all the external parts that aren't covered) to near-instantly get rid of the heat you're generating inside.

The way to keep warm inside a metal structure is to have insulation between the living space and the metal of the structure (i.e. inside). Then, the fuel consumption of your heater will be determined by the rate at which heat flows through the insulation before it reaches the metal. The rate of heat flow through metal is a couple of orders of magnitude (i.e. 100 times or more) greater than the rate through insulation, which is why having a thermal break between your living space and the metal is so important. Insulating outside the bus is basically the exact opposite of achieving a thermal break.

I foamed under my bus. I did it myself. It was a PITA. I have a pusher so I didn’t have to worry about an exhaust pipe or drive shaft. The trans really was no problem. It was the fuel tank that was hard to work around. I wound up filling in the entire gap between floor and tank.

The trick is to cover the floor, gaps, ribs and all. Like this: A28BD223-2D52-4DDE-98AE-B198D06BB60F-1507-000000A771418C88.jpg

Advance Auto parts had a sale on rustoleum undercoating -2 for $5, usually $7+ a can. So I drove around the area and cleaned them out.
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Old 11-04-2019, 11:58 AM   #8
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I really do appreciate the many point of views.
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Old 11-04-2019, 04:54 PM   #9
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Originally Posted by musigenesis View Post
The metal part of your bus is like a giant radiator that has vastly greater capacity to shed heat (or absorb it in summer) into the surrounding air than you have capacity to generate it inside. Spray-foaming a part of the exterior (e.g. the floor and/or the roof) will accomplish effectively nothing for this reason - it would slightly impair the ability of your bus' metal to shed heat, but it would still have more than enough capacity (through all the external parts that aren't covered) to near-instantly get rid of the heat you're generating inside.

The way to keep warm inside a metal structure is to have insulation between the living space and the metal of the structure (i.e. inside). Then, the fuel consumption of your heater will be determined by the rate at which heat flows through the insulation before it reaches the metal. The rate of heat flow through metal is a couple of orders of magnitude (i.e. 100 times or more) greater than the rate through insulation, which is why having a thermal break between your living space and the metal is so important. Insulating outside the bus is basically the exact opposite of achieving a thermal break.
do you have references for that theory?
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Old 11-06-2019, 08:31 AM   #10
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Any images from your under the floor spray foam job... I would think that would have many pro and cons depending on the bus..I insulated somewhat on top of the bus under the solar panels. I can see the benefits.

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Old 11-13-2019, 07:37 PM   #11
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do you have references for that theory?

Read any text book on Thermodynamics!
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Old 11-13-2019, 07:58 PM   #12
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I am not trying to stir the pot, however I do see how insulation on the outside, especially spray foam will create a thermal barrier, and provide very good insulation. as for durability it would need to be protected in some way I would think.

The example I can think of is my own house. It is timber framed, that is large oak timbers pegged together, with stressed skin panels on the outside. a stressed skin panel has foam board bonded to OSB on one side, and often drywall on the other side. We have ceder on the inside. These are tongue and grooved together to create a continuous blanket, and prevent the thermal break that a typical framed wall or roof would have. On our house the foam is 7" thick for the roof, and 5 1/2" for the walls. So with this construction method the entire frame is on the inside and visible.
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Old 11-13-2019, 09:32 PM   #13
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Yeah.. That is nice Ronnie. Thermodynamics has no problem with that.. In general it is better to insulate from the outside so that there is more thermal mass on the inside. That helps with stabilizing the temperature on the inside while it fluctuates on the outside.

As eccb and others mentioned it is hard to do a nice job from the outside.

Good luck. J
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Old 11-13-2019, 10:51 PM   #14
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I am not trying to stir the pot, however I do see how insulation on the outside, especially spray foam will create a thermal barrier, and provide very good insulation. as for durability it would need to be protected in some way I would think.

The example I can think of is my own house. It is timber framed, that is large oak timbers pegged together, with stressed skin panels on the outside. a stressed skin panel has foam board bonded to OSB on one side, and often drywall on the other side. We have ceder on the inside. These are tongue and grooved together to create a continuous blanket, and prevent the thermal break that a typical framed wall or roof would have. On our house the foam is 7" thick for the roof, and 5 1/2" for the walls. So with this construction method the entire frame is on the inside and visible.
The problem is that insulating on top of the roof means only insulating part of the outside. If you insulate the floors and walls of the bus inside, and then insulate just the roof outside, this still allows a huge thermal bridge - heat will flow from your living space into the ceiling ribs (which are not insulated relative to the inside), and from there it will flow along the ribs to the outer walls and windows and floor (which are not insulated relative to the outside) and be radiated away. Insulation on the roof will slow the direct radiation of heat only from the roof, and because the heat still has an easy path to flow to the uncovered parts of the bus (via metal, which conducts heat much faster than insulation or even wood), the net insulating effect will be minimal.

If you insulated the entire exterior of the bus (roof, underside, walls, front and back), it would be effective, in much the same way that the panels you describe on your house are effective. Insulating the whole outside seems a bit impractical, although these guys:



seem to have pulled it off.
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Old 11-14-2019, 07:36 AM   #15
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External foam would probably work but man would it ever be ugly. Much UGLIER than a roof raise.



Charlie, It is nice to hear that from you


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Old 11-14-2019, 08:01 AM   #16
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The problem is that insulating on top of the roof means only insulating part of the outside. If you insulate the floors and walls of the bus inside, and then insulate just the roof outside, this still allows a huge thermal bridge - heat will flow from your living space into the ceiling ribs (which are not insulated relative to the inside), and from there it will flow along the ribs to the outer walls and windows and floor (which are not insulated relative to the outside) and be radiated away. Insulation on the roof will slow the direct radiation of heat only from the roof, and because the heat still has an easy path to flow to the uncovered parts of the bus (via metal, which conducts heat much faster than insulation or even wood),



I am not sure to agree with your analysis. The R factor of 0.5" wood on the inside of the ribs is only 0.5 to 0.75. So the ribs will show up perfectly fine as cold spots... unless you insulate 1 to 2" over the ribs....but that will loose you the precious head space.


Then the surface area of the ribs against the roof and inside ceiling is pretty big being 8 ft long and 2" wide.


Versus the conductivity heat losses thru the cross section of the rib ... say about 6" x1/16" wall thickness.


Then the conductivity heat loss from the ribs to the outer skin where it is not insulated.


So take your poison.



I would think it would be pretty easy to bend 2 layers of 1/2" foam around the bus curves all the way to the windows and tape it with aluminum tape. on the flat section you can add another layer if desired. Putting a thin second skin over it,, rubber or sheet metal could be a lot less complicated then a roof raise... helps with leaks ....save interior space...does not mess with structural strength... lower fire hazard... and probably another couple of reasons....



It all depends on what your goal and your mission is.



I did a small section of my roof with 3/4" aluminum face.. under the solar panels and spare tire .. above the sleeping quarters.. covered it with a thin sheet of stainless .


To the OP.... if you have limited height inside and not ready for a roof raise then I for sure would go for insulation below the floor.




Good luck,, Johan

Later Johan
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Old 11-14-2019, 08:36 AM   #17
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I am not sure to agree with your analysis. The R factor of 0.5" wood on the inside of the ribs is only 0.5 to 0.75. So the ribs will show up perfectly fine as cold spots... unless you insulate 1 to 2" over the ribs....but that will loose you the precious head space.
We are in complete agreement so far.

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Then the surface area of the ribs against the roof and inside ceiling is pretty big being 8 ft long and 2" wide.

Versus the conductivity heat losses thru the cross section of the rib ... say about 6" x1/16" wall thickness.

Then the conductivity heat loss from the ribs to the outer skin where it is not insulated.
I think you're saying that more (most?) heat is lost through conduction from ribs to roof to outside air than is lost from ribs to walls/floor to outside air, which would make insulation on the roof at least somewhat effective. But even if this is true on an uninsulated bus (which of course it is), when you prevent radiation from one area of the skin (the roof) this increases the temperature there - which increases the rate at which heat flows to colder parts of the skin (thermal conductivity increases both with temperature differential and with absolute temperature).

OP, insulating part of the outside of your bus will be a difficult technical challenge with a very limited benefit, if any.
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Old 11-14-2019, 11:25 AM   #18
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I guess we can agree to disagree..... Most builders do not have the headroom to insulate over the inside of the ribs except for 1/2" wood... That means with the outside roof being cold the inside rib being cold as well and only separated by 1/2 wood and some air.



I am not saying that most heat is in whatever system you choose, only that there are more sides to the problem and more solutions to the problem.


and that the solutions depend on what you want to achieve and what your restraints are.


Depending on how thorough or how poorly you insulate on the inside or outside of the box it will do about the same.



We are in the same field.....you like to take on challenging projects with a bus with a lot of rust and spend your time and money there. Others might like to spend there time and money to insulate the bottom of the bus because they do not want to raise the roof( that would be me if it would apply to my bus)
Since we both have to live with our "informed" decisions and not visa versa we can be both happy with our choices..


Good luck Johan
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Old 11-14-2019, 01:53 PM   #19
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I'm picturing the heat loss from uninsulated ribs to be quite minimal compared to what is saved by filling the fields of the roof and wall cavities. Has anyone ever measured the temps of the ribs against the field. I think much ado about nothing.
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Old 11-28-2019, 10:50 PM   #20
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I'm working with a bus that has about 6'4" of headroom. I was thinking about 2 inches of foam board on the floor and 2 inches of spray foam, or whatever thickness I can get and between the roof and the ceiling panel, and then four or more inches of foam on the outside of the roof. I was thinking of the building ribs that I would attach to the existing roof that would mimic the arch of that roof. Then I would spray foam on the exterior of the roof and shape it to the new roof line as determined by the ribs. I would then install a new exterior skin over the those ribs. I think I could finish it out nicely at the front and back ends with custom fiberglass pieces, such that most people could not tell that it was not a factory design. With the original metal skin of the roof sandwiched as I have described it would pretty much be neutralized as a heatsink or radiator.
If I was going that route I wouldn't bother doing anything between the existing roof skins.
My guess is that you might accomplish more with one inch of foam on the floor walls and ceiling from the inside.
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