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Old 10-07-2015, 05:56 PM   #1
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Post Exterior foam roof insulation

After researching this forum, I am starting to lean towards adding insulation to the exterior roof of my bus. There are several posts that indicate this is possible, but I haven't seen many photos or stories of people actually doing it. George Myers is quoted a lot (Bus Nut Online - Articles Of Interest - Super Insulating the Roof - by George Myers) and I have a couple of ideas, but I have a few questions left and would love to hear from anyone on the forum with experience.



Background: 98 Thomas Saf-t-liner, 39ft long, 8ft wide, currently has 6.5ft of interior headroom. I would love to do a roof raise, but it will not be possible this time. (My next skoolie will definitely have a roof raise) Skoolie will likely only be driven 2 times after completion.



Idea:
  • Remove the interior metal roof panels
  • Remove the current roof insulation
  • Maybe fill in or re-rivet the interior rivet holes to make it look nicer
  • Paint the new interior roof (if necessary)
  • Add insulation to exterior roof
  • Cover insulation with Elastomeric Acrylic coating
  • Secure the exterior roof insulation edges against wind

Questions:
  • Should I go with spray insulation or rigid foam or a mix of the two?
  • What rigid foam would be easiest to glue?
  • What thickness would be sufficient? The acrylic coating is supposed to reflect 90% of the light away, and the foam will be a thermal break.
  • What kind of glue would work with rigid foam on metal?
  • How much glue would be required?
  • How much does the rigid foam bend? Can it be scored to bend around the bus wall edges? Should the score marks be filled in with spray foam?
If I went with spray foam, I would probably have to do it myself. Iím quite handy, and luckily Iím only working with flat exterior surfaces spraying downwards, so I should be ok. Still the spray foam costs start to get real high real quick. That is why I keep returning to the rigid foam. I just donít have a lot of experience with it. Any help would be greatly appreciated.
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Old 10-07-2015, 05:59 PM   #2
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Old 10-07-2015, 08:39 PM   #3
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You will never get the thermal break you need by insulating on top of the metal.

Thermal break means your living space inside is isolated from the steel the bus is made from.

The link you posted is old. It's from before the time of spray foam.

If you do want to insulate the top of a bus, spray foam is the way to go.

Commercial buildings use 6 pound spray foam sprayed right over the roofing material. Then they seal the surface of the foam with a UV proof paint.

On a bus, it will need to be shaved smooth for visual appeal. After spraying it looks quite wavy.



Every question your asking have been asked before. There is no way around a roof raise, and doing a proper internal insulation job. Sorry.

Nat
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Old 10-07-2015, 10:38 PM   #4
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I do not think I have presented the question properly, which is probably why I am not getting an answer.
I am not trying to maximize performance, instead I am trying to maximize value. Plus, I have a few immovable constraints, the chief one being I cannot perform a roof raise.
I know that whatever I come up with will not be as good as a roof raise, so now I am trying to find the solution that is the most valuable.
Imaging it costs $300 to go from 5 inches of insulation up to 6 inches. The performace would increase, but not significantly. The ROI of that 6th inch of insulation is not very high. Now the cost of going from zero insulation up to 1 inch is also $300, but in this case, the ROI would be extremely high. As more insulation is added, the performance keeps going up, but the ROI actually decreases.
Here is a second example. If someone only had $200 to spend trying to cool their bus in Texas, I think that painting their bus white would be better than buying $200 worth of insulation and partially insulating it.
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Old 10-07-2015, 10:51 PM   #5
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I imagine that this problem could be described as a curve. On one end, there is the solution of not insulating at all, and just paying a fortune each month in electricity for AC. That solution has low up front costs, but very high monthly costs. On the other side of the spectrum, I could raise the roof and insulate the crap out of the bus. This would be a lot of work and cost up front, but my monthly bills would be much lower.

I am looking for a solution somewhere in the middle. If I only had $1000 up front for the roof insulation portion of the remodel, and I could not lower the inside height of the bus, what would be the best use of that money?

I'm guessing that the answer would be something like I proposed earlier.
-Remove the inner panel which isn't helping much with insulation and lowers the headroom.
- Add 1/2 in of insulation inside the bus just so the metal is not in direct contact with the air. But this is not enough insulation, and the support ribs are a thermal bridge to the outside.
-So add extra insulation outside the bus so that the ribs are no longer a thermal bridge.
-Paint the foam for protection againts UV, and to reflect as much light as possible.

Rigid foam is cheaper and easier than spray foam, but gluing ridged foam is an unknown to me. It may be super easy, it may be hard. The glue might be easy to find, or it may be extremely expensive.

Spraying foam is pretty hard, and according to this forum, it is especially hard indoors, and above your head. There ends up being a lot of waste too. But spraying outside the bus should be substantially easier. Also, I'm guessing there would be a lot less waste too. So spraying foam on the roof may be worth the extra cost.

Any ideas on how to maximize value given these constraints?
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Old 10-07-2015, 11:54 PM   #6
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Foam board could likely be bent with heat. It's polystyrene, a thermoplastic. Poke around online and look for a hot wire acrylic bender; maybe a similar concept could be applied to extruded and expanded polystyrene boards. A bread toaster or hair dryer from a thrift store might be a good source of nichrome wire for the heating element. Many adhesives will dissolve polystyrene so you'll have to read packages and choose one that is compatible.

It seems Myers' objective was only to slow down the heat from sun shining directly onto the roof. I haven't actually corresponded with him at all, but if I may put words in his mouth.. Probably the ambient air temperature was fine; it's just that radiant heat on the roof that he wanted to reduce. The foam cap on top probably did a fine job of it. In the article he doesn't say anything about heating or air conditioning..

My goal is to build a 4-season RV and I believe nat_ster's is to build a 4-season full-timer. With that in mind I'm looking to thermal mass inside to control temperature swings and insulation to control heat gain/loss. The heat "short circuit" allowing heat on the walls to conduct up into the roof, around the exterior insulation, would be a big problem for our builds. If your objective is like Myers', to slow how quickly the sun heats the bus, an insulation cap on top would probably help. Radiant heat gain can be significant. But later in the day when the bus body is thoroughly heat soaked the insulation on top won't be much help and afternoon/evening cooling could be challenging.

Maybe there's enough summer heat left where you are to do an experiment. Try simply shading part of the roof by parking partially under shade or with a sheet of something over the top (important: have space for air to circulate above the roof, below the shade thing). Separately, try forming a sheet or two of foam board onto the roof and compare with the shade result. The foam won't be a loss either way; if it doesn't help on the outside it can be stuck to the inside of the ceiling instead.

We all can agree that top-only exterior insulation won't be the most ideal. But you have constraints that make any other approach difficult. All I can suggest is to try Myers' approach, preferably on a small scale first, and decide for yourself whether the benefit merits the cost.

Removing the interior metal skin and insulating at least to the depth of the ribs would surely be a good first step, and the headroom loss for this is nearly zero (you'd probably want a new ceiling.. plastic sheet, FRP, luan plywood, etc, so there's 1/8 to 1/4 inch loss). Whatever more you can spare to insulate over the ribs too would help.

I'd probably start with painting the roof white and insulating as much as you can "afford" (in terms of remaining interior height) and see where you're at when cooling season comes.

Supposing your walking and standing are all done in the center aisle.. what about keeping the insulation thinner there, and tapering it to be thicker out towards the walls? Kind of a "best of both worlds" approach.
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Old 10-08-2015, 12:38 AM   #7
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I'm out.

I have nothing further to add to this thread.

Sorry I was not able to help you.

Good luck, please post pics what ever direction you take.

Nat
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Old 10-08-2015, 01:33 AM   #8
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I'm having a real hard time imagining how foam on the exterior could last on a bus for more than a few weeks (?) Even if it were coated with something like Gelcoat or fiberglass. And trying to get a reasonable finish is a tough job even for a pro.
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Old 10-08-2015, 08:09 AM   #9
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Have you checked out this thread?
http://www.skoolie.net/forums/f13/in...roof-9099.html
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Old 10-08-2015, 09:30 AM   #10
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Pour hot, really hot coffee into that thin styro cup.
Barely warm to the touch, right? Doesn't take much really.
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