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Old 10-07-2015, 04:56 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, 04:59 PM   #2
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Old 10-07-2015, 07:39 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, 09:38 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, 09:51 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, 10:54 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 10-07-2015, 11:38 PM   #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, 12:33 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, 07:09 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, 08:30 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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Old 10-08-2015, 12:20 PM   #11
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Originally Posted by EastCoastCB View Post
Yes, I did see that thread, and it gave me a lot of hope that this idea was feasible. But on that thread, there were only photos of buildings. I have also seen people spray foam the outside of shipping containers with great results.

So I think the concept is sound.
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Old 10-08-2015, 12:41 PM   #12
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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.
UV light would degrade both the spray foam and the rigid foam, so they would have to be painted with some sort of solar protective coating. The elastomeric coatings are designed to handle temperature swings, rain, and UV light.

The next concern is vibrations when driving. The spray foam sticks to everything and it can handle the vibrations just fine. Also, I only plan on driving the bus twice in 4 years, so I'm not overly concerned with travel. It wouldn't cost too much to repair, or even start over.

As for the finish, I only want to foam the roof of the bus which I doubt anyone will ever see. There are some specialty tools for smoothing spray foam. With those and a belt sander, I should be able to get the roof smooth enough to paint.

I do not want to foam the exterior walls of the bus, I think that would be much harder and would not look good. So my main concern with appearance is the bend between the roof and the wall. That part is visible from the ground.
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Old 10-08-2015, 01:02 PM   #13
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Thanks everyone for chipping in. It really helps to bounce ideas around. So far, I am convinced that insulating the outside, while not ideal, will still be worth the money. It should cost around $600 for the foam, and $150 for the solar elastomeric coating. So with a 4 year lifespan, it only needs to save me $15/month off my electricity bill to pay for itself. I think it will save me much more than that.

But one question that remains, is it better to go with rigid foam or spray foam:

Spray foam advantages:
-Sticks really well
-Proven technology

Spray foam disadvantages
-Higher cost
-More work
-Riskier/more things could go wrong
-Most be done all at once

Rigid foam advantages
-Cheaper
-Easier to do in sections
-Doesn't need smoothing
-Easy to find materials
-Easy to paint

Rigid foam disadvatages
-What kind of glue to use?
-How much does the glue cost?
-Will the glue still hold after temperature fluctuations?
-Do I need a special sprayer to apply the glue?
-Need to cut the foam to fit around the long metal ridges going down the length of the roof.


I lot of this discussion has leaned towards the spray foam, but I still think that the ROI of rigid foam would be higher. I would need 10-12 panels of 4x8 rigid foam at about $10 each. Even if I had to buy 20 cans of spray adhesive, the cost should be under $100. I might need to score the foam to get it to bend around the edges of the roof, and I might need to fill those gaps with Great stuff, but that shouldn't take more than 4 hours and should be fairly easy to trim. Then I could just paint over the whole thing. I would still need to apply insulation to the inside of the bus, but I think that spending $250 on outside insulation would really help a ton.

What are your thoughts?
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Old 10-08-2015, 01:03 PM   #14
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If you are going to use foam board you will want to use several thinner sheets of it with a layer of glue, plain old white glue, in between them. Foam and well many things are more flexible in thinner layers. Basically you want to make plywood-esque foam for the top. you will need to cover it in something or it will deteriorate in the sun. As for attaching it to the bus though I don't know what would stick to the metal and not melt the foam.

If I were to try to foam the outside of a bus I would probably get 1.5x.5 steel square tube and weld them to the roof 16 inch on center (wood 2x2 could work). Then spray foam in between. Cover that with metal roofing and elastomeric over that.

Quick question, how do you plan on using your skoolie? Is it going to be a non moving cabin or are you using it as an on the road RV? How long do you plan on using your skoolie for? Where do you plan on using this rig at?

You say it is only going to be driven 2 times after completion, how long are those drives going to be 50ft or 500 mi?

..shouldn't take more than 4 hours and should be fairly easy to trim.<<<<I seriously doubt this. Take your time and do it right. It seems like you have already made one large time suck by getting a rig that wasn't built for what you wanted and are now trying to make it the way you want.
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Old 10-08-2015, 01:29 PM   #15
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EIFS (exterior insulation and finishing system), "stucco" to most people, often uses foam boards held to a building with screws. They use plastic washers like these to spread the force so the screws don't tear through. Maybe these washers and self-drilling screws could work.

Adhesive might be better because it doesn't make holes in the metal. In a brief search online for "eps adhesive" I'm finding a 3M 78 spray and Liquid Nails LN-604 in a caulk tube. Didn't immediately find a mastic for the job.

If you can figure out who in your area distributes acrylic stucco systems like Dryvit or Senergy, you'll find foam board, tape, and adhesives there too. They might not want to sell to you directly, but at least could recommend specific product names and where to get them retail.
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Old 10-08-2015, 01:53 PM   #16
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Quote:
Originally Posted by CaptainInsaneo View Post
Foam and well many things are more flexible in thinner layers. Basically you want to make plywood-esque foam for the top.
I had thought about this, but I had dismissed it because I had too many unknowns regarding adhesives. But I think you may be on to something. It would be much easier to mold thin foam around the contours of the roof. I did some more research on adhesives and I found one that is polyurethane based and should work very well.


Quote:
Originally Posted by CaptainInsaneo View Post
Quick question, how do you plan on using your skoolie? Is it going to be a non moving cabin or are you using it as an on the road RV? How long do you plan on using your skoolie for? Where do you plan on using this rig at?

You say it is only going to be driven 2 times after completion, how long are those drives going to be 50ft or 500 mi?
I am using the skoolie more like a non moving cabin. Once the renovation is complete, I will be driving in 100 miles to an RV park. I've already checked with them, and I should be good to move in next May. I expect to stay in the skoolie for 2-3 years, afterwhich I may want to drive it to a different state. But if I do move, I will likely have to redo the insulation anyways depending on the climate there. I do not plan on staying in a skoolie long term. It is more of an intermediate step to moving into a house. I'm sick of paying exorbitant rent on lousy apartments. And each year, the rent goes up while the quality goes down.
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Old 10-08-2015, 02:01 PM   #17
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Adhesive might be better because it doesn't make holes in the metal. In a brief search online for "eps adhesive" I'm finding a 3M 78 spray and Liquid Nails LN-604 in a caulk tube. Didn't immediately find a mastic for the job.
Thank you for this valuable information. I was originally thinking more of a spray adhesive, but I think you are right, a construction tube type glue should work well.

Loctite 1390595 is polyurethane based, and it only cost $4.67/tube at Home Depot. I'm starting to envision a new idea. I will sketch it up and post it later today.
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Old 10-08-2015, 06:34 PM   #18
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Ok, I think my solution is taking shape. Thanks again to everyone who helped!

Above is a very rough sketch of the concept. Half of the outer frame of the bus can be seen in yellow, and it includes the long metal ribs going down the length of the bus. Inside the bus, I will take 1/4 semi-flexable foam and layer it along the sections with the sharpest curves (the green part).

I will get thicker foam (purple) and glue it to the inside of the bus. I will take the same type of foam and glue it to the outside of the bus, cutting room for the outer roof ribs. (still purple) Then I will layer 1/4 semi-flexible foam on the outer curved section (blue).

Lastly I will get 9-10 foot sections of foam (or metalic bubble wrap or something similar) and glue it over all the other foam and also glue it to the frame. (red part)

I didn't show that part really well in the drawing, so just imaging the red curve extending a few inches down the wall of the bus so that there will be good contact between the frame of the bus and the outermost layer of foam.
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Old 10-08-2015, 06:50 PM   #19
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Updated idea.



I didn't like the exterior bulge the first idea would create. I should be able to avoid it without losing any headroom or much useful space. Also, I can use less glue if I insert thicker foam next to the windows. I would reserve the thin foam (green) only for the sharp curved areas.
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Old 10-09-2015, 09:48 AM   #20
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What will be going over all this foam? Just paint?
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