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Originally Posted by Andrewclimbs
Yet another derailment. The reading comprehension in this thread has been surprisingly poor! I have gotten some useful information in my private messages though, thanks for that! Understandably regular members of this forum who have actually read my posts and are open to this idea don't want to publicly get on the wrong bandwagon.
1. The windows are sealed
2. The main purpose of the theoretical roof would be more insulation than would otherwise be possible, mostly for the cold months.
3. It would barely be visible from the ground and would add very little weight/bulk
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When everyone is having problems with 'reading comprehension', maybe it isn't the readers that are the problem. Your initial post made it clear your primary concern was the window leaks, and your primary method of addressing the leaks was this roof idea of yours. Now they've magically gone from 'inherently leaky' to 'sealed'

. Backtrack all you want, but understand you're contradicting your orginal post, which means any misunderstandings are yours to own. No-one here gets paid to offer free advice. You might want to keep that in mind as you diss the folks taking time out of their day in an effort to help out a stranger (you).
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Originally Posted by Andrewclimbs
As for the ceiling, that's interesting and does reflect well on your thermal break. In my experience using a half inch thermal break over the ribs the ceiling has not been great at insulating in the winter. I'd be curious to see what temperature reading you get under the ribs versus between them. I think that's been covered pretty thoroughly in other threads though.
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How would you know what impact heat transfer through the ribs, and whatever you have covering them, has on the overall insulation level of your bus? That's like picking one hole out of a sieve and blaming it for the sieve's inability to hold water. Your experience is meaningless unless you have the ability to actually measure or calculate the heat gain/loss through the ribs, while also being able to measure/compare the heat gain/loss through everything else, so you can compare the two and determine whether it's a significant fraction worthy of your time/effort/money to address.
FYI the temperature read off a surface is not sufficient to answer these questions. Consider a 1/2" copper rod lying next to a 12" diameter copper pipe, both at the same temperature. Which one do you think is capable of transferring more heat over a given period of time? There exists no amount of insulation that will prevent the internal temperature from becoming the same as the external temperature (excluding radiant heating effects, etc) given enough time without additional inputs. If you don't understand this, you don't understand how insulation works.
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My main question is whether closed cell spray foam can be injected into a small horizontal space like that. Maybe I should have just said that and left out the details
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Spray foam insulation is not designed for blind recesses / enclosed spaces as far as I'm aware, though maybe there are variants that are. I can't speak from experience but personally I'd be concerned with the potential for unfilled voids, warping/distortion of the materials surrounding the foam, eventual delamination, and possibly inconsistent / incomplete curing. Probably a better question for a professional outfit though.