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Old 04-29-2019, 07:41 PM   #1
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Join Date: Apr 2019
Location: Myrtle Beach, SC
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Spray Foam Under

We have a head room issue inside the bus. Can we spray foam bottom/underneath the original metal bus floor? Havenít seen any posts about this, so figuring there must be a reason why ppl donít insulate from under?

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Old 04-29-2019, 08:23 PM   #2
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The problem is that the foam doesn't last. The underside of the bus is. . the underside of a bus. It gets all the road debris, grime, and water thrown at it that the bus drives over. You've got water, chemicals, and physical damage/stuff kicked up from the road trying to sandblast the foam away. There's also a lot of stuff under a bus, so it's somewhat hard to get good coverage - it only takes a few holes to negate the overall effectiveness of your insulation.

It might be possible to somehow build some type of covering or shielding for the foam - but at that point, I think it's about as easy to do a roof raise (even a moderate one of a few inches.)
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Old 04-29-2019, 09:20 PM   #3
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Crawl under your bus and see what you're dealing with. There's lots of things in the way of getting a uniform coverage .
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Old 04-29-2019, 09:24 PM   #4
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Originally Posted by hjaxsun View Post
We have a head room issue inside the bus. Can we spray foam bottom/underneath the original metal bus floor? Haven’t seen any posts about this, so figuring there must be a reason why ppl don’t insulate from under?
Thanks to the high thermal conductivity of steel (it's basically the opposite of an insulator), the metal floor of your bus will always be the same temperature as the roof and the walls (not to mention the chassis which along with the engine is an even more massive chunk of metal). So if the air outside is 10 degrees F all the metal of your bus will be that temperature as well, and any spray foam insulation on the underside will have no effect on this.

If anything, spray foam on the underside would be a negative in hot weather in direct sunlight, as it would inhibit radiation of heat on the underside - the shaded and (relatively) coolest part of the bus. Fortunately, it would probably not stay attached to the underside long enough to matter.
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Old 04-29-2019, 10:31 PM   #5
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I have thought about using something like lizard skin under the bus to help create a bit of a barrier - I have seen that it was done on some buses - but can't find anyone who has tried it, but I can sure get opinions about it. My floor has two coats of rustoleum, a layer of water/ice barrier, reflectix, 1/2 plywood, 4 coats of paint, vinyl planks.
My floor will generally be close to outside temperature - my walls and ceiling are factory insulated and do pretty well.

For example: It's 34 degrees out right now (I'm in Montana) - my floor is at 51 degrees (including my stairwell - which is a single piece of sheet metal, not insulated) - the carpet areas on my floor are at 61 degrees. My ceiling is 75 in the front of the bus and 65 in the back (I have a wood stove in the front burning). The ribs at the ceiling in the back are at 55.
My walls are at 63 degrees.
The outside metal of the bus is at 41 degrees - but will likely cool quickly as the sun is down.

I am not sure that helps any one - it's just what it is in my bus. I never have a consistent temperature on the bus. The back is always colder in the winter and warmer in the summer. The doorway is drafty. The windows are cold and drafty - except in the summer, they get really hot.

But, it's a bus. And it's home. And I like the variety.

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