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Old 02-06-2022, 10:47 PM   #1
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Insulation question

Hello,

Before you tell me not to use fiberglass please read this. My buddy works for John manville and they make this ducting that has a foil backer. They use it for duct work in commercial applications and it is supposed to have good thermal qualities. Hereís the site for the product. Itís the micro aire one.

https://www.jm.com/en/hvac/duct-board/

What do you guys think for insulation in the bus? Would the foil be a good thermal break? Fill in gaps with spray foam? The stuff is very stiff and comes in 10 foot by 4 foot boards. Thanks in advance. I attached a picture of some of it I have in my shop.
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Old 02-07-2022, 05:04 AM   #2
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This stuff would work OK if you already have a bunch of it. The R-value is listed as 6.5 (for 1.5") so it's about as good as EPS foam board but slightly worse than XPS. The foil backing will have no value whatsoever in a skoolie, and you might have some difficulty getting the material to conform to the curved ceiling. I would guess it's more expensive than XPS so I wouldn't use it unless I already had a lot of it.
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Old 02-07-2022, 07:50 AM   #3
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Thank you for the response! I have a ton of it I got from a left over job. More than enough to do the bus. Iím curious about condensation. I already did my strapping and put a layer of cell foam under it. Is there a way I can use this stuff with maybe like a plastic layer between it and the metal to prevent condensation?
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Old 02-07-2022, 12:12 PM   #4
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Yes, vapour barriers are critical along with good controlled ventilation.
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Old 02-07-2022, 01:04 PM   #5
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Aight so what would be the best for the barrier. Just some plastic drop?
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Old 02-07-2022, 10:11 PM   #6
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Yes but be careful not to rip it.

If you find thicker just as wide not too pricey...
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Old 02-08-2022, 09:48 AM   #7
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Originally Posted by Aamafighter View Post
Thank you for the response! I have a ton of it I got from a left over job. More than enough to do the bus. Iím curious about condensation. I already did my strapping and put a layer of cell foam under it. Is there a way I can use this stuff with maybe like a plastic layer between it and the metal to prevent condensation?
Insulation first! The vapor barrier goes on the inside face of the wall/ceiling, closest to the conditioned air. The purpose of a vapor barrier is to keep moisture from condensing on/within the insulation.
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Old 02-08-2022, 09:57 AM   #8
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Quote:
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Insulation first! The vapor barrier goes on the inside face of the wall/ceiling, closest to the conditioned air. The purpose of a vapor barrier is to keep moisture from condensing on/within the insulation.
I am no insulation expert and I’m very appreciative for the advice. I just want to wrap my head around it. If I put the insulation against the metal and then put plastic under it would it not then allow the bottom of the metal to develop condensation on top of the insulation?
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Old 02-08-2022, 10:08 AM   #9
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I am no insulation expert and I’m very appreciative for the advice. I just want to wrap my head around it. If I put the insulation against the metal and then put plastic under it would it not then allow the bottom of the metal to develop condensation on top of the insulation?

From the outside in your wall system should be metal(skin of bus), insulation, vapor barrier, then finished wall material.

You are heating the inside air of the bus. Warmer air has more moisture. Cool metal lowers the temperature of the air and squeezes the moisture out. Any warm air in contact with cool metal will cause water vapor to condense out onto the cooler surface.

The design goal should be to keep all warm, water vapor-laden air from coming in contact with cool surfaces, so the layer of plastic (vapor barrier) should be on the warm side of the wall system.

[edit] and furthermore there should be no air gaps near the metal surfaces. Stuff everything so there are no free air or air gaps.

Link to this forum's threads on condensation: https://www.skoolie.net/forums/tags/condensation.html
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Old 02-08-2022, 10:15 AM   #10
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Beautiful!!! Much help and thank you!
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Old 02-08-2022, 02:06 PM   #11
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If you are living in places where you expect high ambient humidity, a good rustproofing job of interior surface of a steel skin might be worthwhile, then no harm in doubling the barrier.

But don't seal up the interior surface of a steel skin from the outside, especially when running aircon,

allow that to circulate and dry out when it gets the chance, drip holes along the bottom even.

Also best to use insulation boards that do not absorb moisture.
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Old 02-08-2022, 02:12 PM   #12
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See now I’m confused again. It seems like you are saying a vapor barrier on the bottom of the insulation isn’t good. John would you agree with the above post about metal then insulation then vapor barrier then finishing wood? Or are you saying metal then vapor barrier then the insulation?
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Old 02-08-2022, 07:01 PM   #13
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If you'd like a tutorial on vapor barriers this one is pretty good:
https://www.buildingscience.com/docu...vapor-barriers

Scroll down to the wall cross-sections and you'll see that the barrier needs to go closest to the conditioned living space, to prevent moisture-laden air on the inside from soaking the insulation or metal frame of the bus when the outside temps are low.
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Old 02-08-2022, 07:03 PM   #14
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That’s why I love this site. Thanks all. I’ll update on the final product in case someone else finds this thread.
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Old 02-08-2022, 07:06 PM   #15
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Protecting insulation from the living space humidity is key

> no harm in doubling the barrier

By this I meant - under the circumstances listed - also putting a battier between the outermost skin and the insulation

Most would say overkill
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