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Old 07-08-2019, 02:35 PM   #1
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Vapor barrier confusion

I tore out the floor of my bus. Now I need to clean up the rust spots, cover the several hundred holes from screws and track bolts, and then paint over that. Any advice on paint?



I am confused regarding the vapor barrier, where it goes, and if it will trap moisture and create rust.

What order does the insulation and vapor barrier go, starting from the metal floor? Is it insulation on top of the metal, then vapor barrier, then subfloor/plywood, then floor? Or vapor barrier between subfloor and insulation?



Will moisture be trapped between the metal floor and the vapor barrier?


Do I leave some of the holes in the metal floor so the moisture can drain out?


To cover the holes, I'm thinking of using some kind of heavy duty adhesive and those thin, round, sheet-metal scallop things that are used to nail down over the tar paper.



Is there a specific adhesive for this stuff?


Thank you!
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Old 07-08-2019, 02:40 PM   #2
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Most people put the vapor barrier between the insulation and subfloor. It's mostly to keep the insulation from absorbing water in the form of vapor inside the bus. Also, you will probably want to use spacers between the metal and subfloor, to keep from compressing the insulation.
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Old 07-08-2019, 03:03 PM   #3
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Originally Posted by JustKip View Post
Most people put the vapor barrier between the insulation and subfloor. It's mostly to keep the insulation from absorbing water in the form of vapor inside the bus. Also, you will probably want to use spacers between the metal and subfloor, to keep from compressing the insulation.



Thinking of using 1x2 strips to allow for 3/4 insulation. Or, ripping 2x4s to 1 inch to allow for 1" insulation.
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Old 07-08-2019, 03:26 PM   #4
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After cleaning and using an ospho type rust converter, use Rustoleum rust primer sealer. Normally you want air space between the vapor barrier, not practical in our bus. Paint, vapor barrier, insulation, flooring. Once done I don't think you can put enough pressure on the floor to compress the insulation underneath.
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Old 07-08-2019, 03:38 PM   #5
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Once done I don't think you can put enough pressure on the floor to compress the insulation underneath.
I've always wondered about that. In the videos they usually put down strips of wood to support the plywood, but I kinda doubt that the foam sheets would be compressed by even weight distribution.

I've always seen the vapor barrier put on top of the insulation, to keep the inside moisture from getting into the insulation. Some people don't bother when using insulation that doesn't absorb moisture. Condensation on the metal is the real problem. Otherwise there's no moisture to absorb into the insulation. And that condensation, caused by warm moist air on cold metal (like a soda can or bathroom mirror) doesn't care if there's insulation or if it absorbs. I layed it over the insulation in my trailer build, before putting down the plywood.
I went with 2 foot wide planks of 3/4 inch foam, 3 across in my 6 foot trailer (only 2 support strips)
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Old 07-08-2019, 03:58 PM   #6
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I've always wondered about that. In the videos they usually put down strips of wood to support the plywood, but I kinda doubt that the foam sheets would be compressed by even weight distribution.

I've always seen the vapor barrier put on top of the insulation, to keep the inside moisture from getting into the insulation. Some people don't bother when using insulation that doesn't absorb moisture. Condensation on the metal is the real problem. Otherwise there's no moisture to absorb into the insulation. And that condensation, caused by warm moist air on cold metal (like a soda can or bathroom mirror) doesn't care if there's insulation or if it absorbs. I layed it over the insulation in my trailer build, before putting down the plywood.
I went with 2 foot wide planks of 3/4 inch foam, 3 across in my 6 foot trailer (only 2 support strips)
1" Rigid Pink insulation has a compression of 15psi, or 2160lbs per square foot. Add a harder top floor spreading the weight further. I think compression is a non issue .
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Old 07-08-2019, 05:02 PM   #7
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Yes rigid foam needs no other protection against compression, just a puncture resistant topping.

The "vapor" barrier is also to seal the space envelope against convection losses (drafts), gap filling spray foam and or continuous dropcloth.

Can go just above or below the insulation, depending how hydroscopic it is and where the moisture's coming from. Or both.

Forget about the shiny, also air gaps.
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Old 07-08-2019, 10:27 PM   #8
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OK
That's good advice all around.



My only concern is securing the appliances/cabinets down. I know I can screw them to the sides, but think the floor is also necessary. Maybe a strip running down each side, along where the cabinets will rest?


Also, anyone recommend and adhesive to seal the holes on the floor using those little round plates? There's several hundred holes for the bolts, plus screw holes for the plywood, that I dont want to spend days welding shut.


Thank you!
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Old 07-08-2019, 11:17 PM   #9
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Sikaflex 291 or 3M 4200 sealants stay flexible, are medium adhesive but removable.

BoatLIFE Life-Calk even less adhesive.

Butyle tape least of all.

Anything but silicone based.
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Old 07-09-2019, 01:38 AM   #10
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Originally Posted by john61ct View Post
Sikaflex 291 or 3M 4200 sealants stay flexible, are medium adhesive but removable.

BoatLIFE Life-Calk even less adhesive.

Butyle tape least of all.

Anything but silicone based.



Isn't it best for the the sealant to be as adhesive as possible for that application?
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Old 07-09-2019, 10:45 AM   #11
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If you 100% **know** no one will ever want to remove it, then sure, use 3M 5200 or Sikaflex 292.

Or epoxy, or welding if flexibility is not required.

For many applications, I prefer "good enough" adhesion, don't need to use a chisel or grinder to "undo" the join.
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Old 07-09-2019, 12:52 PM   #12
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If you 100% **know** no one will ever want to remove it, then sure, use 3M 5200 or Sikaflex 292.

Or epoxy, or welding if flexibility is not required.

For many applications, I prefer "good enough" adhesion, don't need to use a chisel or grinder to "undo" the join.



Sounds good.


Thanks!
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Old 08-10-2019, 10:50 PM   #13
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Originally Posted by o1marc View Post
After cleaning and using an ospho type rust converter, use Rustoleum rust primer sealer.



Are you sure that's what it's called? All I can find under that name is a spray paint can, and that doesn't seem practical. Do you know if/where it's available in gallon size that can be rolled on?


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Old 08-11-2019, 09:53 AM   #14
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Are you sure that's what it's called? All I can find under that name is a spray paint can, and that doesn't seem practical. Do you know if/where it's available in gallon size that can be rolled on?


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Old 08-11-2019, 12:46 PM   #15
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I've been using this no.7769. It's great and easy to find here in Oregon, but most stores seem to only order 2 cans at a time, minimum order for both quarts and gallons, and then order more when those sell. So ask a clerk if they have more coming soon.
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Old 08-11-2019, 02:28 PM   #16
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I've been using this no.7769. It's great and easy to find here in Oregon, but most stores seem to only order 2 cans at a time, minimum order for both quarts and gallons, and then order more when those sell. So ask a clerk if they have more coming soon.
That is what I primed my floor with.

I couldn't find it locally so I ordered it online. Plenty available online.
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Old 08-11-2019, 02:39 PM   #17
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I primed my floor with it too. Did a 35 foot rear engine on less than a gallon. And I did not thin it, so it went on pretty thick.

Ace Hardware carries 7769, but it's not always in stock. At least in these smalltown stores, they usually don't order more until the last 2 are gone.
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Old 08-11-2019, 09:41 PM   #18
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I've been using this no.7769. It's great and easy to find here in Oregon, but most stores seem to only order 2 cans at a time, minimum order for both quarts and gallons, and then order more when those sell. So ask a clerk if they have more coming soon.
Getting harder to get oil based enamels in some areas.
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Old 08-11-2019, 09:42 PM   #19
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I primed my floor with it too. Did a 35 foot rear engine on less than a gallon. And I did not thin it, so it went on pretty thick.

Ace Hardware carries 7769, but it's not always in stock. At least in these smalltown stores, they usually don't order more until the last 2 are gone.
Good stuff for the money. I've used it on a ton of projects. My Lowes has gallons of it.
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Old 08-11-2019, 09:44 PM   #20
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Yeah, the new rules about VOCs have something to do with availability issues for sure. I'm not sure what I would do without Rust-Oleum.
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