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Old 10-20-2017, 09:07 AM   #1
1 Rivet Member
1974 Argosy 28
Euless , Texas
Join Date: Jul 2017
Posts: 9
Floor / Belly Pan replacement - Spray Foam?

Hi All,

I've decided to navigate some (from what I can see) uncharted territory... I'm replacing the Belly pan front to back and instead of putting a new shield up; I will be using a foam spray insulation under the floor. Once shaved/sanded to shape underneath, we're going to spray it down with a rubber like material similar to flexi-dip.

Thoughts? Any immediate concerns or issues that this will cause issues?

Personally I think it's rather brilliant (in my humble opinion) The insulation foam will flex a bit and provides very easy access if I need to get into a certain area. (Just cut into it) The rubber seal provides water protection and debri protection from the road.

Someone please let me know if I'm way off base here. Thanks!

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Old 10-20-2017, 09:52 AM   #2
3 Rivet Member
1977 31' Sovereign
Vintage Kin Owner
Sunset Valley , Texas
Join Date: Jul 2016
Posts: 211
I can think of two reasons why I wouldn't do it that way:
1) trapping moisture against the frame and subfloor
2) errant debris gouging holes in the rubber coating
3) When it comes time to service something down there you've created quite the obsticle.

Interesting idea for sure, but if I were you, I spend a little more bucket time looking at how others redid their undercarriage.

My 2¢...


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Old 10-20-2017, 10:02 AM   #3
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1973 21' Globetrotter
Houston , Texas
Join Date: Sep 2009
Posts: 2,194
When I rebuilt my trailer, I added grey water tanks, so with my 21' frame, I only had one center bay that was essentially empty, annd 1 at the very rear that has my dump valve and a bunch of grey water piping going through it. So, had I gone with your plan, I only really would have been spray foaming the sections outside the main frame rails, in between the outriggers.

Believe me, it is hard to rebuild one of these trailers and not go down the slippery slope of "as long as I am here, I might as well..." There is usually a reason why nobody has tried certain things, it it is likely that it is more expensive, more labor intensive, or doesn't function so well. Try not to reinvent the wheel--you have a huge project in the first place, it may as well not take forever to complete.

good luck!
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Old 10-20-2017, 10:32 AM   #4
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1966 24' Tradewind
1995 34' Excella
Lynchburg , Virginia
Join Date: Sep 2009
Posts: 2,444

When I got into my belly pan (Dan’s 66 Tradewind Improvements) the only problem I found was in the area of the gray water tank because it had gotten wet. There is a saying I like “If it is not necessary to change then it is necessary not to change”. I fixed my gray water tank, cleaned and painted the frame, installed new insulation (Roxul and hard foam) and installed a new belly pan.

Another saying I like is “New solutions have new problems”. I would not go there.

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Old 10-21-2017, 10:33 AM   #5
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1967 22' Safari
West Fork , Arkansas
Join Date: Jan 2012
Posts: 221
Personally I like mineral fiber, aka rock wool or Roxul insulation for floor, walls & ceiling. It has very good insulation value, won't trap moisture and is fire proof. At the 2014 Vintage Trailer Academy I heard from trailer restoration experts to avoid foam because it would likely crumble over the years due to vibration.

That just my opinion.
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Old 10-21-2017, 09:07 PM   #6
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1989 34.5' Airstream 345
Saint Petersburg , Florida
Join Date: Apr 2010
Posts: 1,687
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Have you tried Spray Foam?

To be honest, the flexibility you speak of concerning Spray Foam is not present in the products I've used. Once the Foam has 'set', it's very brittle. As well, applying Plasti-Dip to the Spray Foam once it's set, I'm not quite sure that the Spray Foam will 'like' the Plasti-Dip. Both are not cheap products that IMO would allow a test of trying such an endeavor.

As well, the underbelly pan is there for a purpose, to protect wiring and plumbing. I'm guessing Airstream continues to utilize the belly pan is because an Airstream trailer generally 'rides' lower than other Travel Trailers, and needs that extra bit of protection.

As others have mentioned, don't make this even more complicated than it already is.

Good Luck with whatever you decide,
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Old 10-21-2017, 09:24 PM   #7
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1991 34' Excella
1963 26' Overlander
1961 26' Overlander
Central , Mississippi
Join Date: Nov 2006
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As an engineer, I've never understood the obsession with floor insulation. 90% of the heat loss in a structure is 'up' but I see regular posts and efforts to make huge efforts in the floor.
Having done several shell off rebuilds I dread the thought of getting a future project where the floor frame belly pan is a stuck together mass of rotted wood/steel/expanded foam.

Energy saving efforts in our homes are always focused in the ceiling/doors/windows, not the floors.
Hi Ho Silver RV! Vernon, Sarah, Mac the Border Collie -
A honkin' long 34' named AlumaTherapy
and a 26' '63 Overlander, Dolly
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Old 10-21-2017, 09:26 PM   #8
Half a Rivet Short
2017 30' Classic
Carlisle , Pennsylvania
Join Date: May 2017
Posts: 3,298

The dead air between the belly pan and the floor acts as an insulator. Convection across a space shaped like that is not very good. The spray foam needs to be fairly thick to give you the same insulation. As mentioned above, if the spray foam is not very flexible. Net effect is that you stiffen up the trailer. The "flex" of the trailer helps dampen the impact of bumps in the road. Stiffen things up and the cabinets take more of a beating.....

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Old 10-21-2017, 09:53 PM   #9
"Space A" S/O Registry 11
2006 34' Classic S/O
Fort Worth , Texas
Join Date: Feb 2007
Posts: 539
Don't forget if you want to add some extras latter on like solar or front AC sockets or repair plumbing you will have to chisel out pathways. Maybe even go foam mining for fishing wires and pipes.
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Old 10-22-2017, 07:10 AM   #10
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1977 31' Sovereign
Pointe aux Roches , Ontario
Join Date: Aug 2013
Posts: 6
Images: 3
I would recommend to consider 'POR15" product on your steel frame. instead of rubber.
POR15 chemically bonds with metal and harden rock hard to prevent future rusting. I've used it when rebuilding my rear bath undercarriage and on farm equipment. Very impressed with the stuff even 6 years later. As for the insulation, I'm leaning towards bat insulation & away from the spray foam.
My 2cents,
Moe Chauvin
Ontario Farmer
31ft '77 Sovereign
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Old 10-22-2017, 07:20 AM   #11
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1967 17' Caravel
Cadillac , Michigan
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I put zero insulation in the floor cavity of my trailer.
No need for it. Just a water catch.
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Old 10-22-2017, 11:02 AM   #12
Half a Rivet Short
2017 30' Classic
Carlisle , Pennsylvania
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Originally Posted by Dingo Girl View Post
I put zero insulation in the floor cavity of my trailer.
No need for it. Just a water catch.

... and a great place for the mice to hide .... and leave "trace evidence" embedded in it ...

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Old 10-22-2017, 11:31 AM   #13
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1975 Argosy 28
Springville , Alabama
Join Date: Apr 2010
Posts: 424
Many use 2 inch foam board from box stores. Cheaper, removable and does the trick
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Old 10-22-2017, 11:39 AM   #14
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1974 Argosy 28
Euless , Texas
Join Date: Jul 2017
Posts: 9
Not a bad idea with foam board. Luckily this isn't going to a be a road warrior, so I'm not too worried about the rigidity. Mostly for event rentals etc... We're putting in a fold up bar wall and TVs. The occasional road trip.

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