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Old 08-24-2019, 07:06 PM   #1
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1976 27' Overlander
Milwaukie , Oregon
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Using foam board insulation under subfloor

Hey guys. I saw a thread on here where one of the airstreamers used foam board insulation screwed into the underside of the subfloor. I like this idea, and plan to do something like this myself. What I don't want to do is trap water against the subfloor in the event water makes it's way between the wood and the foam board. Would it be wise to use some sort of standoff to create an air gap between the wood and the foam? Would I want to create a way to ventilate said air gap or do I want to seal things as good as possible to maintain the best insulation?

Thanks for any pointers!

(PS - Using 3/4 ACX subfloors with West Systems marine epoxy to cover the entire tops of subfloors, outer edges, and about 10" of the underside edges of the wood, leaving the central underside non-epoxied to allow the wood to breath and dry in event of a leak.)
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Old 08-24-2019, 07:59 PM   #2
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Hi, Iím doing that now on my Safari. We had a vent not closed tight when it was shipped to us last January. We didnít realize the subfloor was soaked until 2 weeks ago. Anyway, we pulled down what Airstream calls insulation to let it dry.

I had the same concerns about putting the insulation against the subfloor. We made ďhatĒ sections to space the insulation board 1/2Ē from the floor. I made them out of 20 ga stainless steel- because thatís what I had (we own a sheet metal shop) and screwed them to the underside of the floor. I put some rivnuts on the flat section so I could screw the foam board to to them. I was worried that sheet metal screws would come loose over time. Screws will have a drop of Loctite on them.

Key is going to sealing around the gap between the floor and the insulation otherwise the insulation wonít be effective. My plan, although I didnít have time to try it today, is to use expanding foam around the parameter of the panel and put it in position before it cures. It should make a form fitting gasket against the subfloor. I hope.

In the photo, you can see the headless screws that will punch through the insulation board when dry fitting it so I can locate where the screws go. That part works great.

Iím traveling all week so it will be next weekend before I get this done. Iíll let you know how it goes.

Steve
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Old 08-24-2019, 08:07 PM   #3
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Steve, great idea using headless screws. Canít go wrong doing that to line up perfectly.
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Old 08-25-2019, 11:05 PM   #4
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Steve, looking forward to seeing what you end up doing! That looks like a great idea.
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Old 08-26-2019, 10:59 AM   #5
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Old 08-26-2019, 12:39 PM   #6
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Old 08-26-2019, 12:42 PM   #7
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Just make sure you have a path for water to get out and put some sort of spacers between the foam and floor. I chose to leave most of my floor uninsulated.



Perry
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Old 08-26-2019, 01:11 PM   #8
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foam board, fiberglass, or spray insulation can all be used.
the issue is that the metal beams are a thermal break and will provide cold spots in cool conditions as they will transmit the heat through
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Old 08-27-2019, 03:01 PM   #9
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No floor insulation

Tough subject that's hard to prove / verify BUT..... what we did was to not insulate under the floor or reinstall a belly pan thinking how much temperature difference can it be without them and is it worth the trouble ?!
We replaced 1/3 of the frame and floor (primed & painted frame with Rust Oleum, plywood sealed) and then sprayed the under floor wood with undercoating & truck bed sealant. The insulation is going to get wet and hold moisture and rot the frame again. Sure the wood & frame gets wet BUT it dries within a day or so.
Anyway...insulation: we camp only when the night temperature stays roughly above 30* and I very much like the idea of spending a little more $ for heat or A/C than letting the smell of wet underfloor insulation (that will hardly ever dry out) permeate the living area.
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Old 08-28-2019, 06:50 AM   #10
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I have to agree. Insulation is going to cause problems if you have leaks. I ended up putting drains in the C-channel to funnel water out of that area above the floor and run it to the belly pan where it will run out. If it is a 3 season trailer, then it is not worth the trouble. I also have not covered my floor so I can find and deal with leaks that might occur.



You loose way more heat to the ribs around the shell since there is no thermal break between them and the outside shell.



Perry



Quote:
Originally Posted by The Twinkie View Post
Tough subject that's hard to prove / verify BUT..... what we did was to not insulate under the floor or reinstall a belly pan thinking how much temperature difference can it be without them and is it worth the trouble ?!
We replaced 1/3 of the frame and floor (primed & painted frame with Rust Oleum, plywood sealed) and then sprayed the under floor wood with undercoating & truck bed sealant. The insulation is going to get wet and hold moisture and rot the frame again. Sure the wood & frame gets wet BUT it dries within a day or so.
Anyway...insulation: we camp only when the night temperature stays roughly above 30* and I very much like the idea of spending a little more $ for heat or A/C than letting the smell of wet underfloor insulation (that will hardly ever dry out) permeate the living area.
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Old 08-28-2019, 12:15 PM   #11
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I can definitely see the argument to insulate vs not insulate. Iím in favor of insulating personally. While I agree that there are areas that reduce the efficacy of insulation (such as thermal transfer via the ribs), Iím of the philosophy that something is probably better than nothing, assuming it can be applied in such a way to not promote rotting of the subfloor. I definitely donít plan on dealing anything entirely though and want my solution to be able to breath when water inevitably makes it in.
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Old 08-31-2019, 08:33 PM   #12
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I finished the insulation under the floor in the rear of our 25í FB. The hat sections I made are approximately the same height as the frame substructure so I could run the panel the full width of the airstream rails. I was going to seal with insulating foam but later decided to use aluminum tape to seal the big gaps. Most of the edges are up against the frame structure but not sealed tight. If water does get between the floor and insulation, there is a way to seep out.

I was able to get nearly the full width between the frames by scoring / breaking the panel to form a shallow V to get it in position then pressing it flat. My panel has foil on both sides so one side acted as a hinge when I scored/broke the panel.

Iím happy with how it all turned out. The top side is sealed with a penetrating epoxy so Iím not too worried about water getting through the floor to the insulation.
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