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Old 05-05-2018, 07:16 PM   #1
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Floor Options - 2X2's or just Insulate and Plywood?

Should the floor have 2X2's to prevent the insulation from being compressed? Afterall they say compressing insulation actually makes it less effective.

Or should you just lay insulation with glue then tongue and groove plywood?

Another reason I've heard to use 2X2's is to prevent the floor from warping/sagging under weight differential on each side of your build.


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Old 05-05-2018, 07:30 PM   #2
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Until someone proves me wrong with data, I think if you have plywood flooring over the rigid insulation you won't ever create enough pressure in any given area enough to compress the foam.
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Old 05-05-2018, 08:53 PM   #3
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Quote:
Originally Posted by BigBlue View Post
Should the floor have 2X2's to prevent the insulation from being compressed? Afterall they say compressing insulation actually makes it less effective.

Or should you just lay insulation with glue then tongue and groove plywood?

Another reason I've heard to use 2X2's is to prevent the floor from warping/sagging under weight differential on each side of your build.


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There are no good reasons to frame the floor, and doing so compromises the insulation.

Let's do the math ...

Compression strength of XPS Foam board ... 25 lbs/sq in

Rough area of shower wall base-plate ... 32x3.5" ... 112 sq in

The foam can support 112x25 = 2800 lbs ... or about 1.15 ton.

And that's ignoring completely the load spreading effect of the plywood sub-floor.

There is literally nothing you can put in the bus that will affect the foam underlayment.
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Old 05-05-2018, 08:58 PM   #4
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Quote:
Originally Posted by Twigg View Post
There are no good reasons to frame the floor, and doing so compromises the insulation.

Let's do the math ...

Compression strength of XPS Foam board ... 25 lbs/sq in

Rough area of shower wall base-plate ... 32x3.5" ... 112 sq in

The foam can support 112x25 = 2800 lbs ... or about 1.15 ton.

And that's ignoring completely the load spreading effect of the plywood sub-floor.

There is literally nothing you can put in the bus that will affect the foam underlayment.
1.4 ton, but who's counting. I agree with you completely.
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Old 05-05-2018, 09:50 PM   #5
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Thank you for shedding some light on this!! I have no idea how building works and really didn't want to frame the floor anyhow
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Old 05-05-2018, 11:25 PM   #6
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Ok, lovely calculations which don't mean much to me. I'm happy not to frame the floor, 1/2" ply over rigid board suits me to the ground. Any concerns about framing walls (probably 2"x2" studs) off this? What are you guys planning on doing for fixing?

Any Damp Proof Membrane (DPM)? Probably a whole new thread in itself, but I assume that everyone is making the metal skin whole again (coins/silicone seems easiest I've seen) so trapping moisture with a DPM beyond this is probably a recipe for rust...?
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Old 05-06-2018, 02:20 PM   #7
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I'd guess that the wall framing would be done by attaching wood to the walls of the bus and building off of that first? But I honestly haven't built yet and don't know a thing about it! XD

Or maybe framing specifically where you're building walls up so that you have a solid piece of wood under to attach to?
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Old 05-06-2018, 02:58 PM   #8
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Quote:
Originally Posted by Twigg View Post
There is literally nothing you can put in the bus that will affect the foam underlayment.
Dow and Owens Corning also do special foams for floor use. Down Highload 100 is rated for over seven tons per square foot.
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Old 05-08-2018, 08:07 AM   #9
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Quote:
Originally Posted by Twigg View Post
There are no good reasons to frame the floor, and doing so compromises the insulation.

Let's do the math ...

Compression strength of XPS Foam board ... 25 lbs/sq in

Rough area of shower wall base-plate ... 32x3.5" ... 112 sq in

The foam can support 112x25 = 2800 lbs ... or about 1.15 ton.

And that's ignoring completely the load spreading effect of the plywood sub-floor.

There is literally nothing you can put in the bus that will affect the foam underlayment.
Nice use of numbers but here's a question for the non-framers, how do you calculate slope?

With rigid board you're pretty safe but I'm going to use spray foam so will use the 2x2s to keep things level.
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Old 05-08-2018, 10:54 AM   #10
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Originally Posted by Brewerbob View Post
Nice use of numbers but here's a question for the non-framers, how do you calculate slope?

With rigid board you're pretty safe but I'm going to use spray foam so will use the 2x2s to keep things level.
Few people ever get the bus completely level, so instead of using slope, measurements are taken from fixed reference points.

Once you get the floor flat, you and use a 90 degree from it to all the uprights.
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