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Old 10-06-2019, 09:50 AM   #1
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Plastic ceiling?

I'm thinking about putting rivnuts into some of the existing rivet holes in my hat channels (either 8-32 or 1/4-20, maybe every 8" or so) and then mounting 0.06" sheets of HDPE plastic (https://www.eplastics.com/HDPENAT0-060SR48X96) as the new ceiling using nylon bolts (https://www.mcmaster.com/plastic-bolts) to maintain the thermal break. Aesthetics notwithstanding, would this last very long? Would gluing the panels as well be necessary or even help? The warning that these nylon bolts absorb water and expand seems to be a concern. Those PEEK screws seem like they might work better, but at $5.50 a pop it's not going to happen.
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Old 10-06-2019, 10:38 AM   #2
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I wanted to use perforated vinyl soffit for my ceiling, but couldn't find any without the molded-in ridges. Sagging between the 28"-32" span of ribs was a slight issue as well.....but solved by running a screw into the insulation. Definitely test it out.

As far as plastic bolts absorbing water, I personally don't think interior condensation wold be enough to affect them.
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Old 10-06-2019, 01:39 PM   #3
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I think testing this is a good idea. I'm going to try putting a small-ish piece of HDPE up on my ceiling with rivnuts this way and see how it holds up. It looks like that plastic bolt place I linked has glass-filled nylon screws that are twice as strong as regular nylon and only cost $1 or so each. I would only do this over the center 4' of the ceiling so there wouldn't be much curvature to deal with. I'm thinking I would initially put it up with steel bolts and let it rest for some time before replacing the steel bolts with the nylon bolts one by one.
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Old 10-06-2019, 11:15 PM   #4
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Worth an experiment. I would also be concerned with the amount of sag in the middle. That is some thin sheets of plastic afterall. Heat may also play an important role in the deformation of the sheets.
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Old 10-07-2019, 12:17 AM   #5
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HDPE has a moderate thermal conductivity. Double that of pine. Easy to clean I suppose.
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Old 10-07-2019, 12:50 AM   #6
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I'd use FRP, i.e. fiberglass reinforced plastic. Stuff like Filon. It comes ten foot wide and is used for everything, such as RV's, semi-trailers, etc.
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Old 10-07-2019, 01:50 AM   #7
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I'd use FRP, i.e. fiberglass reinforced plastic. Stuff like Filon. It comes ten foot wide and is used for everything, such as RV's, semi-trailers, etc.
Seems pretty pricey. McMaster-Carr has 4'x8' sheets of 1/8" FRP for $212 each (vs. $30-something for 1/16" LDPE).
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Old 10-07-2019, 01:53 AM   #8
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HDPE has a moderate thermal conductivity. Double that of pine. Easy to clean I suppose.
Interesting, didn't know that about these plastics. HDPE has a value of 0.48 but apparently LDPE (which I think I'm going with instead because it's cheaper and more flexible than HDPE) has a value of 0.33.
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Old 10-07-2019, 05:25 AM   #9
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Seems pretty pricey. McMaster-Carr has 4'x8' sheets of 1/8" FRP for $212 each (vs. $30-something for 1/16" LDPE).
Check out your local big box store... $32 at Lowes.
https://www.lowes.com/pd/48-in-x-8-f...nel/1000174771

This is what I plan to use to cover my shower stall walls, whenever I get to that phase of the build.

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Old 10-07-2019, 08:23 AM   #10
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following this, seems like a good ideal for someone who like my self might have to retrofit insulation over the celling at some point.
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Old 10-07-2019, 08:45 AM   #11
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I'd sure like to avoid doing 3 inch wide t&g planks on this new bus.. So please keep us updated on this project!
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Old 10-07-2019, 02:32 PM   #12
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The FRP I was talking about is this: https://www.recpro.com/rv-fiberglass...V%20Fiberglass
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Old 10-07-2019, 07:05 PM   #13
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following this, seems like a good ideal for someone who like my self might have to retrofit insulation over the celling at some point.

It can be done with 5mm plywood underlayment as well. We cut it into 23"-ish x 7.5'-ish panels and put a 3" wide trim piece over the seam at each rib. We insulated over the perforated metal ceiling then covered it with the plywood underlayment.
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Old 10-08-2019, 11:46 AM   #14
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It can be done with 5mm plywood underlayment as well. We cut it into 23"-ish x 7.5'-ish panels and put a 3" wide trim piece over the seam at each rib. We insulated over the perforated metal ceiling then covered it with the plywood underlayment.
I think I'm going to end up using this thin plywood instead of LDPE. I just got my sample bits in the mail and the 1/16 plastic is indeed very floppy and semi-translucent (and prone to scuffing). I'd probably have to put some kind of sticky paper on one side of this to keep the insulation from being visible through it. And the 5mm plywood isn't all that much thicker than the plastic that it would really matter.

The nylon bolts are pretty cool, though, and I'll still use those.
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Old 10-09-2019, 04:45 PM   #15
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The 1/16" LDPE sheet is laughably unsuited for use as the ceiling:

LDPE.png

other than the fact that it's super-light. The 1/16" HDPE is stiffer and more opaque but would still sag some over the 26" span between ribs. I have some 5mm plywood and some 2" XPS board now that I'm going to do a test with.
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Old 10-09-2019, 05:27 PM   #16
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Dexter would like it.
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Old 10-10-2019, 02:26 AM   #17
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Borderline transparent!
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Old 10-10-2019, 09:18 AM   #18
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I used ply then veneers to make it look good. Use a quality contact cement for the veneers.
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Old 10-10-2019, 02:30 PM   #19
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do most of ya'll change the ceiling? is there a forum post about pros and cons of changing from standard ceiling to wood or other material? I believe most of them have some level of insulation behind the metal roofing panels, right?
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Old 10-10-2019, 03:08 PM   #20
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do most of ya'll change the ceiling? is there a forum post about pros and cons of changing from standard ceiling to wood or other material? I believe most of them have some level of insulation behind the metal roofing panels, right?
The factory insulation is near-worthless. The metal ceiling panels are connected to the metal ribs and outer skin, so because of the thermal conductivity of metal the ceiling will always be close to the outside temperature regardless of what insulation is there between the ceiling and roof (it's possible that the factory insulation is mainly there because of its sound-deadening properties rather than its thermal ones).

Removing the ceiling allows you to add thicker insulation, but more importantly it allows you to put on a new ceiling which has a thermal break between it and the outer bus skin (this post in fact was mainly about ways to achieve that thermal break).
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