one side plywood, one side fiberglass/epoxy, polystyrene core?

Discussion in 'Materials' started by murv, May 18, 2007.

  1. murv
    Joined: May 2007
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    Location: north america

    murv New Member

    I'm building a lightweight and weatherproof camper on the back of my pickup and have some material questions. Right now I've basically got a 4' tall frame of 2"x2"s (with a 3' over-cab bubble) with the *inside* lined with 1/4" plywood. I'm in the process of filling the gaps in the frame with 2" polystyrene insulation material (pressed up against the plywood interior) plus strips of 1/2" polystyrene along the 2x2 frame. The plan is to wet out the polystyrene with epoxy and then apply a layer or two of 18oz fiberglass roving (painting it for UV protection after it cures).

    What I'm trying to figure out (beyond whether or not I'm crazy) is how best to attach the insulation material. The wood 2x2s should handle the load well, and covering the exterior with fiberglass should provide reasonable puncture resistance (small pebbles and other road material, etc). The epoxy should provide good waterproofing, while the polystyrene insulation should keep the interior warm in the winter and cool in the summer (relatively, at least :)

    I'm not sure how best to go about attaching the polystyrene though - I see four options:
    1) pressure fit the polystyrene between the 2x2s. Then wet out the polystyrene in place and apply the fiberglass to the sides and roof (trimming the fiberglass at the corners/edges and using plenty of epoxy/filler to hold them together)
    2) same as 1, but also use some of that 3M adhesive spray to attach the polystyrene to the plywood and 2x2s.
    3) same as 2, except use epoxy instead of 3M adhesive
    4) wet out the polystyrene sides and roof separate from the framing (e.g. horizontally on the floor) and apply the fiberglass. Before the epoxy cures completely, place the polystyrene sides/roof with the attached fiberglass onto the frame. apply extra epoxy/filler to the edges and trim as necessary. Perhaps use 3M adhesive or epoxy on the "other side" of the polystyrene to bond it to the plywood and 2x2s?

    In my mind, 3 seems the "best", though uses up a lot more epoxy and may not be necessary (?). Options 1-3 have the downside of requiring some hefty fillers in the epoxy combo, since I've basically got to fiberglass several completely vertical 4'x6' surfaces. Option 4 lets me use easier techniques to apply the epoxy more evenly as well, since the sides and roof can be laid flat on the ground. As the epoxy itself won't soak through the polystyrene completely either, I'm not sure what benefits there are to fiberglassing the foam in-place, since it won't reach the other side to adhere to the plywood and 2x2s.

    On the other hand, I'm a novice at this stuff and have only done a small bit of mucking around with epoxy. I picked up a 75yd roll of that roving on ebay for around $150 USD and have a local west marine store for epoxy/hardener/fillers (as well as a local lowes/home depot for cheap 1-2" polystyrene 4'x8' sheets).

    Thanks in advance for any advice! The weather is pretty cold at the moment (40s), but when it warms up to the 60s I'd like to start epoxying :)

    -pete
     
  2. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    I would think the choice of styrofoam thickness should be 1 1/2", and not 2"
    As it is, you could practically heat it with a candle, for all your extra trouble.
    Really, it needs a layer of thin ply outside to be smooth and to dispense with more than pressure-fitting the foam between the frames.
    That said, marine ply isn't needed, just luan with exterior glue. Then put the roving to it. Epoxy isn't needed, really. Polyester won't let go on a camper.
    Much cheaper! Done right, it sticks fine--- not like epoxy, but this is a TRUCK CAMPER.
    The resin would eat the foam, but with plywood in between (which you can afford now that you are using polyester), you have no issues.
    With ply on the exterior, it would be a lot easier to attach things too. It could be air-stapled on in no time.
    ...Is how I (who am not you) would do it.

    Alan
     
  3. murv
    Joined: May 2007
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    Location: north america

    murv New Member

    Thanks Alan - I've certainly been considering wrapping it up in ply and bonding the fiberglass to it, though if possible, I'd like to dodge the 50-75 pound weight involved in the ply (the truck is a little 2.3 liter ford ranger, and the box already weighs perhaps 150lb).

    There's also the trouble with the front - I've got one of those curved over-cab bubbles as you'd see on tractor trailers, and bending the 1/4" ply around that fairly small radius would be tough. On the other hand, if I *need* the ply on the outside, I could go with an angle and could probably drop the entire fiberglass issue altogether - e.g. http://cheaprvliving.com/BuildYourOwnCamper.html

    The polyester vs. epoxy resin benefit doesn't seem overwhelming since my square footage is pretty small (80 or 90 sq ft) and I already have a gallon of epoxy + hardener laying around.

    I'm definitely willing to consider going with a plywood exterior if I need to. Would the epoxy and fiberglass over the polystyrene be sufficient for the sheer forces of bouncing down the road at highway speed?

    Thanks again!
     
  4. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    Regarding the last question first, any amount of glass that will make a solid outer skin will weigh more than ply with a coating of epoxy, and that isn't my primary reason for suggesting ply anyway. You will have to do a lot of fairing to make your skin look flat. That will take a lot of time and epoxy/fairing compound, and maybe you won't have enough epoxy after all, if you ever did.
    1/4" ply would weigh about 30 lbs per cu ft, or 48 1 sq ft layers (more like sixty because it will no doubt be under 1/4" by a bit, but let's say it's a full 1/4"). So the 90 sq ft will be 90/48 cu ft, or 1.875 x 30, or 56 lbs total.
    However, NO fairing (except at corners and joints) will be required, no adhesive for the foam, etc..
    Regarding the nose, over the cab... my suggestion is you chop the corners with 45 degree angles with 2" radii anticipated (4" across the angled face), and get some schedule 80 PVC pipe and quarter it legnthwise, and get two 90 degree elbows for same and cut top corner caps out of them, and the glass over all after fairing.
    A note: Your roving is too thick--- you'll chase it with a lot of epoxy--- get some light cloth and double at seams and save epoxy. It will last.
    Yes, you own the glass. Now, you have a start on a boat. You sound like a misplaced wannabe boat builder but you don't even know it.

    Alan
     

  5. SamSam
    Joined: Feb 2005
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    Location: Coastal Georgia

    SamSam Senior Member

    There's a chart on this thread that will give you an idea on the weight of the fiberglass you use. If you use it over ply, the ply will absorb extra resin.
    http://boatdesign.net/forums/showthread.php?t=8686&highlight=fiberglass thickness chart
    If you do fiberglass on foam (or maybe even on ply) you might have a lot of filling and fairing to do, depending on how good you want it to look. 18 oz WR will have a lot of texture, maybe to the point of being not so aerodynamicly slippery. Sam
     
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