Glassing directly on interior PU/Polystyrene insulation instead of covering with inner skin?

Discussion in 'Materials' started by Eli, Jun 19, 2024.

  1. Eli
    Joined: Apr 2024
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    Eli Junior Member

    Hi,

    I would like to build a plywood epoxy catamaran with 50mm insulation on the inside of the hulls.
    In order to save weight and make work easier I though to glass 200gsm S-Glass 45+/- Bi-directional
    cloth directly on the polyurethane/polystyrene inner insulation foam sheets, instead of covering
    them with thin plywood skin/veneer followed by paint.

    That would be much quicker and lighter than covering with plywood. Using fine peel ply on top of the
    laminate followed by top-build epoxy primer, would spare the need for sanding and provide a ready
    surface for painting. Since the glass glued also to the stringers, it would contribute to hull rigidity
    without being too heavy.

    Being novice, I would appreciate the opinion and advice of the more experienced boat builders here,
    regarding this idea. Especially regarding potential condensation/mold management
    and structural strength sufficiency of the inner skin.

    insulation.JPG

    Best Regards.
     
  2. gonzo
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    gonzo Senior Member

    Laying plywood over foam would be a lot faster. It would not require sanding, fairing, etc. as much as laminating fiberglass; even if you vacuum bag it.
    Plywood is stiffer by weight than fiberglass.
     
  3. fallguy
    Joined: Dec 2016
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    fallguy Boat Builder

    You’ll need to specify, with precision, the shear strength and compression ratings of the foam. Most polystyrene will delaminate easily from the glass. An alternative would be to create a shear web and trap the insulation between plywood and some wood or other web.
     
  4. kapnD
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    kapnD Senior Member

    Portholes leak.
    Why deny yourself access to an area that may need attention in the future?
     
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  5. Eli
    Joined: Apr 2024
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    Location: Tel Aviv, ISR

    Eli Junior Member

    To save weight. However, it is true that even painted plywood is much easier to remove than having to cut fiberglass.
     
  6. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    I'll advise you to actually take a full sheet of foam and glass, primer and paint it. Then hang it on the wall in your smallest room and see if you can live with the result. Make sure to have a fiberglass seam (overlap) on it, so you can judge the effect.

    From a technical perspective there isn't anything speaking against it, it's all about the desired aesthetics. If you can live with it the lightest option is glueing upholstery fabric directly to the foam.
     
  7. Ike
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    Ike Senior Member

  8. fallguy
    Joined: Dec 2016
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    fallguy Boat Builder

    IMG_4816.jpeg

    Here is pink HD polystyrene I tried to use for a livewell bottom about 48x22”. Originally, I figured; it could be a former; make thick glass, etc. But I ran a test. I used peelply and vac for the first piece. I figured if it dented too easy, I’d keep adding glass. But the problem was, it sheared so easily, I had trouble removing cheap peelply. Better aircraft quality peel is much harder to pull. Here, the edges delaminated with the peelply removal despite every best effort and you can see the epoxy passed the test. The foam, however, peeled away with about the force to tear a sheet of paper. I was shocked and switched the core to marine foam.

    Someone may see this and suggest the glass coming apart had some other root cause, but the matrix failing on the edges caused the low resin layup to come apart..

    I rarely disagree with Rumars and respect his posts, but I must reiterate; this idea to laminate over polystyrene is an error. If you had timber every 12”; the foam would only delam to the web, but it WILL delam. And if it is part of the hull; then that delam is basically a hull failure.
     
  9. fallguy
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    fallguy Boat Builder

    You can glass the ply hull to specs; then you can create a web of furring strips say every 2’ and glue or epoxy bond them to the hull. Then you can use polystyrene adhesive and glue the foam to the hull; then you can use 3mm luaun or better as a wall covering and fasten to the wood for a superb looking finish. Here the ceiling was 4mm okume, but results are similar on wall with 3mm luaun. The final coating is a precat epoxy from Sherwin Williams. Hospital paint is easy to wash.. IMG_7979.jpeg
     

  10. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    There's no disagreement fallguy, he will certainly experience what you described if he uses XPS. PU foam is better in this respect, as Ike wrote this is sometimes done on metal boats that have been foam sprayed. The end result always involves a lot of sanding if the surface is to look good.

    But, it's not my place to convince the OP of what he should or shouldn't do. He can do a test and see what he can live with regarding surface finish, impact resistance and other things that might be important to him.
    If my goal was weight and insulation a necessity (no foam at all is even lighter) I wouldn't hesitate to glue some upholstery fabric directly to the foam. Impact resistance would be zero, washing would be difficult, but that's a compromise I would engage in. If the goal includes a nice smooth surface with minimal effort I would use rigid foamed PVC or cellular polycarbonate panels.
     
    fallguy likes this.
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