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29 January 2014

Ply Foam, Ply!

Cross Sections of Arced and Flat PFP Construction

The light is what guides you home, the warmth is what keeps you there.  

~Ellie Rodriguez

Ply Foam, Ply! Looking at Insulation

Over the years, we've come to value insulation.

Not so much to keep us warm - seems simple plywood sides, a good wood stove, long johns and sweaters are plenty for our personal comfort - but to keep condensation at bay.

Condensation occurs when warm, humid air encounters a surface which is cold, relative to the air. Thinnish, dense materials such as plywood radiate their heat away to the outer world, and provide just the kind of surface that encourages condensation. In the cool, moist climes of the Pacific Northwest, condensation is a constant companion. 

And where condensation is like irrigating the spore farm to encourage crop after crop of mildew.

Good ventilation and techniques to draw moisture from the air help, but every time the hatches open in damp weather, we refresh our moisty atmosphere. For best results, these must be backed up by insulation.

Insulation slows the conduction of heat from the interior. This means less fuel to achieve the same internal temperatures. It also means the inboard surface of insulated walls and windows stays close to the internal ambient. Humidity remains as vapor in the air, where it belongs.

Mildew and airflows play a part, too, in our decision as to how we insulate.

Insulation can be made removeable. This is cheap, quick and dirty. I'm immediately drawn to it!

One friend used foamboard cut to shape, then coated with vinyl wallpaper. The vinyl is attractive, easy to clean and removable for cleaning behind / inspection.

Another used mylarized bubblewrap sheet insulation, simply hung along the walls. The effect was surprisingly cozy, and a lit his cabin up from every direction.

In A Warm Dry Boat by Roger McAfee, he suggests hollow walls with inexpensive fiberglass batting, and conducted some shake-it-up tests that sound promising.

The problem with all these, for us, is that that humid air can flow into all of these, more or less unimpeded, merely condensing out of sight. Mildew or worse, black mold, can happily multiply until spring cleaning rolls around.

Or if one DOES impede airflow, then dryrot - another major foe - lies in wait.

I've torn enough stuff apart, around here, to have seen how nasty, fungal flows love to work into any un-bonded space, however diminutive.

A friend introduced us to SIP (Structural Insulated Panel) construction, aka PFP (ply-foam-ply) construction. In  this method, foamboard is laminated between two sheets of plywood, forming a composite devoid of airways. 

There are even fringe benifits: A PFP composite is, itself, a girder, increasing rigidity and strength. And foamboard contributes substantially toward positive buoyancy.

Since much of the framing will be dimensional, 2x stock, 1 1/2in foamboard is convenient. Total R-value comes to about 8.

We've considered three general approaches:
  1. Build the sides as a single SIP - This offers several advantages, chief among which is the ability to assemble, sheath and finish on-the-bench. But also some big drawbacks. The decisive one, for us is that each side gets to be heavier than we can handle without considerable, mechanical ingenuity.
  2. Assemble the sides from SIPped panels - This makes each sub-panel manageable, at the cost of some of the finishing advantages. Construction is complicated by framing which runs the length of the boat, spanning multiple panels... it's possible to assemble, but tricky to get right.
  3. Retrofit insulation and inboard face - This makes for vertical, tedious, but straightforward assembly, with easy-to-handle parts. It eliminates butts on the inner face. Cleats for furnishings mostly double as framing for foam and inner face, and can be lighter where not. Unlike preceding methods, it puts the heavy skin outboard, where I like it.

    It's economical of materials, as one can be (easily) selective as to what to insulate and what not. We can mix-and-match, with inexpensive doorskin (1/8in plywood, used for hollow door construction)  inside cabinetry, and 1/4 inch elsewhere, where we might want to fasten lightly into the wall. Or FRP (Fiberglass Reenforced Plastic) panel in the galley, for easy wipe-down. It takes special glue, but may be worth it.

So it looks like number three for us. 

We'll be insulating the entire interior this time, including holds (unlike SLACKTIDE). We found that even the holds condense. The forward achor well seems to be the only exception... it may condense as well, but very little, and has a high salt content.

So, having waffled away at this for weeks it looks like the following:
  • 3/4in ACX outer face
  • 1 1/2in foamboard + 2x framing of western red cedar 
  • 1/8in mahogony doorskin and 1/4in ACX
  • Waterbased contact glue for foamboard faces with Gorilla Glue around edges (gap-filling) 

So, until further notice, that's the plan!


  1. So, which foam are you thinking of using Dave? There are so many the mind boggles! Polystyrene? White or blue? I know they are all good insulation, or at least I have read that they are! I am a bit dubious though. I have read quite a lot about them all, and I don't much like the level of toxicity produced by most of them, or the possible fire risk., these things seem to burn fiercely. So to be honest, I am no nearer deciding which to use: sheepwool batts? Foam boards, mineral wool,, I don't know! Damn that's started me off again! :)

    1. Hi Joel,

      Blue XPS (eXtruded PolyStyrene) is fully water proof (unlike beaded and open cell foams), and both affordable and available. If we had our 'druthers, we'd go with polyurethane, but it's out of reach.

      XPS is R7.5 at 1.5in (more than we really need).

      Toxicity is relatively low at any given time (I've read all this... factoid alert!). It results from slow out-gassing of insulative gas from within the cell structure. In other words, we want a barrier coating to keep the gas from escaping, both for our long-term sakes and to keep the R value as high as possible.

      Various glues designed to work with foam provide this barrier, so long as it's fully encapsulated... another reason to not leave air voids. Vinyl wallpaper with the right glue works as well, if you're not bonding them in, taking care to cover the backside, as well.

      Glues work, by the way, with foam-and-porous and/or foam-and-NON-porous surfaces. If we decide to use FRP (Fiberglass Reenforced Panels) in the galley, which are non-porous, we'll likely upgrade to a glue which can handle either porosity. More expensive, but less so than ordering two different kinds. I think. Shopping ahead!

      Fire is a threat, though not nearly so much within a SIP. The ply isolates the foam from oxygen until it burns through, at which time it's fully engaged, anyway. In those early, critical moments, foam is isolated or only exposed in a limited area.

      Preventative measures, in our case, include the sheathed stove bay being inset from the sides by 9+inches (woodbox between), and running all electrical inboard of the SIP.

      Batting (fiberglass?) is attractive for cheap and easy. I'd build a mockup and throw it in a tumbler (maybe a cement mixer? Partners don't usually take kindly to using the family dryer!) to stress test it and see how it holds up.

      An traditional approach worth considering is ceiling based ventilation. Essentially hollow walls, open at top and bottom (connecting across the bilge). By day, sun warms one side creating upflow, which sucks downdraw on the dark side, rotating air. When there's not enough differential for rotation, air stops and becomes 'dead air' ventilation (approximately R1 for every transverse inch, plus the R-values of the wood involved). I get nervous with voids of that nature that can't be examined occasionally, or gotten to in a hurry in case of holing. But, with good planning and light screws on the inner panels, this can be gotten around in many layouts.

      Good luck with your waffles! Hope they come out toasty. 8)

      Dave Z

    2. PS. For mockups, we've usually been able to obtain scrap from construction projects that were plenty large to conduct our tests. Crews are usually very interested, helpful, and want to hear how it went. Some of them even like to get involved with the project, and have been a big help!

  2. Posting on behalf of JOHN:

    Hello Dave,
    Could you explain further why panel assembly method #3 has no interior butt joins, but methods #1 & #2 do? Probably has something to do with where frames are -- no more than 8' on center? I also don't understand why #3 allows you to put the heaver plywood on the outside, but #1 & #2 don't? Seems like you could build the panels in any configuration you wanted.

    Do I understand correctly that you are planning to glue the foam to either the inner or outer ply panels? If so, are you gluing them in just to hold them in place until both ply skins are attached (after which the glue is superfluous), or are you wishing to increase the structural strength of the panel by gluing them in? (Styrofoam has little sheer or compressive strength, so there is little point in using the foam itself as a structural component)

    Are you planning to glue/seal the inner plys to the frames and stringers, or will they just be mechanically fastened? I think that gluing will make the panel stronger, and will keep moisture out of the middle of the panel better. But using just mechanical fasteners alone might allow for future panel interior inspection and/or repair.

    Is the use of Gorilla Glue to adhere the foam to the panel frames, or are you just using the glue to seal the foam to the frames? My understanding is that Gorilla Glue isn't very strong if the glue line is over a few paper-thickness thick, plus the Styrofoam itself is rather weak.

    For sealing have you considered spray foam that comes in a pressurized can? Many of those types of foams are open-pore, so will absorb water, but maybe that wouldn't matter if you glue the inner panels to the frames and stringers. Indeed, if the foam fits in the cavity snugly maybe you don't need to foam/glue it in at all.

    It's fun seeing your plans take shape. Good luck!

    1. Hi John,

      RE Panel Order - You're right that we can build in any order we want. But what we WANT is thicker ply up against the bulkheads AND would prefer thicker ply outboard, to stand up to the mean ol' world. Only option #3 does both.

      Generally, our SIP to blkhd options are: internal SIP framing along the bulkheads (more work, material and weight), pulling a tricky (drill through outer, thin ply and foam to outbrd face of inner, thick ply; fasten through inner ply into bulkheads; plug with dowel... lots more work, but no extra fitting and little material weight), or nailing directly through ply into blkhds (preferred... easy and thick ply outbrd).

      The #3 option fulfills our wishes, but eliminates the possibility of contiguous SIP sides, which are broken up by bulkheads and furnishings. Our choice now is to build what we can on the bench, or retrofit in the smaller, rectangular spaces, working between bulkheads and around furnishings.

      In fact, we'll likely do some on the bench, but will avoid creating 'slots' between adjacent, foamed sections... that requires precise assembly of large, unwieldy components that we reckon is not worth it. Every other area, or so is possible, so long as we can avoid crossing over ply butts (which precludes panel by panel assembly).

      RE Gluing Panels - Yes, we're gluing both foam faces to the internal faces of ply. This creates a composite which has no air voids (if we do it right... eliminates fungal intrusion), AND forms a girder structure which is much stronger than the sum of its parts.

      The sheer strength of foamboard is not high, but, added up over a large area, amounts to substantial (try a mockup/destruction test... you'll be surprised!). In the retrofit method, each smallish panel of ply is further fastened and glued. The result is that there should be no sheer load on the foam until the ply itself gives way.

      Stresses travel through the skins. By separating (even with air), a girder is created; the skin goes from one veneer to full ply thickness for a vast improvement in strength. The foam itself is not bearing much load, but serves to support the ply faces, and physically link them. They can't independently flex (until overwhelmed), so the ply is tensioned away from its center (around a greater radius than the outboard ply). Until it fails, it is providing considerable support to the outboard layer. In other words, a SIP composite is MUCH more rigid than a solid wall that was composed of the two ply layers laminated as one, or even than a hollow wall girder.

      Note, too, that the furnishings are arranged as girders, themselves (air filled). They support and limit the areas of potential flex to spaces between bulkhead/furnishing landings).

      As it is, without a SIP or even the second layer, single, 1/2 inch ply is adequate for this hull. The 3/4 inch ply we're going to use gives an extra margin of safety.

      More importantly, it has to do with economy of scale. A 'raft', 'bunk', 'skid' or 'pallet' of plywood has so many sheets. To get the discount, we're doing both bottom and sides of 3/4 inch. I'll get deeper into this later, in a post on Shopping (I hate it!). That and 3/4in ply provides adequate bury for a fastener, simplifying many construction operations.

      LUNA had half inch ply sides (no SIP), and would flex on the dinette side under side slop (partly due to poorly thought out support elements which are easily improved). SLACKTIDE has SIP with half inch ply - foam - doorskin, NO furnishings for a 10ft stretch and NO observable side flex. The difference is dramatic!

      Continued in next reply...

    2. ...Reply to John, continued:

      RE Gluing Inner Panels to Frames - Yes, this is the way we're going for the reasons you mention. I feel that, well done, inspection interior to a composite shouldn't be necessary.

      We do watch for deformity, and go around every year and tap around, listening for dull, thuddy sounds (could be internal rot), but have never run across any (well, there's a rub-rail situation for another time... basically came with rot that we fought until replacing).

      Someday I'll blather about my 'boat as sculpture' vs 'boat as an assemblage' opinions, under which this question falls.

      RE Gorilla Glue - GG adheres only moderately well to foam faces, and its expensive to be non-optimal.

      Being expansive, however, it works well at the edges, getting at least some grip, but mostly filling up any fitting errors. Most of the strength, however, is coming from the ply, which, motion limited at the frames, becomes very stable along the edges.

      GG is very much weaker when expanded (becomes PU foam, in essence), but if you've ever tried to tear off a spilled, expanded, cured lump of the stuff, it ain't easy! It far exceeds the strength of XPS (which is a push-over to crush or break).

      For general lamination where adhesion is good (ply to ply, say) I think even expanded GG is plenty adequate. Picture twisting apart an Oreo cookie... that's pretty much what would have to happen to a laminate to get it to pull apart in sheer. In practice, no forces are acting to torque the hull.

      Peel forces (delamination) are countered by edge fasteners and generally excellent (non-expanded) bond in the neighborhood of ANY fastener. It would take considerable warpage, between fasteners to allow significant expansion.

      Confidence is high!

      RE Spray Foam (I'll add Adhesives) - Andy Stoner used both in MARY ELISABETH. I'm impressed by ease of use, but underwhelmed by cost effectiveness, complete coverage and the sheer amount of nasty, toxic GARBAGE they produce.

      The 3M (contact) Adhesive (designed for foam) worked super well, like all 3M products. But it's expensive per can and takes a LOT of it to do a hull surface. The 'spray one side' option was inadequate, we felt, for marine use, so it only went half as far. I prefer latex based glues that come in 5 gallon buckets (haven't narrowed in on which among many, yet).

      The spray foam worked amazingly well at producing foam. It's downside is that it didn't want to penetrate the edges, so left many (small) voids. The solution would be to allow a gap around the foam and fill with unimpeded spray foam, but that would take considerably more.

      So, our 'greener' solution (so-called) is tight-fit XPS, with GG to expand within miniscule voids.

      It's a thought to spend more time (build a cutting jig?) for a perfect foam fit... slightly tapered, it does crush-fit, and with a generous glue layer inboard and outboard to the ply, should exclude vapor. Thanks for the idea, we'll think on it!

      Thanks for all the great questions!

      Dave Z

    3. Posted on behalf of JOHN:

      Hello Dave,

      Now I better understand your SIP evaluation process. Your putting the heavier ply outboard, and also directly against the bulkheads makes sense to me.

      I still have this nagging idea that a glued ply-Styrofoam-ply panel will provide little more strength than a non-glued ply-Styrofoam-ply panel. Initially, yes, a glued panel it will be substantially stronger, but boat hulls work so much in waves and against docks that soon, I suspect, the foam itself will sheer, I’m going to guess, at the inner ply glue line. Not the glue line itself, but just inside of the glue line within the foam. The foams that are used professionally in boat hull construction have a much greater sheer strength than Styrofoam.

      I think you suggest that you anticipate little or no sheer stress in the foam, so you then would argue that strong sheer strength in the foam is not needed. Consider a deck of playing cards flat on a table, but supported at either end with, say, two pencils. Also on the table and supported at the ends by the two pencils is a block of wood the same shape and thickness as the deck of cards. If you push down on the center of the deck of cards with your finger you can easily bow the cards. But the block of wood could probably support your standing on it without bowing. Why? Both the cards and the block are made of cellulose. The big difference is that the block of wood has sheer strength while the deck of cards has very little sheer strength.

      My concern is that if your structure really is relying on the SIP to provide critical stiffness, you may be disappointed. However, as you suggested, increasing the hull scantlings from ½” to ¾” ply may provide sufficient stiffness in itself to never stress the foam to the point of failure. And of course you could put 1-1/2” wide stringers between the two layers of ply, glued and screwed, to provide the needed sheer strength between the inner and outer layer of ply. I make an issue of all this only because if you are not really going to benefit, mechanically, from your SIP, you might as well save yourself time and expense by not gluing the foam to the plywood. Just fit the foam as best you can into the cavities, and spray closed-cell foam into any cracks that you can’t fit the ridged foam into.

      I’d be curious what type of foam you will use to insulate the bow and stern of the hulls, where the hull is curved? Will you use a flexible foam (like urethane, ethylene, Armor-flex), or will you somehow bend the Styrofoam with heat or by kerfing?

      Keep your pencil sharp,

    4. Hi John,

      Great thought experiment (cards vs plank). I'm thinking, though, that the cards need a little help to bring them up to par. They illustrate the concept, but are structurally quite different than a framed SIP.

      To be made analogous to ply-(cheap)foam-ply, I suggest the following: a) make your outer 'cards' of doorskin, b) frame the perimeter, and c) add some 'non-skid' between each card and doorskin (something non-adhesive, but which raises drag, simulating weak foam... might be easier to just make a scale ply-foam-ply panel). Also, use four pencils, strongly glued around the perimeters.

      My guess is that you will find that it takes a LOT to seriously bend the cards, once this has been done. Until appreciable deflection occurs, sheer forces stay low. Opposing deflection, you've got tensile forces within a flat panel... (arced SIP panels involve compression forces as well), and compression forces in the framing.

      Without the foam, each panel is 'web-frame' construction, as one finds in bulkheads, but doubled. With foam it's that much stronger, and the glue wears the other hat of excluding moisture/fungus laden air (worth the cost and effort, to my mind, at least in the PNW).

      Looking at our layout (hasn't been posted, yet) our largest panels unsupported by structure (counter, platform, dinette table and shelf edges, and guards), are 2ft by 3ft.

      That's 864in2... if a ton of force whops that, it's only 1.5psi. In comparison, each of my feet has about 48in2 of area, and I weigh about 175lbs, geared up, so we get 3.6psi. If I jump or step on the ball of my foot, it halves area and doubles impact. Some of our friends double that. Our SIP decks don't visibly flex at all... backed up by the same observations from SLACKTIDE under water impacts (ST, btw, has much larger unsupported panels).

      Upshot is, I'm pretty confident.

      I'd be reluctant to use cheap foam for a super-light build (1/8 ply-foam-1/8 ply ), but it might well be sufficient. Gougeon Brothers know a lot more about the limits than I do.

      As a reminder, I'm not trained to make these kind of assessments, and could be way off. All I've got to go on is what I've read, seen or experienced. This is complicated by taking 'quick and dirty' paths through the options that others have pioneered. Glues aren't epoxy, no fiberglass is used for structural strength, the hulls are only nominally monocoque. Lots of ways this can go wrong. But so far, so good.

      RE Forming Foam to Curves - We don't, to date. We build our bottoms solid and pretty thick (partially for insulation above the waterline.

      We did look at the SKROWL option, and would have kerfed the foam (filling kerfs with expansive foam) if we'd gone that route.

      Heat forming might be possible, but, unless one is geared up, that could easily go very wrong, seems to me!

      Thanks again for the questions!

      Dave Z

  3. Been following you and your brothers blog for over a year now-really enjoy them-have two questions
    1 what is the thickness of the copper plate you use
    2 what is your thoughts on fiberglass-foam-fiberglass panels as opposed to ply-foam-ply construction. Been thinking of ply-foam-ply panels covered with fiberglass than thought why not leave off the ply- it would be lighter and cheaper.

    1. Hi Linn,

      Glad you're enjoying the blogs!

      RE Copper Plate -- Bottom plating is 3/1326 to 1/8 inch (LUNA's mid-plates at rocker bottom were 1/4 inch). We prefer the heavier plate, but have to juggle expense... as of yesterday ($5.50/lb), that 1/32in difference, spread over the whole bottom, costs an extra $3K! Sides get 1/16 inch. 1/4 inch bronze angle along the chines.

      Weight (ballast) works out to about 15% to 20% of displacement. The lower figure assumes a snug rig low total sail CE, and more-than-usual food and gear secured low in the hull. That being said, however, many sailing barges carried NO ballast, relying on high form stability, despite large rigs. These were professionals, though, and usually didn't have willawaws to contend with. We prefer ballast for more care-free sailing.

      RE FG-foam-FG -- I think this is the best approach to an insulated, FG (FiberGlass) hull, especially if done at construction time.

      One diff to note, in replacing sheathed ply is that the ply is structural, while the sheathing is barely so... to replace the ply, it requires full FG lay-up and much greater quantities of resin. It amounts to a whole different procedure, not just a time/energy-saver. Be sure to crunch the numbers. Probably lighter, not so sure about cheaper. Also, remember that foam must be resistant to whichever solvents are present in your resin.

      Several things keep us away from FG; our aversion to working with resins, the cost and difficulty of remote FG work and repair, and (near) impossibility of using copper plate. If these considerations don't apply, FG certainly can produce a fine hull. Pretty much analogous reasons keep us away from aluminum or steel construction, too.

      Wood is a medium we understand, are tooled up for handling, and can be repaired in the field with found materials. We can actually MAKE our out plywood from driftwood! Being able to copper plate (an advantage over even copper sheathing, Curvy Doggy style) gives us the mechanical protection for heavy grounding. Wood in sheet materials lets us easily fasten the copper, while right angle sections allow simple bronze angle protection (vs fabricating a wider angle which may vary along the chines). Elimination of side curvature allows simple angle forming in two (vs three) dimensions. All these work together, in our favor for how we cruise.

      But the IMPORTANCE of these qualities varies with how one intends to use one's boat. I daresay that MOST folks would have a wholly different set of priorities.

      One cool technique I've found intriguing: Strip-plank the hull from foam strips, fair, then glass inside and out. Seems like a much more practical way to build, DIY and one-off, than building a mold. If we could figure out copper attachment, we might well overcome our other objections!

      Dave Z


About Me

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Anke and I live aboard WAYWARD, our T32x8 ketch. We sail by wind, tide and muscle in the waters of mid- to northern Southeast Alaska. We try to maximize the joys of life, and minimize the chores. ........ We live between the communities of SE Alaska, but drop in to visit with friends. Lately, we've worked, every other winter, care-taking Baranof Wilderness Lodge in Warmsprings Bay. This has given us a window on Web. ........ We're working toward a subsistence lifestyle, somewhat impeded by addictions to coffee, chocolate and cheese. ........ We think TEOTWAWKI is looming, and while we won't be ready, we'd at least like comfortable seats.