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06 November 2014

Insulating the Hull: Retro-fit SIP

Adding inboard layer of 1/4in ply
Next space to my right has fitted foam ready and waiting
Two areas to my right, the inboard face of the hull is exposed

This guy oughtta be wearing his ear protectors!

Insulating the Hull: Retro-fit SIP

Anke and I recently finished insulating the hull cabin spaces, using ply-foam-ply aka SIP (Structural Insulated Panel) construction.

I can think of five, basic options for SIPping hull sides:
  1. Join pre-fabbed SIP panels to make sides
  2. Build one skin, with framing, then add the second skin in one go
  3. Build one skin, with framing, then add second skin in select areas

    ...Join finished sides to bottom and bulkheads.

  4.  Build inboard skin, framed and joined to bottoms and bulkheads, then add foam and outboard skin in one go
  5. Build outboard skin, framed and joined to bottom and bulkheads, then add foam and inboard skin in select areas
We decided the first three had some problems for us.

Building remote, we didn't expect to have the muscle to raise heavy-ish, completed sides into place. Short-handed lifting with mechanical assist takes time and ingenuity.

The first presents framing challenges, since many framing pieces internal to the SIP extend beyond a single panel. No go.

Two is better, and straightforward. But all transverse bulkheads and dividers meet the inner skin...

To join sides to bulkheads, in this approach, fasteners must be on the long side (to reach through the SIP with enough left for 'bury') - expensive. Alternatively, shorter fasteners may be inset and plugged - time consuming. Tape n' glue with judicious use of resin welding (tape n' glue minus the tape) is an elegant solution, but messy, toxic and generates lots of heinous garbage.

Method three addresses these problems, but there's a quandry. Transverse dividers must either:
  • Contact the outer layer (complicated) - SLACKTIDE, with her minimal furnishings, was manageable, but with the many sub-girder furnishings this round, this approach gets out of hand.
  • Attach inboard of the SIP - If using a cleat, then that needs something in the SIP to fasten into (double framing). Resin welding looks preferable, in this latter case, but with resin's downsides.

Method four is intriguing, but we were ultimately uncomfortable with many of its questions and consequences.

For example, which to put outboard; the thicker layer or thin (1/2in vs 1/4in ply)? Thicker outboard makes for better protection against the cruel world. But inboard, it affords more structural integrity and a strong backup to outer skin failure.

Our window framing (2x stock) doubles as structural reinforcement AND we want to see its clear, red cedar in the interior. This implies we want it on the inboard side of the thicker layer.

So number five it is - Retro-fitting foam and ply on the inboard face of a fully functional hull. QED. All framing (2x2 or 2x4) doubles as spacers for 1 1/2in foam (blue-board). Light 'firring' (non-structural spacers) from 1x2 fills in where there's no framing.

A big advantage of this approach is that all transverse panels (bulkheads, partial bulkheads and furniture faces) meet the structural skin of the hull. Not only is this strong, but short (generally 1in fasteners) suffice for the 1/2in ply skin.

Another is that most, if not all inboard panels are relatively small, bite-sized glue jobs. Divide and conquer, with every juncture fully visible until completed (i.e., don't have to try to remember where we put that framing inside the SIP!

So how did it go? Pretty durn well!

In our salon and bunk areas, the surface of seats and the bunks are all the same height. In the galley, all the counter levels are the same height. In a box barge, this implies parallel spaces between framing. Foam and the inner layer of ply can be cut into strips, then lopped off for the necessary length... efficient in both time and materials! Even over the end-curves, we start with the same strip for a given space, and cut it down further to match the odd shape.

We used TiteBond III through the summer months, switching to Gorilla Glue as the days grew cold and damp. This is supplemented by DAP Alex Plus ('siliconized acrylic'... basically a latex window caulk) to fill any larger voids. For whoppers, consider wood or foam shims, and for small ones, consider a bead of (expansive) Gorilla Glue.

TIP: For nice, tight FOAM fits, consider using a tick stick (a stick which is held in place and lengths 'ticked' directly from the structure, then transferred to the material). This is much more reliable than measuring. The tighter the better to exclude air.
 TIP: PLY is easier to fit (and caulk) if it's loose around the edges. Too much accuracy, and you risk wasting time needlessly fighting the material into place. Consider erring on the side of looser.

Standard Operating Procedure is to dry-run and prepare. Take a deep breath. Then...

Slather glue on the inboard face of the hull and framing / firring faces (those abutting the foam). Insert foam working toward one corner to release air (may have to relieve air pressure with a knife, inserted through the foam and given a slight twist). Slather glue on the outboard face (foam side) of the inner ply cover. Nail into framing / firring.

TIP: Consider penciling guides to framing on the inboard face of the inboard layer - 1x along this edge, 2x along that one, nailer line across here, butt-strap along there, etc.. Once you've covered it over and started nailing, it'll be these or your memory to guide you.

We use this order to ensure that plywood faces interior to the SIP are encapsulated by glue, in case of voids. Moving quickly (especially in the case of Gorilla Glue) allows both layers to be completed in one go, avoiding clean-up between layers.

At this point, press outboard near the mid-point of foam areas. If you hear sucky noises as you press and release, you've got a void. Use one or more struts / blocks / wedges (creatively) to provide and hold pressure. Consider that some glues are setting up, and this step must be completed with dispatch.
TIP: Before starting the glue job, consider pre-arranging strut materials to fit the situation. You may not have time to improvise once glue is setting up!
Once the glue is set up, remove struts and caulk edges around the ply with DAP or your favorite trim-in-a-tube.

TIP: It's messy, but we like to fillet the caulk with a blunt (gloved) finger. This forces caulk deep into any gaps, and feathers the edges along the (now concave) 'bead', making for easy cleaning down the road.

E basta!

In theory, the result is a very rigid, insulating wall with no internal voids to collect moisture or dry-rot.

2x2 Framing along top and right edges, 1x2 'Firring' along left and bottom edges
Struts with protective pad and wedge...
Blocking at other end (not seen) adjusts length against opposite wall or furnishing


In these sad and diminished times, little is as it seems. Consider checking your dimensional lumber for accurate sizing (same thickness as your foam). Too thick and you get voids over the foam (plane or saw your stock to size). Too thin and you get voids over the framing (shim or caulk over framing). Either are a major pain where you sit!

Red cedar seems to be an great choice for framing. It's strong, light, glues well and is naturally rot resistant. Works easily and smells good. Be careful of breathing the dust, though... it will sensitize you faster than most (the allergic kind, not the SNAG kind).

Consider laying out your panels on paper, figuring the most efficient way to to get what you need from what you have on hand. It's time consuming, but conserves material and/or $$. Can be done at home of an evening.

Gorrilla glue seems to spread adequately at a rate of about 18oz / 4ft x 8ft / one side. It gets sluggish and inefficient to spread in cool to cold weather. We warm it in warm water to about body temperature to keep it viscous. Still sets down to 20degF (about -10degC)... as low as we've tried it. Feels a little more brittle if applied in the cold, though. Not sure if or how that affects bond. No failures to date, though, on wide-area bonding.

Cutting foam with a table saw produces neat, accurate edges. But when cuts are too wide for our max fence setting, or for short cross-cuts, we use a japanese saw (thin blade, cuts on pull stroke). We walk a quick-square along as a visual guide for near perfect, right angle cuts. The edge is less smooth, but doesn't appear to matter. A whiz might be able to use a circular saw, but I end up destroying one or both sides of the cut.

TRICK: It's easy to lose track of framing that has been SIPped over. Say you want to mount a sturdy row of coat-hooks... can't just screw into 1/4in ply and foam! Before it's covered up, put a galvanized steel nail at each end. Later, when you've forgotten what's in the wall and where, use a magnetic 'stud finder' (they come small) to locate them. Fasten anywhere along the line between them.

TRICK: The method we use calls for a lot of notches in plywood around framing.To lay out notches, keep small blocks of your framing stock handy. Lay onto plywood in the correct orientation and trace one side, then reposition and trace the other. Much faster than squares and measurement, and more accurate.
TRICK: For the deep end of a three-sided notch, cut the two sides with a saw. Layout the backline between cuts on the back face, and slice deeply along from both faces with a sharp knife. Grasp the 'tab' between cuts and bend back and forth until it breaks out. Clean up with your knife, if necessary.
TRICK: If glue sets up and you believe you have a void, drill into it at one edge with a bit sized for your glue bottle, and one or two spots about six inches away. Inject Gorilla Glue until it wells out your other holes. Repeat as necessary, covering the whole problem area. Plug holes after glue has set up (if you plug right away, the expansive glue will likely blow your plug). It helps to cut your nozzle square for this job. Old caps may be saved and used for this.



  1. Dave and Anke,
    I've recently read your Triloboats website(s) from beginning to end with great interest. Pearl and I have often talked of living in SE and if we did, it would likely be similar to the way you're doing it. Engineless sailing, Bolger's AS concepts and your Triloboat designs all resonate strongly, not to mention a love of that part of the world.

    We were amazed to discover that you were undertaking such a large boat-building project in such a remote area. Particularly since we're familiar with Tyee, having visited there briefly in the summer of 2013. Corrine showed us around the old whaling artifacts. Here's a link to our trip blog if that's of interest:

    While Tyee is obviously remote in terms of available boat-building supplies, we also understand that it would have some significant advantages. We were quite surprised when Corrine told us there was an internet connection there and we're glad to see that it's still up.

    We've been enjoying imagining you building a Triloboat there. We occasionally check weather reports for your area and wonder how it's going as winter comes on. Looks like you have some sunny weather forecast for the next few days. A welcome respite no doubt, as it doesn't appear to be accompanied by colder temps.

    We wish you all the best and look forward to following your updates.


  2. Hi Dave and Pearl,

    It would be wonderful to have you in the neighborhood! Sounds like you got a good taste of it on your trip. That was an impressive stretch you covered!

    You probably recognized your picture of the old steam-plant in Tyee in one of the posts on this blog? One more cross reference... your friend Linda from Sitka hosted Anke when she first came to the US in '88 as part of a cultural exchange program. Small world!

    Remote hasn't been as quiet as expected... lots of folks moving through this year for one reason and another. When we're alone, though, we can really focus on building. The downside is that everything had to be ordered and delivered beforehand, with only a few, usually short-notice, chances at resupply. We're in the cabin that Corinne was in, and the internet and phones have been working fine. Two winters ago, we caretook here and the phone tower (near Warm Springs) iced up in November and stayed off-line until April. Guess we'll see...

    Things have been WET this autumn! Glad to hear that YOU hear of sun (we're hearing 'mostly cloudy, chance of rain'). But Tyee runs a little dryer than its surroundings... here's hoping. If we can get the decks on and sealed before snow (crushes our shed), we'll be happy. 8)

    Thanks for the good wishes, and hope to cross paths with you sometime.

    Fair winds,

    Dave and Anke

  3. Seems like the original intent of SIPs still is their strongest allure: mass production of panels then erecting them quickly and sealing the edges. A few with windows framed in. Barge in a complete "kit" and instant boat construction. Probably way too simplistic to work though and only viable for straight sections. Seems it would cut construction time down even on a advanced trilo barge (then insulate the bow or stern sections as the owner sees fit). Really viable on that 40 trilocat! Best of saltwater deliriums to yas.

    1. Hi Robert,

      I'd LOVE to start from prefabbed SIPs... no shortage of straight sections in Tboats. Even the curves could be planked up from SIPs stripped into staves.

      The big problem has been finding ply sided SIPs (vs OSB sided) that are in our price range. Most seem to use 4in foam, too... works well on wider beams, but really shrinks an 8ft interior!

      DIY SIPs seem like they'd be a good way to go, too, but would benefit from a more controlled shop environment. Vacuum bagging looks great for laminating them up, but a 'press' wouldn't be to hard to work out. It'd really pay off in a cooperative situation, where several boats were getting built.

      And pelagic perambulations back atcha!

      Dave Z

  4. Davd and Anke,
    Sorry to get off-topic here (the topic of course being barge building), but I'm not sure where else to contact you.

    Anyway, just wanted to pass along a link to the NOAA pin-point forecast for Tyee.

    Looks like sunshine for you well into the week. Happy building and we're rooting for you to get the deck up before the snow comes down.


  5. Hi David,

    Feel free to drop me a line at triloboats swirly gmail point com. And don't worry, weather is very much on-topic for DIY builiding!

    That's a great link... I had no idea they had such a feature available. We pay attention to N and S Chatham Sound, Frederic Sound, and the Southern Admiralty and Eastern Baranof Island Zone Forecasts. These guys cover a lot of ground, though, and it's we're at the cusp of the three waterways.

    Got sunshine most of today (the 'clouds' that appeared look more like high fog, so it may burn off, tomorrow). A welcome change.

    For anyone interested in weather, I highly recommend Eric Sloane's WEATHER BOOK. More apt illustrations than text and covers the whys and wherefores better than most I've seen.

    Thanks for the link and good wishes!

    Dave Z

  6. Dear Dave, I follow your designs with enthusiasm. I know you live in your boats, but I noticed you hardly refer to them as houseboats. Why not? It seems that "square boats" is a more stylish way to talk about them. And refering to them as "barges", as they really are, brings perhaps a more traditional appreciation of your concept. Do you have anything against houseboats? You could perhaps "tag" your texts a bit more with "houseboat" and attract quite a big niche too. Cheers!!!

    1. Hi Ricardo,

      It's a habit from two reasons:

      First, I just tend to think of a 'houseboat' as more stationary, while if it's mobile, I think of it as a 'cruiser' or 'cruising home'. Not sure why... probably has to do with the order of books I read. 8)

      More seriously, 'houseboats' fall into a different legal/political/social category. Concerned Citizens have often passed legislation against them.

      An example from 'The Last Frontier': Sitka, Alaska, allows a limited number of licensed 'houseboats' in 'The City and Burrough of Sitka'. This includes most of Baranof Island, a good chunk of Chichagof and surrounding waters well offshore. Sitka is the largest city in the world by area, and has more seawater under its jurisdiction than many nations. Sucks. Meanwhile, 'vessels' may anchor where they choose. While still subject to 'move-along' rules that are ever increasing, these are seldom enforced. Yet.

      Just wrote a rant touching on the subject at:

      So, whenever an official form comes our way, it's 'Sailing Vessel' and NEVER 'houseboat'.

      TriloBoats seem particularly prone to the designation, too, as they often resemble a European, standard houseboat. My advice is, avoid the term, outfit with all vessel requirements, and mount propulsive gear (sails, sculling oar, motor, etc.). Even if they're just for show, never admit it!

      "This is a working vessel, my Friend!"

      Maybe we'll get away with it?

      Dave Z

  7. Thanks for the explanation regarding the designation of triloboats as vessels. Very interesting. I love boat building and studied quite a lot about it. (I built a ply-glass kaiak last year). I always focused my studies in cruising boats. But I went to Amsterdam once and went crazy with the houseboats there. Many of them are stationary forever. Some are even built in channels that have lower bridges in both directions, meaning the houseboat will never leave that spot in one piece. I also read a nice book "Handmade Houseboats: Independent Living Afloat" (Russel Conder). He wrote something funny like "some owners prefer houseboats that look more like a boat, it is not necessary to make a houseboat look nautical: it is nautical". Anyway different views and llegal issues about the same design compromises. cheers, Ricardo

    1. Hi Ricardo,

      It's one thing that I love about box barge/scow hulls... they'll float most anything we can dream up!

      And like our selves, we tend to dress our boats up to reflect the dream. I see houseboats that are festooned with decoratives to be a lot like tattoos and earrings. Not necessary, but a way to project one's inner, percieved persona outward.

      Not my style, but I enjoy looking!

      Dave Z


About Me

My photo
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.