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

Notes:

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.