Insulating a Boat Hull – Part 2

This is part two of a three part series. Be sure to check out part one and part three.

Old Boat Insulation

This old vinyl covering is black in places with mold. It felt good to rip it out and disinfect the V-berth.

After identifying the materials that I had at my disposal, the real work began in earnest. The first step was to remove the old vinyl covering that had been glued to the inside of the hull. I’m sure that when the boat was new, this stuff was great, but 20 years later large swaths of it had turned black from mold growth. It felt very healthy to rip this stuff out and douse every square inch of the V-berth with bleach-water.

I showed in the first video the tools I used to get the glue off of the hull. This glue is used in a lot of different construction and is a bane to the home remodeler. Even with the sealant remover and the steel wool, it required a *lot* of elbow grease. Thankfully I didn’t have much square footage to cover (compared to a house).

Once the vinyl, mold, and glue had been removed, I was left with a clean, white fiberglass hull, coated in epoxy gelcoat. Gelcoat is not a very good material to fiberglass to directly. The West Systems 103 fiberglass epoxy I use is designed to adhere much better to the polyester-resin fiberglass underneath the gelcoat. That meant I needed to grab my trusty rotary grinder and gently grind off the thin layer of gelcoat covering it. I needed to be extra-specially careful since my boat is in the water and much of the work I was doing was below the water line.

Fiberglassing V-berth Hull

I use duct tape to hold the studs in place while the fiberglass hardens

I can’t stress enough how careful you need to be when grinding gelcoat. This was hands down the unhealthiest part of the project. Gelcoat turns into a super fine, electrically charged dust when ground off. That means it goes everywhere and coats everything evenly. It’s very hard to keep out of your eyes and mouth, even when wearing a dust mask and goggles. I ended up duct-taping my shop-vac to the handle of the grinder. I used some bilge-hose to create an ‘extension cord’ for the vacuum, so that it could be outside when running. Much of the gelcoat particles are fine enough that they come right out the other end of the vacuum. I also used a plastic drop-cloth to cordon off the V-berth from the rest of the boat. In hindsight, the drop-cloth was a great idea.

T-Nut Upholstery and Insulation Mount

This shows the T-nuts, finishing washers, and machine bolts that I use to mount the upholstery over the insulation.

Before grinding, I carefully measured out sixteen-inch intervals where I wanted to mount the hemlock ‘studs’. The strips of hemlock are ¾ inch wide, so I tried to grind away a 1 to 1.5 inch strip of gelcoat. That left room for plenty of overlap. Between manufacturing blemishes and the grinder, there were plenty of dips and grooves instead of a smooth surface. To deal with this, I laid down two layers of a 1-inch strip of fiberglass cloth between the hull and the hemlock. That worked well to fill in any gaps between the two.

There is also a slight curve to the hull. Most of the hemlock studs measured out to be twenty-one inches long. I ended up cutting them into three seven-inch pieces and fiberglassed them on separately. This made a much tighter fit to the hull. I used duct-tape to hold the hemlock in place while the epoxy cured. After it had hardened, I removed the tape and painted over the surface of the hemlock with another coat of fiberglass epoxy mixed with 403 particles. This coated the wood, sealing it, which will protect it from moisture and mold.

Before fiberglassing the hemlock strips to the hull, I mounted a T-nut in each piece. I had never worked with T-nuts before, but they were much easier to use than I expected. These will allow me to use machine bolts instead of screws for mounting, and will allow easy installation and removal of the upholstery.

Related posts:

Vegetable Soup
Boat Curtains
To Friday Harbor
5 Responses to “Insulating a Boat Hull – Part 2”
  1. Alan K says:

    Hi Chris,
    On an old boat I had I used closed cell foam camping mats. They were relativley cheap. Easy to form. Did not sweat at all and did a good job insulating and of course would not absorb any water. Another thought as well is perhaps use some other wood than hemlock. Its not very rot resistant and even if only damp from humidity tends to mold. Although since your glassing and sealing, it probably doesnt matter. It’s really impressive how meticulous you are . She’s going to be a really comfortable home and keep you safe and warm. A great step toward’s your freedom. Looking forward to seeing how it comes out.
    Alan in PA

    • Great tips Alan. Thanks! I think other people looking to insulate their boat will appreciate them too.

      The Foamular insulation I got cost about $17 for 4 foot by 8 foot sheet, so it was pretty cost effective. I was worried about rot and mold attacking the hemlock as well. That’s why I was careful to hermetically seal it in the fiberglass epoxy.

  2. Brian Stannard says:

    Looks like you are doing a good job.

    A correction to your terminology though. Epoxy is a resin. Polyester is a resin. Fiberglass is a cloth that can be wet out with either resin. Gelcoat is polyester (not epoxy) resin that is basically colored with pigment. Epoxy is a better choice as you have found when attaching items to a fiberglass boat. It can be thickened with any number of compounds from sawdust to colloidal silica to micro balloons. It is easier to use than polyester and doesn’t smell strong. It is more adhesive than polyester ass well as more water resistant.

    • Chris says:

      Thanks Brian! I really appreciate the clarity there, and I think other readers will too.

      I learned something new too. I always thought that gelcoat was an epoxy, but you’re right that it can be both. My understanding is that boat builders prefer to build the hull with polyester resin because it’s cheaper, but then coat it in an epoxy gelcoat because polyester resin absorbs water whereas epoxy does not.

      • Brian Stannard says:

        Epoxy is used for barrier coats under the waterline, but not above the waterline. It is sensitive to UV so needs to be covered with paint. Gelcoat is most commonly polyester and sometimes vinylester. Vinylester is less prone to blistering and is used now by many builders for the outer layers. Epoxy is only used by a few builders of more expensive boats, usually race boats and always painted. Gelcoat as a finish doesn’t adhere to epoxy. Epoxy adheres to polyester very well.

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