How To Insulate a Sailboat – Part 1

This is part one of a three part series. Check out part two and part three when you’re done with this one.


Photo by Jack Amick

This article presents the research I’ve done in anticipation of insulating the hull of my sailboat. I’ll cover the actual work in a future article.

One task I’ve had stored in my winter arsenal of rainy-day to-do projects is insulating the hull of my boat. Despite a lack of rainy days this winter, I’ve managed to plow through the lion’s share of my to-do list and I am now face to face with this semi-daunting task.

A cursory search on the internet for information about insulating a boat brought me to this wonderfull article on insulating a boat. Suzi and Larry of The Frugal Mariner did an excellent job insulating and documenting. At first I wanted to do my insulation job just like theirs, but in the end, decided to do it quite a bit differently. If you are looking into insulating your boat, I recommend you read their article before continuing with this one.

Pricing Material

The Frugal Mariner used two layers of Reflectix and a core material of polyethylene foam. Polyethylene is very expensive and hard to find in a rolls of 16 inch widths. I did find an eBay seller that sold scrap material of polyethylene foam, but when compared with the insulation material I could find at Home Depot, the eBay material was still about seven times more expensive.


I priced out several different insulative materials and here are the results:

Insulation R-Value Thickness qty x y sq ft Price $/sq ft
Reflectix ? 0.25 1 1.3 25 32.5 10 0.307692308
PolyISO 3.2 0.5 1 4 8 32 12 0.375
Polystyrene 3.9 1 1 4 8 32 12 0.375
Polyethelene 3 1 12 1.3 1.3 20.28 45 2.2189349112426
Formular 5 0.5 1 4 8 32 18 0.5625

Each of these materials have their different pros and cons. Here is a quick summary:

  • Reflectix is cost effective in terms of dollars per square foot, however it is sold as a radiative heating barrier and not for its insulation value. My primary concern is heat loss through conduction, and this material doesn’t provide much.
  • PolyISO has a high R value and low cost, but is very stiff. I’m not sure how well it would mold to the curve of the hull. It also seems really brittle.
  • Polystyrene ‘styrofoam’ is the cheapest material and has good insulation value. However, I checked on several boating forums and there are number of problems with this material that make it sub-par for use on a boat. This material is also known as EPS polystyrene.
  • Polyethelene is used in sound proofing and packaging. It’s also the material of choice for the Frugal Mariner. However, it is about 7 times more expensive than polystyrene and its insulation value is not significantly better or worse.
  • Formular is also known as XPS polystyrene. It’s the same material as styrofoam, but it’s pink and the foam is much denser. The insulation value is about four times better while the price is about double that of EPS polystyrene. Boating forums report favorable experiences using this material.

As a result of the above, Forumlar became my material of choice.


The Frugal Mariner used a series of flexible wood strips epoxied to the hull to create a scaffolding along the hull every sixteen inches. I liked this idea for several reasons:

  • It allows insulation to be used in 16 inch wide strips. This reduces the need for complex shapes and allows the use of standardized material.
  • The scaffolding provides support for upholstery, and allows for easy panelization of the upholstery. This means I can easily access the hull for future repairs and modifications.
  • The scaffolding also gives me a mount to easily run electrical wires and route harnesses around the boat.
  • The Frugal Mariner also used several sizes of wood strip and built up the insulation one and a half inches up from the hull. I don’t want to add that much insulation because it would make the cramp V-birth even tighter and because my winter sailing trip to James Island showed I don’t *need* insulation. However, I want to build it in such a way that I can easily upgrade my insulation in the future, if I so desire.

    I took a look at the material available at Home Depot and found an excellent selection of Hemlock strips. Here are material and prices I looked at:

    Hemlock strips x y $/ft Note
    LWM267 1.375 0.25 0.67
    WM239 0.75 0.5 0.59 More flexible, but risky
    WM239 S4S 0.75 0.75 0.73 Very rigid and inflexible
    WM142 0.75 0.25 0.62 Very flexible

    I had hoped to used a strip of 0.75″ x 0.75″ in order to mount a 0.5″ thick piece of Formular and a 0.25″ thick piece of Reflectix. However, that size is very stiff and may not follow the slight curve of the hull very well. I’m deciding instead to use a 0.5″ x 0.75″ piece. This is more flexible, and I can still mount a T-nut to it (more about that in a minute).

    The downside is that I have to nix the layer of Reflectix. However, if I decide later that I don’t have enough insulation, I can always fiberglass another 0.25″ or 0.5″ strip onto to the first. I want to keep the install as simple as possible for now.


    Finally, I have to mount some sort of wall material or upholstery in front of the insulation. The Frugal Mariner used FRP, which I found without any trouble at Home Depot. I thought it was rather heavy and thick. Right next to it, I saw a new material called Numat, which is really thin. Ideally, I’d like to use a material that is in between the thicknesses, but Numat looks like it will be easy to work with and is guaranteed to be mold and mildew proof.

    Upolstry x y sq ft Price $/sq ft
    Numat Plas-tex by Parkland Performance 4 8 32 27 0.84375
    0.09″ FRP 4 8 32 35 1.09375


    Finally, I had to figure out how I was going to attach the Numat to the scaffolding as well as how to adher the insulation to the hull. Again, I want stuff to be semi-permenant in case I decide to change things later.

    I decided to use Liquid Nails polyurethane caulk like the Frugal Mariner did in order to glue the insulation to the hull. I’d prefer not to glue it at all, but the caulk will also make an air tight seal, preventing air and water from condensing behind the insulation.

    One option to mount the Numat to the scaffolding would be to use stainless steel wood screws and finishing washers. I don’t like this idea as every removal will damage the wood strips and I also run the risk of getting too aggressive and screwing into the hull.

    While a screw and finishing water is a backup plan, I am hoping to use T-nuts. These are threaded sockets with prongs that can be hammered into the wood strips. The threaded socket will then accept a machine bolt. That means I can remove the paneling without any damage to the wood strips.

    Mounting Hardware Qty $ Notes
    #6-32 T-Nuts 4 1.25 Installes on 0.75″ x 0.75″ hemlock strip
    #8 finishing washer 50 5
    #8 x 0.5″ SS Screws 50 5
    Liquid Nails Caulk 3  

    To be continued…

    I’ve decided not to insulate the roof as it is fiberglassed with thick core material. It is also carpeted inside, which adds some good insulation. However, I will be making curtains to go over all the windows and hatches.

    Overall, my 27 foot sailboat has about 80 square feet of hull that I’d like to insulate. The hull on my boat is very thin, with no core material, probably 1/4 inch in most spots. US Yachts are strong, sea-worthy boats, but I can see the sunlight through the hull in summer. Insulating it should save a lot of heat energy throughout the year.

    Insulating the boat is last big modification I want to make before moving onto it in April. Now that I have my material selected, it’s game on! I plan to buy an initial batch of material and start work on the V-birth over the next few weeks. Stay tuned!

    Related posts:

4 Responses to “How To Insulate a Sailboat – Part 1”
  1. This is on my bucket list for Andromeda and it’s going to be a huge job. I’m considering using a marine insulating coating that you have to spray on. It’s not cheap, though, and you have to prep the surface and spray it just right. It’s going to be a huge job if I do it right. There are other insulating coatings made for roof tops that I would like to try first because they can be rolled and brushed on. I just don’t know yet how they would stand up to a marine environment. But they are really cheap and might be worth a try.

  2. Sarah says:

    This is great, how did your project turn out? I am looking to do this to, its a bit intimidating so thank you for the information. I can wait to have a warm cozy boat. Can you tell me the downsides you mentioned of the styrofoam board?


  3. julie says:

    Hey there! I live in Port Angeles and am working on insulating my boat! Where did you find the Formular?

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