Boat Window Insulation

Written by Dan Smith, Sailrite employee

A common problem for anyone living aboard a boat is keeping the heat out during the summer and keeping the heat in during the winter. The problem grows exponentially with larger windows, like those found on trawlers and houseboats that seem to have a built-in convection draft that sucks air right out of the boat.

As a live aboard myself, I’ve observed several different methods for insulating a window over the years, but none seemed to offer an optimal solution. Standard curtains or drapes provide some relief but are not sufficient on really cold or windy days. Thin cushions, made from ½” Ensolite foam and sized to fit into the windows, cut down the draft but kill any light coming into the boat and require drilling holes in the interior of the boat for the mechanical fasteners needed to hold cushions in place.

While helping a fellow live aboard and Sailrite customer, I came up with an effective (and creative!) solution using sheets of random fiber expanded foam packing material (found at any post office). This translucent foam material is a great insulator and is only about 1/16″–1/8″ thick allowing for more light and less bulk. Watch this video for step-by-step instructions on how to make these window covers:

Use the foam material as a liner between (or behind) panels of Sunbrella Shadow (preferably light colors like Snow or Sand). Covering the panels with Sunbrella on just one side will let more light through, but the window will not have as finished a look when viewed from the outside. Fold the outer edges over twice and sew to create a rubbed seam or bind with a 1″ premade binding to finish the edges.

Hang the window panels with the new adhesive-backed YKK® SNAD®. A SNAD is a low profile nylon molded male or female snap held in place by a high tech 3M adhesive. Install a button snap in all four corners of each panel. Then attach a male stud SNAD to each button. Pull the paper backing off the SNADs and adhere directly to the glass, smooth fiberglass, or varnished wood surface.

The result is an attractive window cover that provides both insulation and privacy while still letting in quite a bit of light. Your neighbors will be impressed and probably want to know where you got them!

A summer alternative to the foam material is Sailrite’s Thermozite material. Thermozite is a very good insulator and when installed with the shiny side facing the window also reflects sunlight to keep the heat out. The obvious drawback is that Thermozite is opaque and will block all light. Use SNADs and female snaps as described above to hold the panels in place. See this video for instructions on how to make Thermozite Window Covers:

If I were refurbishing my boat, I would create window covers using Sunbrella Shadow and create ceiling panels with Thermozite insulation. (Boater lingo is a bit funny as “ceiling” panels refer to the vertical sides on the inside of the hull and “overhead” to the horizontal surface above your head.) Adding Thermozite Insulation to the inside of the hull will help keep the heat in or out when oftentimes the only thing between you and the elements is less than ½” of fiberglass.

This clever energy-saving project is simple and the concept could be applied to window covers and sun shades for homes, RVs, and cars.

Project Materials List:

  • Sunbrella® Fabric
  • Insulation Material (foam packing material or Thermozite)
  • Snap Fastener Buttons
  • Thread
  • Sunbrella® Binding (optional)

Thanks Dan for coming up with such a great idea! Visit to get these products.

  1. Cassie said:

    From Joseph:

    If you are trying to keep the heat out in summer it is much more effective to put your window cover outside the window.

    There are two advantages. Once the light gets through the window it is converted to heat in the boat, and with interior barriers that heat is already inside the boat. If you have a reflective material then the light/heat/UV is reflected back out through the plastic speeding the degradation of the port-light and the energy not reflected becomes heat in the boat.

    Any covering that shades the port-light outside keeps the heat outside and protects the acrylic from damaging UV rays.

    Joseph s/v Prestissimo

  2. Cassie said:

    From Mark:

    I use a type of cling film, which is secured to the frame with double sided tape and then tightened up by using a hair dryer. This cuts down on both condensation in the winter and heat transfer in the summer.

    There are a number of films that can be applied to windows some working on the outside, some from the inside, and some suitable for both applications. However, most of these are designed for glass and do not work very well with acrylic, and not at all with polycarbonate. Theses films can act as mirrors or be entirely transparent, whilst cutting thermal transfer significantly. Those on the outside last up to 4 years whilst those on the inside last up to 7 years. My windows are very curved and thus applying a film would be a problem.

    What a lot of people use is what is known as shade cloth, secured on the outside of the window. This not only reduces the thermal transfer into the boat, but also helps to protect the acrylic from UV damage. This would provide a good sailrite series.


  3. Learning how to make window insulation is a wonderful idea. You can save more money at the same time light can pass through it.

  4. Hello there, You have done a great job. I will definitely digg it and personally recommend to my friends. I’m confident they will be benefited from this web site.

  5. Great info. Lucky me I came across your blog by accident (stumbleupon). I’ve book marked it for later!

  6. Gregory King said:

    Greetings, etc. -consider Velcro. Attach the ‘felt-side’ of the Velcro with silicone directly to the metal frame surrounding the window. Sew the ‘hooked-side’ of the Velcro to the edge of the insulating fabric. The purpose is to make the insulation (almost) leak-proof. For hard-core insulation, add 1 inch styrofoam cut to fit the window snugly -then attach the fabric insulation in place.

  7. the facts have been discussed is really important. keep doing that!

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