Shopping for Foam

by William Watt - Ottawa, Ontario - Canada

With the growing popularity of small boats made out of rigid foam insulation I thought I’d see what information I could find about it. Foam used in houses has to meet standards so I looked up the standards to see if they contained anything useful. There is one standard for the United States and another for Canada where I live so it’s a bit confusing although both seem to use the same testing procedure. Both measure the same qualities and they are min or max, products may exceed the standard. The “compression resistance” tells how hard the foam is, the “flexural strength” how bendable, and the “density” how heavy. I’d use them to rank foams for comparison. There are three kinds of foam. The familiar bead board is EPS for expanded foam; the closed cell is XPS for extruded foam; and the not so familiar ISO for polyISOcyanurate which is usually sprayed or faced with foil The XPS is probably the best for making boats covered with fabric although I used ISO and removed one face. The EPS is being used for heavier boats covered with fiberglass. The US standard for EPS and XPS is called ASTM C578. In Canada it’s CAN/UCL S701.

The following table shows the US standard and where one manufacturer’s products fit in it. Type IV has been used for foam boats featured in Duckworks articles.



Here is the relevant Canadian standard, showing how the products can be compared. Since this table was printed a denser Type 4 foam has been added but I was not able to find a table which included all four types.

The Canadian and American standards are not standards for load-bearing structural material so the measurements are not useful in that sense. I think for boatbuilding we need to know how much it weights and how much it bends so I went to three home improvement stores to weigh and bend foam. The foam I measured was made in Canada to the Canadian standard but can be cross referenced to the American standard using the tables above. To measure the bend I pushed on the short edge until it stopped and measured the change in length, in effect measuring the length of the chord of the arc it made. I went by feel so it was at best an estimate. Ideally I would buy a bunch of foam and put weights on them and measure how far they deflected before they broke but for that I’d need a research grant. My method was free. I calculated the height of the arc by substituting the curved arc for the straight hypotenuse in the Pythagorean formula so the height and the corresponding radius of curvature in the table are slightly less than the actual numbers. Consider it a small margin of safety.  To measure weight I used a kitchen food scale which goes up to 10 pounds. Note that 1/5” lauan and okoume plywood weigh 1 lb per sq ft for comparison. The numbers went into a computer spreadsheet. I would use them to compare and to rank type and thickness of foam. The “radius” column is the radius of curvature of the arc in the foam. It could be compared to the radius of curvature of plywood when thinking of substituting foam for plywood in a small boat design as I did for small foam canoe I made. The thicker the foam board the stronger it is but less bendable to the shape of a boat, just like plywood. Note on cost: the Canadian dollar was worth seventy-five cents American when the cost was calculated.

I hope the numbers will help fellow experimental boatbuilders choose the best foam for small, lightweight, durable craft.

Bill in his foam boat


  1. Question! Did you remove the clear sheet of poly on the clear foam before it was tested? That film would significantly affect the bending test.

    • Nothing was altered for the tests because the sheets were in the retail stores and not taken home. On the boat I made I removed one facing to get the foam to bend to the shape of the boat. After the boating season is over I plan to submit article on the boat to Duckworks.

  2. Bill, good job and very accurate information. I am from the EPS Industry Alliance – the source of the CAN/ULC S701 table in your post. We represent the expanded polystyrene (EPS) industry in North America. Type 4 CAN/ULC is only extruded polystyrene (XPS) so we did not list that in our rendition of the S701 table. For the record, EPS and XPS are both closed cell foams. It is great to see people using foam for these applications. EPS is extremely versatile and can be molded in different densities. Higher densities are often better for fabricators using hot-wire cutters or rasps to make complex shapes. Look us up if you have any questions.

  3. Willam, Thanks for the info on rigid foam. That info may prove useful on some planned projects. I have built a Sawfish 12 from the 2″ Formular pink foam, and am now planning to convert that to a small trimaran using the same foam for amas. My longer term plan is to make a 16′ two-seat tri using pink foam with some plywood reinforcements to distribute the forces of the mast and the akas. We shall see….
    Jim Brown, Sweetwater TN

    • Nice. I have been trying to imagine how the boat I made would bear up under the twisting stress of a small sail and how to provide lateral resistance. Would be interested in your results.

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