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Tips for using 2 Part Urethane Foam

Urethane foam is a commonly used 2 part expanding foam excellent for gap filling, insulating and flotation. It uses a simple 1:1 mix ratio. Uses for urethane foam include:

  • Male mold forms or plugs for laminating over. Once fully cured this foam can be laminated over with any type of polyester, epoxy or vinyl ester since the foam will not melt with exposure.
  • Excellent thermal insulation and sound deadening, useful for soundproofing and insulating in household and marine applications.
  • Flotation foam- it cures to a closed cell foam that is excellent for marine applications. Closed cell means it resists water absorption for many years.
  • Expanding foam- will shape to cavity or form it is poured into. Great for blind access voids such as below deck.
  • Urethane foam suffers from very poor UV resistance. If used in areas exposed to sunlight it should be painted or laminated over. Urethane foam is not meant for structural load-bearing, as it alone possesses poor strength- it will crush under foot. If used under decks, the decks must be supported independently by stringers. While urethane foam cuts easily, it is messy and difficult to sand.

    Urethane foam is great for insulating and adding permanent flotation. Closed cell means it will maintain buoyant pressure even after prolonged exposure to water. It has the water resistant properties to keep a capsized vessel floating for long periods of time. It can also be used to make buoys and floating markers provided they are encapsulated in UV resistant material. Urethane foam is also used to make light-weight core blanks for sailboards and surfboards, provided that sufficient lamination strength encapsulates it. Urethane foam does not offer much strength for structural applications.


    Before mixing this product, several considerations must first be addressed:

    This product should be mixed and applied in room temperature, about the temperature you'd feel comfortable wearing a t-shirt in. 75-80° F is optimum. In too cold an environment the reaction is inhibited. In such cases the urethane will shrink back after rising, losing much of its insulating and flotation strength.
    This product must be poured immediately after a quick mixing. The area to be filled must be accessible and prepped ahead of time. This may require drilling holes etc. in advance. Surface should be clean of contaminants for proper bonding.
    overflow relief-
    Urethane foam will expand tremendously, 20:1 by volume to be exact. In doing so it can exert as much as 5 psi of pressure in contained areas. This is enough pressure to lift the deck off a boat when poured into cavities without proper venting. It may be necessary to cut overflow vents over large surface areas such as decks. This can be easily accomplished with a drill powered hole saw.

    Flotation foam expanding in kayak bow. In forward compartment, flotation foam has just been poured and expansion has not started.


    You've opened access to the area and completely cleaned surface as best possible. The weather or room temperature is comfortable, t-shirt wearing conditions. It's now time to mix up a batch and apply. Protective eyewear and gloves are required when applying foam. Assure good ventilation if indoors. Use a jiffy drill mixer, the same kind used for mixing paint for best results. The mixing window for this product is short, therefore a drill mixer is key. Keep a cup of acetone onhand to place the mixer mandrel in immediately after mixing. Mix for approximately 25 seconds. There should be no visible swirls in cup when mixing is complete. Rising begins almost immediately and is dramatic. Urethane foam has 400 second rise time (just shy of 7 minutes) before reaction is complete, so there is no second chances once mix begins. Geltime is 4 minutes. The thermal reaction generates heat which can reach temperatures nearing 130° F. Be sure the substrate you are pouring the foam into will not be compromised by this heat. Since this heat is momentary, this poses no problem for polyester, vinylester and epoxy laminate materials. Pour the batch into the void and leave undisturbed til cured. If filling large areas, pouring consecutive batches before the first gels will maintain a chemical bond between layers. Urethane foam will also adhere mechanically to it's cured form, so don't sweat it if consecutive pours are impractical. Care should be taken in estimating the proper amount. For calculating purposes, 5 lbs of material will yield 10 cu ft of volume under normal conditions.


    The cured foam should harden to shape with minimal shrinking. If the foam sucks back in, this is likely due to a bad reaction. Cause is either too cold for proper reaction or incomplete mixing of parts.

    This 30+ year old foam eventually degraded from exposure and age and now retains water.

    Removing old foam from voids below deck.

    Urethane foam will degrade over time and if exposed to certain chemicals. After decades of use in older vessels, particularly where exposure to gasoline and other chemicals occurs, this foam can actually retain water. It is common practice to evacuate and replace decayed foam in restoration process.

    Contributed by Michael Reardon
    Information provided by Craig Thompson, North American Composites

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