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How to choose closed cell spray on insulation

GBA Editor | Posted in Green Products and Materials on

My building is 800 SF timber-framed with ship-lapped sheathing on walls and ceiling. The insulation will go over the sheathing, then the metal roof and siding. The space for the insulation is made with engineered joists 24″ apart. They will support the roofing and siding.

I need to decide whether to choose the contractor using Bio-based 1701 or the one with Corbond, both closed cell spray on insulation. I think they are comparable in R value, but I can’t tell whether they are equally benign environmentally.

I also would like to know whether 4 inches is sufficient on the roof as the Bio-based 1701 contractor says that more than 4″ is “overkill”. And the other contractor said that after 6 inches there is little advantage. We thought that we were all talking about 9 inches in the Fall, so we constructed spacing of 9 inches for the insulation.

Thank you for your help.

Betsy Calhoun

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  1. GBA Editor
    Martin Holladay | | #1

    It's time for me to trot out the usual questions:
    1. Where is this house located? What's your climate?

    2. Do you know anything about local minimum code requirements for ceiling insulation?

    3. Four inches of closed-cell spray polyurethane foam will provide about R-26. That isn't much. According to the 2006 IRC, the minimum prescriptive requirement for ceiling insulation is at least R-30 everywhere in the country. In climate zone 4 and all colder zones, the prescriptive code calls for at least R-38 or more.

  2. Riversong | | #2


    Neither option is environmentally benign. Petrochemical plastics are one of the greatest threats to our environment and our health, since they only perpetuate the destructive fossil fuel industry.

    And you have been suckered by the hype of the spray foam industry: that anything more costly than what they think they can sell you is "overkill" or has diminishing returns.

    If you have designed for 9" wall and ceiling cavities but have not yet installed the I-joists, then apply an air barrier material to the outside of your sheathing and fill the insulation cavities with dense pack cellulose - the only truly green insulation on the market in the US. That will give you an R-34 envelope at less cost than the inadequate foam.

  3. Betsy Calhoun | | #3

    The house is in Putnam County, New York with R38 required for ceilings, R13 for walls. We have 5750 heating degree days, We have already installed the I-joists, with roofing paper between them.

    I very much appreciate your responses and advice.

    Betsy Calhoun

  4. Perry525 | | #4

    The thing to remember is that heat always moves to cold.
    The key things for keeping your home comfortable are stopping heat loss due to wind suction and conduction through the fabric of the building.

    The best way to do this is, to isolate the warm interior from the frame by lining the inside with tightly butted polystyrene or Blue Board or similar. Then use the finish of your choice.

    Fill the gaps between the frame with more polystyrene carefully cut to size with a knife or saw. (a saw makes more mess) Remember holes and gaps = heat loss or gain.

    Six inches of polystyrene will do the trick.

    Make sure you treat the ceiling and floor joists exactly the same way.

    The floor needs to be floating on top of floor grade polystyrene.

    Built like this your home will require very little heat, if any.

    Take a look at Passive House standards, Germany has the highest standard, with one cubic metre of air loss and minimum heating requirement. Also look at the United Kingdom level six.

  5. Perry525 | | #5

    The way to look at this is, the heat on the inside always moves out towards the cold.
    The passing wind sucks the air out through every crack and gap, the heat inside moves out via the very fabric of the building by conduction.

    You will be the ones that produce water vapour within the home by washing, cooking, breathing and sweating.

    The whole of the outside sould be covered in tightly butted water proof plywood, covered with a almost water vapour proof membrane.

    The framing should be filled with tightly (friction fitted) polystyrene or Blue Board or similar, then the inside of the framing should be covered with water proof plastic sheet and another two inches of polystyrene laid across the inside of the frame to prevent the heat from the room escaping through the frameing.

    This inner layer should cover the walls, ceilings and floors.

    The purpose is to produce an air tight box, to keep your expensive heat in.

    Then you need to consider the water vapour.

    You must have extractor fans in the kitchen and bathrooms, fans that have heat exchangers, outside shutters and humidistats are a must, or you need separate sealable air vents, that can be opened when the extractor fans are working.

    The air outside is nearly always drier than inside (not always) and running these fans will remove the water vapour and smells, and keep the home dry.

    Spray foam while slightly better insulation, it seals all the gaps, is very expensive, sheet material will do the job.
    It is closed cell, wind proof and waterproof, it can get wet, freeze, dry out and still provide the same level of insulation. It has been in use for fifty years, I have used it for almost forty years and it is as good today, as the day it was first used.

  6. GBA Editor
    Martin Holladay | | #6

    Your suggestions from the U.K. are likely to appear peculiar and idiosyncratic to North American readers.

    In general, we don't fill stud bays with rigid foam board. It takes far too long and is far too fussy; moreover, most stud bays include wiring, making foam board a poor choice to fill all of the stud bay's nooks and crannies.

    Moreover, your advice — "The whole of the outside should be ... covered with a almost water vapour proof membrane" is potentially dangerous, especially since you are also advising that "the inside of the framing should be covered with water proof plastic sheet." This is insane. You are enclosing your framing with two vapor barriers -- one on the exterior and one on the interior. This wall assembly had zero drying potential.

    It's amazing to me that the laws of physics apparently don't apply on your side of the Atlantic Ocean.

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