The tale of airtight spray foam insulation

Leakages on a spray foam insulated roof
Blower Door Testing in a spray foam insulated house without additional membrane. The screwings for the insulated boards are highly visible through the thermal imaging. Every blue spot is a potential air leakage.



First of all, I need to state that I usually have no issue regarding spray foam insulation. It definitely has advantages and disadvantages as every insulation material.

I´m just concerned about statements of some spray foam companies and manufacturers regarding the airtightness of their product.


The photo above shows a pitched roof area where spray foam has been installed in between the rafters and insulated boards were screwed on top. A membrane wasn´t used, because of the "airtight statement" coming from the spray foam installer. I do agree that spray foam might can be airtight as long as it will not processed afterwards. But mostly all installers have to trim the spray foam because it´s impossible to create a smooth insulation layer without overhang just by spraying. Unfortunately this trimming work weakens the cell structure of the foam and can create cracking between the rafter and the spray foam filling.

That´s the main reason why I believe, that spray foam insulation, without using a membrane on the inside, is not permanent airtight.


And there´s still another important fact for using a membrane in addition to the spray foam insulation, which is called vapour control. Without having a membrane on the warm side of the roof installed, you will gamble with the risk of condensation in your insulated structure each winter.


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Comments: 4
  • #1

    Mike Trench (Saturday, 05 November 2016 14:03)

    I wholeheartedly agree with you on the level of air tightness claimed achievable by the installers of these products is misleading at best. As a NSAI certified air tightness tester I can verify that none of these products alone give adequate air tightness results.

  • #2

    Brent Hamilton (Thursday, 10 August 2017 02:02)

    I agree with Mike. None of these products alone give adequate air tightness results.

  • #3

    James o Connell (Thursday, 25 January 2018 20:30)

    Interesting reading the above. Would you classify icynene spray foam in same category as above. It claims to be breathable which is why it's a lot more expensive than other options but say that it works out cheaper than metac optionwhen you don't have to encounter costs of airtight membrane. Interested in hearing people's thoughts on this.

  • #4

    prodomo Ireland (Friday, 26 January 2018 01:25)

    Hello James, thank you very much for your response. Currently I can´t find NSAI certificates on the Irish website of Icynene, but even in the shown BBA approvals, you´ll find plenty of detailled requirements for the use of additional VCL. If you want, you can have a quick look here:
    The most common version of a VCL is a membrane, which delivers a high standard of air tightness as well.
    The biggest issue of all spray foam installations in regards to the air tightness is not the foam in between the joists or rafters - it´s the fact that the foam can´t cover the small joints in between two joined joists or between the joist and the wall.
    My favourite sampe about the air tight performance of spray foam can be found here:
    I bet that I get a similar structure and dimension, as used in the test, to an air tight level of less than 0.3 ACH (Air Changes per hour), by using the conventional membrane & tape. They used another brand of spray foam but the result would be similar with the Icynene.