Q&A: Is spray foam insulation a good idea?

18 July 2023 by Jason Quinn

Q. What’s your opinion about spray foam insulation for New Zealand homes?

A. Spray foam can be used with good results if properly designed and installed well—but a successful outcome requires both of these or a lot of luck. In the UK, there have been enough problems with bad installations of spray foam in retrofitted roof insulation that it impacts homeowner’s access to mortgage and equity release products.  

It’s enough of an issue that earlier this year, the UK Royal Institution of Chartered Surveyors published updated guidance about using spray foam to insulate residential properties.

Spray foam is essentially foamed polyurethane glue. It is more accurately considered a family of insulation products, which can behave in different ways in different applications. The formulation can be readily varied, mostly with regard to the percentage of blowing agents. R-value and vapour permeance values can vary a lot and need to be matched to application. The distinction between CCSF and OCSF—closed or open cell spray foam—is useful as they have different properties.

Spray foam is made on-site by mixing two chemicals and then spraying the resulting product onto or into the building cavities. The results heavily depend on mixing the two chemicals precisely and then applying it in thin enough layers that it does not shrink as it cures. The worst case—admittedly a very, very rare one—sees the foam not cure properly because it’s been mixed incorrectly.

When installed well, spray foam can deliver really good R-values and improve airtightness. The best approach with wall cavities is to over-fill them and trim the excess back. This ensures the entire cavity is filled to capacity. If a cavity is unevenly filled or the thickness varies, then it is reasonable to use nearly the thinnest spot to estimate the installed R-value.

Spray foam is much less vapour permeable than fibreglass insulation or even plywood. Closed cell spray foam performs similarly to XPS foam insulation. This means that assemblies need to be designed for this low-vapour permeance. If this is not done correctly the results can be tragic. There are many variables to consider. For example, timber stud walls in New Zealand need to be able to dry to the outside but even a thin layer of spray foam in the wall could prevent this. 

Please make sure that the blowing agent for the spray foam is one of the low Global Warming Potential (GWP) blowing agents. Otherwise the embodied carbon in the blowing agent would outweigh the energy savings from the insulation, even over the long term.

Comments 3

  1. Hi Jason,

    In this article you mentioned “timber stud walls in New Zealand need to be able to dry to the outside”.

    Is there a range for suitable permeability? It seems using external sheathing (plywood/ plasterboard / fibre cement etc.) will limit the ability for the wall to dry out, when compared to a flexible underlay.

    1. Yep, external sheathing can definitely be problematic if there’s no internal control or option of drying back to the inside. BRANZ research showed condensation forming on the back face of external sheathing in almost all situations they tested. They considered it still acceptable on the basis of moisture buffering by the sheathing itself that could then dry. Not sure I would draw the same conclusion they did from the test results!

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