Spray Foam – good or bad?

Closed cell-spray foam used as insulation, has been on the market since about 1979, and has gone through a number of evolutionary steps.  The product is a solid plastic, formed by the chemical reaction of two primary components:  an “A” side which is isocyanate, and a “B” side which is usually a mixture of oils, stabilizers, fire retardants, blowing agents, and colouring agents.  The reaction is exothermic (ie, generates a lot of heat), and the materials form a solid within seconds. The blowing agents are the compounds which produce the low-conductivity gas that forms the bubbles in the foam, and thereby form the primary insulation.

When applied to a minimum thickness of 50mm (slightly under 2 inches), the material acts as  insulation, vapour barrier, and an air barrier.  Newly-installed foam has R-values of R-6 to R-7.5 per inch, but this diminishes to about R-5 per inch over time as the insulating gas dissipates.  Compared to other insulation products like fiberglass bat or mineral wool bats, the product has more insulating value per inch, and resists the loss of heating value that sometimes occurs with loose insulation due to air convection with extreme temperature differences.

The ability to prevent air and vapor movement is generally a good thing, except in situations where moisture can enter a wood structure, and then cannot get out.  Therefore the short answer to the question in the header is “spray foam is good when PROPERLY installed, and BAD when installed in inappropriate places or in an incorrect manner.  There are also issues of the impact on the environment, and potential impact on the health of the people living in the homes where the product is used.  The rest of the post touches on some aspects that inform whether the installation is good or not.

Safety – short and long-term


The products used to make the foam on site are very reactive chemically, and the workers using and installing these products should be trained in the use, and in the proper application.  They also should be properly protected with appropriate masks, eye protection, and body protection.  In Canada, operators installing these materials need to be licensed (See the CUFCA website here: Canadian Urethane Foam Contractors Association – HowTo page).  Simply put, the unreacted products are dangerous, while the solid foam that results from the reaction is not.

This requirement for the products to be properly mixed and reacted, when not followed, gives rise to horror stories such as documented in the CBC Market place story (see the website here: CBC Marketplace Blog of Friday Oct.25, 2013, and the uploaded YouTube video CBC Marketplace video uploaded Sept.10, 2014).  There are some reactions to those stories in rebuttals in forums (one such forum here:  RedFlagDeals Forum thread on Spray Foam), and by some installers (see Georgian Insulation Systems rebuttal).

The companies I have used to carry out such work have emphasized that for the spray foam work to be done properly, a number of precautions have to be taken (the list below is in no particular order, and is NOT exhaustive):

  • The equipment must be clean and the mixing ratio must be closely monitored so that proper mixing of reactants occurs, and no unreacted material is left.
  • The occupants of the house should be out of the house for at least 1 or even 2 days, to ensure that any outgassing that happens dissipates.
  • The areas to be sprayed are properly prepared, dry, and at the proper temperature.
  • The ambient air temperature should be within a specific range as does the humidity.  If it’s too hot, too cold, or too humid then the spraying must be delayed.
  • Because the reaction is very exothermic (meaning that it generates a LOT of heat), it is important that the substrates are not going to melt, or even flash into flames (which is why proper preparation is important).  As well, any significant moisture can be heated to steam, which then will try to escape through the foam, and create pinholes which will work against the goal of having a vapor and air barrier.
  • Continuing the above, the material must be applied in layers, with time allowed for the heat generated to dissipate, before the next layer can be applied.
  • If the foam is applied in confined spaces, there is particular need to monitor the temperature of the foam and surrounding materials and to ensure that it cools down sufficiently before applying additional layers.

Therefore, when done by a company that does the proper preparation, waits for the right conditions, uses trained and properly-equipped operators, and installs the right type of foam to the right thickness for the intended use, the results are generally excellent.

Medium to Long-term:

If the spray foam was not placed in the right places and in the right amounts, then new problems can crop up, including the trapping of moisture in unventilated places (which can lead to rot and mold), and reduction of interior air quality (due to lack of air exchange).  Another problem that sometimes arises, is that the foam becomes a very warm home for burrowing critters if care is not taken to make sure that animals such as raccoons, squirrels, mice, bats, and some birds don’t have access to the foamed places.  Let’s look at some of these issues, and how to avoid them.

Some Typical Installation Errors.

Failure to ensure sufficient thickness.

In actual practice, the foam can be applied in various ways:

  • over the ceiling of a dwelling (ie, inside the attic or space in the cathedral ceiling where the insulation should go), and be the primary insulation, or be the warm-side layer which supplies the air-and-vapour barrier,
  • on the insides of walls,
  • on the underside of the roof deck,
  • and on the upper side of a roof deck (when an old roof deck is insulated from above as part of a new roof assembly).

It is relatively common to apply the foam directly on the underside of the roof decking.  If the thickness is sufficient (say 5-8″ depending on the type and brand of foam), there is virtually no residual heat reaching the decking, and the decking stays at outside ambient temperature.    If the thickness is not enough, then residual heat WILL reach the roof decking and CAN melt the snow that is on the outside of such a roof.  If ice dams form on such a roof, then water will accumulate against the roof covering, push against the material, and enter the wood deck through any waterproofing weakness.  Now the decking will be wet, without any air circulation from the underside to dry it out.  Keep it wet long enough and it will rot.  Unfortunately, this will be virtually impossible to detect since the underside of the deck is covered by polyurethane foam, and the top surface is covered by roofing materials (shingles or other).

Spray Foam_9896

Approximately 2″ of polyurethane foam sprayed directly against roof decking and walls.  Upper arrow points to mushroom growing from wood.  Lower arrow points to mold seeping in between rafters.

In the above situation, polyurethane foam was sprayed on the inside of a knee wall which was not ventilated or insulated and suffered from persistent ice-dams on the outside surface (the floor of the knee-wall is just above the soffit cavity on the other side of the roof surface).  The homeowner was persuaded to add insulating foam on the underside of the roof deck and along the walls.  While this reduced the amount of heat loss, it was not sufficient to completely stop the heat loss, which therefore continued to melt snow on the roof, and contributes to ice dam formation.  This resulted in water entry into the roof decking, and since there was no ventilation of any kind to allow the moisture to evaporate, it leads to mold and fungal growth.

Failure to seal roof deck from above

If, as in the above case, the insulating foam is applied to the underside of the deck, creating an air-and-vapour tight layer, the installer MUST ensure that the wood forming the deck is also sealed against water entry from the outside, so that the wood deck stays dry.  If the sealing cannot be guaranteed, then some form of air channel and air circulation MUST be provided above the roof deck to ensure that any moisture is able to be evacuated or dried by the passive air circulation.  In the above case, the outside of the roof deck was left unchanged – ie, a standard application of asphalt shingles over tar paper underlayment.

Failure to ensure the entire surface is sealed

If you examine the above photo, you will see that the supporting rafters were doubled-up, leaving a gap between the rafters.  The roof deck between the rafters was covered by foam, but the space between the doubled-up rafters was not.  This space extends to the underside of the roof deck, and therefore to the outside, and becomes an open channel for air or water entry.

How to fix such installation issues?

Unfortunately, this kind of problem will require the structure to be disassembled, all the wood rot and mold removed, the rafters verified as to their integrity, and then the roof has to be properly designed and rebuilt.  Design, in this case, has to include the fire-stop (usually gyprock), the support structure (rafters, joists, etc.), the vapour and air barriers, the roof decking, an appropriate air space and adequate venting, underlayment and waterproofing appropriate to the roofing materials to be used AND to the specific conditions that this roof has to deal with, and finally a new roof covering.  Therefore, IF you are trying to improve your home, consult with someone who looks at the entire roof system (instead of just certain components), and puts together a comprehensive approach which addresses ALL the issues of the existing structure.  Failure to do so will result in all kinds of unpleasant and undesired unintended consequences.

(c) 2017 Paul Grizenko



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