Using spray foam with a metal roof. What is the options?


Using spray foam with a metal roof. What is the options?

Asked by Leon Cruywagen

We just bought a log cabin that had a very bad shingle roof. We have pulled off the shingles and replaced the roof with a metal roof. I left the old wood deck in place and covered it with a vapor barrier before putting down purlins on which the metal roof is attached. There is not much insulation in the roof as it is. It currently has very old compressed fibreglass batons laying on the ceiling. I am thinking of using a spray foam to replace what is there but have questions around the best use of spray foam. Should I apply the spray foam directly to the inside of the roof decking and seal the whole addict or should I just apply the spray foam on the ceiling the same way the current fibreglass is applied?

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Harold Remlinger, AIA, LEED AP, NCARB's picture

Location of insulation is primarily dependent on the location and climate zone of the home. Typically in cooler climate zones, you want the insulation on the ceiling so that the heat is concentrated near the occupants. As for warm climate zones, you insulate the bottom of the roof decking so that the attic becomes conditioned space and allows the heat to rise up higher and away from the occupants. Either way is acceptable by most building codes. If you apply the insulation to the underside of the roof deck, use a closed cell or open cell spray foam. Refer to you local building codes as to the 'R' value required. Cost will be a determining factor. Close cell foam costs upward of 2.5 times the cost of open cell foam. If you intend on applying the insulation above the ceiling, I would recommend applying 2 inches of closed cell foam and the balance of insulation to be either loose fiberglass or cellulose to achieve the required 'R' to meet code. The 2 inches of closed cell foam will create a vapor barrier and seal all penetration through the ceiling where heat can be lost or air infiltration can occur. This approach will keep cost down and achieve the performance you are looking for. Prior to insulating, verify (if there are any installed) that the recessed ceiling fixtures are IC rated. This stands for 'insulation covered'. If they are not, you will need to allow air movement around the fixtures so they do not over heat. However you can purchase insulated boxes which can be installed prior to the application of the spray foam so that the proper clearance can be maintained at the recessed lights. As always, I would recommend that you contact a local insulation contractor or energy consultant to help you properly assess all of the available options.