Hansen Pole Buildings’ Designer Rachel recently had an interesting discussion with a client. The gist of the discussion was the client had heard spray foam insulation will corrode the steel and void the warranty of the steel.
Rachel did some research and found this article: https://www.greenhomeguide.com/askapro/question/can-i-apply-spray-foam-insulation-directly-to-the-underside-of-a-metal-roof.
When I added the external elevator shaft to the rear of our steel covered post frame home, my choice of insulation was closed cell spray foam. Although I knew it was going to be more expensive than other choices of insulation, I was (and remain) convinced of it being a superior R-value, as well as completely sealing the system. In the case of our addition, the steel roofing was applied directly over the wood roof purlins, without any solid sheathing or other barrier.
So, will spray foam insulation actually corrode the steel?
Highly unlikely, as from the research I have been doing there appear to be no chemicals in the spray foam which would react with the steel or the galvanized or galvalume protective layer over the bare material. Most steel roofing is factory finish painted, which adds yet another barrier surface in the interior primer paint coat which further isolates the steel from the spray foam.
There are some cases where I could see some challenges.
One would be if someone went on the cheap and used open cell spray foam, rather than closed cell. In this case moisture could get through the open cells and be in contact with the underside of the roof steel.
The other could occur if there was a leak in the roofing or the ridge cap which would allow moisture to get trapped between the roof steel and closed cell foam.
As to the warranty discussions – steel warranties primarily cover fade and chalking of the exterior finish of the steel. Personally I am hard pressed to see how it is the application of closed cell spray foam insulation on the interior of the steel roofing, would influence the life of coatings on the exterior.
Of course everyone looks for an “out” when it comes to warranties, and the reality is a good warranty protects the seller/manufacturer far more than it protects the consumer.
If I had it to do all over again, I would still closed cell spray foam my own steel roofed building. Check back with me in another thirty or forty years and see if my opinion is yet the same.