A suggestion for a project from a contractor this morning: “there is a new rigid insulation called cold board that xxx is exploring that will allow us to apply the roofing to the rigid without having the layer of xxxxdeck which should allow xxx to bring the cost down even more”
Does nyone have some knowledge of the performance of this new (cheaper) material?
I think it is a composite insulation with a laminated OSB, glass-mat gypsum board, or cellulosic-fiber-board facer. Thus, it eliminates the labor of installing a cover board after installing the insulation, since both are installed at the same time.
This product has been around since the mid-1990s. Atlas makes several varieties (including a plywood-faced product that is kinda buried on their website but which is accepted to go in Miami-Dade assemblies). I have tried to get it incorporated into a number of projects but it has been VE’d out. It is more expensive than the cost of the components, but a lot of the roofers don’t really want to consider labor cost in figuring the assembly cost.
Another consideration (which does have merit) is that doing the insulation and coverboard in several layers permits staggering of joints which minimizes thermal shorts caused by joints through from membrane to steel deck.
We have rejected this type of product in the past but it was based upon the thermal shorts mentioned by Mr. Jordan and not the product itself. Staggered joints appear to make a huge difference in the performance of the system in our area. It also appears to make a difference in the life of the roof membrane as well. (Less thermal shock to the membrane? we don’t know for sure)
I’ve specified this many times (whether or not it was actually installed, I don’t know). I avoid the the thermal shorts by requiring a minimum thickness (usually 4 inches) with no less than two layers of insulation provided. Each layer must be offset from the previous. Of course, only the top layer has the cover board facer.
By requiring the minimum two layers and the offset, the thermal shorts are minimized.
I like to specify this with a veneer plywood face for a asphalt shingle roof or a metal roof, I have given up trying to specify it for a low-slope roof.
Ron, I like your idea about stggering the layers. Did not know you could get it this way and am going to look into it.
Peter, I don’t think the composite insulation boards intended for low-slope roofing are suitable for steep-slope applications. I assume you’re referring to composite nail base insulated roof sheathing, which typically has an OSB facer but is offered w/ plywood by some mfr’s.
An important distinction is that composite insulation for low-slope roofing typically has square edges, but nail base sheathing panels usually have T&G or a similar type of edge detail. This is critical to avoid telegraphing of panel joints through asphalt shingle roofing. Nail base panels have a tendency to warp in roofing conditions, so the ones w/ the most positive interlock at edges will minimize the chances of problems. I imagine this could also be true w/ metal roofing.
Nail base panels are not typically intended for multi-layer application, so offsetting joints is not an option.
As I think about it more, I think you are correct. Got carried away in the moment. Still, when I tried to specify the composite panels on low-slope applications 10 years ago, the contractors always came back wanting to use individual components.