The system of installing metal roofing over the trusses or over purlins is typical. It gets more complicated when you look at adding the foam into the equation. You've got the metal, the foam, and the interaction between them to consider.
- With the complexity, I've enlisted the help of my designer partner, Jeff Frost, who also contacted Mike Uniake, a foam insulation expert at Advanced Insulation, and MBCI, one of the largest metal roofing manufacturers whose products Jeff frequently uses.
- Our assumptions are that you are doing this to the roof of a home (versus a commercial building or an open barn or shed) and that your metal roofing assembly is of an architectural grade that will not cause safety issues if it is not supported by sheathing.
Let's start with the foam side of the equation first.
An open-cell foam (about 0.5-pound density) is the most cost-effective way to insulate, but any opening over 1/16" could cause the foam to expand out onto the roof. It would be better to use a closed-cell (2-pound density) foam because it doesn't expand as much as open-cell and is more likely to fill voids but not go through them. In our neck of the woods, closed-cell foam costs about 56% more than open-cell.
Closed-cell achieves about twice the R-value per inch (open being 3.5 and closed being 6-7), but the density is 4 times higher and therefore you are using a lot more material to cover the same area. Closed-cell foam also contributes additional rigidity to the roof (while remembering our minimum safety assumption). Closed-cell foam is a vapor retarder, while open-cell is more vapor-permeable. This is important because you do not want water vapor within the foam condensing at the juncture where it contacts the metal.
Since you are insulating at the roofline, you will want an unvented attic, bringing your attic space within the conditioned building envelope. Therefore, you should employ planned fresh air exchanges and dehumidification, preferably also utilizing an energy recovery ventilator.
Now, what about that metal roof?
For metal roofing, it is fairly common to use architectural-grade roofing without sheathing in commercial projects, and it is certainly doable in residential. If you are using wood purlins, the cost will probably be a little cheaper than full sheathing, but you lose some of the shear values associated with full sheathing.That should be considered in the engineering of the house to make sure it adequately resists lateral loads. If you are using metal purlins at today's metal prices, it is probably going to cost more than wood sheathing. The embodied energy is likely to be much more, as well.
Combining foam and metal roofing
The challenge comes when you combine systems, spraying foam onto the metal roofing itself. Metal roofing manufacturers typically extend a 25-year warranty. The concern of manufacturer MBCI is over who is going to verify that the foam will not cause corrosive damage over time to the roofing.
- Most foam companies are not willing to provide written guarantees that their product will not cause problems and that they will stand behind any damage that may happen.
- Most roofing companies will void or waive their warranty under these circumstances.
Because of this issue, be cautious of any local roofing contractor who says that they will warranty this type of application. As contractors are able to come and go faster than manufacturers, make sure the manufacturer backs up any contractor representations in writing.
So, while minimizing materials is a good green cause, adding sheathing is a good measure not just for shear requirements but also for creating the separation between metal and foam, helping to create a good system that does not involve speculative material interaction performance. Add an underlayment over the sheathing as a water barrier before putting on the metal roof. An example of a complete unvented attic assembly such as this can be found at the bottom of this page. Also consider sourcing Forest Stewardship Council (FSC)-certified sheathing, thereby ensuring that it comes from sustainably managed forests. From an air-quality standpoint, look for sheathing with no added formaldehyde (although exterior-grade wood typically uses less-emissive phenol formaldehyde rather than urea formaldehyde), and look for a spray foam insulation that uses water-based blowing agents. And don't forget the air exchanges and humidity control. You'll need them for your tight envelope.