Waco Home Inspection Part Two

Foam Roof Insulation

About a year ago, I was contacted by a client in West, Texas, concerning a new home construction. It had already been dried in, but I found a few areas of concern. If you have read my blog on phase construction inspections, you are already aware that even a new home may have defects. However, this piece is not directly concerned with that so much, so please, read on.

My client contacted me again about two months ago with a concern about the appearance of her roof at certain times of the day; in short, you could see several of the plywood seams, which was unappealing to the eye.


While trying to help determine the cause, I focused for a time on the spray foam insulation. Since I was not too familiar with this relatively new type of insulation, I had to do a great deal of research, some of which I will share with you today in a condensed form.


Before going into the differences between the two types of foam insulation, a little bit of how it works is necessary. Foam insulation can and does provide R-value, but that is not what really makes it effective. What makes it effective is that it provides an air barrier. If no air is moving through, neither is temperature. That is why this system is not ventilated, typically. The idea is to have a complete separation of air from inside to outside. Air also carries water vapor, and I will come back to that later on. Both closed and open-celled foam can effectively seal out air loss, and this is where the differences begin. Closed-cell requires about two inches, where open-cell takes about six to do the same thing. Based on varying sources, closed-cell foam is more rigid, and can add structural integrity where it is applied. The down side of this is that since it is more rigid, it may be more susceptible to movement, and can become separated from the substrate it is applied to, which is not a good thing. In my opinion, open-cell foam is better suited for most residential structures for that reason; it is more flexible. On the other hand, closed-cell foam is more resistant to moisture and water vapor than open-cell, which leads to another issue about using foam insulation.

Did I mention that water vapor travels with air? To make a long story short, air can carry quite a bit of it, then, depending on temperature, it can condense into moisture or water on a given surface. Now, let us imagine that air moving through a house and into an attic that is sealed air tight. Where might the moisture develop? The likely answer is the attic, where it is basically stopped by the foam insulation. We might call this a problem caused by a solution. Much greater efficiency, but new problems related to moisture. I will provide a list of excellent references at the bottom of this article for those of you who want to read on and become better informed.

Back to what led me to the research. As it turned out, foam insulation was not my client's problem; the OSB sheathing was. The seams of the sheathing were visible at certain times of the day, but I came to the conclusion that this was due to lighting conditions and was actually a permanent feature linked to swollen edges. I wasn't there for the framing, so I can only guess; I think the sheathing got too much rain before getting installed or covered over. In my opinion, it was mostly a cosmetic issue, but a darn thorny one!

Sources for information:

http://www.greenbuildingadvisor.com/blogs/dept/building-science/joe-lstiburek-spray-foam

http://www.greenbuildingadvisor.com/green-basics/spray-foam-insulation-open-and-closed-cell

 http://www.buildingscienceconsulting.com/presentations/documents/2010_Straube_Vented_Unvented_Roofs.pdf

http://www.specialty-products.com/pdf/articles/FineHomebuilding0311.pdf

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