Hi dnb5660,
Welcome to the community and asking a great question regarding foam spray insulation. I have heard and seen my customers and co-workers in my store discuss this form of insulation quite a bit. I'm glad you did your research of the new way people are insulating their homes. Open cell foam uses 0.5 lb per cubic foot while closed cell uses 2.0 lbs. a cubic foot. As you stated, you would have to spray several times to achieve the desired R rating needed for your ceiling. Whether you choose closed or open foam, consider costs, how its installed and overall performance between the two....a compare/contrast if you will.
With the open-cell vs. closed-cell issue, there are two major factors to understand and consider. The first is the nature of the foam. It is either open-cell foam, where the tiny cells of the foam are not completely closed. They are broken and air fills all of the “open” space inside the material. This makes the foam weaker or softer feeling than closed-cell foam.
Notice the differences between the two foams above and below.
The advantages of closed-cell foam compared to open-cell foam include its strength, higher R-value, and its greater resistance to the leakage of air or water vapor. The disadvantage of the closed-cell foam is that it is denser, requires more material, and therefore, is more expensive. Even though it has a better R-value, typically the cost per R is still higher than open-cell foam. The choice of foam can also be based on the requirements for the other performance or application specific characteristics such as strength, vapor control, available space, etc. Open spray cell foam typically has R-values around 3.5 per inch, the same as standard insulation. Closed cell foam typically has around 6 to 6.5 per inch by comparison. Closed cell spray foam uses blowing agents that create millions of tiny bubbles that are filled with gases less conductive than air. The gas inside the closed cell foam works on the same principle as the inert gases used between the panes of glass on a high-performance window, giving it a higher R value.
So where to use them? You typically would not use open-cell foam below grade or in places where it could absorb water; this would make it fail in its thermal performance because water is a poor insulator compared to air. Closed-cell foam would be a good choice where small framing sizes need the greatest R-value per inch possible. Closed-cell foam would be used for roofing applications.
So in closing yes, dnb5660, you would have to spray several times really with any foam, cellulose, or blown batting to achieve a higher R value for your home insulation, regardless of type you pick out.
I hope this has answered your question, and again if there is anything we can assist you in please ask.
aboveaveragejoe
