Understanding Polyurethane Foam

Updated October 8, 2007
by David B. South

One of the characteristics of polyurethane foam is density. Density equals how many pounds per cubic foot it weighs. So, when we say we want two pound density foam, it means we want two pounds per cubic foot of foam weight. This can be a little confusing. When we spray foam, the density actually goes up. For instance, if we were using two pound density foam, we would expect to get six board feet per pound. (a board foot is one foot square, one inch thick. There are twelve board feet in a cubic foot, hence, six board feet would equal one pound).

When you are spraying polyurethane, there is a certain amount of the foam that doesn't rise. It is generally against the edges. For instance, if we are spraying on something cold the foam will cool off before the blowing agent can heat up enough to expand and we lose all or most all of the expansion. The top layer of the foam will be against the colder atmosphere and will cool off without nearly as much expansion. Therefore, we wind up with a (skin) on the top with a much higher density than the rest of the foam.

When you are in the polyurethane foam business one of the serious questions you always ask is, "What kind of yield am I going to get with my urethane?" If you can get four board feet of finished foam in place using two pound density foam, that is considered really good. For the most part we estimate 3.5 to 3.8 board feet per pound as a method of calculating how much chemical we will need.

Urethane foam comes in many densities, 0.5, 1.5, 1.7, 2.0, 2.5, 3.0, 4.0 6.0 and even higher. The very light density foam such as the 0.5 pound per cubic foot foam is primarily used for packing, if you have something you want to ship, you don't want it broken, you put it in a box and fill the box all the way around the device with half pound packing foam.

A few people are using ultra light density foam as insulation for houses. It is kind of a sales gimmick. You can get way more thickness with the same amount of chemical but you don't get way more insulation. The insulation factor goes down. Two pound density foam is what we recommended for Monolithic Domes. It is good and strong, yet it is still light enough to be economical. If we get lighter than two pound densities we run into troubles getting enough strength to hold the rebar and help with the shape of the building. On the other hand, there is very little to be gained by going with heavier density foams. They don't give you a better R-Value, and they cost more because the yield is goes way down.

The higher density foams are a little bit tougher, so they give you a little stronger outside surface, but if surface strength is an issue a shotcrete or stucco coating over the airform is a far better solution.Usually when this topic comes up someone suggests, "Lets put some four pound density foam next to the Airform to give us a real tough surface and after that is in place then we will spray the bulk of the thickness using a two pound density foam." Our experience shows the two foams will tend to separate and give you big problems.

Recently I visited a new gymnasium we had built. It is a beautiful, beautiful gymnasium. One thing that I did notice is there is just a little bit of foam anomalies showing on this building. This is not an unusual thing. And few would notice. But I do. I don't think it is possible to build a Monolithic Dome building without foam anomalies showing.

An anomaly is a variation in the urethane due to its chemical and physical nature. We all expect surface variations. But with the Monolithic Dome the surface of the sprayed foam is covered by the concrete. What we see when we look at the roof is the “back side” of the form. Urethane foam as it cures has a tendency to pull or shrink just a little bit. If it is sprayed on metal or concrete it can not move the substrate and we can not see it. When it is sprayed on the fabric we can see its backside. Then we see the effects which often show up as surface roughness (smooth roughness if that makes any sense).

We have to be sure and tell our potential customers that this isn't machined, this is sprayed and because it is sprayed, foam can and will have variations. In this case some foam had collapsed. I tell customers it is like a man troweling a surface on concrete. No matter how careful the workman is there is always some places done better than others. In a way this is our situation with the urethane – except there is no going back over a spot. Once it is sprayed it is sprayed.

Rebar Hanger Placement and Final Foam Application
Polyurethane Foam Application
Foam As A Fire Hazard
Foam as a Fire Barrier
Sprayed Polyurethane Foam Insulation -- A Little Known Alternative With Big Benefits
"R" Fairy Tale -- The Myth of Insulation Values

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