Understanding Polyurethane Foam

Foam density

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 1’ square, 1" thick. There are 12 board feet in a cubic foot, so 6 board feet equal 1 pound).

When you are spraying polyurethane, a certain amount of the foam doesn’t rise. It’s generally against the edges. For instance, if we are spraying on something cold the foam cools before the blowing agent can heat up enough to expand, and we lose all or almost all of the expansion.

The top layer of the foam will be against the colder atmosphere and will cool without nearly as much expansion. Therefore, we wind up with a skin on the top that has a much higher density than the rest of the foam.

A serious question

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 4 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 0.5 pound per cubic foot, is primarily used for packing. If you have something to ship that you don’t want broken, you put it in a box and fill the box all the way around the article with half-pound packing foam.

Foam as insulation

A few people are using ultra-light density foam as insulation for houses. That’s kind of a sales gimmick. You can get more thickness with the same amount of chemical, but you don’t get more insulation. The insulation factor goes down.

For Monolithic Domes

Two-pound density foam is what we recommended for Monolithic Domes. It’s good and strong, yet it’s light enough to be economical.

If we go lighter than two-pound density we run into problems getting enough strength to hold the rebar and help with the building’s shape. 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 goes down.

Higher density foams are a 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 better solution.

Usually when this topic comes up someone suggests, “Let’s put some four-pound density foam next to the Airform to make a tough surface, and after that is in place we will spray the bulk of the thickness using a two-pound density foam.” Our experience shows that the two foams tend to separate and create big problems.

An anomaly

When we talk about foam, an anomaly is a variation in the urethane due to its chemical and physical nature.

Recently I visited a beautiful, new gymnasium we had built. In looking it over, I noticed a bit of foam anomalies showing on this building. This is not unusual, and few would notice, but I did. I don’t think it’s possible to build a Monolithic Dome without foam anomalies showing.

We all expect anomalies or surface variations. But in a Monolithic Dome, the surface of the sprayed foam is covered by concrete. What we see when we look at the roof is the back side of the foam.

As it cures, urethane foam tends to pull or shrink just a bit. If it’s sprayed on metal or concrete, it can’t move the substrate and we can’t see it. When it’s sprayed on 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 must tell our potential customers that this isn’t machined. This is sprayed and because it’s sprayed foam can and will have variations. I tell customers it’s like a worker troweling a surface on concrete. No matter how careful the worker is some places will look better than others. In a way this is our situation with the urethane, except that there is no going back over a spot. Once it’s sprayed, it’s sprayed!

Updated: October 8, 2007

Editor’s Note: For more information on foam, we invite you to review Urethane Foam: Magic Material and the Best Kept Insulation Secret by David B. South.