Low Pressure Air Forms
Do they work? Are they worth the cost?
There’s more than one way to skin a cat.
That’s an old saying whose literal application is truly out of date. Nobody is any longer out to skin cats.
However, many people want to do something – like build a dome – their way and hopefully improve on current technology. The problem is that the their/new way is usually an old way long ago discarded.
For example: many very large thin shell domes were built by piling dirt to form the shape, then pouring concrete over the earthen form. After the concrete set the dirt was removed. That method still works, but new technology has replaced it.
The Monolithic Dome was a new way of improving on old technology.
The old technology used air forms made of light weight plastic – often as light as polyethylene sheeting taped together. The old way was to low-pressure inflate the lightweight form and then very carefully apply layer after layer of polyurethane foam, until a dome was created that stood totally on its own. The poly-skin was then peeled off and the dome could be painted.
A later iteration was to apply a fireproofing layer, perhaps plaster, to the inside. The old way worked and still works, but it has some very serious problems that needed solving.
The urethane has to be strong enough.
Increasing the strength of the urethane means using up to 6 inches of foam (often 3-pound to 6-pound density foam). At $2.08 per pound (current 2009 cost) the material alone for 6 inches of 3-pound foam is about $6.00 per square foot. Double the $6.00 to account for the cost of in-place application and you have over $12.00 per square foot of surface area.
Think about this $4 per square foot cost increase of the urethane foam:
- $12.00 per square foot for 6 inches of 3-pound foam for the low pressure system
- $4.00 for 3 inches of 2 pound foam for the Airform system
The increased amount of urethane resistance to heat loss goes up from 95% to 99.4%, but at what cost? The thermal diffusivity of the concrete mass of the Monolithic Dome more than makes up for the extra insulation – by double.
The coating has to be strong enough.
There are no coatings as effective as a fabric reinforced membrane. If the lightweight air form is peeled off, the urethane foam is exposed. Unless very expensive coatings are used and a very good maintenance program is in place, the urethane foam will fail.
This failure is proved every day all over the country in the urethane foam roofing industry. In general, excellent coatings cost more than a good Airform. Either the Airform or the excellent coating needs to be protected as it ages or as failure occurs.
Learn from the past
In the early 1970s, builders used a lightweight air form and applied a lighter layer of urethane. They then applied concrete. The weight of the concrete broke the bond of the urethane to the light air form and collapsed the structure. This release of the form and collapse of the dome were very serious problems.
Look at what Wallace Neff did. Neff inflated an air form and shoveled the concrete onto it during the second World War to build munition storages. It worked for a while. Many of these domes have since been torn down to make way for modern buildings and homes.
Or look at Dante Bini. He poured concrete on the air form while it was laying on the ground. He then inflated and smoothed it. He built many domes, but no longer does.
Lloyd Turner came up with the foam dome or what some are calling low pressure air forming. The system worked but was confined to small self supporting foam domes. The costs were high because the urethane and the coatings had to be extremely strong. The technology moved on.
Fiberglass domes in all of their variations have been used over the years. The domes work, and in some cases have been quite effective. But time and size are their enemies, and so far fiberglass domes have not made great inroads into the structure market.
They reflect lessons learned by past builders. The Monolithic Dome stays strictly within present codes. Building codes are in place to save lives. The codes are not always best, but neither are they in place foolishly.
Many of the new ideas are outside of the codes. The cost of getting code acceptance is very high. No individual project can support the cost of getting the tests for acceptance. Many new materials do not stand the test of time.
It is tough to beat concrete for domes. It has stood the test of time – nearly 2000 years in the case of the Pantheon.
The Monolithic Dome and its technology will allow us to build bigger, faster, more economical and greener buildings than ever before.
originally published June 16, 2004, updated August 1, 2009