an Giffin, the Boss Nut, asked me to reminisce concerning foam insulation. I have to qualify my expertise about insulation. Being an old airplane designer, and I emphasis the word old, I may be a bit obsolete in the prevailing technical knowledge. After all, it wasn't too long ago that we found a substitute for dope on our wings - metal.

Well now, it's my opinion that there are at least two forms of insulation. One is very thin and is slathered on tail pipes and exhaust nozzles and fuel lines and stuff you don't want to get too hot to touch. We used to use a slurry of asbestos but we all know what happened to asbestos. Later we came up with various combinations of ceramics and fiberglass. One that comes to mind is Fiberfrax. At this point I might mention a new product called Temp-Coat, a liquid ceramic insulation coating. For more information try http://www.hiwaynet.com.

Getting back to the subject, BossNut posed the question, "Is it possible to put the walls and ceiling in place, leaving a gap of 2 or 3 inches at the wall/ceiling intersection so a foam insulation company may spray into the gaps and still do an effective job given the expansion and drying characteristics of spray foam"? The answer is, "No"! Now that we have satisfactorily answered that query, next question please.

With respect to insulation foam in place, half a century ago we made control surfaces mostly of foam. Control surfaces such as rudders, elevators, flaps, fins, slats, ailerons and that sort of thing. We would make a wedge of .012 polished aluminum skins and stand them in the upright position in a jig with a spar of .032 aluminum channel with lightening holes. We would mix a urethane foam and pour it into the cavity. At that time we used a resin and a foaming agent with a curing agent. Now, all this stuff is premixed. We mixed it in a Dixie cup and poured it into the lightening holes. As it expanded, it eventually bubbled past the holes and when it cured we would slice it off.

Now that we have digressed sufficiently, let us examine the procedure of applying foam to a bus for insulation. One technique is to use block foam. It is easily sawn and stuffed into the spaces. The disadvantage of block foam is it does not take advantage of the unique properties of blown in foam. Urethane chemistry has incredibly strong adhesive characteristics. Just get a little liquid foam on you and it makes a believer out of you. By apply blown in foam, you get a bonus of not only insulation but added strength.

Most of you have heard of sandwich structure. This is a structure where the full advantage of the strength of material can be utilized. For example, a very thin sheet of material may have exceptional strength in tension, but just try to put a compressive load on it and it will buckle, wrinkle or just fold over. A beer can, for example, that you can smash with your foot could withstand a tensile load of about 2,000 pounds. If you could stabilize that material so it would not buckle, you would need a 2,000 pound foot to smash it. One technique of stabilizing the materials is to attach a whole bunch of little strings or wire to the sides so the material can't move out of the way due to elastic instability (I love to use that term). Well, this is basically what we are doing when we inject foam into a thin walled structure. The stabilizing resistance will be equal to the adhesion of the foam. The point of all this, is that our bus is immeasurably stiffened and you may hold a dance on your roof if you wish.

Now, to the technique of applying the foam to our bus walls and ceiling. A really slick way to do it would be to build a jig to mount our bus so that we can rotate it about a fore and aft axis. This way we could drill holes periodically so we may pour the foam into the holes and it will expand and cure. Then all we have to do is shave the excess off. Now lets talk about how far to space the holes. If you have a cavity of unit thickness, space your holes at 4 times that thickness with a diameter of one-half that thickness. So for a 1½ inch cavity, use ¾ inch diameter holes spaced at 6 inches apart. I suppose I'll hear complaints that some of you backyard conversion specialists don't feel like fabricating a jig to rotate your bus (!), so we'll discuss another approach.

The common method of applying foam insulation to the inside of a bus is to spray it in. This is done with a mixing gun and metering pumps. The equipment is probably a little expensive for a one time usage, plus you know your going to screw up the first bus or two, so it is strongly recommended you hire someone to do this job who has already screwed up a few buses. The spray device is a mixing gun fed from two drums with their own metering pumps. When it is shot, it looks like cottage cheese being shot out of gun. It sticks to the substrate and expands. The foam application specialist sprays a layer about 1/8 inch thick and it expands to about ¾ inch thick as it cures. A couple of passes and it will fill most cavities.

Urethane foams cures with an exothermic reaction (another great word). This simply means that in the process of curing (drying or hardening), a chemical reaction takes place which causes heating which accelerates the process. You follow that? Anyhow, the temperature can exceed 150 degrees Fahrenheit. Because of this heat, flat metal may distort. As we all know, metal expands when subjected to heat. If it is restrained because it is attached to frames and ribs, it will buckle, or oil can. If this happens in the process of the foam curing and setting up while still hot, permanent deformation will occur and our bus will look funny. Several answers to this perplexing problem have developed. One thing many converters do is use block foam in the flat side walls to obviate this condition. I forgot to mention that our curved roof skin will not deform because of the inherent stiffening of the curvature. Another approach is to apply a thin insulation block, say ½ inch thick, thus shielding the outside skin from the heat.

Due to the nature of the foam-in-place technique, it is impossible to spray the material to a specific thickness. It is, therefore, applied so that it will be slightly over the desired thickness, say, an inch or two. I know many of us would like our cabinets to fit with a closer tolerance than an inch or two, so a high precision tool is used to trim away the excess foam. The trimming process is generally done at least a day after the spraying to allow adequate curing of the material. This high precision tool is often a piece of plywood cut into a disk so it will fit into a hand polisher or sander. The business side of the plywood disk has a whole bunch of nails sticking out about an inch so it looks like the proverbial East Indian bed of nails. The specialist dons what looks like a space suit with a hooded mask and goes at it. The average bus ends up with about 12 garbage bags full of scrap foam. I know of one guy who saved all this scrap for packing material thereby saving the expense of buying professionally manufactured popcorn. The fact he never shipped anything was unimportant.

Getting back to BossNut's question, to try and apply foam over a 48 inch space using only a two to three inch opening at one side, would fail because the foam would begin to expand and cure before it reached the full depth of the cavity thereby blocking any further material from filling all the way. So, random voids would result. Now, voids are OK if you can control them but since you can't see a void, you don't know where it is. OK?

For further reading. . . . . . Zzzzzz.

(Editor's note: Indeed, for further reading, please refer to Chapter 29 of Dave Galey's book, The Bus Converter's Bible. This chapter further delves into the exciting (!) topic of insulating your bus conversion.)