Determining the efficienciency of any wall construction/insulation combination is more complicated than just measuring the temperature of the skin in sunlight. Let me offer some information about heat transfer that I think is correct information:
Three types of heat transfer:
1.)
Radiation (Infra-red) - heat transfer directly from one entity to another by means of direct energy transfer in the infra-red spectrum. A good example of this type of heat transfer is what happens to the skin of an AS sitting in the sun. The source of the heat is, of course, very far away. This type of energy will cause the skin of the AS to warm up depending on how much of the energy is absorbed vs. how much is reflected away. The principle reason that the plain aluminum in Andy's experiment does not rise in temerature as much as the aluminum with the other coatings is exactly because shiney aluminum is a better reflector than even the white surface.
2.)
Conduction - energy transfer directly through a given material. Aluminum is a very good conductor and foam insulation is a very bad conductor. When you hear about the "R" value of a particular type of insulation this is referiing to the materials ability to transfer heat by convection. In the case of typical insulation materials you want the "R" value per inch of material to be as high as possible so that heat flows through the material as slowly as possible.
3.)
Convection - heat tranfered from entity to another by means of the movement of air. The heat source warms the air and the warm air transfers some of its heat to the things it is in direct contact with. A good example of this type of heat transfer occurs between the hot air outside of an AS and the surface of the skin. Another good example is the type of heat transfer that occurs between the heating element in a typical AS heater and the air that flows through the heating ducts. The air is warmed and the things inside of the trailer that the warm air comes in contact are warmed.
Heat always transfers from hotter things to colder things:
Pretty much all of the objects around us are warm to one degree or another (pardon the pun). A warm object always trys to transfer energy to colder objects nearby. The sun is obviously a lot warmer than an AS so the flow of radiant energy is always from the sun to the AS during the daytime. Radiant energy transfer is by far the way that most heat transfers from one object to another. I have read statistics that indicate that 65% to 85% of the heat that passes from a warm wall surface to a cold wall surface or through an attic is by radiation.
Anatomy of an AS:
The factory original construction of an AS with regard to heat transfer works something like the following when it is hotter outside than inside:
1.) The reflective aluminum surface on the outside turns back a lot but not all of the radiant energy from the sun.
2.) The highly conduction aluminum surface of the AS conducts some heat from the warm air outside of the AS into the wall cavity and through the wall framing to the inner aluminum surface of the AS.
3.) The fiberglass insulation in the wall cavity resists the flow of heat by conduction through the wall cavity to the inner skin of the wall.
Why an SOB might be better insulated:
Other brands of trailers may very well be better insulated than the factory original AS is. While I do think that a polished alumuminum skin will do much better in the direct sun than a white fiberglass skin relative to reflecting radiant energy from the sun the story does not stop there. Here are some reasons the rest of the trailer might perform better:
1.) Fiberglass skins most likely do not conduct heat as readily as aluminum skins do.
2.) Many of the other brands have a wooden frame which also does not conduct heat as well as the aluminum frame on an AS.
3.) It is becomming increasingly common for other brands to use solid foam insulation instead of fiberglass hence better results.
4.) The roof framing of a typical SOB is usually thicker than te 1-1/2" we have on our AS's. Ther is more room for solid insulation in there.
5.) The inner skin of most other brands of trailers is typcially not aluminum. These other types of skins generally do not conduct heat as well as the aluminum skins of our AS's.
Possible changes and their effect:
1.) Any additional coating on the outside of the outer aluminum skin will help block the flow of heat by conduction to the wall cavity and to the frame members. Unfortunately it may also reduce the ability of the skin to reflect away radiant energy from the Sun as evidenced by Andy's test results. Rubber roof coating would block more heat flow by conduction than paint would both because it is thicker and probably has a higher R value.
2.) Any additional coating on the inside of the outer aluminum skin will also help reduce the flow of heat by conduction to the wall cavity but not so much to the frame members. Any vulkem between the body panels and the frame will help block heat flow to the frame. Rubber roof coating for example can help here. I think this explains why panel 4 in Andy's experiment is slightly hotter than panel 1. The extra coating prevents some of the heat from bleading off the bottom side of the aluminum panel into the foam insulation.
3.) Different types of solid insulation in the wall cavity can help reduce the flow of heat if they have a higher "R" value than fiberglass insulation does. Foam insulation, for example, has an "R" value in the neighborhood of about R4 to R7 per inch depending on the type of foam while fiberglass is more in the range of R3. This means that the R value for 1-1/2" of AS wall can be in the range of R5 to maybe R10 maximum.
4.) Using radiant foil insulation between the inner and the outer skins of the trailer instead of solid insulation can dramatically reduce the amount of radiant energy that still manages to transfer through the walls by radiation. Supossedly a single layer of rlective foil insulation spaced between the inner and the outer wall surfaces has an effective R value of R16. For more informaion about how foil insulation works check out the following site:
http://www.tvmi.com/rfoil/physics2.htm
I also wrote up some things about reflective foil insulation in the thread titled "What should I use to insulate the floor?".
The bottom line:
For tests about the relative merits of the different types of AS surface treatments (and insulation types) to be entrirely valid they must measure the amount of heat that actually gets all the way through the wall to the other side.
It is my intent to modify my AS as follows:
1.) Add a rubber roof coating to the inside of my outer skin while I have the inner skins off. I would really like to know how effective this would be at reducing the amount of heat flow from the outer surface to the inner surface. If Andy could put his heat gun on the bottom side of the test panels this might help me know that.
2.) Use reflective foil insulation in the walls and roof spaced between the skins so that there is an air gap on both sides of the reflective foil. I have in mind to use narrow strips of foam insulation around the edges to help center the foil in the middle of the cavity. I would really like to do a heat flow test on complete wall panels of different construction styles (orginal and with my modifications). I may very well put such a test together myself just so I can find out for sure if what I am proposing will work effectively.
Would anyone else interested in the results of such a test?
Malcolm
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