Hello All. Thanks for the input Jerry. As usual I'm about to say too much about too little so grab some chips and a cool one 'cause here I go.
I read up on how heat can be transferred-- the usual, radiation, conduction and convection and then I began reading about insulation and how it works. I found many interesting articles and scientific studies of insulation in of all places the camping/backpacking forums. Two themes seem to repeat themselves: trapped air and re-radiation of heat energy within the insulation. The best insulation for our purposes at least, seems to be one that traps air in a fine matrix of material in such a way that the ratio of matrix to air favors air. This apparently reduces heat loss through conduction (small amount of matrix material) and increases heat retention by re-radiating heat energy within the matrix thereby keeping it from passing to the exposed outer surface of the insulation only to be lost via convection. And thats "ducky". I'm sure others on the forum can explain all that better than I did and please do.
Enough already. I wanted to know what happens in the bus world. I began my study by fabing up a constant temp heat box constructed of a cardboard box and a 25 watt lamp. I did my little experiment in the confines of my garage where I could maintain the room temp at 68 degrees F for an extended period of time. I allowed the heat box to come to its peak temp which turned out to be 83 degrees F across its upper surface while the surface was exposed to ambient garage temperature.
As an aside, I used a China Freight IR thermometer to track heat change. I have learned that the thermometer gives faulty readings when aimed at reflective surfaces (like chrome) and that it seems to heat saturate if it is used repeatedly in rapid succession to measure the temp of a single location. The heat saturation problem can be eliminated by treating the thermometer to a cold measurement after each hot measurement. I fooled with this for an hour or so and found that I could get readings to exactly repeat themselves time after time. Therefore, I made my measurements on patches of flat black electrical tape and cleared settings before each successive reading.
I constructed test panels one each of 1/4" thick bubble wrap, two layers of 1/4" polycarbonate sheeting which looks like plastic cardboard box material, 1/2" thick single vinyl faced foam and a section of faux down comforter made of polyester. I then measured the insulating capabilities of each by sequentially placing the panels on top of the heat box and taking a series of temp readings -- a couple of hundred or so. Each test panel was allowed 20 minutes to come to equilibrium with the heat box and the ambient air prior to taking measurements. I found the results surprising.
[/b] This is the box and lamp setup.
[/b]
I made temp measurements at 6 places on the top surface of the heat box. I later used these readings as baseline info.
The first measurement was on bubble wrap.
The next was on polycarbonate. [\b]
Next was the foam rubber the pic of which I cannot find!?!. Oh well.
The final measurement was made on the polyester comforter.
I'll take my findings with a grain of salt. I have no training in thermodynamics--but I did stay at a Holiday Inn!
I simply looked at the ratio of the temperature on the surface of the heated box and that of the surface of the various insulation panels at the upper surface/air interface and expressed the result as a percentage of the heated box surface. For example, had the hot box been 100 degrees at its surface and the test panel only 95 degrees at its surface/air interface, I defined the result as a 5% decrease in heat loss (a heat savings I suppose). This is what I discovered:
One 1/4" layer of bubble wrap------------------------------------2.4%
Two 1/4" layers of bubble wrap-----------------------------------3.1%
One layer of 1/2" thick foam rubber (auto uphol grade)-------5.5%
Two layers of 1/4" polycarbonate---------------------------------8.4%
One layer of polyester comforter---------------------------------9.8%
Here are my thoughts on the results. The higher the % the better the insulation. Looking at the physical makeup of the bubble wrap it seems that the ratio of trapped air to matrix material is about one to one.This results in a large surface area of matrix which lends itself to conductive heat loss which lessens its effectiveness as an insulator.
I was surprised that the foam rubber did not do better than it did. The ratio between matrix and air looks to favor air. Perhaps the wall thickness of the matrix leads to conductive heat loss. I looked at a cross section of both the foam rubber and a gob of cured "Great Stuff" under my microscope and found the air pockets of each to be similar in size but the matrix of the foam rubber to be many times as thick as the walls of the Great Stuff. Though I didn't test Great Stuff, we know that it has a high r value. So for whatever thats worth-----------
I tested the polycarbonate because that is what the pop up on my bus is made of. While the material is too rigid for what I want to use it for, I was pleased to see how it stacked up with the other materials.
Of the materials tested, the faux down was clearly the most effective insulation. An added advantage of the faux down is that it can be heavily compressed for storage without damage to it's structure and it is washable. The particular comforter I tested was "tube stitched". The stitching makes the surface of the comforter look like plump hotdogs lying next to each other. The stitched areas reduce trapped air to zero at the stitch thereby creating a location for conductive heat loss. A "V" stitched comforter would therefor better control heat loss by conduction.
I couldn't resist testing the samples with a sheet of shinny foil between the heat box surface and the bottom of the test panel. The results in all cases was a major reduction in the effectiveness of the insulation. I can only surmise that any positive gains in insulation due to re-radiation were more than negated by the thermal conductivity of the heated foil
I don't know why it should have come to me as a surprise, but the heat box required cooling between test panels. I got sidetracked with that for a while and discovered that the heat increase in the heat box is almost the reciprocal of the "heat savings" attributed to the insulation. Too much fun! Jack