You can absolutely follow through on your best case analysis. Finding another homeowner with a similar home and the same product installation will tell you nothing about your best case, only about a possible similar outcome. Unless the other house is in the same area of the world, same tree cover, same type of use of the house (# of times the doors open during the day, etc.), same kind and efficiency level of HVAC equipment, etc. it will be impossible to do an apples-to-apples comparison.
You have to do a little work but the arithmetic is trivial. Or use a site such as EnergyUseCalculator - dot - com ( http://energyusecalculator.com/electricity_centralac.htm ) . Check the labels on your AC equipment and your electricity bill to get the numbers you need.
For instance, if you normally use your AC for 6 hours per day for a solid 3 months every year and your AC uses 3500W of power and an electricity rate of $0.10/kwh, then your best-case (no AC use at all) would save you 10.5 years to hit the break-even point.
The research you did is based on a flawed understanding of the thermodynamics of roof structure, because the most prominent info on the web is from sellers/installers of radiant barrier films or paints (competing products). I was a physicist long before I got into roofing. Like I said a few msgs ago, I’m not going to dive into this debate, but info that discounts heat barriers out of hand is just simply hogwash. The only thing that matters in the end, however, is not the science or theory but whether it makes the home more usable, comfortable, or cost efficient.
I previously explained that the choice of shingles is important in this situation as some are more sensitive to under-ventilation than others. Ventilation is BY FAR the most important choice you will have to make and there is no way whatsoever to know what type you need to use (or even can use) until you take off the shingles and several sheets of decking to inspect the structure.
So, run your AC cost analysis, strip the roof, pull some decking, document what you find, replace the decking, dry it all in with “felt”" (use synthetic underlayment, not asphalt-impregnated paper) and then you can sit down at your kitchen table and figure out exactly what options make sense and which ones don’t. Then you can pick your shingles, decide on the ventilation, decide on the heat barrier, order it up and do the project. If the house is dried in properly (pay special attention to any valleys), then it will be fine for it to sit for a few days while you sort out the options. The only downside that I can see is that you might end up paying for underlayment plus heat barrier, instead saving the cost of the underlayment, but you’re only talking about one man-hour of labor and $100 in materials, so it isn’t that big a risk.
Well worst case would be that it makes absolutely no difference.
Best case would be that it makes 10° of difference which would be huge, and I probably wouldn’t need to run the air conditioner hardly ever.
But at this point in time there is no way to know about the best case scenario, once I can find someone with a similar roof I’m similar mobile home as mine.
So what I’m going to do is use the contractor with the ridge vent, which triple the venting capacity and bringing it up to San Jose city standards which is based on the 150 formula.
What I meant was I didn’t have that 2k for a very uncertain cooling gain.
And after doing some more research on the reflective barriers.
What I had read is basically that there has to be some type of air gap between the reflective surface and the shingles.
So the best place to put it would be on the underside of the attic rafters if it was possible to get in there which it isn’t.
I would be convinced if I could find a referral of a similar type mobile home owner as myself that has used it and has noticed a noticeable cooling effect with it.