Found this product completely by chance. White Double Bubble Reflective Foil Insulation, read a lot of reviews on this stuff and a lot of super detailed blogs. Most of what I read made perfect sense, it's not R8 unless properly installed, it's really only R5 when put straight up against the block.
You're right guys, that is kind of misleading, but I could easily read that straight on the manufacturers website. I could even find where they stated it would be R5 if applied straight to the block. Don't see anything really misleading about them giving you actual numbers, as long as you read directions. Of which, I don't know why a person would not read directions before working with a new tool or new material.
So lets get down to what the so called experts missed. If you have the gap between the insulation and the wall and can get R8. Then lets put the insulation against the block, then leave our gap. We are already at R5 by being against the wall, the gap after should still take us to around R8 (probably a slightly higher, since the air gap is dealing w/ warmer air this way anyway).
Now the experts claim, that this R5 just isn't worth it, since with bat insulation you're going to achieve a much higher R value. Wrong!!!!!! I'll say this again, you (the experts) are wrong!!! bat insulation installed correctly or even incorrectly, will leave some small amounts of air through. There's fiber in there, which means air can pass through it (albeit slow enough that you're probably not going to feel it). Lets consider the seams though, where you staple the insulation to your studs, this too is an air space which can allow air to pass through (again it's a small gap and you likely won't notice it).
Lets then look at this stuff, it's a solid sheet wrapping your wall. One seam in the middle, which you should be taping to another sheet. You're probably going to staple it up top and likely will be using some Construction Adhesive to adhere it to the wall. Leaving only the gap at the top of your wall and a gap at the bottom (instead of a small gap every stud, plus the same top and bottom gaps). So you've reduced potential air heat exchange quite drastically.
The Experts will tell you that your wall itself will stop this air exchange, which it will, in your room. What about in your wall itself? Are your walls cooler to the touch? Can this really be what you want?
If you wrap your block first, then frame out your wall, then use bat insulation, then put your wall up. Guess what you've done? You've created an air stop at your block, reduced airflow to your bat insulation, and thus reduced air flow through it, reducing the amount of cooler air actually reaching your interior wall. How can this be a bad extra step?
I get it, it's not a requirement, it's a step that could be skipped, but it's a step that shouldn't be. It's not going to cost you a whole lot more, it'll even save you a bit in the long term. Will this be enough to pay for itself, probably not in normal winters. Imagine though if you will, the winters where it's negative temperatures for the entirety of January (I seem to remember having one), now imagine it being very windy (again, I seem to remember this), nothing was preventing this air from coming in, my furnace ran constantly. With this type setup, I'd wager my furnace won't be taxed nearly as much if this happens again. Best thing is it'll still probably cover half it's cost in it's life cycle, during normal winters, but if a cold one hits, it'll pay for itself outright. 140$ I'm willing to put that little bit into my project, to potentially save my furnace if the weather gets completely insane for a month.