@AsktheBuilder I think he is referring to modern insulation boards or foam solutions. With foam I would i'm sure 2/3's is possible in a place where insulation is difficult to apply like thin studs or around pipe etc.

add any additional batts, just blow more insulation. It's even better for sidewall use. My professional opinion; have a nice day.

Blown cellulose insulation into an open joist (or even a kneewall attic, with the help of some sort of holder) is the best choice. I've worked with a guy who's been doing it for 25+ years and nothing about any kind of health problems have arisen to his knowledge. The cellulose they make now has fire retardents, bug repellants, and if installed correctly filles all gaps and voids. Plus if you live in a colder climate and need to have your attic insulated to a R-38 or higher you won't have to..

Read my past columns about cellulose insulation at my AsktheBuilder website for the answer and more. Use my search engine to discover the articles and more.

Because it is made of the same material as paper, blown cellulose insulation apparently has added to it a fire retardant. Could this fire retardant conceivably represent a cancer risk, a few decades down the road? After all, while it is placed into the attic, or the walls, I'm sure that particles of this material will eventually make their way throughout the entire home. Have you ever heard anything about this possibility?

You can't get away with a comment like that without hard proof. Can you share the location of the independent test data that supports what you say? I'd love to know about this magic material.

Improvements to inslation have the potential to cut energy needs by two thirds

Good video - There's just one thing I would change. In milder climates a 6 mil polyethylene vapor retarder is not recommended. The kraft paper or even no VR is good.

Vapor barriers should face the warm side of the wall. But ceilings should not have a vapor barrier. Read my past columns about attic insulation at my website to understand why.

I just bought a house that has the insulation installed in the attic upside down. The vapor barrier is facing toward the inside of the house. Should I flip all this over, or is it not a big deal?