Quote Originally Posted by jadnashua View Post
You're missing the point...obviously, adding insulation helps. But, putting blown in over a batt type compresses the batt. Your total insulation level wouldn't be as high as if you'd either used all blown-in, or put the batts on top of the blown in. IOW, you didn't get the full benefit of what you installed.
Not necessarily. Low density batts (the typical goods, not the high density "cathedral ceiling" batts) need a topside air barrier to actually achieve their rated R value performance, and in most attic installations the tops are left bare. But if you blow over them with a higher density product like cellulose, it actually performs BETTER, despite being slightly compressed. (And with only 6" of overblow it's a VERY slight compression indeed.) By filling in all the gaps and voids, even low density crappy blowing wools will improve the performance of the batts, but not as much as cellulose.

With any batt installation layed down on top of blown (any density batt) or other batts you have the issue of the gaps allowing convection to rob performance.

What I WOULDN'T use is cheap low-density fiberglass blowing wools- I'd do rock wool before that, but cellulose is even more air-retardent still. A 3" overblow of cellulose can "restore" low density batt performance to near it's ASTM C 518 test rating, but not low density blowing wools. The test plates used in the ASTM test become air barriers during the test which is why in many ways there's a labeling issue when batts or blowing wools are used in attics without an air barrier on the top side, as is usually the case.) Testing those products in the fixture with an added air gap also doesn't cut it, since the gap isn't anywhere near as deep as an attic space, and isn't vented to the great outdoors. (And attic ventilation rates will vary, making any arbitrary vent rate in a test fixture wrong.) But blown cellulose hits pretty close to it's rating, even without a topside air barrier.

To get best results it's good to set up depth gauges and rake it all dead-even, since a few thin or bald spots can rapidly become the thermal bypass for the rest.

I've occasionally blown insulation as a 1-person DIY, and I can testify that it's a scrambling, tiring. and slow way to do it.