My house has 5.5" of 52 year old rockwool and about 6" of fiberglass on top of that. In many places the fiberglass is quite compacted from people working up there. I would like to add more to bring it up to R-50.
I have thought about getting one of those vacuum trucks that does duct cleaning to come out and remove all the old insulation since I don't know how well the old rockwool insulates. We just had it tested for asbestos because it has a lot of suspicious debris in it. The lab said it is 10% wood chips, 2% mica and 88% rockwool, I thought the mica could be vermiculite.
From what I have read fiberglass will not stop air movement but rockwool will. Would it be advisable to blow more rockwool on top of the fiberglass or would it's weight just compact the fiberglass making the whole thing an exercise in futility?
Putting more insulation on top may compress the goods below, but that doesn't take it's R-value to zero- the compressed layer will have about the same R value per-inch, just slighly fewer inches. It's the settled/compressed depth that determines the R-value of the layer, but it'll still have the same R/inch. A 6" layer of blown low density fiberglass is worth ~ R18-19, but compressing it to 4" still yields ~R12-13. It won't start going negative on a per-inch basis until it's an inch or so (which would take quite a bit of weight.)
Rock wool is higher density, and does outperform fiberglass on convection losses, but still undeperforms cellulose in that regard. No fiber insulations "stop air movement", but on a good-better-best in open-blown goods, cellulose wins in the the air-retardency category. Blowing as little as 3" of cellulose over low-density fiberglass is enough to "restore" it's convection-related losses in R-value. If you use "stabilized cellulose", it contains an adhesive that keeps it from settling, or being moved around by high-winds through the soffit venting as well. JM Spider super-fine fiberglass has a similar adhesive, but needs to be installed at 1.8lb/cubic-foot density
A combined 10-12" of rock-wool + fiberglass yields something on the order of R30-R35. Blowing 6" of cellulose, or whatever it takes to get a uniform depth of ~17-18" would get you to R-50. Both rock wool and old-skool blown fiberglass yield ~ R2.5-R3.5/inch of depth, cellulose will give you at least R3/inch in a low-density open-blow (more like R3.5/inch or more, in wall-cavities at higher density.) At 18" of total depth, you're there, with a little bit of margin for settling/compressing over time.
Many installers will use a series of cardboard gages distributed around the attic as a guide, then rake it all smooth at the end.
It is true that the pros will often install it cheaper than a DIY, but there are good/better/best installers, and "fluffing" fiberglass for higher depth/lower density was an all too common practice in some regions 20 years ago (don't hear too much about it today). When they do that the total R is lower, and the convection losses even higher- packing that down with a higher-density overblow IMPROVES the performance of a fluff-blow by quite a bit. It's a little harder for them to cheat that way with cellose or rock wool, but not impossible. Unlike fiberglass, fluffed cellulose doesn't take a very big initial performance hit, in R/inch but will lose considerable loft- like 25-30% in it's first 5 years of service, as opposed to 5-10% over 2 decades (or centuries) for proper-density goods. But stabilized-cellulose (aka "wet sprayed" ) doesn't have that issue- stays pretty much at it's initial installed depth unless you walk on it or something.
Before insulating it's always a good idea to air-seal the attic floor/conditoined-space interface, with spray foam/caulk/boxes over recessed lights, etc, since the as-installed performance of the insulation depends on it. You may need to pressurize/depressurize the house to find & fix them all, but you can count on there being quite a bit of air-flow into the attic on a previously-untreated house. Beyond the ubiquitouse nightmare leaks like recessed lighting cans, concentrate on plumbing, vent & flue penetrations, electrical penetrations, un-plated partition wall framing, etc. Hiring pros to air-seal the whole house with before & after blower-door verification of their work is probably more cost-effective than adding 6" of insulation in your situation.