Insulating the attic, or above the ceiling, is a bit more difficult than insulating walls but done right everything will go smoothly. Here are a few hard learned tips...
1) Cover up - Protect Yourself
Be safe! We covered this in a previous post (see it here).
2) Use Lots of Staples
The weight of the insulation is supported by the vapour barrier until the drywall is installed. The weight can be substantial PLUS there is added pressure as you are moving and placing each batt of insulation. Before the drywall is in place the only thing holding the vapour barrier to the ceiling joists are the staples. Use lots of staples. We found that one every 3 inches seems to work really well. Anything further apart than 6 inches just wasn't secure enough. Trust me I learned the hard way...
3) Overlap Vapour Barrier by one Stud
Another trick was to overlap the vapour barrier by one ceiling joist pair. This means that every 6 feet or so there is a double thickness of vapour barrier. This provides extra holding strength for the insulation -- mostly because you are doubling the number of staples in that section.
4) Lots of Tuck Tape
Tuck tape is the red stuff in the picture above. It is used to seal the joints between the sections of vapour barrier and around the holes for the electrical openings. Not only does this ensure a complete vapor seal, it adds additional holding strength. Use lots of tape! It is not that expensive and an extra roll or two is not going to kill the budget.
5) Use a Bread Knife
With mineral wool, the brand we used is called Roxul, the best way to cut it is with a bread knife. And I recommend using a real bread knife. You can recognize a bread knife by its unusual shape. A bread knife has the tip pointing down, and has a serrated blade. Cutting Roxul is really just like cutting a loaf of bread! Other knifes which are better for cutting glass wool insulation just do not work well with mineral wool.
I actually keep two knifes when working with Roxul a bread knife plus a standard box cutter which I use to open the bags of insulation.
6) Change Directions
It is generally recommended that you approach an R40 value when insulating an attic. This will mean using two layers of insulation. To get the best coverage place one layer between the ceiling joists, and then one across this layer perpendicular to the direction of the joists. Because the ceiling joists are mostly 2 x 4's in our attic we used R14 for the first layer, as it fit perfectly. We covered this with a layer of R22 across the joists. In total we have R36.
It is worth the effort...
With the above tricks and tips you can save yourself a lot of grief. It takes a marginal amount of extra time but in the face of the consequences a great insurance policy.