Use lowes, get 20 bags, they give you the machine for the day. Bring back what you don't use.

Love the precision in the above post!

The US Department of Energy provides this calculator by Zip Code.

This calculator shows payback period, ie. how long it takes the savings to equal the initial outlay.

Dr. Lstiburek of Building Science Corp. recommends minimum R60 in attics anywhere north of the Mason-Dixon line.

As important as insulating is sealing all entries between the attic and house.

You should put in at least the recommended R value for your region.

If you are DIY, there will be a sign right next to the bags that says x inches = R whatever. There are several different types of blown in insulation and they have different R values per inch.

If you pay a professional, they will do the calculation for you. We recently took this route and consider it money well spent. It was only a couple hundred more than doing it ourselves.

As someone who has spent the better part of two weekends removing blown in insulation in my attic...by all means, make sure you seal off any gaps first.
I found my kitchen soffits were totally exposed up in my attic and were right next to an outside wall where cold air was pouring in.

The question really can not be answered on a forum. That's why I answered it the way I did. If they buy 10 bags they need to rent the machine. If they get 30 bags and only use 20 they have to transport and purchase all the extra material. For DIYers 20 bags will be enough for them. If they need more they can go back and get some the same day.
Its not rocket science.

gzec- I found your original post VERY helpful, thanks.
Paul

I'm getting ready to add more insulation to my attic.

It's 680 square feet. I'm in Northern Virginia.

My calculations are that I'll need between 55 and 65 bags of insulation to bring me up to R-50.