I own a 40 yo Cape in MA. Until this year I thought our attic was sufficiently insulated. Years ago we had fiberglass blown in. Subsequent to that we decided we would like to use the attic as storage space. We had attic pull-down stairs installed and I nailed down 1/2" plywood over about 80% of the attic floor joists.
For ventilation, we have 2 gable vents and a ridge vent but no soffit vents. Based of a severe ice dam (and consequent dining room ceiling stains) we have decided to add soffit vents and have cellulose dense packed under the plywood flooring as well as over the non-covered areas of the attic.
I have 2 questions:
1. How effective is dense packing when there is already insulation in place (I have been told by 3 contractors that they will drill holes in the plywood and that the preocess is very effective), and
2. Everything I have read says to install soffit vents and seal off the gable vents to avoid short-circuiting of the air flow from soffit to ridge One of the 3 contractors says it's okay to leave the gable vents open.
Imo, you really need to find a better place for storage. A major component of attic insulation is air sealing all of the penetrations (chimney, vent stacks, junction boxes, top plates, etc). Then, you really want as close to R-60 as possible (at least R 38) for your region of the country. Dense-packing under the plywood won't come anywhere close to this (Even if you have 2x8 rafters, you'd be in the R 20 range at best). Personally, I'd rip up the wood, air seal, add sufficient insulation (for those R values you are looking at 16-20") and then if you must have storage, build a platform... As for ventilation, I'd seal off the gable vents and add soffit vents.