Re boards: Get a circular saw and set the blade depth to the thickness of the wood. Cut in the direction of the joists. Make your first cut between the edge of the board and the next joist. That should let you pry up the small section. Once you get a small section up, it is easier to expand out from that, but if you get stuck, you can keep sawing.

Adding height - use 2x4's perpendicular to the current joists. They make special joist hangers for this or you can just toe nail in if you have the skill. If you don't have one, renting a nail gun will make this job dramatically easier.

For the spray foam, if you are doing the whole ceiling, you should hire it out. If you are just doing the plates and holes, Great Stuff works fine. They make a "Large Gap" version.

Also I want to increase the depth of the joists by 3 inches to be able to add add'l insulation.

There is no point in putting in thermal bridges. Blow in cellulose across the floor. Or, if you insist, criss-cross with batts.

I don't know that the effort of pulling up the floor boards is worth it either, though it will enable you to eliminate the convection currents between the boards and the settled insulation. (Unless you run across the "expert" opinion that calls for currents so the insulation can--wait for it: "breathe".)

The point of "thermal bridges" is so you have have an attic as storage space. If you want to put plywood down, you gotta have something other than insulation to support it.

A sawzall will cut the nails beneath the boards, but you must have an exposed edge to start with.
I see no reason why the plywood couldn't be fastened through sheets of rigid foam laid over the floor joists. It's storage space, not human habitation. The continuous sheet foam would be a full thermal break.
Casey

The point of "thermal bridges" is so you have have an attic as storage space.

True. But I haven't seen one of those since I was a kid growing up in a 19th Century farmhouse.

The continuous sheet foam would be a full thermal break.

Nice idea.

You haven't seen a house with stuff in the attic??? Maybe it is regional, but I thought everyone had at least the christmas decorations up there.

As for the sheet foam, it is an excellent idea while building a home, but not many people can get a 4x8 sheet up into their attic once everything is finished off.

Another compromise solution - a 2x4 platform base in the center of the attic and just piling up insulation around the edges. The outer areas usually aren't great for storage anyway.

what about a nail puller like this one?

http://www.kk.org/cooltools/archives/001231.php

It was recommended to me in this gardenweb thread:
http://ths.gardenweb.com/forums/load/oldhouse/msg0112111816369.html
(skip the long bits and scroll down to the entry by la_koala)

And here is a direct link to a thread with a picture of the nail puller.
http://ths.gardenweb.com/forums/load/oldhouse/msg081804475852.html

Here is a link that might be useful: and another type of nail claw/puller

Foam also comes in 2x8 t&g sheets, IIRC. But anyway, you could rip it down to any convenient size and tyvek tape the joints.
Casey

>I want to remove the present old fiberglass insulation >that is presently in my attic floor

Why?

>which is covered by nailed down wide floor boards. I have >>tried to remove these boards already but the wood is very >old and it is impossible to pry up the nails. I cannot get >at these nails from the side. Does anyone have any helpful >info on removing them?

Get a bigger pry bar.

>The only thing I have come up with is possible drilling >>the nails out with a metal bit but not sure if this is the best way to go.

The bit would skip off to the side and drill everything but the nail.

>>Also I want to increase the depth of the joists by 3 inches to be able to add add'l insulation.

If you want more R value in the same space consider a commercial spray foam contractor.

>I thought adding 3" strips the same width as the joists

Why not just use standard 2x4s laid on edge?

You haven't seen a house with stuff in the attic???

I'm sure there must be many here. This is a Greater Metropolitan area of six million people. But I've not come across them in my 30 years+ as a renovator, builder and real estate broker. Late 19th Century homes here--and I've renoed/owned a couple dozen--had no unfinished attic spaces. Instead, there were narrow steep stairs to a completely finished set of rooms with steeply sloped ceilings. Basements, which are the standard here, were where all the excess "stuff" got stored.