You have insulation at the roof line and under foot up there????

you would increase the insulation value of
the attic floor by blowing enough inches
to meet requirements in your area.
less than 8" isn't much for your climate.

best of luck.

Ionized: yes. We pulled out all the old insulation above the attic floorline -- it was breaking up, sagging, falling down, etc. What's under foot is the unknown quantity and quality under the floorboards, which seems to be a few inches of blown-in, resting on top of paper-backed batts. There is blown in at the eaves. It appears that various previous owners pursued overlapping theories on insulation. Somebody evidently believed "the more the better," but I'm reading in some places that it is a mistake to insulate between the rafters. I'm inclined now to leave it all open and see how we survive the winter.

"...but I'm reading in some places that it is a mistake to insulate between the rafters."

It depends on the climate and the house. Sealing the attic and insulating at the roofline is a good thing for a lot of houses in the hot, humid South US.

Ionized, insulation other than foam is often
used in cold climates. just not here as the high
temps in summer & high RH have educated code
to NOT do this here.

OP..I agree that going thru the season is the best
option at this point.

again check into code insulation Rvalue requirements.
code is minimum..so upgrading to higher R-value is
a good idea.

insulation at roofline & insulation at attic floor
it is either one or the other. the air space between
roof & attic floor is huge. so the combination doesn't work.
even 1/2" (inch) gap reduces performance & lowers R-value.
just food for thought.

when you wrote overlapping of insulation..this is
blown over batts?

best of luck.

Energy_rater, OP has insulation at the house ceiling/attic floor, a (poorly) vented attic and MIGHT have insulation at the roofline. In the OP's second post it says " above the attic floorline" which is not the same as the roofline. Maybe it does not matter since it is gone. I am just trying to figure out what was going on up in that attic.

DC is pretty hot and humid in the Summer, but they have to deal with a little ice and snow. I am quite aware of the historical insulating strategies there and much further North since I have lived in the mid Atlantic. What I don't know is if sealing the attic is a good thing in that area. If it is, he might consider it.

If the attic is to be left vented, it needs more ventilation than one 18" square vent. As Energy_rater wrote, you need more insulation and gaps are not good. I think that you have little choice but to pull up the floor to add more insulation. Pay attention to the weatherstripping on the attic stair door and insulation around the encloses stairwell too.

ionized: what exactly do you mean by 'the roofline?" I will say this, that there was insulation between the rafters for many years and there is no evidence of mold, moisture damage or condensation anywhere. If I let the attic get cold this winter, will I be increasing the chance of condensation? I'm not sure it's "poorly" vented. It's a 75-year old house and it's dry as a bone up there--or has been up to now.

bppark you and ionized are talking about the same
install. just our climate Louisiana...putting
insulation that absorbs rather than seals moisture
at the roof line..between roof rafters is
not done. with our high humidity & hurricane
damages roofs it is against code (which is minimal
allowed by law) to put batts, cellulose & blown
fg (bibs).

building practices are climate specific.

but a standard is to air seal attic floor to
living space below..all climates.
properly vent bath fans, stove vents.
in your climate the lack of proper venting
& air sealing leads to ice dams.

blown insulation over batts is a good thing.
but remove any wood, or spaces between the two.
otherwise the insulation performance is reduced.

check buildingscience.com for cold climate info.
worth the time.

best of luck.

Nice job pulling the insulation from the rafters, it was useless. I sounds like you may have blown or poured rock wool under the boards, you can have blown in added under the boards by pulling just a few boards. Then I would recommend 10 to 12 inches added on top of the boards. My recommendation is blown in cellulose.

"ionized: what exactly do you mean by 'the roofline?"

I meant between the rafters, under the roof deck. If your attic is vented, there is no point to having insulation there.

I'd not put cellulose but a blown fiberglass.
1930's houses were not built tightly.
cellulose contains small fibers that
enter the house via holes in ceilings.

put cellulose in a house with a lose air
barrier between living space and attic
(ceiling plane) is going to have
homeowner dusting borate particles often.

chose an insulation that won't easily enter
areas around lights, bath fans, stove vents,
and the dreaded recessed lights.

better yet...improve that air barrier
by sealing penetrations before adding
insulation.

best of luck.