Isn't 450 degrees with everything closed and 450 degrees with the MPD open a little, still 450 degrees?
I still insist you have no insulation. Someone mentioned R factor without explaining what it is actually. AN R factor is calculated with a temp source of 75*. It is the amount of heat that transfers through a VACUUM at that temp. Across a certain distance.
When the temp outside is 0*, and you want it 75* inside, you are well beyond the parameters of what an r factor is. You do not have a vacuum in your wall, you do not have anything trapping air like fiberglass (which I hate), but it is better then nothing. You have a somewhat dead air space, depending on the outside sheathing and siding, and you have 1/4 inch of foam and foil, which I believe was more correctly stated to be about 1.45 R factor. You have built a traditional plaster and lathe house from the early 20th century with a piece of foil behind the lathe. Crank the stove up like the old timers did and buy more coal.