True dimensional 2x8 insulation question

[Dean Klein]

Hey everybody! I've got a duplex built in 1888, and have a couple of cold rooms, upon inspecting the basement, there is no insulation under the floor of these rooms. The floor joists are true dimensional 2x8. I've been doing alot of research but still have questions. It looks like R-25 Batts would be ideal being 8" thick to fill the space however R-25 is not readily available from what I can tell. What is the recommended R-value to use? R19 being 5.5" won't fill the cavity, and the 9.25-10" that R30 comes in would mean that it would have to be compressed a bit. Any help would be greatly appreciated! 

[Marcus Auerbach]

I think you are looking at the wrong places, insulating the floor wouldn't change much. 

How are the box sills insulated? On a pre-1900 build I often see them filled with mortar, at least here in Milwaukee. This is one of the biggest insulation issues second only to the attic. You can use bat insulation or spray foam, which has the added benefit of giving you an air-tight seal and also keeps the bugs out. 

Windows would be my next check-point (less than R1) and then wall insulation: I would not be surprised if they have no insulation inside, but you can do blow-in, which only requires a small hole.

A home looses heat in two ways: heat conduction (which you can slow down with insulation) and drafts, which in old houses is usually the biggest culprit. Box sills, windows, doors, outlets, switches, lack of house wrap under siding etc.. 

In a pinch you can check drafts with a lighter and see if the flame flickers. Or buy a thermal camera to check for heat losses, they have gotten really cheap.