Half inch asphalted fiberboard sheathing is worth about R1.3, three-quarters fiberboard is about R2. Asphalted fiberboard is extremely moisture tolerant, T1-11 not so much. A housewrap or #15felt will protect it somewhat, a crinkle-type housewrap (eg Tyvek Drainwrap, Obdyke Hydrogap) is substantially better. The housewrap or felt has to be properly lapped with the window flashing to redirect any water that makes it to the flashing to the exterior side of the T1-11.
With 2' overhangs the windows are fairly well protected from direct wetting on a single story house except when it's windy, so your house is going to be a bit more forgiving of sloppy flashing & housewrap details than a house on a windswept island, but it's still worth paying attention to those details.
Best practice: If you have the space, strips of 3/8"-1/2" CDX ripped to 1.5-2" through-nailed to the studs to create a drain space between the felt or housewrap keeps any windblown rain moisture that blows by the siding or water directed to the exterior by the window flashing from being retained by the assembly, and gives the T1-11 path to dry toward the exterior. Use strips roll mesh venting use for ridge vents and bug screen to avoid turning those thin cavities into critter-condos. This type of assembly is referred to as a "rainscreen", and is required by code in the dank wet western slopes of British Columbia, but still a good idea anywhere that it rains or snows. It also qualifies your fiber cement over T1-11 as "
Vented cladding over wood structural panels" in chapter 7 of the IRC that allows the use of standard interior paint as the interior side vapor retarder in most US climate zones.
It's nearly impossible to air seal a T1-11 clad house so tight that it would require active ventilation without a full gut of the interior and obsessive caulking & taping. With the structured channels of the faux board & batten T1-11 even a fully adhered housewrap such as Henry Blueskin or Delta Vent SA won't really buy much in air tightness either. While CDX sheathed house can be tightened up by by detailing the housewrap as an air barrier, it's impossible with T1-11.
R11 batts are extremely low density, and will not appreciably slow down infiltration air- it's barely an air filter. Every seam and crack and electrical/plumbing penetration of the walls leak air, and in most 1970s houses is a MUCH bigger cumulative hole than intuition would tell you. Drilling into each stud bay on the exterior every 4' or so and blowing it full of cellulose will compress the R11s and deliver about R13, but more importantly it cuts the air infiltration through the wall by 90% or more. Caulking the bottom and top edges of the T1-11 to the framing with polyurethane caulk may be possible, which brings down the infiltration even further. Also, the tighter you make the house, the better the HVAC design can work, since it reduces the random & unknown air paths using "the great outdoors" as a return. The indoor air quality usually improves, along with comfort.
Most single stage box store blowers use 2.5" hose with no reducer nozzles. While you could just drill big enough holes to accommodate the hose, a reducing nozzle to something like 1.25" would be better, drilling 1.5-1.75" holes, which allows you to direct the blowing direction a bit. (Use a cut-off sleeve from a sweatshirt as a held-in-place gasket to keep the blowback out of your face.) When calculating the amount of material, assume it will be about half the volume of the cavities (yes, the R11s will compress that much), at about 2-2.5lbs per cubic foot density if using a rental single stage blower.
