These should only be insulated with batting or blanket manufactured for vertical installation, or polystyrene with extruded sealant around all perimeters. Loosefill/sprayed insulation is not suitable for unlined walls. Wherever possible full the cavity with insulation, or fit hard against the internal lining.
The Total R value (installed) will be considerably less than the R-value stated on the bale, due to excessive thermal bridging through timber framing. In most houses the timber bridging in a wall averages 30% and NOT the 15 – 20% stated by goverment orgainisations.
To achieve a Total R-value (installed) greater than R2.2 with 90mm framing include a thermal break (eg 15mm polystyrene) between the framing and either the external or internal linings, or increase wall cavity width. Batting for unlined walls are made from Glasswool, Polyester, Polystyrene, Sheep Wool/Polyester blend or Sheep Wool.