In some cases it can be advantageous to use split insulated wall assemblies; that is, assemblies where insulation is provided both in the stud cavity and on the exterior of the sheathing (or less commonly as the sheathing itself in commercial buildings). Usually these are used as they can provide the necessary R-value in a relatively compact (i.e. thinner) assembly. As one might guess, these walls essentially provide a mixture of the performance of stud cavity insulated and exterior insulated wall assembly, but there are a number of important considerations.

As noted previously, the addition of exterior insulation will keep the sheathing closer to interior temperature. In a split-insulated assembly, the more insulation placed outboard of the sheathing compared to the insulation within the stud cavity, the closer to interior conditions the sheathing will be, and consequently the closer these walls will be to exterior insulated walls. This concept is often expressed in terms of a nominal-outboard-to-total- insulation ratio. For example, if R-6 of rigid insulation is placed outboard of R-14 batt insulation within the stud space for a total insulation amount of R-20 nominal, the approximate ratio is 30% (or 3:10) of outboard insulation excluding other materials. Overall, the lower this ratio, the closer the performance will be to an insulated stud wall, and the higher the ratio, the closer the performance will be to an exterior insulated wall.

Given that split-insulated walls fall somewhere between insulated stud wall and exterior insulated walls with respect to performance, there are important considerations with respect to vapor diffusion. Generally, the exterior insulation will keep the sheathing closer to interior conditions, but if it is a relatively small fraction of the overall insulation in the wall, the sheathing will still operate at conditions similar to the exterior. In this situation, the vapor permeability of the insulation should be considered.