Can you insulate a brick veneer house by adding exterior layers of insulation?
Problem #1: When installing insulation, you should think “envelop”. That is to say, just applying slabs of insulation to exterior walls does not “help” the energy conservation of the building, if there are gaps between the slabs, around the edges, etc. there will be NO insulation qualities.

Problem #2: Moisture Barrier. I’m sure there is an existing moisture barrier. (Note: moisture barrier is different than vapor barrier. Let’s discuss moisture barrier first.) Moisture barriers are installed to keep OUTSIDE moisture from penetrating the exterior walls and causing dryrot, mold, etc. When you add another exterior layer, like insulation, it requires another moisture barrier. If moisture gets past this new moisture barrier it can get trapped and create dryrot, mold, etc.

Problem #3: Vapor barrier and Dew Point. Vapor barriers keep vapor from exiting the living spaces and getting into the insulation. Once in the insulation it can change into moisture through a process called “Reaching it’s Dew Point.” The placement of vapor barriers is important because vapor can change into moisture at the Dew Point and create dryrot, mold, etc. in the walls and ceilings. When there are two moisture barriers, this will often occur between them, depending on the amount of existing insulation in the wall. 

Dryvit is a manufacturing company that has a system of installing insulation on the exterior of walls and then troweling on a coat of waterproof plaster. If you google it, you’ll see about a hundred lawsuits for these identical problems. 

Be careful adding new “systems “ over existing systems.