Lack of breathability can be considered as resistance to water vapour transmission. Very air permeable fabrics have excellent breathability and very windproof/waterproof fabrics have low breathability. This link demonstrates the difference by forcing air through eVent, Gore-Tex and Nikwax Analogy.
So what about layering a waterproof over a windproof?
If the windproof is 90% breathable and the waterproof is 80% breathable, the combination is 90% times 80% or 72% breathable. And every other layer further reduces moisture vapour transmission.
Please note: this only relates to conventional breathability and not wet weather breathability.