1. Assume your building costs $150/sq.ft. to build, and that the
double-skin is 10% of that cost, or $15.
2. Assume that energy for the building costs $1.25/sq.ft./year, and that
the double-skin saves 25% of the total energy budget, or $0.31/sq.ft./year.
(In many buildings, lights and plug loads comprise about half the energy
cost, and this analysis assumes the double-skin has no impact on either. So
this guestimate is based on the double skin saving 50% of the HVAC costs,
which seems quite optimistic.)
3. The simple payback is then $15/$0.31 = 48 years.