I'm sure this is a late response, but adding insulation is ALWAYS cost effective. I'm in the HVAC business, and we always say "you pay for it whether you install it or not". Meaning - either you pay for the insulation, or pay for more fuel. Insulation always reduces your energy usage - that's a fact. It's like the question of starting and IRA when you are 18 or later in life. The earlier the better - always. I think most people question the "pay-back" period. But it's simple, the more insulation, the faster the payback.