This is a financial decision more than anything else.Open cell costs less to apply, allows you to see leaks sooner and cost less to make repairs and fix open cell foam.....
You may consider whether or not your duct work is in your attic space or not.
- If you have your air handler and/or duct work in the attic, it is worth it to condition the air space.
- It should keep your system from working harder than it has to....as a duct surrounded by 120 degree air bleeds more than a duct surrounded by 80 degree air.
- Allows your home to cool faster, remain cool, and if you do spray foam the attic and make it completely conditioned space, you can set up your system to run cool to lower the temperature and then just the fan to recirculate the cool air throughout the house....huge energy savings by being able to run the fan without cooling the air. HUGE!
Imagine your home so efficient the cooling only came on once an hour because the fan kept moving the air and keeping the house cool. Either way, it is your choice.
Keep in mind all costs and then amortize the difference over 10 years the useful life of your HVAC system. Upgrade your HVAC system to an 18 SEER, have the builder do a Manual J Calculation to determine the CORRECT size for your home....if this is not done, it can cause serious problems....bigger is not always better