The "greenest" way to heat and cool the space would be to
- zone and thermostatically control the areas separately,
- reduce overall energy use, and
- minimize material costs.
Because we are looking at ethically designing your heating and cooling system, I would rather refer to the approach as "sustainable" vs. "green."
It is my opinion that a variable rate volume (VRV) HVAC system would suit your requirements best.
Variable rate volume (VRV) HVAC system
Here is how a VRV system can achieve all three of the design considerations I listed:
Zoning: The VRV system distributes refrigerant to individually placed units throughout the home. This will allow each room to be thermostatically controlled and monitored and each room can also be fitted with an occupancy sensor which controls the operation of the unit.
Energy reduction: The VRV system has the possibility to use a smaller condenser for the size of the demand of the units. How this operates is that it only sends the required volume of refrigerant as needed at the particular unit call for refrigerant. Also the condenser(s) ramp up in amperage as the demand increases vs. a typical condenser which has a large surge amperage load when it starts and then reduces to its normal operating amperage. The ramping effect reduces the amount of electricity used by the unit.
It also reduces energy usage by use of the occupancy sensors which I mentioned above. These sensors will allow the temperature to increase or decrease depending on the season when the room is vacant, thus again reducing electricity used by the condenser and room-mounted unit.
Minimize costs: Because individual units are being installed at each room or small ducted systems are being installed for multiple rooms, you will be able to reduce or even eliminate the cost for ductwork, insulation around the ducts and labor to install the ducts and their insulation.
Considerations of an attic-mounted conventional ducted system
Insulation and ventilation
Attic-mounted ductwork needs to be insulated and sealed. If it is not, you may experience excessive heat loss leading to overheating of the attic during the winter months, creating ice dams at the eaves and possible water intrusion in the home's attic and walls. This water intrusion will lead to failure of your insulation, mold and mildew growth and costly repairs.
Insulating the ductwork is not the only concern. Your attic must be properly ventilated to handle heat produced by the unit itself. Again, improper ventilation will lead to the same outcome as poorly insulated ductwork.
Noise and structural vibrations
HVAC units installed in an attic should be properly isolated from the structure. Noise from attic-mounted units can travel long distances through framing by means of structural vibration. Use vibration dampeners on the unit as well as flexible collars at the connection points between the ductwork and the HVAC unit itself.
Maintain proper clearance at the unit to access the filter so it can be changed frequently. Minimum clearances given by the manufacturer sometimes do not account for alterations in the equipment or even the ability to replace a damaged unit without major construction.
Work with an experienced HVAC contractor
Since there is not one cure-all for every situation which arises, I highly recommend that you discuss your specific requirements and issues with a knowledgeable mechanical contractor in your area.
A green hvac contractor should perform an on-site observation of your home, check the balance of the existing unit and make a variety of recommendations which can be implemented to achieve your goal.
For more information:
Also, read Jesse Terzi's Q&A "How can we prevent car fumes from polluting our house through the attached garage?"