Ducting Systems and Air Ventilation - New Installation

All:

First time home buyer here, handy but not a pro by a long-shot.

We bought a cape w/ a roughed in second floor, so the return and supply stacks are into the attic (center of the house), but the branches to the two bedrooms and bathroom are not there. This seems like it is a deceptively easy installation. I'll outline my plans and thoughts and hopefully learn from you all on pitfalls and tricks!

House is forced hot air, propane. No Central air, but probably coming someday.

The return line is closer to the hallway, so I had the thought of putting the return vent there, probably 12 x 12" or so, rather than run to each bedroom. Would this suffice? or should I plan on 16"x16" OR one in each bedroom (To go to the opposite side of the rooms from the planned supply vents, we're talking a 15ft and 18ft run....

The supply branches should be fairly short, 5-6ft and ~10ft to get to each room comfortably.

The rooms are ~12'x12' or so, not including closets, and was thinking 6" flexible insulated ducts. Should I go with 7" or 8" instead?

is there a significant difference in efficiency between R6 or R8 tubing to justify the cost? How else can I add to the insulation on the ducts? Attic will be spray R44 or 49 cellulose when I'm done up there, but the ducts will be above the joists and not covered...

I'll have some questions on tying into the main trunks in the basement, but that can wait till I get the upstairs done!

thanks, Bryan

I would say that the system you have now is under sized to heat or cool the 2nd floor. You will also need a heat load done of the 2nd floor to know what size duct you need to go to each room. This is a do-able project but you should have a manual J and D done to get it right.

Thanks.

Undersized, meaning another unit in the attic?
House is only 800sqft...

What do you mean by J and D?

thanks for the response!

Manual (book) J is a heat loss / heat gain calculation and manual D is duct design to ensure the supply and return ducts are the proper size to allow for the proper airflow to the various rooms and back to the furnace/air handler.

Your furnace might be sufficient to heat this space but there is no way to know without doing the calculations.