The speed with which heat moves from a warm space to a cold is proportional to the temperature differential. Moving air in a duct located in a larger space will maintain that temperature differential and therefore guarantee the maximum possible heat transfer although if the air temperature and volume is adequate the rooms may still be heated and cooled adequately but at a higher fuel cost.

The unconditioned space should be insulated so it is included in the conditioned space of the house even if it is not directly heated.

You're in a marine climate.

The best approach would be to run the ducts in conditioned space making use of joist spaces, chases and boxing in portions of the attic. Reflective insulation in the attic will reduce the cooling load. www.bira.ws/files/Crawlspace.pdf (See HVAC System Location, line 18.)

Energy Star recommends R6 or higher insulation on ducts running in unconditioned space.

As the linked brochure details, careful attention to proper duct sealing is critical. Run from any HVAC contractor who tells you otherwise.

This post was edited by worthy on Sun, Dec 2, 12 at 12:16

"The unconditioned space should be insulated so it is included in the conditioned space of the house even if it is not directly heated."

Once the duct has leaked heat outside of its surface (either by radiation or convection) it is lost which relates to system ineffeciency and wasted energy. Locating HVAC ducts outside the envelope in today's construction is unacceptable. Conditioning the duct space to remedy duct losses is similar to putting a cup under a 5 gal. bucket of water with a hole in it. In a short period on time the cup runneth over and you have emptied your bucket. If you disqualify the duct-in-unconditioned space option, you will certainly be capable of engineering an appropriate solution. Watch your duct sizing, minimize your offsets and bends. Stay within the capacity of the HVAC unit(s).

Our ducting will be R-8, and sealed "with a ducting mastic" "as is required these days".

I can't run these ducts in joist spaces, as they will mostly be running perpendicular to the joists. They will either be in very small attic space, or eave space of our cape cod style home. Should I just plan to drape them with extra insulation/batt?

There will only be 3 duct runs (1 to each of 3 bedrooms).

I'm just wondering if it is adequate/the best choice versus putting interior ductless heads in each of the 3 bedrooms.

The total floor space on this floor (upper floor) is about 900sq feet, so 3 heads, even at 6kbtu each, seems like too much...

I just plan to drape them with extra insulation/batt?

To be effective in preventing condensation and retaining heated/cooled air in the duct, the insulation must be tightly wrapped and the joints taped.

You might want to avoid bubble-wrap insulation.

While ducts in conditioned space is ideal, in your climate, it isn't a terribly big deal. Sure R-8 helps a lot.

My audit still had about 15% loss by being in unconditioned space. Sure 15% is a lot but can be mostly overcome by going from Seer 13 to Seer 15.

Doing ductless just for this portion of the house sounds like spending $5000 to save $30 a year. You can always figure a way to get ducts in conditioned space if you try hard enough for less money.

RE: "Doing ductless just for this portion of the house sounds like spending $5000 to save $30 a year."

We aren't choosing the ductless to save money as much as to choose something to heat our house that isn't an electric baseboard (or in wall) heater (and therefore save money). Those would be our other options. While most houses in our area have natural gas, ours does not, nor does it currently have a furnace or ducting of any kind.

Also 1/2 of the square footage is "new" square footage--never had anything for heating/cooling. So, paying ($7k) for us, ductless, gets us heating and cooling in a space that previously had nothing at all.

RE: "You can always figure a way to get ducts in conditioned space if you try hard enough for less money."

OR, maybe, do you mean, I shouldn't spend the extra money for the 3 headed system if I can just do the ducted ductless for this space?

Tyhe low mass of theflex ducts means tha\ey caot hold much heat, so the penalty for cooling or heating them when the system comes on is not large.
They also have enough insulation to limit heat movement for the time they are delivering conditioned air under most conditions.

Unico used to have the smaller ducts available in a couple insulation thicknesses.

It is NOT as efficient as having them in conditioned space, but they are designed for retrofit systems when other options will not work.

If you are putting them in new work, keep the ducts in conditioned space.

Keep in mind the area inside interior walls is conditioned space, ad if you put them on the interior side of any insulation in a cavity it is also conditioned space.

Comparing the cost of lost energy from three short runs of insulated ducts in a mild environment vs. the cost of two more mini-splits seems like a no-brainer to me. Glad to see a salesperson was trying to save a client money, not waste it. Two minutes every couple of months to change filters is not onerous. (Though not to the owners/renters whose homes I used to inspect where the furnace filters resembled thick felt blankets.)

Thanks all!
It sounds like, on this forum, no one is worried about a ductless unit being able to supply these three bedrooms via ducts (that is what one salesperson was trying to tell me--he'd not ever use these with a duct run longer than 6 feet, and certainly not 3 duct runs...)

In fact he said,
the ductless system is "rated at only .2"w.c maximum. Typical duct systems are designed at a .5"w.c. static pressure, the low static units are best suited to having the air outlet directly against a sidewall and the air inlet directly below the unit,..."

This is where it definitely goes above my understanding, but the salesperson trying to sell the ducted ductless system says they just size up the ducts so there isn't as much resistance on the interior head.

I just wish there was a lot more information available (in English) on these systems and how they have been used well.

kirk....go over to greenbuildingtalk.com There is a guy there that knows 'everything' about mini splits. His name is Dana.