I am finalizing plans for new house and need some help. I was trying to keep A/C ducting in conditioned space since I am in zone 2, but have been told by she who must be obeyed no dropped ceilings or soffits. The walls are 2x6 with roxul and 2" iso on the exterior, could i do similar on the roof? Put 2" iso on the roof desk and 12" blown cellulose on the ceiling. The cellulose would cover most of the ducting and the attic would be partially conditioned non vented. The insulation job will be DIY to cut costs so foaming under the roof deck is not an option.

thanks.

The danger in partially conditioned space is that warm inside (or outside) air will enter it and become high humidity (or even condensing) air as it cools off. On the other hand, a dehumidifier will help dry it out whenever attic temperatures are well above freezing. And with good air/vapor barriers, there won't be much warm air/moisture entering the space.

jonr, since I am in zone 2, N. Florida not many days where the attic is below freezing and I am going to use ADA for sealing the home so there won't be much conditioned air leaking into the attic.

thanks

Be sure to also air seal the exterior. You don't want moist outside air getting in there and then cooling off (summer with A/C).

In zone 2 Florida that stackup on the roof works, no vapor retarders needed. You'd have to go with thicker exterior foam in zones 4 & higher.

It's technically a code violation to split the R between the roof and the attic floor, but it's not a real problem. If you want it to look more like code, R30-38 batts under the rafters and no floor insulation works.

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

If splitting the R between roof & floor, air sealing the attic space to the exterior as jonr suggests is critical, since the moisture drives are primarily summertime outdoor air humidity which can produce deep moisture cycling in the floor cellulose (causing it to settle quickly & deeply ) if it's leaking a lot of air. With the attic space air sealed to the exterior the dew point of the attic air will roughly track that of the air-conditioned space, keeping it all nice and dry. Wintertime moisture drives are of little consequence in your climate (unlike zones 5 and higher.)