Im not a pro but I would want to know where your HVAC unit and ducting will be located. You could do both (radiant barrier shouldnt be expensive and should help reflect some heat). I would go with foamed roof line if you have your ducting in your attic. You could also foam the exterior of your roof like you do the walls.

As noted above, the answer depends on the location of your HVAC. And also, what climate region you're in. See map below. (This is not the same as plant hardiness zones.)

Other than in a hot zone, radiant barriers are not effective; in Florida, you can expect a six to seven year simple payback provided there is no insulation in the attic, according to the Florida Solar Energy Center.

If the HVAC is not in the attic, the best use of foam would be to seal the floor of the attic, then use a fibrous insulation; forget the radiant barrier.

For HVAC in the attic, foam under or over the roof (or both) is effective.

This post was edited by worthy on Wed, Sep 18, 13 at 20:26

If you do use a RB on the underside of the sheathing there must be at least an inch air space below it for it to work at all.

There is a cost for the RB so it is a matter of how long it will take for the energy savings to pay for the RB configuration.

Since estimates of energy savings of RB's are often exaggerated by using comparisons with poorly insulated houses in very hot climates, the break even point may occur after you sell the house.

Thanks ya'll for the great info. We are in Austin, TX, so on the map that worthy provided (thanks), we are in the Hot/Humid area. The HVAC will be in the attic. We are planning to stay in the house at least 10 years.

So based on the responses, it looks like we have two options: 1) foam AND rb, or 2) just foam. But that we should not do just RB by itself. Am I reading that correctly?

For a new build with HVAC in the attic seeBuilding Science Corp.s profile for Houston.

Dr. Lstiburek is not a fan of radiant barriers or vented attics with HVAC in hot climates.

This post was edited by worthy on Wed, Sep 18, 13 at 20:25

Duplicate

This post was edited by worthy on Wed, Sep 18, 13 at 20:18

I agree with Joe. if your ductwork & equipment are
in the attic, use foam to create an unvented
semi conditioned attic space.
130 degree attic with R-8 ducts surrounded by
this extreme temp air & all the humidity we have...
condensating equipment, plenum & ducts.
I see it everyday.

with mid to high efficiency equipment the air is much
colder entering the plenum (vs air handler....two stage
units). so in a hot unvented attic the equipment condensates, and the first 4' of the plenum and any
ducts taken off in that first 4' condensate.

I'd never foam the walls & not the attic..well I'd never
foam the walls period. foam sheathing on exterior of
walls yes...but not foam in wall stud bays. big waste
of money.
our heat gain is via the roof...where the sun shines
all day...not the walls that get sun at different parts of the day for certain periods of time.

that said...I have a radiant barrier. But no ducts
no equipment in attic. my air barrier @ ceiling is
near perfect. all ducts are in fur downs inside living
space & equipment is in interior closet.
this was planned for a long time...not just
a last minute choice.

invest in the foam in the roofline.
educate yourself about foam sheathing on exterior
of walls, conventional insulation in wall stud bays,
and air tight drywall approach for sheetrock to interior.

and understand that you still have to reach R-value
requirements. 2-3" of foam doesn't meet code requirements.
inches of foam X R-value of foam = code requirement.
don't buy into the whole 2-3" performs as R-30
it is bull...and will not pass code inspection.

open or closed cell? we use open cell 7" in roof
rafters & covering faces of rafters, here in La.

as Renovator * pointed out...radiant barrier & foam...
RB has to have minimum of 3/4" air space between it
and anyother product. waste of $$ to foam over
tech shield.

best of luck.