We live in Fort Worth Texas and are planning to complete our bonus room that is over our garage.  I've read that bonus rooms are very difficult to heat and cool if the proper insulation is not used.  The floor, walls and celing are still opened (no wood) so I can easily install the insulation.  The celing rafters are your typical wood beams most are 4" and the main core is 6".  Reading various postings, it sounds like moister is a big concern.

So, has anyone installed insulation in a bonus room  in Texas with limited celing/roof space?  if so, what are your recommendations?  Foam, Radian Barrier, traditional insulation, etc.... also, how did you address the moisture concerns?

I am new to the do it yourself, but am a fast learner :)

thanks in advance

To get a reasonable R value in a sloped ceiling with 2x4 rafters it takes 3" of closed cell foam (the expensive stuff, 2lbs/ft^3 or denser) between the rafters, with 1/2-1" of XPS or fiber-faced (not foil faced) polyisocyanurate under the sheet rock is about as good as you're going to do. In Ft. Worth the summertime humidity is high, and any vapor retarders (like 2lb foam) are best applied to the exterior of the structure to limit condensation potential on the AC-cooled framing structures. XPS is semi-permeable, and any moisture incursion into the cavity will dry to the air-conditioned interior, while still creating a thermal break at the rafters.

Radiant barriers won't buy you much here, but low solar absorption/moderate to high infra-red emmissivity cool-roof materials can, particularly at roof pitches of 2:12 or less. At higher roof pitches the roofing convection cools fairly well, and the benefits of cool-roof materials is marginal at best. If roof deck soffit/ridge ventilation is required by code, using foil-faced radiant-barrier for the chutes for the ventilation spacing under the 2lb foam buys back some of the cooling season R value that you lose with the reduced foam thickness though.

Thanks Dana1 for the reply. when you say expensive stuff... what kind of cost should I keep in mind? Also, I am not sure what is a 1/2-1" of XPS or fiber-faced (not foil faced) polyisocyanurate under the sheet rock?

To make sure I understand your recommendation... I only would need the 2lbs/ft^3 foam material with the XPS reference... but will not need to have air flowing between the roof and foam to address condensation?

We did 5 yrs ago what you are planning now.  However, our walls are all ICF, not stick.  That being said, here's what we did.

We used Sealection500 open cell spray foam in all our attic floor areas, and we did we did same between the rafters in our vaulted ceiling in the main living area and so that's what we used between the rafters in the media room above the garage (what you are calling a bonus room). There is no attic in the media room, either.  Our rafters are 2x6.

The 2x4 wall between the media room and the rest of the house upstairs (an open loft and a bedroom w/ batch) we filled w/ fiberglass bats for some sound deadening.  Finally, we spray foamed the underside of the media room floor from the garage, before the garage ceiling was sheet rocked.

Finally, I would not use closed cell foam, as it hides leaks. We have a metal roof (from Mueller) that had a leak in the valley where two roof sides meet.  It took 3 heavy rainstorms to finally resolve.  We saw the leak about 10-15 minutes after each storm started.  I hate to imagine the ugly mess inside the ceiling if we had closed cell and only discovered it months later.

Make sure that you have adequate A/C returns in that room. If the supply air can't be removed adequately, it will never be comfortable.  We are very happy with our result.  Good luck!