If the attic floor/ceiling can take the additional weight, in Dallas you'll get better bang/buck out of adding 6-10" of cellulose on the attic than foam or radiant barrier on the roof deck. RB can seem pretty cheap, but the benefits accrue mainly to homes with less than R20 between the joists, with the joist-tops exposed, and attic ventilation. (Contrary to mr. radiantbarrier, it does NOT reduce the amount of heat entering the structure, only the heat transfer from one hotter portion of the structure to the interior. This raises the temp of the roof deck, which allows it to radiate more heat out. This is better achieved with "cool roof" shingles, which reject the heat at the exterior surface, keeping the roof deck cooler.) If you can cover the joist-tops by at least 3" and you have at least R30 center-cavity the benefits of rb are already becoming marginal. Once you get to R40 & up it's somewhat academic.
If yours is the typical R19 fiberglass batt job between the joists the penetration of radiated heat into the insulation layer renders it roughly half the rated R when under a 130F roof deck- (the insulation will be hotter than the attic air temp about an inch or so into it!). A 3" overblow of cellulose would fix that, and add another R11 or so, but 6" would add another R20+. Adding the same R20 in foam at the roof deck is significantly more expensive, but may still be worthwhile if you have ducts & air handlers etc in the attic. In your area insulating the roof deck with sprayed cellulose or a high end high density spray fiberglass (eg. JM Spider) would also work.
Attic ventilation serves more function in cold climates, and can actively work AGAINST you in places as humid as FL or Houston (and probably even Dallas when the dew points hang in the 70s for days on end, as they have been recently.) Attic ventilation has very little effect on the cooling load in hot weather, but purges moisture during cold weather that would otherwise condense & cause rot issues. But insulating the roof deck from below works in cold climates too, as long as there's sufficent vapor retardency that moisture can't accumulate rapidly in the roof deck via air & vapor diffusion through the insulating layer. In Dallas it's just not cold enough in winter to be concerned about attic condensation, with average outdoor temps well above the dew point of reasonable-humidity conditioned space air. But in the summer when dew points are in the 70s the ceiling of the air-conditioned room can be below the dew point of outdoor air, and attic ventilation can result in excessive moisture condensing in the insulation or on top of the gypsum board. Sealed attic would be the lowest-risk way to go in your neighborhood. Insulating it with cellulose would also serve to moderate the moisture levels in the attic, since condensation that starts within the cellulose is rapidly redstributed, then slowly released as the temperature & humidity levels allow. (The effect is sometimes referred to as a "hygric buffering".) It can take on a substantial amount of moisture without damage (~25% by weight)
Foam can get you a perfect air seal though, and if the attic is leaky under a blower-door test, even 2-3" of open-cell foam at the roof deck, taking care to seal it at the soffits & ridge can be a significant benefit if the attic floor can't be more cheaply sealed by other methods. If the attic floor is pretty tight, you'll derive more benefit out of converting those insulation dollars into deeper cellulose. Common penetrations that break that boundary are plumbing stacks/chase, ducts, chimneys, recessed lighting, electrical penetrations, etc. but those can often be air-sealed cheaply, allowing you to just heap on the blown cellulose. If you think that's an issue, see if you can't find an air-sealing contractor that will test, seal, then re-test to verify the result. Cellulose will resist air flow much better than fiberglass, but is no substitute for real air-sealing. Unlike colder regions, air leakage isn't likely to create rot & mold problems due to condensation, but infiltration losses add to both the heating & cooling loads. |