I live in central FL and need to replace the roof on my house - it's a hip roof.  The house has soffit vents all the way around but no other vents anywhere.  We often get late day showers here in the summer that cool the air temps down considerably but the attic remians quite hot.

Because it is a small house the actual top ridge is quite small (again it's a hip roof), so a ridge vent would be very small and probably not vent much air.

What is the best most cost effect way to get that hot air out of my attic.

First, you have to be absolutely air-tight at the attic floor, then a solar-powered attic fan will lower your cooling load by a few percent. Grid powered attic fans tend to not be cost-effective operationally (they use more power than they save in air conditioning.)

If you already have R30+ of insulation in the attic there's almost no point to cooling the attic with ventilation, as the net drop in cooling load from ventilation would be difficult to even measure. If you don't have R30 in the attic, it's almost surely cost effective to raise it to R30 by blowing in more insulation.

If you're trying to use the attic as additional storage/living space, it won't be cheap, but insulating the roof deck would be ideal.

So you're saying that if I have proper insulation between the ceiling and attic it's not worth adding additional venting to the attic.  BTW the attic is not usable space.

Posted By bmancanfly on 12 Mar 2010 11:21 AM
So you're saying that if I have proper insulation between the ceiling and attic it's not worth adding additional venting to the attic.  BTW the attic is not usable space.

That's what I'm saying. 

The Florida State Solar Energy Center has studied this one to death.  Their best-case scenario real-world testing showed that in an R19 insulated attic a solar powered ventilator could save as much as 6% on the cooling bill.  But doubling the R-value to R38 would reap double-digit savings.

Searching their archives for "attic ventilation" yields this list.  Skim a few of those papers.  In FL, R30 is fine without ventilation if the solar reflectivity and infrared emissivity of the roof is high enough. If you have a dark roof, R30 at the attic floor with radiant barrier anywhere between the roof deck attic floor and passive venting (like you have) is plenty. 

If you have ducts in the attic, insulating the roof deck and sealing the attic has benefits, but it takes a sharp pencil to figure out when/how a retrofit is cost-effective.  R19-20 (5.5-6")  of half-pound foam on the underside of the roof deck, sealing the vents is pretty common in FL, as is doing it with R12 (2") of 2lb foam (which glues the roof deck & rafters together forming a structural monocoque for hurricane resistance.)  But it's not cheap.  Blowing 6-8" of cellulose on top of whatever attic floor insulation you have is cheaper, but leaves ducts exposed to the attic heat. Sealing & insulating ducts in an unconditioned attic is usually cost effective. 

Attic ventilation is of little value in cooling dominated climates. It's often used for ice-dam control in snow zones

If you decide to price spray foam then be aware that all companies do not charge the same amount per square foot.  For instance, at a trade show in Birmingham AL today the price for insulating between the rafters (from inside the attic) with closed cell foam ranged from $1.95 to $2.50 per square foot 2.5" thick.  So, in other words, like most things it pays to get more than one bid.