Replies

1. 
   user-2310254 | May 30, 2019 12:12pm | #1

   Bill:

   I think you find this discussion helpful: https://www.greenbuildingadvisor.com/question/air-sealing-vented-vs-non-vented-attic-and-a-new-hvac-system-possibly-in-the-attic

2. Expert Member
   Dana Dorsett | May 30, 2019 02:10pm | #2

   Attic venting is about removing moisture, not about cooling the roof.

   Vented attics in FL bring in more moisture than they purge. In an insulated attic in FL it's usually better from a moisture management point of view to close off the venting, even if it raises the peak attic temperature by a 5-10F. Vented attics always make it into top 10 lists of "The dumbest way to build a house in the south", usually a bit ahead of vented crawlspaces, but behind putting ducts and air handlers in the attic above the insulation:

   https://www.buildingscience.com/documents/insights/bsi-015-top-ten-dumb-things-to-do-in-the-south

   It looks like all of those made Matt Risingers top 5, vented attics scoring #1, on his personal stupid-methods list:

   https://www.youtube.com/watch?v=Vwos211XlXo

   If you are now installing the air handler and ducts in a closet, inside the insulation and pressure boundary of the house there is no point to installing closed cell foam in the attic other than what it takes to for air sealing around penetrations. It would then be safe to install the code minimum R30 of fiber insulation on the attic floor and install a vapor retarder (or true vapor barrier) on top of the insulation. That vapor retarder could be foil faced rigid foam, or unperforated radiant barrier, or even polyethylene sheeting, just make sure it's reasonably air tight as well, to prevent wind or convection from bringing moisture laden air from the vented attic to the cooler ceiling gypsum.

   Sometimes (but usually not) it's easier/cheaper to close off the attic venting, install an inch or so of closed cell foam (minimum of R5) on the underside of the roof deck, along with R23-R30 of high density batt snugged up to the roof deck, and use PERFORATED radiant barrier stapled to the underside of the rafters to keep the batts in place, which would bring the attic into conditioned space.

   Closed cell spray polyurethane is one of the least green insulation materials in common use, made far worse if blown with the industry standard HFC245fa, and extremely powerful greenhouse gas (~1000x CO2 @ 100 years.) There are greener variants blown with HFO1234ze (~5x CO2), but they all have a high polymer weight per R, and are nowhere near as low-impact as rock wool or fiberglass, and at least an order of magnitude higher impact than cellulose. If using closed cell foam, keeping it to the minimum amount necessary for moisture control is nice to the planet, and usually nicer to your wallet too.

   Whether in a closet or in a conditioned unvented attic, the cooling load will be lower than it was when the ducts and air handler were in a 150F attic, and that needs to be factored into the sizing of the replacement unit. The reduction is at least a half-ton, often more than 1 ton. The best place to start is to run an aggressive (not conservative) Manual-J type load calculation, and when in doubt, down-size to the nearest half-ton. Load calculation tools have built-in margin, and bigger is definitely NOT better. In most cases being slightly undersized is more comfortable in humid climates than being oversized by a ton or more. If you take a stab at running your own Manual-J using a freebie online tool such as loadcalc.net (recommended, even if it's only for sanity checking proposals from HVAC installers), do NOT oversize by even 1/4 ton.

   Air handlers come in all sorts of configurations upflow, downflow, as well as horizontal flow. Some can be installed all three ways, but most have a preferred orientation. Pick the one that works best for you. Depending on your load number fall there may be a modulating ducted mini-split solution that works.

3. 
   Jon_R | May 30, 2019 03:26pm | #3

   It's not clear which Rheem model you are considering, but give preference to variable speed AC.

4. 
   BillBerit | May 30, 2019 04:30pm | #4

   Thanks guys for the great info. You shot all my ideas about AC in the Keys to H!
   What it looks like is I should attack and commandeer the local Tiki Bar and homestead it, forget AC and go native after installing 10 layers of No-Seeum Screen and a 12 foot diameter slow moving ceiling fan. In reality it is scary that there is no real way to build anything viable in the Keys climate.
   Thanks again
   BB

5. Expert Member
   Dana Dorsett | May 30, 2019 05:18pm | #5

   I didn't mention it, but an UN-vented attic lowers the upward pressures on the roof deck in hurricane force winds (that never really happen in the keys, right :-), and that as little as 3" of closed cell foam on the underside of the roof deck glues it to the rafters sufficiently to form a structural monocoque, making it far less likely to lose the roof entirely in a storm.

   That makes for a pretty reasonable rationale for using 3" of HFO blown foam (R21) with R15 high density batts snugged up to the foam, keeping the air handler & ducts in the now-conditioned attic.

   Or you could just do the Tiki Bar invasion thing and take your chances. :-)

6. 
   Jon_R | May 30, 2019 05:35pm | #6

   It appears that the 2017 Florida building code (406.1) allows you to use insulation on the attic floor (cheaper, less heat gain, fine with no attic ducts) but block the natural ventilation openings. This provides the flexibility to be smarter about the use of ventilation to control humidity. For example, a humidistat controlled fan occasionally pulling in small (eg, 30 CFM) amounts of drying air from the house.

   As Dana says, use more insulation and a radiant barrier to deal with heat flow (not lots of attic air flow). And keep moist attic air away from your cool ceiling.

   You can achieve low cost, low heat transfer and a dry, mold free attic. And reduce hurricane attic pressure. All without spray foam.