I am going to build a home in Melbourne, Florida, climate zone 9B or 10A depending on the source. The home will have wrap-around porch varying in depth from 6 – 10 feet around the majority of the exterior. I will be installing a white metal roof and cold formed steel trusses. I have read a fair amount of literature on the topic of attic/roof configuration, including papers, reports and forums. There are two configurations which I believe would perform the best energy-wise. The first is a vented attic with ductwork contained in the conditioned space below and insulation above the ceiling. The second is an unvented, conditioned attic with open cell spray foam insulation under the roof deck and which contains the ductwork. I have not found any detailed information comparing my two desired configurations (both with a white metal roof) and I'm not sure which simulation tool would be the best for comparison. Does anyone have insight on which configuration might perform the best thermally? If I chose an unvented attic, I'd try to reduce the surface area to insulate and reduce the volume to condition. With that in mind, I would not apply insulation in the attic space above the porch and garage, but instead add wall sheathing to extend the home exterior wall plane to the roof deck and apply the insulation to that vertical surface. I am wondering though if this would cause any thermal performance problems due to stagnant air being baked in those unconditioned, unventilated volumes during hot summer days. Would this approach cause any thermal issues? Although there are a number of other considerations, including moisture control and cost of installation, I'd like to get some advice on which attic configuration would be the best thermal performer. Any feedback would be appreciated.

Thermally, a vented attic will allow you to put in more insulation at less cost. And it will have reduced surface area and volume. Structurally is a different question.

Posted By trevdawg122 on 30 Jun 2013 07:35 PM
I am going to build a home in Melbourne, Florida, climate zone 9B or 10A depending on the source. The home will have wrap-around porch varying in depth from 6 – 10 feet around the majority of the exterior. I will be installing a white metal roof and cold formed steel trusses. I have read a fair amount of literature on the topic of attic/roof configuration, including papers, reports and forums. There are two configurations which I believe would perform the best energy-wise. The first is a vented attic with ductwork contained in the conditioned space below and insulation above the ceiling. The second is an unvented, conditioned attic with open cell spray foam insulation under the roof deck and which contains the ductwork. I have not found any detailed information comparing my two desired configurations (both with a white metal roof) and I'm not sure which simulation tool would be the best for comparison. Does anyone have insight on which configuration might perform the best thermally? If I chose an unvented attic, I'd try to reduce the surface area to insulate and reduce the volume to condition. With that in mind, I would not apply insulation in the attic space above the porch and garage, but instead add wall sheathing to extend the home exterior wall plane to the roof deck and apply the insulation to that vertical surface. I am wondering though if this would cause any thermal performance problems due to stagnant air being baked in those unconditioned, unventilated volumes during hot summer days. Would this approach cause any thermal issues? Although there are a number of other considerations, including moisture control and cost of installation, I'd like to get some advice on which attic configuration would be the best thermal performer. Any feedback would be appreciated.

No doubt unvented is the best for hot-humid climates
HVAC ducts stay in conditioned space,
sprayed foam to the underside of the deck helps plywood in hurricane winds
unvented prevents wind / water from being blown in soffits during hurricane force winds (this was a problem during hurricane Andrew)

Posted By jonr on 30 Jun 2013 09:45 PM
Thermally, a vented attic will allow you to put in more insulation at less cost. And it will have reduced surface area and volume.

http://www.fsec.ucf.edu/en/ FLORIDA SOLAR ENERGY CENTER has done many studies with these systems

trevdawg,

Since you plan to build in Melbourne, why not look at the NexGen Framing System with super insulation (445 Stan Drive, Melbourne, FL).  I saw their display at the SEBC show in Orlando.  Very good way to use metal for the walls and roof structure.  See   http://nexgenframing.com/   Ask them if they have a crew that will install the walls and roof and whatever else you want done.

HVAC ducts stay in conditioned space,

Chris, I think you missed that the OP is not going to be putting ducts in a vented attic. Any data that includes that (significant) factor as a plus for unvented/conditioned doesn't apply.

If you are going to not vent the attic, consider making it a cathedral ceiling - you might as well see the space that you are paying to condition.

I have a preference for no wood in places that have many termites.

Posted By jonr on 01 Jul 2013 09:59 AM

HVAC ducts stay in conditioned space,

Chris, I think you missed that the OP is not going to be putting ducts in a vented attic. Any data that includes that (significant) factor doesn't apply.

If you are going to not vent the attic, consider making it a cathedral ceiling - you might as well see the space that you are paying to condition.

I have a preference for no wood in places that have many termites.

jonr,
if you read the OP he is stating 2 choices and my reply is that the unvented/conditioned attic is the best choice for hot/humid climates

Jon, Chris, Alton, thanks for your insight. Jon, in my vented attic configuration, the ducts would run within the conditioned living space, so the unvented attic duct energy savings would be voided in comparison. I have read about the other benefits you've mentioned and they are also worth careful consideration. Chris, I have read some literature from FSEC and they are a very good resource. From what I've seen so far though there hasn't been any experiments comparing my two configurations exactly. Alton, it's interesting you mention NexGen because I have met with them several times and they are a likely candidate for my trusses and interior walls. I believe the exterior walls will be cheaper using concrete block with exterior rigid foam insulation, though their exterior wall system is very attractive.

I am still curious whether the attic space above the garage and porches, being mostly sealed but not insulated, will have a negative effect on the adjacent conditioned attic space. If the air in the unconditioned attic cannot flow, will it heat up and conduct into the conditioned space to a thermally detrimental level? It may be that the white metal roof would prevent this potential scenario.

Thanks, Trevor

Trevor,
You might consider insulating over the garage area as well, just to make it more comfortable

the porch areas are not an issue if they are open, because there will be good air flow

If the air in the unconditioned attic cannot flow, will it heat up and conduct into the conditioned space

If you have some conditioned attic space and then some adjacent non-conditioned attic space, you definitely need insulation between them, air flow or not.

Posted By jonr on 02 Jul 2013 09:50 AM
If you have some conditioned attic space and then some adjacent non-conditioned attic space, you definitely need insulation between them, air flow or not.

jonr,
not only will it be needed, it is a code requirement