I am building a  house in Central FL. I plan on using a Polyurethane spray foam insulation to the underside of the roof deck.  I have a few questions I can't seem to get answered. I will have a silver Metal roof.

1. Do I use Open vs Closed cell?
2. Is there an issue with condensation and humidity due to the attic being sealed? Won't humidity eventually get into the attic? Where does it go if there are no vents, etc...?
3. Does the type of duct material I use matter?

Thanks

I don't happen to be an expert on insulation, I will tell you that steel duct is preferred buy often not employed due to other convieniences such as insulation requirements or poor access which might make duct board or insulated flex more economical to install.

I would vent the attic to the interior to address #2.

Posted By sphingers on 30 Mar 2014 02:34 PM
I am building a  house in Central FL. I plan on using a Polyurethane spray foam insulation to the underside of the roof deck.  I have a few questions I can't seem to get answered. I will have a silver Metal roof.

1. Do I use Open vs Closed cell?
2. Is there an issue with condensation and humidity due to the attic being sealed? Won't humidity eventually get into the attic? Where does it go if there are no vents, etc...?
3. Does the type of duct material I use matter?

Thanks

1>  Open cell is fine in a central FL (US climate zone 2), and meets the definition of air-impermeable insulation for roof decking insulation under IRC 2012 R806.5.1:

http://publicecodes.cyberregs.c...sec006.htm

It's also cheaper per R/foot than closed cell foam, and 1000x less damaging to the climate, since it's blown with water instead of HFC245fa, which has a very large global warming potential (about 1000x CO2.)

While it is far more water-vapor permeable than closed cell foam (which could be a problem in Minneapolis), it's not a problem for the roof deck in central FL. 

Like any foam that is exposed in an attic, it needs to have an ignition barrier at a minimum (intumescent paints are usually accepted in my neighborhood), but would require a timed thermal barrier like half-inch drywall if it was a fully used space for say, storage purposes.

2> In FL the primary moisture source is from the outdoors, not the interior (though you need to vent bathing & cooking peaks. The roof deck is (with rare & fleeting exceptions) going to always stay above the dew point of the conditioned space air even in winter.  But in summer the dew point of the outdoor air is often above the temperature of  indoors, putting the indoors at risk of accumulating moisture from outdoor air infiltration. As a rule, venting the attics in that climate INCREASES the moisture content of the wood and an other moisture absorbing materials that may be stored up there.  Venting the attic also increases the likelihood of condensation forming on air conditioning ducts and dripping onto the attic floor.  In an unvented attic the dew point of the attic air tracks that of the conditioned space- the air inside the duct is essentially at the same dew point as the outside of the duct, and no condensation occurs on either the inside or outside of the duct (it condenses on the air conditioning coils instead.)

3> Hard piped metal ducts simply last longer and are harder to damage, but are more expensive to install. Sealing all seams & joints with duct-mastic is important for them to work as-designed, and there is more air-sealing to be done than with flex-ducts, but hard-piping is still the right way to go.

...the air inside the duct is essentially at the same dew point as the outside of the duct, and no condensation occurs on either the inside or outside of the duct ...

Only on houses with no air infiltration (very rare) or separate dehumidifiers. But interior air does typically have a lower dew point than outside air and is therefor less likely to condense on ducts.