[Shophound]

What can lead to grossly oversizing HVAC on a foam insulated house:

• The fact that with a foam insulated roof deck, the ducts will be in conditioned space, even if they remain in the attic. Most standard load calcs account for duct heat gain in ventilated, overheated attics.
• infiltration rates for foam insulated homes are lower than conventionally insulated equivalents. Guessing at infiltration rates, as is commonly done with conventional homes, will lead to a gross over-estimation of infiltration rates for foam homes.
• foam insulated walls generally perform better than a fiberglass batt equivalent because the foam, properly applied, allows very little air circulation within any given wall cavity.
• Typical thermal bypass blunders made with regular insulation methods are typically not in play for foam, since everything below the roof deck is technically within conditioned space, and sealed from outdoor air and wind.
• Stack effect, and its summertime brother, reverse stack effect, are much less in play in a foamed house due to minimal air leakage through the envelope.

All of the above is based on an assumption that the foam insulation is properly mixed and applied. A bad foam job can cause many setbacks and heartburn. Coinciding with your push to find an HVAC pro with a foam house brain, find an equivalent pro for the foam insulation.

 

[Shophound]

I also agree with Motoguy that it would be smart to consider reflective roofing even with the foamed roof deck. While a foamed roof deck will mean a much cooler attic, the end result for many house designs can be an overall greater heat flux through the ceiling beneath the attic than through a well insulated attic floor. This is because the square footage of a roof is usually more than the ceilings below it, and a much wider temperature difference between the superheated shingles and the cool attic. It can be argued that the drastic reduction of duct losses offsets this, but if you want the best setup for DFW, it's hard to beat a reflective roof/foamed roof deck combination. Your house with such a setup will be very easy to keep cool and dry over our long hot muggy cooling season.

 

[Shophound]

I was under the impression that radiant barrier didn't work on a foamed house. (I don't recall why) I'll look into that.

Radiant barrier is not the same as a reflective roof. The latter is a roofing material that reflects a lot more of the sun's rays than it absorbs. It is part of the roofing material itself, not underneath the roof deck like a radiant barrier.

With a foamed roof deck you cannot use a radiant barrier. But you can use a reflective roofing material, also known as a "cool roof".

 

[Shophound]

Teddy and Mark are spot on: a foamed house is typically much more airtight than a regularly insulated house, which is why both of them are recommending a ventilating dehumidifier. For our area an energy recovery ventilator (ERV) isn't appropriate, as it can't control humidity adequately.

I applaud your efforts to learn more about energy efficient house construction. Many people fly into house building without a single clue how complex and involved it is. The Green Building Advisor Dot Com web site is chock full of information about energy efficient construction, as well as Building Science Dot Com.