Check to see if there is an Energy Rater near you. We hired one, and one of the first things he mentioned was that designing a house to breathe may not be necessary. Note he doesn't have any partnership with the foam guys or anything. He is strictly an Engineer that is an Energy Star rater and consulatnant. We are going to use spray foam, california corners, etc. You can also use spray in cellulose also, but I've heard about settling issues after a period of time. Just some thoughts... by no way am I an expert! But the Energy Rater has given us a lot of good information. And he also will go out there while the insulation and HVAC or geothermal, and windows are being installed to confirm that it's being done correctly.

A house does not need to "breathe". The best built house is a tight house that is properly ventilated. Two different things entirely.

I wouldn't preclude using fg. But unless it is combined with a less air permeable insulation and tight construction, it's not going to be thermally effective. FWIW, when I have a say in building, I combine fg with XPS sheathing, lots of tape, caulking, gasketing and spray foam.

Until contractors and builders and the guy taking orders at your typical lumber yard get over the idea that "loose" construction is desirable, there won't be much progress towards tighter homes.

The best information on homebuilding widely available to the general public is from Building Science Corp. See link. Incidentally, they also write much of the similar info at the Department of Energy EERE.

Here is a link that might be useful: Building Science Corp. on Insulation

"We live in the South, where it is hot in the summer, and can be cold in the winter".

You have to be real careful about getting a house "tight" in the deep south.

We live in the SW LA, we have closed cell foam in the exterior walls and open cell under the roof deck (unvented attic). We have an energy recovery ventilator (ERV, we keep the fan on all the time so there is fresh air.
We have been in our home a year now and are very happy with the indoor temperatures and low energy bills.
Yes. it is expensive and perhaps 2X6's and rock wool may have been more cost eficient. (Learned about this later)!

What is meant by a house "breathing" is that it allows moisture to pass through the walls which is not necessarily appropriate in all climates. It has nothing to do with air movement although some builders think it does.

Since open-cell foam (like Icynene) is not necessarily a vapor barrier it might be good. Rigid insulation board on the exterior of the sheathing might also be good. Dense-pack cellulose (not loose or wet sprayed) might also work. It all depends on the climate, the building envelope, and the HVAC system; there are no rules that work for all conditions.

Thank you VERY much everyone! Lots of food for thought. Great idea to hire an energy consultant - I had not even thought of that.

Here's Dr. Joe's annotated list of the"Top Ten List of Dumb Things to do in the South" when building homes.

No you don't have to be careful about building too tightly
anywhere. you just add fresh air.
build tight..ventilate right.
Put your closed cell foam boards (1" foil facing out)'to
exterior of walls. Tape all seams, but first make sure foil
is dry and clean. when trades make holes..foam or tape them.
this is added insulation ( R-6.7 for 1" closed cell) to the wall and provides a thermal break to minimize thermal
bridging thru wood studs. insulate with conventional insulation and use ADA (air tight drywall approach)
with sheetrock inside. The exterior foam will make walls
air tight from exterior and ADA will continue the air tightness from the interior. Insulation loses its R-value
when air passes thru it.

Open cell in attic on roofline, if your mechanicals
are in the attic. if you put your heating system inside
and ductwork inside then use the open cell on the attic floor. Buy ICAT (insulation contact air tight) recessed lights if you use these lights. It is best to put the
things that heat and cool your home inside the conditioned space. If you decide to install them in the attic then
the roofline install makes a huge difference. With this application the attic becomes semi conditioned.

Hvac system can be downsized, get a load calc and stay away from that rule of thumb sizing (usually 500 sq ft per
ton) Oversizing of hvac systems is a huge problem because
they don't run long enough to remove humidity..and high Relative Humidity and mold are issues.

Louisiana has a program called HERO Home Energy Rebate Option through the dept of natural resources.
I've been a rater for 10+ years and don't know everthing
but do know what works in most cases.I admit that I have
good resources when my clients ask something I am unsure of
I tell them that I need to check it out and get back to them.

visit www.buildingscience.com site and you can
review the information for our climate.
Joe knows. And he shares,,you can email him...call him
and he gets back to you farily quickly..as his schedule allows.

best of luck,.