Brock,

The moisture in the dead air space was brougth up on another thread and has me somewhat concerned.  But, it seems a vapor barrier on the interior wall and tyvek on the exterior will allow the wall to breathe should moisture enter the dead air space.  (?)

As for the cost, a 2x6 is nearly double the cost of a 2x4.  And, there would not be double windows, each window would be trimmed out with jamb extensions similar to a SIP house. 

I looked at foam boards on the exterior, but you are looking at a significant price increase (about $1 per sq ft versus 25 cents per sq ft for r 13 fiberglass). 

Just thinking out loud and trying to find the weak points before I go through with this crazy idea... : -)

I'm in Kansas City.

Fiberglass has taken a beating compared to foam, but is it really that bad?  Take away air infiltration and one insulation is no better than another...?  With fiberglass being so cheap and the price of wood down, one could insulate to a much higher r value as compared to foam.  Not really looking for an argument, just looking for the best bang for the buck.  And considering fuel costs, maximizing insulation has never been more important. 

l will break ground on my home later this month.  The basement will be ICF.  If things go well, the first floor will be ICF also.  However, if I have problems with the basement, I want a back up plan, thus the double wall with fiberglass.  We built with SIPs a couple years ago.  I liked the product, but am looking at various options.     

Going with my gut, I would agree with you.  But, considering the price difference, I want to fully investigate the possibilities with fiberglass.  Take away air infiltration and what makes an r20 foam better than an r20 fiberglass?  Certainly, one would need a deeper cavity for the fiberglass to acheive the higher r, but...

As I said, if all goes well, I will go with ICF on basement and 1st floor, then fill the attic with an r50 cellulose.  But I can't help but wonder about the value of fiberglass. 

Thanks for your replys.

Dug,

I hope you can find someone to build what you want.  In my experience, builders tend to stick with what they know.  When you ask for something different the price goes up much higher than it really should.

If you choose to use fiberglass insulation, you might consider using closed cell spray foam to seal the structure (as others have mentioned).  If you do the heat load calculations (or have someone do it for you), the air changes per hour (ACH) value has a huge effect on your heat loads.  Fiberglass provides no air sealing whatsoever, and loses most (if not all) of its insulating properties with a little air flow.  Build tight.  Ventilate right.

Check out the status of our project... perhaps the most energy efficient modular home ever built - www.GouinGreen.com.

Good luck with your project.

Dug, here are a few comments about your plan:

Insulation and air sealing are the best investment you can make right now. You can cut your usage by 40%, which is quite a pay off, but when skyrocketing energy prices are factored in, the payoff is huge. Don't skimp. But you don't need to throw your money away either.

Exterior foam will do everything you need cheaper than spray foam. 1-1/2 in. of rigid foam on the outside of your walls will eliminate the thermal bridge, make a nice air barrier, keep the interior framing warm, and act as a drainage plane behind your siding. Skip the plastic vapor barrier on the inside -- it won't help and it may hurt in your climate (during AC season). As Dr. Joe says "Houses get wet in summer and they rot in winter" because AC units pull exterior humidity inwards, the plastic traps it there and the wall can't dry out in the winter. Vinyl wallpaper counts as a vapor barrier, incidentally. (http://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers/view?searchterm=vapor%20barrier)

With a good air barrier fiberglass batts CAN POTENTIALLY work. But everything has to be perfect -- 2% voids can slash performance by up to 40% (surf arounf the Oak Ridge Nat'l Lab web site for a lot of info on insulation performance). It's much easier to get excellent installation with a blow-in type insulation like cellulose, BIB, or Spyder. Exterior foam and dense pack cellulose would be a super good and fairly inexpensive choice.

Double walls use way too much wood for what you get. And it doesn't seem like the greenest choice, using twice as much wood as you need to. If you really want deep walls, you could do what the people at e-colab (www.e-colab.org) do: use I-joists for studs.

The "envelope house" referenced earlier is sort of a gimmic. Read about it at Environmental Building News. Why try to reinvent the wheel? theree are plenty of ways to build super efficient houses.

The comment about builders liking to stick to what they know is true, but there are some who are ahead of the curve. Fine one of those guys to anything other than standard, leaky, coinstruction. Remember, code is the worst you can build without going to jail.

Good luck,
Dan
www.thegreenbbuildingadvisors.blogspot.com

Ebo,

You loose a lot of the intended sheer strength of the osb sheet when the nails are going through the foam. The osb needs to be against the stud.

Patrick T