Hi All...found this website a few weeks ago and it has been a great resource. I run a custom home building company and am trying to steer it in an energy efficient/green direction. I'm using myself as a guinea pig to test out many of the innovations spoken about on this website, as we're about to start building my own home. Have definitely decided to use icf walls and buy the best windows I can afford. (Still haven't decided what to do with the roof but am leaning towards a sprayed ceiling with celulose). So my question is this....if I spend so much effort and money to build hunking big concrete walls with a say, 50 R value and assuming I get the roof really tight also but then I install windows which even if I spend a bomb might get me up to a R7 or 8 (U factor 0.14/0.125), isn't it like building a super tight envelope and then leaving a door or two wide open? Or am I focusing too much on R-values and the more important issue is how well the envelope is sealed? A secondary question is, if I do get the envelope super tight does it make less sense to use geothermal (which I want to do) because the payoff is so much longer since you're not using it nearly as much as you would with say, a stick built house? Thanks in advance for any light anyone can shed.

Your location (climate) will determine the correct answers to many of your questions.
So, where are you located?
Also, I noticed a reference to 'R50' icf walls. This reminds me of the 'R50 equivalent' marketing tactics that were very detrimental to the advancement of ICF usage with the general public. Please refer to your ICF mfg specs for the exact R values of the forms you intend to use. Most of the forms commonly used have and R value in the low to mid 20s. If you intend to promote an R50 wall to potential clients, make sure that your forms are justly rated at R50.

Hey Wes..we're located in Northern Virginia...coldish winters (but not too much snow) and hot, humid summers.

Considering your location's climate, I would think that standard ICF walls with 6-8" cores would provide sufficient R values. If you have read the forums, you know that I am a believer in spray foam insulation and insulated attics. Your window choices should be specified by the project. Take into account the location, and particulars of each home.
By taking advantage of the positives of each location, and designing a home to fit that location, including room and window placement, overhangs, etc., you can reduce the detrimental effects of windows and doors on the overall R value of a home. From a practical point of view, if you use a good quality, well built, lowE/Argon filled insulated window (which should have a rated R value of about 4), set in an ICF wall, the more important point is not the R value of the window, but how well it is installed, weather proofed, and sealed.
As to geothermal usage, it is becoming less of a positive now, with the development of air to air heat pumps with SEER rating of 16+, and the difference in costs still so great. If we can ever get the costs more competitive, then geothermal would be worth looking at.

Nova,
Wes is right about the core size, 6 or 8 will give you the best bang for the buck.
However, the geo and icf combination is great if the dwelling is larger than normal.
If the house is under 2500sqft the return on investment will be quite longer. Since the building is air thight windows don't need to be the absolute best but not the worst either. One could find a medium and still keep a good seal. Instead take the extra cost you would apply on windows and put it in a good hrv to ensure the indoor air quality.
Cheers!

<!--[if gte mso 9]> Normal 0 <!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0in; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} @page Section1 {size:8.5in 11.0in; margin:1.0in 1.25in 1.0in 1.25in; mso-header-margin:.5in; mso-footer-margin:.5in; mso-paper-source:0;} div.Section1 {page:Section1;} --> <!--[if gte mso 9]> Geothermal heat pumps are also improving in performance.  The heat pumps cost is almost the same.   The difference is the cost of the ancillary equipment needed to make them work.  With the 30% no cap on the rebate for geothermal in addition the rebates and grants that may or may not be offered by the State and local governments suddenly geothermal becomes a great deal.  Add it all up coupled with the savings = a fairly short ROI.  Done in a commercial setting combined with ICF the ROI can be as short as 18 to 24 months. 

Gentlemen,
Thankyou very much for taking the time to reply. I think I will still go with the geothermal, esp. since the house is on the large side. Thanks again for your insights.

No, and No. You can mitigate the Heat Loss by using insulated drapes or blinds at night. R-Values are important, and you are on the right track. The R-50 thing? Ah, never mind, Wes brought it up ;)....