We're investing in the envelope of our house with full ICF construction and good quality windows.  My question regards the 2x12 P.T. lumber we're using for the window bucks.  Since the bucks are flush with the ICF forms inside and out which can't be insulated, isn't that a significant thermal bridge which kind of negates the idea of using foam insulated fiberglass window frames and creating a weak point in our envelope??  Is there a better way?  Thanks in advance for any ideas.

 If you calculate this you'll see that your 2x12 buck is going to have an R-value of at least 12. This is probably close to three times the R-value of your windows, and double that of your doors.

The vinyl window-buck proponents will offer their comments. Fpr me there are too many advantages with wood bucks to consider vinyl. STALA brand IFAs (steel bucks) are a very good option if you don't like wood or vinyl: try David Morris at 502-424-2509

Good luck, Felix

There are number of ways to avoid the thermal bridge across the 2x12" -- wood typcially performs about .6 to 1 R-value per inch or probably at best an R10 for a 2x12". This is still much better than even high end windows. Perhaps the easiest way (while staying with wood and not utilizing specialized bucks like Stala or V-buck) is to create what you might call a hybrid wood buck. Use treated 2x6" buck lumber for your buck for a 6" concrete wall. The buck would sit inside the concrete core for the opening and have full ICF foam on both sides. Thus your rough opening for both the ICF and the buck would be the same. Other variations exist to insulated one side or the other with the icf and not both sides. That said, I like your thought process as you keep looking for week liinks in the structure but I don't think this is going to make much of a measureable difference for such a small percentage of your building. Keeping in mind that if the building was stick as much as 20% of the building would have the R-value of a 2x6". You might also consider some type of hybrid for your top plate as they are alot easier to implement than the internal buck method. Regards.

P.S. much more important to your overall performance is how the windows are chauked and sealed -- any convection (or tiny leak) will result in R0.

Nova,

There are several ways to make this work -- even including utilizing longer screws through the foam.  Again, benefits are very small, but if I were to do it  I'd do something like the following detail attached.  You will notice that you will still have some weak spots around the window in terms of wood thermal breaks but this approach does protect the thermal mass somewhat more from outside influences.  Regards.

Posted By thagreen on 10 Feb 2010 02:55 PM
This is if you use the foam buck. If not you'll defenetly need to go with p/t.

Not necessarily. IRC requires p/t in contact with concrete only when it's less than 8 inches above ground. But because of thermal mass differences between concrete and wood there could be a condensation problem where wood contacts concrete. A moisture barrier is all that's needed. Non-p/t wood is better quality and less expensive than p/t. I'd suggest the OP use construction grade doug fir, or similar, and face it with roofing felt, poly sheet, or sill foam where concrete is going to contact it.

Posted By dmaceld on 11 Feb 2010 11:22 AM
Oh, oh. We have a problem with this new (upgraded??) forum software. Quoted text is not surrounded by a box in the msg compose window. Subsequently all the reply gets included inside the box when posted.

Posted By Bigrig on 11 Feb 2010 07:00 PM

Posted By dmaceld on 11 Feb 2010 11:22 AM
Oh, oh. We have a problem with this new (upgraded??) forum software. Quoted text is not surrounded by a box in the msg compose window. Subsequently all the reply gets included inside the box when posted.

Actually you just have to make sure you click below the quote box when you "reply". An easy error to make with a new forum setup.

The LiteForm ICF manual has a detail for installing the bucks inside the forms, i.e. with 6" core forms, a 6" wide 2x (2x8 ripped down to 6" since a 2x6 is only 5-1/2" wide or scab a 1/2" plywood strip on the side of a 2x6).  The foam forms are screwed to the bucks inside the cavity with long screws and large plastic washers.  Pressure treated lumber is typically used for these bucks in contact with the concrete.  The 1-1/4" strips that are ripped off the 2x8s can be used along the bottom of the bucks to leave an opening for filling the area under the sill with concrete. 

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After the pour is complete, there are several options for attaching the windows and interior finish to the bucks.  One way is to just use long screws through the window nailing fins through the foam and into the bucks.  This provides the most thermal insulation and best thermal break, but can make interior finish material a little tricky to attach.  Another way I have seen it done is to allow an extra 1-1/2" width and height to the window rough opening, then case inside the bucks and foam with 3/4" plywood cut to fit which doesn't tend to cup like a 2x12 will.  This provides a nailing base for trim and such.