Sizing windows to correspond with advanced framed window openings
I’m designing a house and struggling a bit with window sizing in regards to advanced framing. I’ve noticed that more than one manufacturer offers standard sizes, in addition to custom-sizing their units in 1/8th inch increments. The standard size widths are in 4 inch increments, which accomodates the standard building practice of using two studs on each side of a window. This holds for off-the-shelf as well as catalog sizing.
As an example, the widest Anderson that will fit into a four foot wide (nominal) opening in an advanced-framed wall is 3’8″, leaving a sizeable gap. The next window size up is a little over 4′, which is a bit large.
I’m wondering how folks are dealing with this detail, which seems to require either extra framing around the window as a non-structural filler on which to mount the window (if so, what sorts of interesting techniques are out there), or super-special ordering the exact size (which I priced and found to be costly for just a couple of extra inches of glass). Is it a caveat for advanced framing that it raises your window costs?
Thanks for your responses
W.M. Freeman
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Myles,
To me, your question is more important for a tract home builder trying hard to remove 4 extra studs per home than for the average builder.
There are lots of reasons why windows might not line up evenly with stud spacing: a window sometimes needs to be centered in a room, or to line up with a hallway or doorway, or accommodate an appliance. In short, it is rarely possible to choose window locations by popping the windows between the studs. Other factors affect window placement.
But the bottom line is -- if the jack studs bother you, you'll have to spring for custom windows.
I think that the energy savings attributable to advanced framing are actually pretty small. What really saves energy is exterior rigid foam or exterior mineral wool. More information here: The Pros and Cons of Advanced Framing.
+1 on the minimal thermal benefits of advanced framing. The whole-wall R of a 2x6 wall with R20 cavity fill with 25% framing fraction (typical, for 16" o.c. framing, doubled top plates) is only about R1 lower than an AF-framed 2x6 wall with a 15% framing fraction.
That R1 performance can be made up with an additional 1/4" of additional thickness on the exterior foam (or by using 1/4' XPS siding underlayment as the WRB on a code-min home.)
But it does save on lumber costs, and when planned well, reduces the scrap fraction.