Replies

1. GBA Editor
   Martin Holladay | Mar 17, 2013 07:38am | #1

   Kevin,
   Your framing factor will vary, depending on how many windows you have, and whether you are using advanced framing techniques. Typical framing factors range from 14% to 27%.

   Here is additional information on framing factors:

   A New Whole Wall R-value Calculator: "As a result, the amount of structural components is increasing. The most current study performed for California Energy Commission (Carpenter 2003) demonstrated that Framing Factor (fraction of the opaque wall area represented by solid wood used for framing) for residential walls is close to 27%."

   “Couple Secrets about How Framing is Effecting the Thermal Performance of Wood and Steel-Framed Walls” by Jan Kosny, David Yarbrough, and Phillip Childs: “The framing factor for hot-box tests is often between 11% and 14%. In reality, however, the framing factor is often much larger. According to a 2002 report, framing factors up to 27% can be found in residential walls in California in 2001. A similar study performed by ASHRAE in 2003 found an average 25% framing factor for US homes.”

   The main concern about a high framing factor is the energy penalty associated with thermal bridging. If you are building a double-stud wall, however, the insulation between the two rows of studs goes a long ways towards eliminating the energy penalty associated with thermal bridging -- so I'm not sure why you care about your framing factor.

2. 
   dickrussell | Mar 17, 2013 09:07am | #2

   To expand on Martin's last paragraph, let's use an example of a double wall with 2x4 studs and 12" cavity depth, and assume insulation at R 3.7/inch and wood at 1.1/inch. For the cavity, the U*A (or A/R) is 12 x 3.7 = 44.4. For a framed slice, ignoring heat transfer not perpendicular to the wall surface, the U*A is 2 x 3.5 x 1.1 +5 x 3.7 = 26.2. Now, for a 25% framing factor, the whole wall R will be:
   1/[0.25/26.2 +0.75/44.4] = 37.8
   For a 20% framing factor, the calc gives 39.0
   To these numbers, add R for sheathing, external foam and siding, and interior sheetrock. But the bottom line is that for a double wall structure, the framing factor doesn't affect the results much.

3. 
   kfree | Mar 17, 2013 01:17pm | #3

   i will be building the second 2x4 wall an inch outside of an existing wall. that wall has a cdx ply, 3 1/2 inch of dense packed cellulose, and one inch foil isocyan (part of an interior remodel ten years ago, when this was considered 'the place' for vapor barrier/retarder in zone 5). i plan on removing the cdx (to improve drying potential to the out, build the second wall with advanced frame type, dense pack with cellulose, then add an inch and half thermax (unfoiled isocyan), then zip system (wrb, and outer air barrier), rainslicker, shake siding. i already have calculated the transmission for the existing wall, i am replacing the windows so i will have to change those values, when i builld the second wall, i will be staggering them to offset them from the interior wall. Without really over complicating this too much, i'm really curious how the offsetting of the frame change the way in which you calculate the frame area. does that make any sense?

4. 
   kfree | Mar 17, 2013 01:23pm | #4

   also when i came up with the transmission loss on the first wall, i actually sketched out the wall to get
   get a more accurate area of frame. im not really sure about how to use frame factors in calculation. i was hoping to use the same method on the outer second wall and then overlay the first wall in terms of u.
   it might be little more time consuming but seems to be intuitively more accurate for me. maybe not though?