Learned another thing....

Lbear: I know what you're saying, but yours is a very different usage of the term "whole wall R" than is commonly used by the building science types, who treat wall assemblies and window assemblies separately. In their terms, "whole wall R" means the average thermal performance of the finished structural wall area only, after factoring in the thermal bridging of any internal structural elements of differing thermal conductivity, such as wood framing (in the case of stick-built structures), or the bottom/top plates, splines, and window & door framing elements of structural insulated panel construction, while adding in the thermal performance of the siding and interior finish materials. This is done to correct for the much higher R values that the insulation-R alone might imply.

The term is never applied to the average performance of the whole side of the house, including window & door U-factors, air leakage, solar gain, etc.

Also, the 15% glazing fraction is the typical ratio of window area to FLOOR area, not the fraction of gross WALL area that is filled with window. The IRC specifies a minimum window/floor ratio of 8% for daylighting (4% operable window for ventilation) of habitable space. Local codes in cooling dominated regions often set a max of 15% to limit the unwanted solar gain/cooling load. LEED specifies an optimized 18% ratio for both enhanced daylighting & desired solar gain, optimizing the U-factor & SGHC for the orientation of the window.

In US climate zone 4A (southern Indiana) the IRC 2012 prescriptive limit for window SHGC no more than 0.40, but there is no SHGC limit for 5A. It's fine to go over that for south facing windows shaded by overhangs (or north facing windows), but for east or west facing glass keeping the SHGC under 0.25 is better.

http://publicecodes.cyberregs.com/i...sec002.htm

The fenestration draft proposal for changes for IRC 2015 is to keep the SGHC limits where they are, but to lower the maximum U-factor to U0.20 for zones 4 and higher.  That would be a low-E triple pane (or low-E vacuum insulated double pane), and quite expensive compared to a U0.28 low-E double-pane, let alone a current code-max (for zone 4) U0.35 window.

Posted By Dana1 on 17 Feb 2015 03:37 PM
Lbear: I know what you're saying, but yours is a very different usage of the term "whole wall R" than is commonly used by the building science types,....

The term is never applied to the average performance of the whole side of the house, including window & door U-factors, air leakage, solar gain, etc.

The fenestration draft proposal for changes for IRC 2015 is to keep the SGHC limits where they are, but to lower the maximum U-factor to U0.20 for zones 4 and higher.  That would be a low-E triple pane (or low-E vacuum insulated double pane), and quite expensive compared to a U0.28 low-E double-pane, let alone a current code-max (for zone 4) U0.35 window.

I wasn't the first one to use the term in that example. It was actually started by building scientists from Journal of Light Construction and Hammer and Hand. The latter being the top leading green building firm in the USA. If someone needs a scolding, it would be them, but they are top dogs in their field so some offense will be taken by them ;)

I understood what they were stating with their R-50 wall vs 15% glazing example. The point they were trying to show is that 15% of R-3 windows on a R-50 wall would drop that wall assembly down to R-15. Nothing more, nothing less. Now if you take into consideration thermal bridging, wood studs, etc, that R-15 number would drop even more but that is not what they were trying to get across.

The term "whole wall R" value as defined by Building Science Corporation:

"R-value for the whole opaque assembly including all additional structural elements (such as double studs), and typical enclosure interface details, including wall/wall (corners), wall /roof, and wall/floor connections."


As far as the 2015 IRC, I highly doubt the 0.20 U-Value will pass. Most municipalities are still using 2006 IRC and the one municipality that I know of adopted the 2012 IRC but did NOT adopt the 2012 IECC and therefore the energy codes of it do not apply and they still hold to the 2006 IECC. So they hold to 2012 IRC but 2006 IECC. Where I live they are still slamming 2x4s together and stuffing R-13 fiberglass batts in-between, just like they were doing in the 1970s.