Replies

1. GBA Editor
   Martin Holladay | Jan 11, 2011 01:19pm | #1

   Lucas,
   As far as I know, since 2004, the window area limitations have been completely eliminated from the prescriptive paths of the IECC and IRC.

   So, all-glass house, here we come.

   Needless to say, the decision to eliminate window ratio limitations from the International codes was controversial. The change was enacted as part of the code simplification efforts introduced with the 2004 codes.

2. 
   LukeInClimateZone7 | Jan 11, 2011 01:30pm | #2

   Martin,
   Many thanks for your reply!
   This is very helpful, even if it means that the only thing keeping the project from becoming a greenhouse (check your stones at the door) is my articulation of the challenge in providing comfort in such an envelope on a cold Michigan night. I'm not sure triple-glazing is in the budget yet.
   :)
   Here we go,
   --Luke

3. 
   Riversong | Jan 11, 2011 01:39pm | #3

   Lucas from California building a house in Michigan?

   Yes, it's an absurd reality that there are no prescriptive window-to-floor area ratios in the IECC residential section, though the commercial standards limit fenestration to 40% of any wall.

   But the fact that the performance standard has been reduced from 18% to 15% window-to-floor area ratio, that Energy Star 2011 has reduced their standard from 18% to 15% to agree with IECC, and that some code jurisdictions are proposing a 15% limit in the prescriptive standards - all indicate that this is broadly considered a reasonable target. 15% is also the practical limit for passive solar design.

4. 
   LukeInClimateZone7 | Jan 11, 2011 01:59pm | #4

   Robert--
   Thanks for weighing in too... Geez, two of best experts I could think of (among the many great contributors to this forum).
   Yes, strange but true-- a Californian working on a Michigan project. It's just conceptual design at this point.
   As for Passive Solar-- the house is in the woods, so any solar aperture we have would be quite effectively shaded by the surrounding trees.

   --Luke

5. 
   Riversong | Jan 11, 2011 01:59pm | #5

   Window-Floor Area
   ENERGY STAR Qualified Homes National Program Requirements
   Version 3.0 Effective 1/1/2011

   For Prescriptive Path: All decorative glass and skylight window areas count toward the total window area to above-grade conditioned floor area (WFA) ratio. For homes using the prescriptive path that have a WFA ratio >15%, the following additional requirements apply:

   a. In Climate Zones 1, 2, and 3, an improved window SHGC is required and is determined by:
   Improved SHGC = [0.15 / WFA] x [ENERGY STAR SHGC]
   Where the ENERGY STAR SHGC is the maximum allowable SHGC in Exhibit 1, ENERGY STAR Reference Design, for the Climate Zone where the home will be built.

   b. In Climate Zones 4, 5, 6, 7, and 8, an improved window U-Value is required and is determined by:
   Improved U-Value = [0.15 / WFA] x [ENERGY STAR U-Value]
   Where the ENERGY STAR U-Value is the maximum allowable U-Value in Exhibit 1, ENERGY STAR Reference Design, for the Climate Zone where the home will be built.

   For Prescriptive Path: Up to 0.75% WFA may be used for decorative glass that does not meet ENERGY STAR requirements. For example, a home with total above-grade conditioned floor area of 2,000 sq. ft. may have up to 15 sq. ft. (0.75% of 2,000) of decorative glass.

6. 
   Riversong | Jan 11, 2011 02:01pm | #6

   the house is in the woods, so any solar aperture we have would be quite effectively shaded by the surrounding trees.

   Oh, good. So it's all heat loss and no gain.

   As my mother used to say (rest her soul) "Homo Sapiens who reside in transparent edifices should refrain from hurling geological objects through the atmosphere."