I plan to build a 3500 sq-ft workshop next year and am not clear about applicability of Title 24 window requirements. The site is in CA, climate zone 16. The structure will be an insulated metal building on a concrete slab, fully detached from my existing house. The site is at 5400 ft elevation where I will need heat but not cooling (have never wanted an AC at the house). I plan to use hydronic heat and insulate with R38 for the roof, R19 in the walls, R17 for overhead doors, and R10 under and around the slab. And double pane windows. My question relates to what appears to be a counterintuitive requirement under title 24 to use low E glass and to limit windows on the west end to 5% area. In the site there is considerable solar heat gain available in the winter but if I limit glass area in the west (the other walls will have some glass but are heavily shaded by trees) and use low E glass, I will have to get nearly all of my heat from propane. Is there any flexibility under title 24 where solar heat gain is a good thing and AC will never be installed, or should I be planning to add another 1000 gallons or so worth of propane tanks? thanks

The short answer is yes, there is flexibility, but the extent of that flexibility depends on your construction assembly choices.

When meeting T24 energy codes, you can choose the performance method, which will allow you to use more than the 5% west facing (and 20% overall, btw) as long as your performance report shows it uses no more energy than the "standard design" for your climate zone.

Trade-offs are made between the energy usage of the different systems on the building. Depending on what you plan to use for your insulation, mechanical, water heating, glazing types, etc. the report can dictate how much glass you can place on your west wall.

Word of advise: If possible, try and get plans submitted before the end of 2016. The new Title 24 energy code, effective Jan. 1st 2017, is expected to toughen residential T24 compliance by 30%