ASHRAE 90.1-Envelope Fenestration & Skylight Fenestration area

I would like for you to be able to tell your client that. I will leave it up to you if you want to do so however, neither value is a mandatory provision. They can be exceeded in the design but the baseline model cannot exceed these values. So for projects with high amounts of glass they pay a penalty in energy performance but this can be traded off against other energy saving strategies. In our experience for most projects the sweet spot is about 20%-30% window/wall ratio. This is usually enough for good daylighting design and not too much to adversely affect the energy use. This number probably varies a bit by climate and other factors but is a good starting place. Over 40% in any climate zone is just bad design in my opinion and in the opinion of the authors of 90.1.

None of the ASHRAE 90.1 components are guidelines as I understand the term. The 20% to 30% value I mentioned above is a guideline. There are mandatory and prescriptive provisions in ASHRAE 90.1. All of the mandatory provisions are contained in the X.4 portion of each section. So the building envelope mandatory provisions are in section 5.4. The envelope tables are in section 5.5 and are therefore available for trade off against other prescriptive provisions. All LEED projects must comply with all of the mandatory provisions. These are the only provisions of the standard which the project must design to meet. The 40% and 5% glazing area factors are in Appendix G. Appendix G just defines the baseline energy model and must be followed precisely when creating the baseline model.