The Fenestration Game

By Brian Redlein

Your humble Filing Rep has been spending much time coordinating with the DOB Energy Code team as of late and the single biggest Energy Code comment the Department likes to harp on has to do with the sheer amount of clear glazing that applicants want on the side of their buildings while trying to get out of the Energy Modeling requirements.

A couple months ago, I wrote on how more than 40% clear glass above grade requires Energy Modeling and frankly it has for a much longer time than just this year. The New York City Energy Conservation Code does not exist in its own bubble and is really based off of the State standards.

The issue is that the State standards, like all the standards based off the International Energy Conservation Code, don’t really cover buildings that have more than 40% clear glazing above grade.  As I said in June, this requires the Job be done to ASHRAE standards with specific regard to Chapter 11 of 90.1 – the Energy Cost Budget Method which means Energy Modeling.

The reasons for this is that glass does way more harm than good from an energy conservation standpoint.  Single pane windows have practically no insulating properties and without fritting or tinting can let massive quantities of heat and light into a building.  Adding panes and air gaps helps lowers the thermal conductivity (U factor), but the simple fact is light generates heat and with too much light entering the building the design team is forced to make trade-offs between systems – hence the ASHRAE Energy Modeling requirement.  Glazing also creates issues with air sealing as windows have to be practically air-tight post-installation to meet Code – so getting the glazing right is the single most important element in the building envelope.

To that end, the DOB now takes the Window-to-Wall Ratio (WWR) as seriously as it does Zoning Floor Area deductions.  When doing a curtain wall building, applicants should be extremely attentive to the amount of clear glass on the curtain. Specifying curtain wall practically begs the Department to come back and ask for an Energy Model.  The other problem with curtain wall is as they are typically custom made, they lack corresponding technical documentation for the applicants to use in their Energy Analysis.

Currently, the DOB really only likes to allow applicants two places to get their values from for use in an Energy Analysis: the ASHRAE 90.1 Appendix A or the manufacturer’s specifications.  If something isn’t in the Appendix A, or the values provided in the Appendix A cause the building to fail Energy Analysis because they’re default values, then the Department would like to see the manufacturer’s numbers.  This is easy when doing a home remodel, but when building skyscrapers with multi-million dollar curtain wall systems it becomes a complete nightmare.

So how does the design team get around this?  Candidly we’re still figuring that out here.  The National Fenestration Rating Council (NFRC) rates windows for sale in the US and with smaller jobs it’s easy to spec a window or place-holder window that has NFRC documentation to support the Energy Analysis.  With big curtain wall buildings, we seem to have entered a twilight zone as NFRC can only certify a custom-built curtain wall after it’s physically installed in field.  Where exactly to get Energy Analysis values from for something that hasn’t even been built and tested yet is still something this Filing Rep is working on with the Department.  Stay tuned.