Getting to Know the ASHRAE 90.1-2013 – Building Envelope

By Brian Redlein

With the make or break date of October 3rd  fast approaching for the new NYCECC we are going to continue looking at the differences between the new and old.  As usual the updated Code allows RAs and PEs a choice of actual Codes to follow – the IECC based NYCECC, or ASHRAE Standard 90.1 as modified by the NYCECC Appendix CA (not just Appendix A anymore).  In a previous newsletter I touched upon some of the changes between the IECC 2015 and the IECC 2012 and said I would follow up with a look at the new ASHRAE 90.1-2013 vs the old 90.1-2010 requirements.  Before we go further I should clarify that the 90.1-2013 only applies to commercial buildings as defined by the Energy Code, which includes residential structures over three stories.  Now that we’ve had a chance to get to know the new ASHRAE it is safe to say making the choice between the two is now more nuanced than ever.

With this month’s newsletter we’ll be focusing on updates to the building envelope established in Chapter 5.  To begin they seem to have clarified (or muddied, depending how you look at it) the general exceptions regarding roof replacement / recovering.  The old rules seemed to indicate that if I exposed the roof deck and/or insulation under my proposed scope of work, I’d pretty much have to update that roof area under my scope-of-work to comply with the roof insulation requirements established in Table 5.5-4.  Now “roof recovering” is exempt from Chapter 5 in its entirety [5.1.3 Exception 5], but be aware “roof recovering” isn’t a defined term so there’s still ambiguity about what forces compliance or not.  On the other hand, Exception 6 to that same section was clarified to say that if I remove and replace a roof membrane, provided the roof is insulated in some form, I don’t have to re-roof the whole thing to comply with the new Code.  The old version of exception 6, exception e, said that if I so much as exposed the roof insulation I now have to make the roof comply with the Code and now I’m putting down 4” of polystyrene where once I may have had significantly less.  This is all a good thing, as the new ASHRAE now requires roof decks to be insulated to R-30 just like the NYCECC.  In plain English that’s six inches of continuous polystyrene foam insulation on the roof deck now [Table 5.5-4].

Air leakage provisions were updated under section and now include provisions for “vehicular access doors”.  Said doors cannot leak more than 1.3 cubic feet of air per minute per square foot of area (cfm/sf) whereas the old section did not have explicit provisions for vehicular access doors.  That said, metal coiling doors of all types serving semiheated spaces, like most vehicular occupancies, do not have to comply with air leakage requirements and are still excepted from  Also added to the air leakage requirements was a brand new exception, Exception 3, which states “products in buildings that comply with whole building air leakage rates of 0.4 cfm/sf” can be excepted from air leakage requirements.  So if my entire building doesn’t leak more than 0.4 cfm/sf I don’t have to worry about making sure all my doors and windows are certified for air leakage.  This might work well for new buildings in NYC between 25,000 and 50,000 square feet as they’ll now have to test for air leakage anyway regardless of which Code, NYCECC or ASHRAE, the designers choose to follow [NYC specific new 90.1 section].  Provided of course those buildings do actually comply once tested, otherwise there’s an awful lot of doors and windows to be replaced or air sealing work that will need redoing.

Now I have to touch on vestibules.  We do so much storefront replacement at Metropolis the vestibules come up all the time on so many of our jobs, large and small.  Now the Code has always allowed me to replace entry doors in kind without providing a new vestibule, but rip the entire storefront out and I can’t claim Exception 7 to 5.1.3 anymore (old exception f).  The key change to the language in section, which governs vestibules, now limits their size to “the greater of 50sf or 2% of the gross conditioned area for that level of the building”.  Also a pair of exceptions were added for semiheated spaces (these weren’t explicitly excluded in the old ASHRAE) and doors off parking garages that access elevator lobbies at the building entrance.  All that said, the one local modification to the IECC vestibule requirements that allowed air curtains to replace vestibules was not carried over to the local ASHRAE 90.1 modifications.  That exception alone may greatly influence whether or not to go ASHRAE depending on how big a deal the vestibule is for your space.  Before we move on from vestibules, ASHRAE added one more section, that really only affects big box style retailers and a handful of other occupancies that may have entry floors over 40,000sf.  While big box buildings over 40,000sf are rare in NYC compared to our suburban surroundings, vestibules serving those buildings must now be a simply massive sixteen feet deep.

So moving on to the tables in section 5.5, and specifically 5.5-4 which governs the climate zone NYC is in, we can see they’ve made some changes it seems to everything but opaque above grade mass walls.  Opaque mass walls must still be below a max U of 0.104 or be insulated with R-9.5 continuous insulation for non-residential buildings or be below U-0.090 or have R-11.4 continuous insulation for residential buildings.  Key changes to the tables include:

  • Thicker or better continuous roof insulation (R-30) that we already touched upon
  • foundation insulation applies to all occupancies except semi-conditioned space and is now more stringent for residential buildings
  • mass floors over unconditioned spaces and slabs on grade basically have to be more insulated than they used to be
  • All opaque doors serving everything but semiconditioned spaces must hit a U no higher than 0.5 (non-swinging doors used to able to be as high as U-1.45 for non-residential occupancies)
  • Vertical fenestration Us have gotten more stringent
  • Skylights, which used to be broken out amongst multiple types, are all now subject to the same rules with a U no higher than 0.50 and are limited to 3% of the roof area, down from 5% for many skylights (although there’s daylighting witchcraft that can be employed to get it to 6%)
  • New column was added regarding Visual Transmittance (VT) to Solar Heat Gain Coefficient (SHGC) ratio as fenestration installed in conjunction with daylighting controls must have an minimum VT/SHGC of 1.10.  To recap VT lets more daylight in, while SHGC keeps the infrared light out that contributes to heat gain.

Finally, there were also some updates made to minimum skylight fenestration areas and fenestration orientation requirements.  Daylighting has been mandatory in the Energy Code for some time now and for spaces over 2500sf with high ceilings there are provisions for minimum skylight area to let some daylight in and keep the lighting bill down.  That said this doesn’t mean you have to figure out a way to install a skylight on say, the ground floor of a building where you have no roof.  The updated ASHRAE now requires more space types than before to provide skylights if they can, such as library reading rooms, book stacks, and court rooms.  Fenestration orientation, which governs glazing on exposures that get more sun on the east and west sides of a building, also had a new exception added that allows said east and west exposures to be excepted form, provided the window-to-wall (WWR) ratio on those exposures is no greater than 20%.

Well that about wraps it up for the updated envelope requirements.  As usual check in with us again in the coming months to find out about updated mechanical and lighting requirements under the rest of ASHRAE 90.1-2013.