Building envelope - or building enclosure?

In October, I published "Tegularity," a discussion of the proper term for acoustic ceiling panels. (The title came from the name for a specific type of panel edge.) Shortly thereafter, in December 2016, I received a question from Anthony Capkun, editor for  Electrical Business Magazine and former editor for the Construction Specifier. He asked, "What is the correct term these days: a) Building Envelope or b) Building Enclosure?"

I responded that I had always used building envelope, and that that is the term I hear most often. But, having learned a long time ago that always hearing a term used in a particular manner does not mean that that is the correct term, I decided to investigate further.

One of the first places I go for this type of question is Google's Ngram Viewer. This is a handy search tool that charts frequency of appearance of words or terms, based on sources printed between 1500 and 2008. Although it has its problems, it's a convenient way to get a feel for the relative uses of similar terms. In this case, the results suggest that my experience is probably common, with building envelope being used far more frequently than building enclosure.

However, in our line of work, we don't rely on popularity contests, so I turned to the experts - published standards and leaders in the subject.

I started with the National Institute of Building Sciences (NIBS), self-proclaimed "Authoritative Source of Innovative Solutions for the Built Environment." NIBS has several committees, one of which is the Building Enclosure Technology and Environment Council (BETEC). In 2004, BETEC and AIA established the Building Enclosure Council - National (BEC-N), which now has 26 chapters (BECs) in various states. BETEC has other committees, some of which use "enclosure" in their titles; none use "envelope."

Next on my list was recognized guru Joe Lstiburek, PhD, PE, ASHRAE Fellow, principal at Building Science Corporation. Joe is blunt about his view. In BSI-024: Vocabulary, he said "They are building enclosures—they are not building envelopes. You put letters in an envelope not people." The same document defines only building enclosure. It has been reported that he also said, "Envelopes are for FedEx. Enclosures are for engineers." It's interesting to note that he wasn't always this certain; in 1999 he wrote a paper titled "Air Pressure and Building Envelopes."

A search of the Whole Building Design Guide shows some documents that use building envelope, while others use building enclosure. ASTM and ASHRAE use both terms, and Wiki defines building envelope only, yet has a discussion of building enclosure commissioning.

Our friends to the north have the National Building Envelope Council of Canada. As you might expect, building envelope is widely used in Canada, but building enclosure also appears. Because they've been more concerned about weather barriers than the US has, for a longer time, my initial inclination was to follow their lead. Unfortunately, Joe Lstiburek and his buddies muddied the water, deciding that building enclosure was better than building envelope.

I was not surprised to find that I was not the first to try to find the better of the two terms. In October 2012, Allison Bailes III, PhD, owner of energy vanguard, posted "Building Envelope or Building Enclosure Which Is the Better Term?" in the energy vanguard blog. After discussing the debate and stating a preference for building envelope, he ends by saying, "Both are perfectly adequate, but the existence of two terms for the same thing will create unnecessary confusion. Such is life." About a month later, he posted a follow-up titled "'Building Enclosure,' Not 'Building Envelope.'" In this piece, he discusses additional information and states, "Precision of language matters. The building enclosure is one of the most fundamental concepts in building science, and it does make sense to use a single term to describe it. I'm now a convert to 'building enclosure' and will use it exclusively."

I sent inquiries to a few of the standards organizations, asking if there will be an attempt to agree on a single term. Even if they do, it will take at least a couple of years to change their standards, as they would undoubtedly wait until the standards were due for updates.

As for me, I'm going to follow the lead of NIBS and Lstiburek, and use building enclosure.