Building Envelope or Building Enclosure: Which Is the Better Term?
If you travel down into the building science vortex, you start discovering that people argue over a lot of things that most people never think twice about. Wait. Most people probably never think once about these things. Earlier this year I wrote about the stack effect and used the expression 'heat rises.' Whoa! I was nearly excommunicated on my way to Sunday School. I had awakened the Building Science Fight Club.
I like these discussions, though, so let's open up another: building envelope versus building enclosure. Most people I know use the former. Joe Lstiburek prefers the latter. In a paper on vocabulary, he began, "If we don’t call things by their right names we don’t really understand how things work." I agree. Language matters, and not just when it comes to sales.
Diving deep into definitions
But envelope versus enclosure isn't a slam dunk like the term 'control layer,' at least not in my mind. According to Merriam-Webster, an envelope is "something that envelops," and an enclosure is, "something that encloses." But an enclosure could also be "something enclosed," as a photograph in an envelope. That muddies the waters a bit.
Even muddier they get, though, when you consider that a fence around a pasture is an enclosure. It's completely open on the top. It'll keep the cows in but won't keep deer or gophers out. That sense of the word may work for a lot of existing homes, with their swiss cheese ceilings and floors, but I think most of us would prefer buildings to be completely enveloped by appropriate control layers.
At Building Science Summer Camp this year, I asked Joe about these words. He responded, "Envelopes are for Fedex. Enclosures are for engineers." In the vocabulary article, he wrote similarly, "They are building enclosures—they are not building envelopes. You put letters in an envelope not people." Neither comment explains why he prefers enclosure to envelope.
A bit of history
That photo of books at the top of this article shows that the guys who did a lot of the mid-twentieth century building science work in Canada preferred the term envelope. Those are older books at Joe's house. As I understand it, Professor Eric Burnett introduced the idea of enclosure as a replacement for envelope.
Martin Holladay, the Energy Nerd at Green Building Advisor and another stickler for using proper language, discussed this in a comment of his article on Green Building Vocabulary Disputes. (See comment #4.) There he quoted from a 2003 article he wrote when he was editor of Energy Design Update:
“Burnett proposes replacing the term ‘building envelope’ with ‘building enclosure.’ At the recent Building Science Symposium in Westford, Massachusetts, Burnett said, ‘The term “envelope” does not imply the inclusion of the below-grade enclosure. It comes from curtain-wall construction, and it connotes something thin.’
That seems like splitting hairs to me. It also ignores the basic definitions of the two words, as discussed above. An envelope envelops completely; an enclosure encloses but maybe not on all sides.
According to the December 2003 Energy Design Update, Burnett also proposed replacing weather resistive barrier (WRB) with the term, "“The exterior membrane to the exterior sheathing.” Uh, yeah. Let's replace one already-too-long term with an even longer one. I'm sure that's gonna happen. Any day now.
What are people saying?
According to my unscientific poll on our Facebook page, 10 out of 12 people prefer building envelope to building enclosure. One prefers "enclosure-envelope. Leave off the building, please." The other doesn't care what you call it as long as his pipes don't freeze. In conversations I have with other folks, most use the term envelope.
Then there are groups like the Building Enclosure Council. I'll be doing a workshop called How to Be a Control Freak with Building Enclosure Control Layers at the Raleigh, NC chapter in January and have adopted enclosure for that event. They've clearly taken a position, and now the schism has moved out of our discussions and is embedded into our industry's organizational infrastructure.
On the textbook front, Building Science for Building Enclosures, of course, takes Burnett's term, since he wrote it with John Straube. But Green Building: Principles and Practices in Residential Construction, an introductory level textbook by Carl Seville and Abe Kruger, sticks with building envelope.
I generally use the term building envelope. I've tried building enclosure a couple of times, and it makes me feel smarter, part of the elite crowd. It's like putting on a tuxedo when I say it. Sometimes it makes me want to smoke a pipe while wearing a tweed jacket with elbow patches. But I grew up more in a working class family, so I think I'll stick with building envelope* for the most part.
The adocates for building enclosure have made enough progress that I don't believe that term is going away. Those of us who still prefer building envelope are numerous enough to prevent it from being washed away. It seems that the two terms will have to coexist. Both are perfectly adequate, but the existence of two terms for the same thing will create unnecessary confusion. Such is life.
*If you prefer envelope to enclosure, you then must decide on which pronunciation you prefer: en-velope or ahn-velope. Saying the latter almost gets you back to the tweed jacket with elbow patches, but without the pipe. Since both of my parents died of lung cancer, I'll happily forgo the pipe.
Green Building Vocabulary Disputes by Martin Holladay at Green Building Advisor
Vocabulary by Joe Lstiburek at BuildingScience.com