Building Enclosure Blog

Building Envelope vs Building Enclosure.

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Summary: The terms’ building envelope’ and ‘building enclosure’ are being used interchangeably in the building design and construction industry. This post gives a short overview of the history of these terms, differences, and why ‘building enclosure’ appears to emerge as the preferred term. 

Introduction

You are reading the ‘building enclosure’ blog, so it should come as no surprise that I call the thing surrounding the inside of a building the building enclosure. You might call it something else (skin, wrapper, facade, shell), but likely the building envelope. The discussion regarding whether to use ‘envelope’ or ‘enclosure’ has been ongoing for several years, and I suspect it will not stop soon. This post looks at the history of this discussion and the current state of the ‘envelope’ vs. ‘enclosure’ debate.

What does it matter?!

Those outside our industry might find it strange that highly educated and intelligent (presumably) individuals would debate (argue) about which term to use. Those familiar with the industry are surprised that there are not more of these debates. 

At the very least, we should ask ourselves, does it matter whether we use one vs. the other? Yes, it does, and it seems that people much smarter than myself have the same position.

While there is a tendency in Architecture to make up words to convey some concept or idea, we need to be more careful about the words we use when it comes to Building Science. This article does a good job further elaborating on this point. So, can we agree that accuracy matters? Excellent.

How did we get here?

While trying to get to the bottom of this question, I found articles addressing this issue dating from 2006, 2012, 2017. 2006! That is quite a while, considering that many of the readers of this blog were barely walking in 2006! 

Since I was not around either when North American Building Science was born, I would appreciate it if somebody can better explain why the term “Building Envelope” was originally used. (Email me or comment below, either is fine).

I’m assuming they used ‘envelope’ since it adequately described the ‘thing’ that contains or ‘envelops’ the inside of a building, and building ‘wrapper’ or building ‘container’ just didn’t sound right. So, the founding fathers and mothers of the fledgling discipline decided on ‘envelope,’ and as the definition suggests, it made a lot of sense: 

Envelope: 

In short, the word ‘envelope’ communicates the idea of something fully wrapped up, folded in, or contained in something else. 

While I’ve heard the term ‘building wrapper’ used before, I agree that ‘building envelope’ is a better choice. Perhaps, in the future, we should consider using building integument, especially when discussing green architecture, biophilic architecture, and such…

Historically, and understandably so, it seems that ‘envelope’ became the predominant term referring to the part of a building that contains the inside and separates the outside from the inside.

In his 2012 article, Bailes makes a similar point when he says, “…a lot of the mid-twentieth century building science work in Canada preferred the term envelope“. Architectural Details for Insulated Buildings (1990), the $890.00 ‘Canadian’ book we previously reviewed, also uses ‘Envelope’ as the standard term when referring to the enclosure.  

So, in the years following the birth of the field, the use of ‘envelope’ became widespread and remained so. (see graph presented later in the post.)

So why the objections?

Excellent question.

I was looking hard for the single convincing reason why one would prefer enclosure over envelope and could not find one. (If you know why envelope is a terrible, no good, awful, very bad, will get you in trouble, sued, or worse, laughed at, word, please let me know so that I can share it with our readers in a follow-up post).  

In the 2006 article, John Straube notes, “The term building enclosure is preferred to the term building envelope largely because it is considered both more general and more precise.” (Now, I’m not known as the sharpest tool in the exterior garden enclosure, but that was not as helpful as I had hoped it would be).

He further states, “… note that the building enclosure may contain, but is not the same as, the so-called thermal envelope, a term that is used to refer to the thermal insulation within the enclosure”. (Now that made much more sense, and I agree – more on that later, see my ICC cop-out comment at the end of the post).

Another criticism of the term states that “…the term ‘envelope’ does not imply the inclusion of the below-grade enclosure. It comes from curtain-wall construction, and it connotes something thin…”. I can see how a 36″ concrete slab might not appear very ‘envelopy,’ but I’m still not convinced.

Lastly, we have experts in the field noting: “They are building enclosures—they are not building envelopes. You put letters in an envelope, not people.” Again, I understand why he makes that point, but I want more darn it, give me more! (Side note: This is a good article, so consider taking some time to read it especially if you like learning things).

So again, if you have any clear reason why ‘envelope’ is a really, really bad word, please let me know. I might even put it on a t-shirt.

What is an enclosure?

 Moving on to the definition of ‘enclosure,’ we find the following:

My first frustration with the term ‘enclosure’ (apart from all the chicken coops, bullpens, and gerbil shelters the search engines return) is the impression that enclosures do not need to ‘fully’ enclose something to be considered an enclosure. 

However, putting that small issue aside, ‘enclosure’ seems to work well in generally defining an environmental separation; and to John Straube’s credit, it is both more general and more precise.

More general, since it can refer to the environmental separator between the interior and exterior environments and also to the separator between various interior environments. More specifically, enclosure refers to a generic category of wrappers, while envelope appears to communicate thinness, as noted above. I guess he was right.

So why do we prefer ‘enclosure’? Let’s say it is the generic specificity or specific genericity (this word actually exists!) of the term ‘enclosure’ that convinced us. That and the fact that many other people agree:

Why we are convinced that enclosure is the right word

While the list is partially offered for your reading pleasure (and to increase our search engine rankings by providing high quality, yet contextually selected links ), it does demonstrate a key consideration for the use of ‘enclosure’ over ‘envelope’. The industry as a whole seems to move towards the use of ‘enclosure’.

Don’t believe me?

Look at this unedited screenshot of a fancy chart. (Thank you for Sheldon for making me aware of the Google NGram tool found here https://books.google.com/ngrams/). The graphic below clearly shows how ‘building enclosure’ and ‘building envelope’ phrases have occurred in a corpus of books over the selected years (or something like that – It’s Google, so who knows what it means…). 

Google NGram tool

From this undeniable graph, it is clear that ‘envelope’ is trending down while ‘enclosure’ is trending up. 

As my financial advisor always says, it is good to put all your eggs in the basket attached to the arrow that points up. And we agree. I rest my case.

Where does that leave us?

If, after all that I’ve said above, you still decide to use envelope, remember what I said before. You are among friends, and we don’t judge (too harshly). However, have your reasons for selecting one over the other.

To wrap this up, there remains nothing else to do other than to provide two of my favorite definitions of ‘building enclosure’. I’m hoping to write a future post digging into these definitions more, but for now, this is it:

Building enclosure: The system or assembly of components that provides environmental separation between the conditioned space and the exterior environment. Note: The enclosure is a special type of environmental separator. Environmental separators also exist within buildings as dividers between spaces with different environmental conditions. (buildingscience.com)

Building enclosure: The portion of a building that provides a continuous environmental separation between interior space and the exterior environment. A building enclosure comprises a coordinated set of construction assemblies and the connections between those assemblies in order to protect occupants and their property from the harmful and uncomfortable effects of local climate and other ambient conditions. Control of the passage of heat, air, and moisture through the building enclosure determines the critical performance requirements for building enclosure design. We, therefore, refer to control layers in describing assemblies that comprise the building enclosure. (AIA Building Performance Knowledge Community (BPKC))

Last comment for future discussion. After I wrote this post, I wandered over to the ICC website and took a look at the incidence of envelope vs. enclosure in the new 2021 IECC and IBC. Interesting…

This is it for now. Please comment below if you have something to say. We might even add your comment on the back of an envelope as we think about ways to push the envelope without stuffing the envelope (too soon?).

References, credits and, articles for further reading.

Usually, I try and provide a link of all the references, but since I linked to them in the text, (some more than once) please follow those links for further reading.

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