ASHRAE Journal - October 2009 - 65

Building Sciences Taming the Terminology By Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAE W hat we call things tells us a great deal about how much we understand them. If we don’t call things by their right names we don’t really understand how they work.* If we don’t understand how things work, how can we prevent problems from happening? Or, how can we fix problems when they occur? How can we possibly make things work better? We are all guilty of “bad-name-itus,” but the model building codes take the cake.† They are examples of bad wording and misunderstandings. We mostly put up with it because they are the law of the land and, therefore, scary. In one chapter of the International Residential Code, the same enclosure element is called three different things, none of which are defined. Why this occurs, particularly in the model building codes, is unsurprising if you understand how they are made, modified and adopted. That the codes work as well as they do is attributable more to the good will of the code officials‡ than the brilliance of their terminology. I will attempt to do something completely arbitrary, unilateral, and annoying to others because we must start somewhere to clear up the mess. I will attempt to straighten out the terminology and try to get everyone to agree.§ Each term needs to be defined with a performance metric. This is not easy, but it’s necessary. Here goes. They are building enclosures, not building envelopes. You place letters in an envelope, not people. Building enclosures need four principal control layers: a water control layer, an air control layer, a vapor control layer and a thermal control layer. These control layers can be combined in one material or be separate. For example, a liquid-applied water control layer can also be the assembly air control layer and vapor control layer (Figure 1). In some assemblies closed cell high-density (2 lb/ ft3 [9.76 kg/m3]) spray polyurethane foam can be the assembly water control layer, air control layer, vapor control layer and also thermal control layer (Figure 2). In a typical residential example, shingle lapped asphalt saturated kraft paper or asphalt impregnated felt is the assembly water control layer. The fiberglass batt cavity insulation is the assembly thermal control layer. The glued and gasketed gypsum board is the assembly air control layer and the paint on the interior surface of the gypsum board is the assembly vapor control layer (Figure 3). Stop with the water resistive barrier, weather resistive barrier, house wrap, building wrap and building paper. Enough already. What is this layer supposed to do? Control the water. It is a water control layer. End of story. Where did weather come from anyway? What part of the weather are we resisting? We don’t need the word “resistive.” It serves no purpose except to make sentences longer. And, spare me from “wrap.” What are we doing? Wrapping Figure 1: Institutional Wall. The wall has a combined water control layer, air control layer and vapor control layer in a single material. The thermal control layer is located exterior to the other control layers. * Thank you, Jim White. You first beat this into me in 1982 in your office at the Canada Mortgage & Housing Corporation in Ottawa where you were one of the big dogs of Canadian housing research. It hit home, and this terminology mess has irritated me ever since. I was just a kid in 1982, and you had time for me. Thanks for that. You never forget stuff like that, and it taught me to always have time for others, especially the kids in our business. † ASTM is another shining example. WRB means “water resistant barrier” in one committee and “weather resistant barrier” in another. ‡ That’s not a typo, I really mean “code officials.” Do you have any idea how difficult it is to administer and enforce a less than perfect document? One which everyone has an opinion on because they saw something on “This Old House” or because they stayed in a Holiday Inn Express once. § Agree with me because only I am right. And, I am the right guy to do this because I am such a famous guy: the best known building scientist in Westford, Mass., and the second best known building scientist in my firm. Also, a few folks know me at the lumberyard. October 2009 ASHRAE Journal

ASHRAE Journal - October 2009

Table of Contents for the Digital Edition of ASHRAE Journal - October 2009