There are a number of ways to build a wall. Just ask any two building professionals and chances are you’ll find yourself in a spirited debate. While opinions and new ideas abound, most will agree that designing the perfect wall for high performance and longevity comes with a set of best practices developed by Joe Lstiburek, PhD, P. Eng, ASRAE Fellow – Building Science Corporation, a.k.a. the “Father of Building Science.” Yet since that paper was published in 2010, there have been some code changes and technology updates. ECHOtape sat down with Building and Construction Expert Colby Swanson to discuss Building the Perfect Wall in 2016.
Risa Edelstein: What are the most critical elements when building the perfect wall?
Colby Swanson: Wall function is always the top priority, specifically: #1. Structure; #2. Moisture; #3. Air; and #4. Heat. From there, the simpler, the better. The fewer “layers” you have, the less potential there is for installation mistakes. Likewise, the fewer attachments, such as nails, you have connecting the layers you have, the better. Less is more!
Also, I always like to remember that improper installation will trump proper design and/or premium materials every time. Details matter.
So, what are the most common errors when building a wall?
CS: The most common error I see is Installers and site supervisors not understanding the critical details that link layers. Either they don’t understand how their piece fits into the bigger puzzle, or they don’t bother to think it through. After that, the second most common error is the lack of continuity within various layers.
I always hear the saying: “An air barrier is not a vapor barrier is not a vapor retarder.” What does that mean exactly? Can you clarify?
CS: Air leakage is FAR more likely to lead to building failure than vapor diffusion, so in all buildings an air barrier is highly recommended. On the other hand, Vapor Barriers/Retarder are usually not needed in a building, although there are some exceptions where a vapor “throttle” is necessary to slow down instances of high vapor drive.
When we talk about vapor barriers, all materials have a permeance rating which determines how easily or hard vapor can move through a material. When designing the perfect wall, one must take into account the vapor permeance of the materials used, such as cladding, brick, sheathing, insulation, house wrap, paint, etc.
When we talk about air barriers, it is less about a rating — how air-tight a material is — and more about the continuity of the material. Are the seams sealed? Are penetrations air-tight? Are fasteners air-tight? Proper air barriers are “continuous”.
So is there a different set of best practices for vapor barriers?
CS: Understand that vapor diffusion is much, much lower on the priority scale than bulk water and air leakage. Many folks wrongly believe that vapor diffusion thru materials moves LOTS of water. It doesn’t. Installing a vapor barrier, such as a polyethylene sheet, on the interior of a wall is a bad idea in most cases except cold climates like Zone 7 or 8. In these very cold climates it is imperative to manage interior humidity with spot ventilation.
You mentioned cladding… since rain is one of the most common external moisture issues, what is the best cladding combination to control rain?
CS: The best cladding system is one that is back-vented with a rain screen to decouple bulk water from the main building structure and enhance drying when system gets wet. When it comes to building the perfect wall, surfaces behind the cladding should be water resistant and shed ALL water down and out … this includes flashing at windows and doors, and through wall penetrations such as plumbing and electrical.
With the 2012 IECC and 2013 CEC changes in air barrier codes, how have the best practices in air barrier construction changed since Lstiburek’s 2010 paper?
CS: Due to code requiring building envelope air tightness testing, more and more “air barrier” products have been hitting the market, for example the seemingly endless supply of house wraps.
Is wood still the best option?
CS: That depends on so many different factors, one of which is region. I prefer to think about wood as one building material option and weight the pros and cons. The PROS being that wood is a rapidly renewable material. The embodied energy is significantly lower than steel and concrete. And from “earth friendly” standpoint, wood is better than steel/concrete. The CONS are that wood is susceptible to biological degradation of material, such as mold or rot. It’s susceptible to termites. You have to consider the expansion and contraction of the type of wood.
I believe that wood is the most “flexible” construction material and can last nearly indefinitely IF it’s kept dry and warm.
What about pre-fab walls and joints? How do best practices change to accommodate this newer technology?
CS: Off-site construction is very interesting right now, and there’s so much to talk about it. But for right now, I think it’s important to say that the weak link in pre-fab walls or panels is their joints, where air leakage can occur. Joints must be air-sealed to prevent premature failure. Joint treatment is best when redundancy is used to treat seams, gasket/sealant between panels, then tape/sealant on surface of panels.