Insights from John Straube: Ensuring High Quality from Design to Construction
We interviewed John Straube, PhD, P.Eng, principal at RDH Building Science and professor at the University of Waterloo, to get his insights on a topic that’s critical to the building industry: ensuring the highest quality. In particular, we discussed product substitutions and why they need to be handled with care.
Q: There’s no doubt, in your career you’ve seen your share of building failures and successes. Thinking back, what stands out to you about comparing quality? What makes a critical difference?
A: Well, a few things spring to mind. High performance can be thought of in three parts:
- Design
- Products
- Installation
Too often there is a focus just on workmanship, but using top-quality products is also a critical factor. Selecting high-grade products is a prerequisite, but a superior design means you need to know what you do with those products. If you’re a designer or a specifier, you should be asking:
- Do you understand the materials you’re choosing?
- Do you know why you’re choosing them?
- Have you matched the materials’ characteristics with the location you’ve placed them in?
Hear John explain the risky business of substitutions or using alternatives without fully understanding the material’s technical background.
Q: As you mention, product substitutions are quite common. Should we be trying to slow down and have fewer substitutions?
A: It’s not that I’d say you shouldn’t substitute; we should always be doing our job and asking, “is there a better product, or the same product at a lower cost or easier availability to the contractor?” But we have to know the products involved and think carefully, because substitutions made on the fly routinely end up with a problem.
The reason we get into trouble is that people are substituting product A for product B without a full understanding of the characteristics and performance levels that led to the original choice of product A. You must know what the critical performance attributes are for your particular project, considering the different control layers in the assembly and how they work together, the external and internal environmental conditions you’re dealing with – all the knowledge that goes into good building enclosure design.
Q: Can you address the confusion between air, water, and vapor barriers?
A: Vapor permeance is a very clear performance attribute with an easy-to-understand and definitive measure of performance: perms. Just swapping a permeable membrane for one that is vapor impermeable without careful evaluation of that decision can make a wall or roof fail. If you’ve designed your roof or your wall to be able to dry through a membrane, and then it can’t because a vapor barrier was substituted…it seems like it should be obvious that that can become a big problem.
In fact, this may be one of the most common product substitution problems in modern construction. The advent of high-performance “peel-and-stick” vapor permeable air and water barriers has enabled enclosure designs with the membrane outside of some or all of the insulation. Contractors are still often only familiar with the older vapor impermeable peel-and-sticks and hence can wrongly substitute one of those. The result: trapped moisture during cold weather that damages sheathing, framing, and fasteners.
John continues with the difference between barrier types and what you need to know to avoid negative performance.
Q: You mentioned product compatibility as a concern. Can you expand on this thought?
A: The point is that an enclosure is a system, so a designer may specify a particular membrane for how it works in the overall assembly, including how it works with other specified products. For example, if we’re talking here about things like differences in adhesiveness to the substrate—maybe a specified sheathing membrane, let’s call it Product A, sticks well to the OSB that’s being used on a particular project. A proposed Product B substitution may not stick as well. Or maybe you have a WRB that’s known to be compatible with the sealant being used around the windows — Product B may not be compatible with that sealant. Another example occurs if there are accessories available to handle penetrations at pipes, balcony corners, air conditioning ducts, and so on – and when you switch to product B, you now don’t have those accessories. Any number of things can influence the choice of a product.
One category of characteristics that can be overlooked are the conditions during construction. This becomes an issue when people substitute a fluid-applied for a preformed membrane product. They go “Oh, with fluid applied I don’t have to worry about lapping seams and all that, I just smoosh it on.” But in some climates, it seems easy, but suddenly you’ve created a whole problem of being more weather sensitive. In colder climates, or rainy climates, a preformed sheet membrane may have been chosen exactly because it has much less weather sensitivity during installation, and substituting a fluid-applied product, even one with the same or superior water hold-out or airtightness, etc., means that now you have to wait until the weather allows you to install it, and that doesn’t always align with the construction schedule.
Q: Are there any key points in the design and construction process where knowing material characteristics is especially critical?
A: Substitutions most commonly happen in shop drawing review, early construction, or just before construction – the contractor/sub-trade is submitting drawings for the designer’s review, and they submit an alternate. And usually it’s the architect, but sometimes it’s the general contractor making the decision about the equivalency of the products. If you are making that decision, it’s not good enough to say “well, they’re both insulation and the R-value looks the same” or “well, they’re both water-resistive barriers” or “they’re both sealants.” Those are just broad categories. There was a reason why a particular insulation, WRB, air barrier, or sealant was chosen.
Except, of course, when there wasn’t a reason. Which brings me to the problem of “cut-and-paste specs,” where a product was used on a past project and it gets used again, just pasted into the specs, even though the design of the building is different or it’s in a different climate zone, or new and far superior products exist. In that case, a substitution may be a very good idea.
Either way, wherever the question of material choice arises, the person making the decision has to know what function the material serves and what material properties are needed to serve that function. If you don’t know, find out.
Q: Good advice. Any closing comments about superior building design and construction?
A: There are no shortcuts in life, or in building science. If you want a building to perform well, you have to know what you’re doing. Know your climate, know what components in your design are controlling water, air, vapor, and heat. Choose materials that can actually perform the function you’ve assigned to them. And talk to other people—on your design team, on site—to make sure you are all on the same page.
