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How to Prevent Your Worst Building Assembly Fears from Coming True

Expensive callbacks and lawsuits can result when you don’t attend to the assembly details.

Directional drying is designed into high-performance buildings, and all three control layers must continuously manage water, air, and heat. Note how the air barrier is primarily accomplished at the interior and how difficult it is to prevent thermal bridging at structural framing if exterior rigid insulation is not used.
Photo Credit: Steve Baczek Architect

We’ve all heard the nightmare scenarios: water leaks that mar the finest architectural features of a new building; air leaks that cause hidden mold or rot inside the walls; thermal bridges that compromise occupant comfort and energy performance.

Money on the line

These scenarios have two things in common: first, they could all land you in court. Second, they are all preventable if you’re giving each building assembly detail the time and attention it deserves.

During a recession, most firms are already working with knifeblade-thin margins, so it can be tempting to cut corners. While the “extra” work required to get the details right might seem expensive in the short term, it’s a good long-term investment.

In this month’s EBN feature article, Peter Yost and I take a look at how industry leaders are changing the way they practice architecture in response to the increasing complexity of—and increasing demands on—our buildings and building assemblies.

Energy demands put the pressure on

We all want energy-efficient buildings, but there are tradeoffs: when you decrease the energy flow through a building assembly, you have to be a lot more careful about every little air leak and thermal bridge; otherwise you risk moisture problems.

This challenge is even greater because more sophisticated, multilayered building systems may have properties we don’t know about or forget to check. For example, all building materials—not just the ones we call retarders or barriers—affect vapor movement to some degree; serious moisture problems can occur if the permeability of each assembly component is not accounted for in assembly design.

Tricks of the trade

In our article, we go through the four essential features needed to ensure good hygrothermal performance, and we’ve included lots of beautiful, detailed cross-sections from leading residential and commercial building science experts to give you ideas for solving common problems, such as:

Also Read

How "Smart" Vapor Retarders Work

Hidden Seam Failures? We Put Flashing Tapes to the Test

Sustainable Sealants: The Problem of Predicting Service Life

  • Continuing thermal, air, and bulk water barriers at the parapet of a low-slope commercial roof
  • Achieving thermal barrier continuity where a residential wall meets the slab
  • Allowing water drainage at a commercial foundation without compromising the thermal barrier
  • Continuing the air barrier at window heads and sills in a deep residential exterior wall
  • Creating a continuous air barrier at a residential eave
  • Minimizing thermal bridging through a commercial balcony assembly

Special building science issue

As a matter of fact, this whole issue of EBN is chock full of building science know-how and includes:

Upcoming report

I’ll just leave you with a little teaser: our whole editorial team is currently expanding on all this work in a new report on high-performance building assemblies. It’s due out just before the holidays—so put it on your wish list, and watch this space for more details soon!

Posted by Paula Melton on October 29, 2012


Wrong Direction?

The graphic, which seems to illustrate a cold-climate envelope, shows the primary wall drying direction to the inside. That means that most drying will occur in the summer months when envelope temperatures are well within the range for mold and decay organism growth. I prefer to allow drying to the exterior in the winter months when materials are too cold to mold, as well as to the interior in summer. But that requires higher permeance exterior layers and makes exterior rigid foam problematic.

The caption states "Note how the air barrier is primarily accomplished at the interior and how difficult it is to prevent thermal bridging at structural framing if exterior rigid insulation is not used."

I agree that the interior surface is the ideal location for a cold-climate air barrier, since its primary purpose is to prevent warm, moist interior air into the envelope where it will cool and condense, leading to potential moisture problems. But the increasingly predominant exterior foam or air-tight sheathing approaches ignore this.

And it's not difficult to eliminate thermal bridging with a double-frame system, such as double studs or a truss wall, and those systems also allow drying to the exterior if properly detailed, without the need to resort to petrochemical foam materials (with their very high global warming contribution).

Direction isn't as important as drying

As always Robert, thanks for your insights.

1. drying to the interior in cold climates: yes, this assembly is designed to do just that and it's important to mention that this summertime drying is enhanced by not having active air conditioning during the conventional cooling period. I don't have a strong preference for which direction an assembly is designed to dry; I have a strong preference that it be designed to dry in at least one direction. If we could get all builders and architects to do that, I would be really happy.

2. location of air barrier(s) - we get this question all the time, as I am sure you do too: which location is better, exterior or interior for the air barrier. And again: one, regardless of location, is better than none, and two (exterior and interior) is ideal. I think that given the choice, I prefer an exterior air barrier because it is an easier location to get continuity and it does a better job of preventing wind washing at corners, but I also agree with your point: an interior air barrier does the lion's share of keeping interior wintertime moisture out the exterior assemblies.

3. spray foam blowing agents: a very hard one to argue. Closed cell spray foam will hopefully soon have a blowing agent other than the high global warming potential HFC 245 fa, but until then, you pay a big penalty right out of the gate. And with no vapor retarder, great care must be taken in balancing reduced thickness of the spray foam in comparison to the depth of the air and vapor permeable cavity fill, in cold climates (the first condensing surface of this assembly is the interior surface of the spray foam).

Best  - Peter

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