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BS* + Beer

The BS* + Beer Show: All About Rainscreens

Building scientist John Straube explains the role of rainscreens in keeping walls dry

This episode of the BS* + Beer show features industry giant John Straube talking “All About Rainscreens.” There are so many variables involved in correctly detailing the air gap behind cladding, and John lays out many of them—including climate zone, siding type, substrate material, and, most importantly, objective. He addresses the questions: How much of a gap is needed for drainage? How does the potential for wildfire influence gap size? Is the expense of the assembly worthwhile or would that money be better spent elsewhere? What is a pressure-equalized rainscreen? How much of a concern is interior vapor diffusion? What is the role of a rainscreen in preventing paint from peeling off wood siding? Should a rainscreen ever be vented into an attic? Does the furring need to be treated wood? Suffice to say, it’s an information-rich episode (and full of some good chuckles too).

Enjoy the show!

 

Join us on September 17 from 6 to 7:30 p.m. ET for a conversation about “Flashing Details” with guests Doug Horgan, Mike Guertin, Aron Jones, and Bryan Uhler. The first rule of building science is keep bulk water out, which requires good flashing details. As simple as that sounds, if you’ve ever remodeled a house, you know that it’s rarely done as well as it should be. Hear from builders—you might say they’re the Big Dogs of the flashing game—who know how to do it right.

Bios

Doug Horgan is vice president of Best Practices at BOWA, the Washington, DC, area’s premier custom and remodeling builder. In this training, quality-monitoring, and troubleshooting role, Doug’s goal is to reduce construction defects through knowledge sharing. Doug’s 30 years of experience in the fields of carpentry, warranty-troubleshooting, and instruction, along with thousands of photos taken along the way, are the foundation for visually rich presentations on how to build well and avoid construction problems. Doug is a frequent contributor to The Journal of Light Construction/JLC, Pro Trade Craft, Fine Homebuilding, Remodeling, and Professional Remodeler, and he has presented at Remodeling, JLC Live, NESEA Building Energy, the National Home Performance conference, and local events. His credentials include BPI Building Analyst and LEED AP – Homes.

Mike Guertin is a builder, writer, and educator based in Rhode Island. He has 40 years of experience tackling every facet of residential construction—from excavation and foundations to interior finish and tilesetting. He shares his expertise regularly, writing articles and contributing to books, videos, and live events. His work has been featured in Fine Homebuilding, The Journal of Light Construction/JLC, and Professional Deck Builder magazines.

Aron Jones is a partner at Big Dog Construction on Grand Manan Island, N.B., Canada. Aron declared August 26, 2021, as the First Annual #nationalflashingawarenessday (and #internationalflashingawarenessday), which took social media by storm. Aron is passionate about teaching the next generation of apprentices (#apprenticeshipisanobligation), and considers himself a BS “nerd-in-training.” In his free time, Aron enjoys shooting clay pigeons.

Bryan Uhler is part of the second generation of Pioneer Builders, which builds custom, high-performance homes in and around Port Orchard, Wash. He is a contributor and brand ambassador for Fine Homebuilding magazine.

Use this link to register for The BS* + Beer Show

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Kiley Jacques is senior editor at Green Building Advisor. She can be reached at [email protected].

10 Comments

  1. Expert Member
    Malcolm Taylor | | #1

    Great episode.

    One thing that stuck me was John Straube seems to think that it wasn't worth modifying details to address pressure equalization, but rather it was a useful byproduct of building in good air-sealing.

    That'a a bit different than Christine Williamson's approach where she suggests modifying how you install windows (not foaming between the frames and surrounding opening) specifically to equalize the pressure.

    I'm curious where others stand on this (perhaps minor) issue.

    1. Expert Member
      Kohta Ueno | | #5

      Hey Malcolm! These two data points are actually consistent--the term we generally use is "pressure moderation," where the inner seal is airtight and the outer seal is leaky, to allow for this moderation effect.

      Straube's research (going back to his, um, PhD thesis I think?) showed that in full-wall rainscreen assemblies, due to the variations in wind pressures, you're never getting a "zero pressure difference" across the outer side. There's just too much surface/volume going on for that to work.

      But pressure moderation in joints, such as the window-to-wall joint, or precast panel joints, is a different creature, due to the relative size.

      Joe writes about these topics here:

      BSI-004: Drainage, Holes and Moderation
      https://www.buildingscience.com/documents/insights/bsi-004-drainage-holes-and-moderation

      This is the basis of the current ongoing Canadian Civil War between pressure equalization and ventilated and drained claddings. Old school engineers like me were taught by Old Masters like Handegord who pointed out that drainage was the key to life in general and pressure equalization—if it applied at all—was for joints and not for entire wall assemblies. Claddings were to be drained. Ventilated if you must, but first and foremost they had to be drained. Whereas joints in insulated glazing units, pre-manufactured windows both residential and commercial, joints in precast panels and connections between windows and wall assemblies could be pressure equalized but regardless they had to be drained. This view is not universally accepted—but I remind you dissenters that if you disagree with the Old Masters you do so at your own peril.

      1. Expert Member
        Malcolm Taylor | | #6

        Thanks Kohta, That's very helpful. At get risk of putting you in an awkward position: where you would stand on the utility of moderating pressure around windows in small residential installations vs being able to fully insulate these joists?

        1. Expert Member
          Kohta Ueno | | #7

          My first-cut preference is to leave the joint between the inner and outer seal empty... this is less about pressure moderation, but more about drainage. I.e., "when water gets in, can it just drop down and drain out or not?"

          Filling the entire cavity with low-expansion foam adds some R-value for that gap, but eliminates both the drainage and the pressure moderation effects of the air cavity. So that's not my preference... I've seen it done lots of times, but it is not my favorite.

          If somebody is all worried about the R-value of the gap, or if the rough opening-to-window gap is exceptionally wide, I would be okay with partially filling that cavity with mineral fiber or similar.

          1. Expert Member
            Malcolm Taylor | | #8

            My own preference is to reduce the size of the rough opening so that the gap is around 3/16". No gap to insulate reduces the problem and makes air-sealing at the interior face easier.

  2. John Woloshyn | | #2

    I haven’t been able to see this video......it shows as “unavailable”. Is anybody else having this issue?

    1. Expert Member
      Michael Maines | | #3

      John, you can watch it here: https://www.youtube.com/channel/UCvAmHSecglp-u-DTFQlU8Tw/videos

      1. John Woloshyn | | #4

        Perfect! Thanks Michael!

  3. Mark Brooks | | #9

    John mentioned the power of the sun to help drive the drying in the drainage/air gap with warm air rising. I'm thinking about the north side of the house which won't have direct sun. Would it be fair to say that having the drainage/air gap vent into the attic is advantageous in that it would promote more air flow on that north side, and thus increase the drying potential? Would that south side warm air rising through a well ventilated attic help "drag" air up and through the north side drainage/air gap as well?

    1. Expert Member
      Malcolm Taylor | | #10

      Mark,

      RDH did studies on rain-screen gaps here in the PNW and suggested that in this climate, if the two are connected, the amount of moisture transferred with the air from a rain-screen into the attic outweighed the benefit of increased air-flow.

      Now that might not be true everywhere, but I think in the absence of any data showing benefit to connecting them on the north side of a building, it's probably best not to.

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