The Perfect Wall, Roof, and Slab — Building Science Podcast

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The Perfect Wall, Roof, and Slab — Building Science Podcast

Dr. Joe Lstiburek talks about enclosure design principles for energy-efficient houses. Insulation, air conditioning, heating, and forced-air delivery systems have all changed the physics of how houses work. Houses didn’t used to rot, but too many of them do now.

Posted on May 6 2009 by Joe Lstiburek, GBA Advisor

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This podcast series is excerpted from a two-day class called Building Science Fundamentals with Drs. Joe Lstiburek and John Straube of Building Science Corporation. For information on attending a live class, go to BuildingScienceseminars.com This week Dr. Joe talks about enclosure design principles of energy efficient buildings


Let’s start with smart things

The building enclosure has four functions. In order of importance, they are:
1. Rain control
2. Air control
3. Vapor control
4. Thermal control

Thermal control is the easiest to specify, calculate, and measure, so that’s what codes focus on. Codes typically ignore the most important layers because they’re the most difficult to specify.

The vapor control layer is easier to specify than the air control layer, so codes obsess over specifying the vapor control layer and ignoring the air control layer.


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The most important factors are often not considered in design, construction, and regulation; and the unimportant ones tend to have an overly enthusiastic and detailed amount of specs associated with them.

The Perfect Wall has all of the structure to the interior and all of the control functions to the exterior. Let’s start at the outside of the perfect wall with the claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. .

Cladding provides three functions:
1. Aesthetics,
2. Protection from UV light
3. Physical, mechanical protection of the other control layers.

Aesthetics matter because people don’t take care of ugly things. Ugliness is not sustainable. The longer something is around, the more resources it consumes, so the more resource efficient it is, and the fewer resources it uses over its lifetime. We want a beautiful building that lasts a long time and is ultra-efficient.

Claddings should be completely open — we want air circulation behind the cladding system. The more air circulation, the better the system works. Sealants are purely aesthetic, they’re not functional. If the sealants fail, the primary air, thermal, vapor and rain control elements are not affected.

If we take the perfect wall and lean it, we get the perfect roof. From the inside to the outside, the control layers are:

  • Structure
  • Vapor control membrane
  • Insulation
  • Cladding

Some of the old-timers will recognize this type of roof as an IRMA — Inverted Roof Membrane system. If you replace the ballast with dirt, grass, and a goat, you would get a green roofRoof system in which living plants are maintained in a growing medium using a membrane and drainage system. Green roofs can reduce storm-water runoff, moderate temperatures in and around the building (by providing insulation and reducing heat island effect), as well as provide a habitat for wildlife and recreational space for humans. When properly constructed, green roofs can increase roof durability because the roof assembly’s air and water barriers are buffered from temperature fluctuations and UV exposure.. (That was a joke.)

Flip the roof and you get the perfect slab:

  • Dirt and stones
  • Insulation
  • Vapor
  • Concrete (structure)

The physics of a foundation, wall and roof are the same (this is an Ah-Ha! moment). When we look at a section of the perfect roof, wall, and slab, and we get the other Ah-Ha! moment — the important parts are the corners. You have to connect the rain control element of the foundation to the rain control element of the walls, the air control element of the foundation to the air control element of the walls, the vapor control element of the foundation to the vapor control element of the walls, the thermal control element of the foundation to the thermal control element of the walls… Pretty fundamental stuff.

Most failures occur where roofs connect to walls
Tip: Buy multi-colored pens
Whenever we do design reviews in our office, we tell the youngsters to take a colored pen and trace the rain control layer around the building enclosure. If the pen has to leave the paper, they’ve identified a discontinuity that needs to be addressed. Use a different colored pen for each of the control layers. Whenever the pen leaves the paper, you’ve identified a flaw. It’s as simple as that. We find that the flaws are concentrated at the connecting elements.

Windows complicate the perfect wall
Now these are pretty easy, but it gets complicated. In the real world, someone pokes a hole in the building and we call that a window. Windows have to do everything that a wall does, and more. It has to control water, air, heat, and vapor; you want to be able to see through it, and every so often someone is going to want to open it too. Windows can actually do all of that stuff, which is pretty amazing. No wonder they’re so expensive. All we have to do is connect the rain control element of the window to the rain control element of the wall, the air control element of the window to the air control element of the wall, the vapor control element of the window to the vapor control element of the wall, the thermal control element of the window to the thermal control element of the wall.

The reason we’ve been having so much trouble with window-to-wall connections is because we’ve been relying on one person to do all of this: His name is "By-Others." Mr. By-Others shows up on all of these specs and you have to make sure he is not going to be responsible for all of these connections. Someone has to be responsible. The window industry doesn’t do us any favors either — they don’t tell us in their window system which part of these windows systems are responsible for controlling water, air, vapor and heat. In the absence of guidance, we have to assume that the innermost component of the window is where all four of those functions collapse. So we wrap the window openings and make the connection at the back — so that if the window should fail, the water will go to the outside.

That’s how you design a building: water continuity, air continuity, vapor continuity, thermal continuity. It can’t be that simple, right?

Well, the answer is, “Yes it is.”


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