The give-and-take of high tech wall enclosures
By Rick Reynolds
While walking with a neighbor along an old stone wall separating their properties, Robert Frost, in his poem, “Mending Wall,” ponders:
“Before I built a wall, I’d ask to know what I was walling in or walling out.”
That’s a reasonable question. After all, that’s the way we think of walls. Unless you’re Humpty Dumpty, you’re either on one side of a wall or the other.
When it comes to our buildings, however, high tech walls are not so simple.
Now swap out Robert Frost with air molecules, and then imagine frosty/warm temperature differentials on either side of the wall, and you’re entering the building science of high tech, panelized, wood-frame, wall fabrication.
In Chinese philosophy, “yin yang” imagines how seemingly opposite or contrary forces may, in fact, be complementary, interconnected and interdependent – even giving rise to one another as they interrelate. That’s a lot of “inters,” but without getting too technical, inter-wall thermo-dynamics and Open-Built® walls are what this post is about.
Bensonwood began manufacturing proprietary wall panels in 2000. Over the past seventeen years we have refined these panels into the OB PlusWall®, an R35 wall that is designed to manage water vapor, minimize thermal bridging and ensure a tight thermal envelope. For all its dynamic properties, BuildingGreen.com named the OB PlusWall one of the top ten green building products of 2010.
These standardized, dense-pack cellulose-insulated structural panels come in either 12-3/4-inch or 8-3/4-inch thicknesses and are sealed with gaskets and specialized tapes at every joint. More recently, we’ve developed a variation of the OB PlusWall using Steico wood fibre insulation boards to further reduce thermal bridging and better manage water vapor in the wall assembly. Aside from being made from a renewable resource, the Steico boards are vapor-permeable. As with the dense-pack cellulose insulation, their ability to both accept and release water vapor is key to the dynamic functionality of our walls.
So how do these walls actually handle water vapor? Despite the considerable air sealing details, weatherproofing and rain-screening properties of our walls, all walls, including ours, do allow the building to “breathe” some air (containing water vapor) by means of air diffusion through the wall assembly. We mitigate this air movement by meticulously taping and sealing the envelope. As diligent as we are, there is some microscopic movement of air through this air barrier, and thus the wall at large, driven by thermal and vapor pressure differentials on either side of the wall.
In poorly insulated walls with free air cavities, outside and inside temperature differentials can move that air around within the wall itself setting up a situation known as “convective looping.” When this happens, colder air descends and warmer air rises in a looping motion. Convective looping can reduce effective R-value significantly, and if the exterior sheathing is cold enough, can create a condensation situation within the wall cavity. Convective looping does not occur in our walls due to the dense-pack cellulose preventing free air movement.
Dewpoint calculations indicate that condensation occurs within the wall cavity at a location, known as the “preferential nucleation point.” When it is cold outside, this location is the backside of the sheathing, and the more moisture (water vapor) the air in the wall contains, the sooner condensation will happen. In order to condense, moisture needs a non-gaseous surface to make the transition from a vapor to a liquid. In effect, the cooler backside of the sheathing is that surface, acting as the dehumidifier for the wall cavity. But, when insulated sheathing is used (like Steico) the backside of that sheathing is warmer, and thus the temperature is kept above the dew point and the wall stays dry.
As a double layer of protection against wall condensation, we use cellulose as our standard insulation because it has the ability to absorb some of the moisture from the air in the wall without becoming “wet,” and thus it inherently lowers the risk of condensation. The moisture the cellulose “soaks” up during the cold seasons, is stored in the cellulose fibres and released during the warmer seasons as the insulated Steico sheathing is vapor open and allows the moisture to, in effect, be “exhaled” from the wall cavity.
Some have asked why we use dense-pack cellulose insulation in our walls, thinking that the material, made from recycled newspapers, would absorb and hold water, leading to mold. Actually, quite the opposite occurs. First off, dense-pack cellulose is just that: it’s blown in under pressure, which eliminates gaps and virtually eliminates air movement. Beyond that, as previously mentioned, dense-pack cellulose has the ability to absorb and release water vapor, unlike other insulating materials (e.g. fiberglass and foam) which can hold moisture and trap it against the surfaces that contain it, potentially leading to both mold and rot.
When this give and take, “yin-yang” property of walls is ignored, and the naturally occurring moisture condenses into trapped water leading to rot and mold, unlike Humpty Dumpty, “…all the king’s horses and all the king’s men, can’t put your walls back together again.” So the health, insulative properties, and ultimately, the durability of walls is directly related to their ability to handle moisture.
Bensonwood’s best walls are designed with Passive house and net-zero goals in mind. But despite their ultimate application, thanks to our panel technology, a typical Bensonwood home is over 50% more energy efficient than a home built to current code and over 80% more efficient than the average American home.
“Something there is that doesn’t love a wall”
Last but not least, Frost called his poem “Mending Wall” because his old stone wall had collapsed in parts and restoring it would have required great effort. Bensonwood’s “disentangled” Open-Built® wall, on the other hand, anticipates change so you can avoid alterations due to future lifestyle choices. With the OB PlusWall approach, the wiring and plumbing run through accessible channels, or chases, so they do not compromise the structure and insulation of the home. Beyond this, their disentanglement from mechanicals can prevent having to mend walls due to inevitable modifications in living preferences and evolving technologies. Simple changes in wiring and plumbing can be made without calling in carpenters, electricians, plumbers and painters to excavate, fix and mend the wall. What’s not to love about that?
In conclusion, metaphorically speaking at least, no wall is impermeable: not the Great Wall of China, not the Berlin Wall, not the Trump Wall and not the walls of our buildings. To varying degrees, walls, no matter how tight they are, are always breached by the laws of thermodynamics and diffusion. To ensure their longevity in our buildings, however, we should always ask what we’re walling in or walling out, and then build them accordingly.
PS – There is more stone in the walls of New England than in the Pyramids of Giza. (From Wessels’ Reading the Forested Landscape)