Buildings fail. It's a harsh reality, but they do, and for many different reasons. Two pretty common ways are by falling down, and by making the people in them sick. There is a long list of other reasons, but this post is focused on these two.

I am going to share how we are preventing these failures in all the homes we're building, by using a well-known "perfect wall" concept for the design of the walls, roof and floor, to create an uninterrupted perfect enclosure. I will illustrate how we achieve the perfect enclosure, by building homes on a raised foundation (concrete, masonry, or driven piles are all good options), so we can literally wrap the entire structure, every wall, roof and floor surface, with identical and continuous control layers ((The four control layers are Water Control, Air Control, Vapor Control and Thermal Control. More on control layers further down in the post)), giving us continuity everywhere in the enclosure, especially at corners, one of its weakest links.

Tiny House Continuous Building Enclosure

Why Buildings Fall Down

In grad school, we read a book on why buildings fall down. It focused on the structure, the common failures causing buildings to fall, and how to prevent them. Essentially, if a structure can not hold up its own weight, along with everything inside it, it is likely to fail. That's not OK. To help prevent this, we turn to our structural engineer, Brendan Crowley, who prefers a belts-and-suspenders approach to help ensure this doesn't happen. Just like my step-father taught me!

Another reason buildings fall down, which has nothing to do with Brendan and the sizing of beams or columns. It's a think called water, in all it's many forms. Whether it's bulk water or water vapor, if it's not controlled it can get things wet that shouldn't be. This causes decay, which causes failure.

Why Buildings Make People Sick((Buildings don't actually "make people sick", but I'm sure you understand what I mean. Something withing the building enclosure, is causing the people inside it to get sick. Mold, mildew, etc.))

People need to breath, but buildings don't. ((To be clear, the enclosure of a building needs to dry out, but that's not the same as "breathing")) There's no disputing that. In fact, we want the buildings to "breath" as little as possible. ((Yes, and provide adequate fresh air ventilation)) By this, I mean we do not want the building enclosure to allow unwanted air in, and desirable air out. As a mentor of mine, Joe Lstiburek, Ph.D., famously says, "The perfect wall is an environmental separator - It keeps the outside out, and the inside in."

The Bottom Line

When the bad stuff that's in the outside air gets inside, we breath it, and it can make us sick. When water gets in, either because it's vapor that's forced in by air, or it's bulk water that finds it's way in through cracks, gaps or holes in the enclosure, things can get wet, and cause nastiness((This is a highly technical term, for which I'm not going to take the time to define. But, for the purposes of this discussion, let's just call it mold, mildew, and other growies that can make us sick if we breath them in)) to grow on within the enclosure. If/when we breath any of that, it can also make us sick.

As much as I'd enjoy it, I will not be launching in to a discussion on Indoor Air Quality (IAQ), this time. Instead, I will focus on keeping the outside out and the inside in, which you can read even more about in Dr. Joe's document, BSI-001: The Perfect Wall.

Also, if you feel you must geek out on IAQ before reading further in this post, read anything that Max Sherman, Ph.D. has to say about the subject. He knows. OR, "Radio Joe" Hughes, and Cliff "Z-man" Zlotnik cover lots of IAQ related topics on their IAQ Radio show, with great guests. Very geeky and interesting stuff. Their most recent guest happened to be Dr. Sherman

The perfect wall is an environmental separator

To be effective as an environmental separator, the Perfect Wall controls Water, Air, Vapor and Heat, and in that order of importance. In other words, it has these four control layers (for more on control layers, go here and here), that are assembled in a way that keeps the outside out and the inside in...perfectly. All the layers are in the "right" place. Figure 1 illustrates the concept, which I will elaborate on later in the post.

perfect wall control layers

The climate conditions on the outside, and inside of the house, determine the "perfect" formula for the enclosures individual components, and as a whole. For example, the vapor control layer is not going to be exactly the same for every house. In fact, until recently, most perfect wall assemblies included a true vapor barrier ((Class I, less that 0.1 perm)) as the vapor, air and water control layer. In his most recent document on the topic, BSI-091: Flow-Through Assemblies, Dr. Joe explains that vapor open membranes, Class II and III((For more on Vapor Retarder Classification, read this document from Department of Energy)) are just as appropriate for this application as vapor closed.

So, how do we know what is "right" for our climate zone? ((This is a good resource for determining your climate zone)) Insulation is the other variable. It not only controls comfort, but it can have a very significant impact on controlling water, in any form. For example, not enough continuous insulation on the exterior of the structure, can make ideal conditions for condensation on the sheathing layer. Too much continuous insulation on the outside, can make it difficult or costly to install cladding, as well as create conditions in the wall that allow interior moisture to build up on the sheathing layer. Finding the balance is where our work is cut out for us.

A Tiny House Example

One of the tools we use to determine what assembly is appropriate, in terms of performance, for the walls, roof and floor, is to use a modeling software, called WUFI Plus. We use it for heat and moisture simulation (hygrothermal analysis), as well as simulating the indoor environment to model comfort and energy consumption for each home.

Below are the results of the hygrothermal (drying potential) analysis of four (4) different wall assembly options for the High Performance Tiny House, which is in central Florida, or Climate Zone 2.

WUFI Tiny House Wall Assembly
WUFI Results for Tiny House, Relative Humidity

We ran many different simulations, but the results above show us what happens to the relative humidity in the assembly when we change the thickness of the continuous insulation layer, from 1" up to 3". Option 1 (blue) is the 1", and Option 4 (green) is the 3". The orange line is the threshold of water content. The air and vapor control layer, which includes the OSB sheathing ((ZIP System sheathing is an OSB sheathing with an integrated vapor retarder. Together, they also act as the air control layer)), varies depending on temperature and relative humidity. It's permeability ranges from 12-16 perms.

The simulations indicated that overall, the exterior side of the sheathing layer will experience higher humidity levels than the interior side of the sheathing. All options demonstrated appropriate humidity levels. These levels did not pass the threshold on the interior side of the sheathing layer; and only slightly surpassed the level on the exterior side for short periods of time.

Overall, the high levels of insulation on the exterior are allowing for conditions to remain within a reasonable threshold. The levels noted are not indicative of potential risk due to their short duration and high drying rate. This is due to the high vapor permeance of the insulation materials which promote drying to both the interior and exterior. In the occurrence of water penetration, the high permeance and hydrophobic characteristics of the insulation will limit the potential to hold moisture.

The results are based on a very tight building enclosure, and since vapor brought in through air leakage, and rain water brought in through the gaps and cracks are a key contributor to the results in these calculations, we must ensure that the enclosure is perfectly sealed.

Below are section details at the floor-to-wall, and wall-to-roof connection of the Tiny House, to illustrate how the perfect wall assembly literally turns the corner. The amount of exterior insulation will vary, for sure, in the projects we're designing and building in Climate Zones 3 and 4. As for the other components, we'll let the budget, WUFI and the architecture ((Sometimes, the the levels of insulation can have a dramatic impact on the architecture of a home. We may want more reveal at the windows, so we may push for more exterior insulation. But, we must make sure the rest of the assembly works with it for optimal performance)) help us decide that!

zip system roxul perfect wall and floor
Floor-to-Wall Detail, High Performance Tiny House
zip system roxul perfect wall and roof
Wall-to-Roof Detail, High Performance Tiny House

That's all for now. In future posts, we will discuss the mechanical systems, high performance windows, and other components of all the homes we're building with the perfect enclosure.

LG Squared would like to thank the following research and development teams for their participation and contribution to the perfect enclosure of the High Performance Tiny House. Together, we are all going to be publishing installation practices, modeling, performance testing, data and other best practices, through our respective social media and blog channels. Stay tuned.

  1. ROXUL Insulation: We chose 2" ROXUL Comfortboard IS for the continuous exterior insulation, and 3.5" ROXUL Comfortbatt insulation for the cavities
  2. Huber Engineered Woods: We're using ZIP System Sheathing for all structural sheathing on the floor walls and roof, as well as Advantech sub-floor sheathing for interior side of the floor. We are also using all ZIP System tapes and liquids to air sealing and flashing all penetrations. Look for ZIP System Liquid Flash and Stretch Tape to be used a lot!


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