Super ‘Control Layer’ Freak – a primer on the two most important control layers in your building assembly.

Before we begin, I feel I may have some explaining to do.  When I was in the ideation phase of writing this blog, I was going to title it, “Control Freak”, which is an ode to my type-A personality traits and the topic of ‘control layers’. Then, every time I began to think about the word, “freak”, immediately the late, great Rick James set up shop in my head.  And so, I hope you enjoy the evolution. 😂

Anyway…

Want to know why I like this topic? Because it just takes a little good sense and a dash of BS (building science) to understand the control layer concept.  There are layers of protection in our assemblies and THEY’RE IMPORTANT.  The layers of our walls, roofs, and foundations are what actually facilitate the comfort we expect from our indoor environments. Sometimes one building material can serve multiple purposes, but can we all agree that we want to keep water outside and keep clean, mechanically cooled or heated air inside?

Can we also agree that something that impacts the durability, resilience, and comfort of your healthy home also deserves time, attention, and a healthy budget? The control layers are so important, in fact, that you need to #1 understand the necessary control layers in your climate zone to speak to them intelligently, and #2 dig into these details when you are interviewing potential builders to see ensure their processes and the materials that they’re familiar with using resonate with what you will soon learn is arguably the most important part of your project (assuming you are embarking on a custom home journey with the expectation of ending up with a “forever home”).

Fantastic. You already “get” this, but let’s discuss.

The control layers that make up your building enclosure (or envelope) are what keep the outside out and the inside in and actually protect the more vulnerable building materials within the structure.  What’s outside that we want to separate ourselves from? All the things that aren’t inside (hopefully), of course—rain, snow, bugs, pesticides, the elements, heat during the summer, cold during the winter, dirt, mold, condensation, humidity—pretty much everything that makes us want to leave the outside and go inside.

What’s inside that you don’t want to get outside?  Energy that you paid for (your mechanically produced hot or cold air), your clean, purified, and dehumidified (in the hot/humid climate zones) air… the things that make you comfortable and that you’d like to be able to reliably control.

Imagine an 85 degree day in your home state. You crank your AC down to 70 degrees, but the temp never reaches 70 inside, despite running all day and night.  Assuming your mechanicals are working properly, sized properly, you don’t have leaky ducts, and yada yada, this could be a sign you’re losing energy, maybe via windows, doors, service penetrations.  Or, maybe even via the assemblies themselves. You probably also have a humidity and condensation issue or two, but that’s for another blog post.

Next, imagine you see bugs. Creepy crawlers—ants, roaches, etc. That means they’re traveling on a path to get inside, which means they have an access point. If bugs can fit their nasty creepy selves through a hole, you better believe air (and all that comes with it) is getting in as well. Does that mean you spray the shit out of the inside and outside of your home with toxic chemicals? No. That means you solve the REAL problem ahead of time by ensuring a continuous air barrier system.

The exterior walls of a home are the largest surface area exposed to the elements, and therefore the most vulnerable to energy losses. If a wall system isn’t functioning properly, it can be the cause for up to 30% of the total energy losses in a home.[1]

Control layers are the EXTREMELY important for healthy homes. Got it.

Ok, so what ARE these “control layers” I speak of???

Listed in order of importance:

  • Water Control Layer

  • Air Control Layer

  • Thermal Control Layer

  • Vapor Control Layer

Let’s discuss the two most important control layers: The water control layer, and the air control layer.  We’ll leave the other layers for another day.

The Water Control Layer

The Water Control Layer is your enclosure’s last line of defense from bulk water; it’s a drainage plane.  It’s located outside of the building framing and includes siding, windows, doors, and flashing. It’s the layer protecting the most water vulnerable elements of your building assembly, like interior cavity insulation and drywall, among others. 

Wait, does that mean that water GETS BEHIND THE brick, Hardie® Board, stucco, insert your preferred exterior cladding choices here___???

Yes, yes it does. 

This is where you PLAN for, and provide, a path.  Water needs a path to travel DOWN AND OUT of your wall assembly.  If it doesn’t travel down and out, the outside will eventually come in from the rot of your building assembly, for example.

Gravity takes care of the “down”, your Water Resistive Barrier (WRB) + rainscreen + weepholes takes care of the “out”.

The WRB does not just need to be present, it MUST be detailed correctly and integrated with all flashing of service penetrations, walls, windows, and doors (all the holes we punch in our beautifully sealed enclosures).  We know that water travels via the path of least resistance, so then the logic follows that any weakness in the integration of the WRB with the flashing will be an easy path for water to travel back behind that part of the assembly, which will result in a deterioration of building materials that are not meant to handle bulk water.

Some people think that the best bet is to “just prevent” any water from making its way inside the building assemblies. Time has tested this theory and, as it turns out, Mother Nature always wins. It’s not “if” water gets to a place it’s not intended to go, it’s “when”.

Since water is the #1 killer of buildings[2], I think my point is well-made that the water control layer is the MOST important of all the control layers.

I like this visual from Fine Homebuilding of a wall assembly from the outside, in. Note the siding is lapped so the majority of bulk water can hit the cladding and run down the cladding, or the most exterior layer of the wall. Thanks, gravity. The furring strips create a gap (hopefully at least 3/8”) to allow for adequate ventilation behind the cladding for that errant water that will ultimately get past the cladding. Air will create a convective loop back there because you’ll also want gaps at the top and bottom of the wall assembly. This allows for drying. The green board could be Zip Sheathing, which has an integrated WRB (you might also have seen Tyvek® over plywood or OSB. Tyvek® is used in a lot of production builds I’ve seen in S. TX, it is difficult to keep continuous, and is susceptible to failure as a WRB. All seams of the sheathing (here the green board) need to be taped to ensure full continuity of this layer of the wall so that water can run down and not in. If the seams are not taped, water will run further in to the wall assembly, forced back there by the pressures of wind, stack effect, or pressurization created from the mechanicals. Behind the sheathing the framing, cavity insulation, drywall, and in this case latex paint (which is a class III vapor barrier).


The Air Control Layer

Let’s move on to the second (maybe first and a half) most important control layer, the air control layer. This is the layer that keeps the building “airtight” (hopefully). I’m not sure who gets credit for the phrase, “Build tight. Ventilate right”, but I know it’s not me, unfortunately.


The concept is sound, though. Because of today’s building codes, there is a level of airtightness that must be achieved, outlined generally by the building code adopted by the municipality in which the home is being constructed.

According to the 2015 International Energy Conservation Code (IECC), for example, the code that my municipality builds to today (yes, not adopting the most recent code is absolutely possible), the air changes per hour that a home must achieve in climate zones 1 and 2, is an air leakage rate of less than 5 air changes per hour (ACH) (or, 5 ACH50).

Where I am, deep south Texas (HOWDY, Y’ALL *tips cowboy hat*, JK, I don’t own a cowboy hat…or a horse. Sorry to disappoint), we’re comfortably in the dreaded “hot/humid” climate zone, or climate zone 2. In climate zones 3 through 8, 3 air changes per hour or (3 ACH50) must be achieved.[3] These levels must be tested and verified with a blower door test, which you can and should request access to.

High-performance builders usually far exceed this requirement. A home built to Passive building standards, for example, may achieve ≤0.6 ACH50. Wow. Standing ovation stuff. Am I right?! So good. Just go check out @ubergreenspaces Instagram page, for example. Sy regularly hits these numbers and makes it look like a piece of cake (it isn’t, but he’s just that good).

The airtightness level, or blower door score, that a building needs to achieve is something that absolutely MUST be discussed and agreed to with your builder ahead of time, BEFORE you agree to terms and sign the contract (what if their tradespeople aren’t familiar with high-performance building standards and they simply don’t have the means to achieve the level you desire?). It is impractical to begin a home build project with the expectation of a “forever home” if you do not agree to performance standards before the build ever begins. If you want a home that is comfortable, durable and resilient and reflective of your financial investment, achieving a code level blower door test is probably (most likely) not going to get you there.

-Me.

Another side note, you say “forever home”, in high-performance builder speak that SHOULD mean a home that is durable, resilient, comfortable, and thermal bridge-free. This is because only homes that are durable, resilient, comfortable, and thermal bridge-free are actually capable of lasting most of a lifetime, which is mostly (sort of, “forever”). A topic for another blog post, but worth mentioning here, is that a “forever home” does not last forever by luck. It also requires “forever maintenance”, which is where you, as the educated, informed, and empowered homeowner, have complete control.

Anyway, back to the topic at hand, air control systems—John Straube has an excellent write up on the importance of controlling air flow through building assemblies, which you can find here. In order to get the air control system working as designed (and to meet whatever specs you agree to ahead of time with your builder to achieve—see what I did there?), special attention has to be paid to the continuity of that system and how it all comes together at penetrations (service penetrations, windows, doors, chimneys, etc.)

The biggest transporter of water through building assemblies is via air pressure changes and a path, like a hole, puncture, or gap in your air control system. 

Take a look at the visual below. Vapor diffusion is like when you have a Bounty paper towel and you start soaking up a spill.  The water spreads through the paper towel from the source of the spill and outwards. If the amount of the spill exceeds the ability of your paper towel to absorb, then the towel is soaked and can’t absorb anymore. If your towel has the ability to take on and distribute the spill (doesn’t completely soak), then it’ll probably also dry itself out if you allow for air flow as well. The “real” scienc-y definition of vapor diffusion, according to Green Building Advisor, is this:

Water vapor diffusion is the movement of water vapor through vapor-permeable materials. Vapor diffusion happens through a solid material even when the material has no holes.[4]

When water travels on an “air highway” provided by a 1x1 hole provides, for example, 30 quarts of water will move through the assembly.  Compare that to 1/3 quart of water trasmitted through an assembly via just diffusion, without a hole or “air highway”.

Even when we’re not talking about water, we’re talking about water.  Water is the #1 killer of homes and we must protect the assemblies that protect us, as a priority.  A home that does not provide for a fully integrated WRB is susceptible to water damage, rot, and mold. A home that is not airtight provides many highways for water to travel to vulnerable building materials that are not meant to manage water, leaving you with a mess inside your assemblies, and definitely not the resilient, durable, or comfortable home you hoped to achieve.

🙌So, let’s hear it for the powerhouse control layers: Air and Water🙌

Are you ready to discuss your project specifics and get set up for success in your own home build? Head over to my calendar and book a call with me.

Let’s build something (tight)!

Stay conscious and curious, my friends.

♥️C




[1] "Build tight. Ventilate Right", Energy.Gov

[2] According to basically everyone who knows, but definitely Steve Baczek, Architect, and his colleagues at “The Build Show”

[3] "2015 IECC Updates", Energy Conservatory

[4] "All About Vapor Diffusion", Green Building Advisor

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