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Control layers fall into four distinct categories, listed in order of importance. The importance is based on the amount of damage produced if a given layer fails. There's no point in making your house super insulated if you can't keep the rain out.

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1. Water Control Layer

2. Air Control Layer

3. Vapor Control Layer

4. Thermal Control Layer

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1. The water control layer is the most crucial control layer. It's the first line of defense against bulk water intrusion in any form (rain, snow, flooding, etc). If these water sources are left uncontrolled when they meet your house, they can lead to significant problems like mold, rot, structural damage, and poor indoor air quality. For these reasons, ensuring your home’s exterior is properly sealed against water intrusion is vital.

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How does the water control layer prevent water intrusion?

Through a variety of properly installed materials that help drain the water down and away from your house such as:

• roofing, roof overhangs, and siding

• layers directly under the roofing and siding like water resistive barriers (aka WRB; most commonly seen as house wraps or Zip System sheathing), flashing, and roofing underlayments

• rainscreens

• windows and doors

• gutters and downspouts

• site drainage and sloping the ground plane away from your foundation

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​2. The air control layer manages uncontrolled air leakage in and out of your house though cracks and openings in the envelope. 

 

Why is it important to control air leakage? 

• It stops moisture laden air from entering the wall and roof cavities which can produce mold, rot, structural damage, and poor indoor air quality. In fact, air movement accounts for 98% of wall water vapor movement in building cavities.1 

• It increases energy efficiency and reduces your utility bills. Any unwanted air movement in and out of your house makes your HVAC system work harder and can account for up to 30% of your heating and cooling costs.2

• It increases your comfort level by keeping temperatures more consistent inside your home. 

• It reduces allergens and pollutants from entering your home creating better indoor air quality. 

• Big bonus here: bug control! (For any of you who have a roach phobia like me and lived in old houses with zero air sealing, this is huge!)

 

How does the air control layer block air leakage?

Through the application of properly installed materials that block air from entering the wall and roof cavities and interior spaces of your home:

• the layer over your wall and roof sheathing known as an air barrier (i.e. house wraps, taped sheathing, and roofing underlayments) 

• tapes, sealants, caulk, gaskets, weatherstripping

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Ideally the air control layer should be continuous all the way around your building envelope to keep it as air tight as possible* to avoid the issues described above. The most critical areas of concern are the transitions and joints between materials. 

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* BIG CAVEAT HERE:

Air tightness, especially in new homes needs to be blower door tested after installing your air control layer at framing/rough-in penetration stage. Waiting till the end of the project makes it difficult to fix any air leakage problems since so much has been built on top of it. Your blower door test must also be coupled with a properly designed HVAC system including appropriate ventilation and dehumidification requirements specific to the home. When I say a "properly designed HVAC system" I mean more than just a basic square footage calculation that many HVAC subs use to size your condenser or exhaust fans.  This is not a one size fits all approach. Sick homes and occupants can happen if the other factors are not taken into consideration.

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3. The vapor control layer is essential for regulating humidity and preventing condensation in your home. High humidity levels can lead to issues like mold, rot, structural damage, and poor indoor air quality. 

 

Water vapor can still pass through materials with a water and air barrier, so we look at the permanence of materials to understand how much water vapor can pass through the envelope of your house. Each material has a perm rating that tells us how vapor permeable or vapor open it is. Conversely, it can tell us how vapor impermeable or vapor closed it is. Understanding what that rating means and where it is located within your wall and roof assemblies is essential for it work correctly. Getting it in the wrong order can lead to very serious and expensive consequences. Controlling vapor prevents materials from getting wet and promotes drying, ultimately giving you a more comfortable and healthier indoor environment. 

 

4. The thermal control layer refers to the insulation and materials used to maintain comfortable indoor temperatures by preventing heat loss in winter and heat gain in summer. It’s crucial for ensuring energy efficiency, lowering utility bills, and keeping your home comfortable year-round. Factors you should consider are:

 

• What type of insulation do you have?

• How thick is the insulation?

• Where is the insulation in your walls and roof? Is it between the studs, rafters, or ceiling joists, or do you have a layer of continuous insulation on the outside of those structural elements?

• How well was it installed?

 

The answers to these questions affect the thermal comfort and energy efficiency of your home. The answers can also cause potential condensation issues resulting in mold, rot, structural damage, and poor indoor air quality.

 

In conclusion, it's critical to not only understand how each of these control layers works independently, but also how each layer works in conjunction with the others, your HVAC system, and the climate you live in. The layers, designed and installed correctly, are the foundation for creating a home that works in harmony with the environment, enhances your well-being, and stands the test of time. ​​​​