This post first appeared on the Andersen Windows ProViews Blog.
Shelter is a human necessity. A place to be warm and dry. A place to be safe and healthy. These are the main purposes for every new home that gets built. As our species has moved out of caves and into, first, stone structures, then concrete and mass wood structures, and finally to what we live in today, our expectations for health and comfort have increased. We want our homes to stay dry, but not too dry. Temperatures are expected to be comfortable, and sometime consistent with little variation throughout the home. The quality of the indoor air should be healthy to breathe. And we want our investment to last for a long, long time. A newer expectation is that we do not want to spend a lot of time, effort or money maintaining the structure.
If you dig into the different assemblies and individual pieces that make up a home, you see each piece or combination of pieces has a purpose. For instance, the foundation supports the weight of the structure and keeps it tied to the earth. The structural shell of the home is there to hold the walls upright and keep the roof over our heads. What about the parts and pieces used to keep the rain from entering the home, or the wind from blowing through? What keeps the comfort (temperature/humidity) you just paid to create from leaving? These duties are the job of the control layers, the topic of this blog post.
Building science has created an order of importance for control layer, of which there are four:
- Water control
- Air control
- Vapor control
- Thermal control
Each of these layers should be identifiable in a set of construction plans.
Water Control Layer
The purpose of the water control layer is to keep bulk water out of the structure. It’s a system of several different products and assemblies working together to keep water moving down, out, and away from the building. It starts at the roof, moving to the walls, and then extending to and below grade. The exposed roofing materials and cladding or siding are the first line of defense with other materials under those finish materials adding a belt and suspenders approach to managing moisture. Felt and synthetic papers, and sometimes peal and stick membranes back up the exposed roofing materials. Felt paper and plastic membranes, also known as house wrap or water resistive barrier (WRB) are located behind the exterior cladding and protect water sensitive building materials in our walls. Other materials may also be present behind the finish materials, products that improve the draining and drying of water (rainscreen) are sometimes included to improve the building durability.
Air Control Layer
After water control, the second most important control layer is air control. Small holes and cracks in our building envelope allow outside air (along with dust and critters) to leak in and conditioned inside air to leak out. This constant infiltration and exfiltration can affect comfort, indoor air quality and even the durability of the structure. Operating costs, especially in hot and humid and cold climates can also be influenced by air moving through an assembly, as a matter of fact, up to 40% of the heating, cooling, dehumidification or humidification costs associated with creating comfort inside our homes can be attributed to air leakage.
Air contains some level of moisture, which can be a double-edged sword when it comes to the wetting or drying of building materials. When air containing moisture comes in contact with a cold surface, the moisture in the air can condense on that surface. This, over time, can lead to mold, rot, and affect indoor air quality and building durability. But, when air is dryer than the surfaces it contacts, air movement can promote drying.
Vapor Control
The vapor control layer, though much less of a concern than the water and air control layers, still needs to be given consideration, especially in hot and humid and cold climates. Water in vapor form can move through many common building products. This process is called diffusion, we measure diffusion by a term called permeance. Permeance is how easily water vapor passes through the material. A permeance of less than 0.1 perm is considered vapor impermeable. Materials in this class of permeability would be metal, glass and some types of plastics. Materials with permeance ratings of 10 perms or more are considered vapor open. Most fibrous insulations, unpainted drywall, and many house wraps have permeances greater than 10.
Water vapor wants to move from someplace more, to someplace less. This can result in wetting of building assemblies, but the process can also help dry a material. In cold climates, vapor retarders are often installed on the interior or warm in winter side of wall assemblies to prevent humidity from moving into a dryer wall cavity where it can condense (freeze) on a cold surface. This same vapor retarder may restrict water vapor from drying inward, into the living space. This drying may be needed if a bulk water leak is present, wetting the cavity during a summertime rain event. The biggest consideration in vapor control is to be aware that much more water vapor is transported by way of air leaking into and out of the home than by vapor diffusion. Air control is more important than vapor control in most instances.
Thermal Control Layer
The final control layer is thermal control. The primary purpose of the thermal control layer is to maintain comfort and reduce heating and cooling costs. We do this by adding insulation to our floors, walls and roofs. If insulation is installed in the right location and made continuous, such as outside our walls and roofs, we reduce our risk of condensation forming inside walls and roof assemblies. Again, we are keeping moisture sensitive materials dry. We measure insulation effectiveness by its resistance to heat flow, or R-value. The higher the R-value, the more resistance to heat moving through an assembly.
So, how do windows fit into the discussion of control layers? Well, windows are one of the parts and pieces that make up a building’s envelope, they need to be connected to all four control layers to operate correctly. When effectively connected to the water control layer, they keep water out of the building assembly. They need to keep the wind from blowing into the home and help keep the air you just paid to condition, in. If you’ve chosen the right window, with the right performance characteristics (and maintained a proper indoor humidity level), the risk of water vapor condensation on windows creating damage is much less of a concern. And lastly, a window is part of the thermal comfort package of the home. Understanding the control layers is helpful to assure your window installations are effective at creating the needed continuity with the control layers.