June 16, 2016

In an effort to return to the more technical side of our discussion let’s tackle the most obvious of features with a less obvious definition: the building envelope. Also referred to as the building “shell” or “enclosure,” it is literally the walls, roof and foundation of your house, or that which separates the exterior environment from the interior environment. Seems straightforward and for all intent and purposes it is; however, we need to look a little deeper when talking about its efficiency, comfort and durability.

Essentially, we are most concerned with three key aspects:

  1. Penetrations in the envelope (holes or bypasses)
  2. The thermal boundary (where the insulation is)
  3. The pressure boundary (where we control airflow to and from the interior environment)

Penetrations in the envelope also may seem straightforward because holes in your house are bad. But wait, not all of them are. For example, exhaust fans, chimneys and dryer vents are intentional holes and good ones for arguable reasons. It’s the undesirable penetrations that we want to focus on to improve the envelope’s comfort, health and durability. These can include bypasses into crawlspaces, attics, walls and internal cavities, and their impact can be quantified by a blower door test and dealt with accordingly.

Think of the thermal boundary as where the insulation in your house is located. Then take into consideration how you use the house. Insulation has to be in the right place and installed correctly. Insulation is designed to slow the transfer of heat. Whether it’s keeping heat in (heating a house) or keeping it out (cooling a house), where the insulation is located represents the area of the house that you will be heating or cooling. Would you want the insulation in your attic to be on the inside of the roof slope, or on the floor of the attic? If it’s on the inside of the roof slope, you are paying to heat or cool the entire attic. Maybe you want to do that… but if you don’t, your thermal boundary is in the wrong place. This is true for every other area of your house – crawlspaces, walls, basements, attached garages and so on.

The pressure boundary is where we control unwanted airflow. If air is allowed to pass through, around or by insulation, the insulation’s effectiveness drops off the map. This also reduces comfort, increases bills and allows air from undesirable places (attics, basements, crawlspaces) to enter your lungs. The pressure boundary and the thermal boundary should be soulmates. Always in alignment, at the same location and constantly working together.

Building Envelope – a Haiku

Walls around your house
Floors under foot could be too
This blog is over

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