What Is The Building Envelope (or Building Shell or Building Enclosure)?

by Erik North on September 19, 2011

What Is The Building Envelope

 

I talk about it all the time but what the heck is the building envelope (or to use the more appropriate modern term, what is the building enclosure)? Why is the building envelope important?

As a side note, I’m going to use the terms, ‘building shell’, ‘building enclosure’ and ‘building envelope’ interchangeably. Building scientists prefer the term ‘building enclosure’ but as long as we’re all talking about the same thing…

And why do I get all worked up over chimney chases and modern shower framing (because they’re usually big a– holes in the building envelope / shell / enclosure).

Put as simply as possible, the building envelope is that part of your house which separates the indoors from the outdoors. It is the roof, walls and foundation structures which control the water, vapor, thermal and air flow differences between the conditioned air inside your house and the nasty, wet, windy unconditioned air outside.

If you can imagine a building or your home wearing a giant invisible blanket, then you’ll be able to understand the building envelope. While it can’t be seen, it is still an extremely important aspect of the building’s energy usage. The building envelope protects you from weather, and ideally keeps you warm in the winter and cool in the summer. However, it can also be subject to the most wear and tear, so you’ll want to keep up on your home’s outer maintenance.

Parts of the Building Envelope

The building envelope is comprised of all of the parts that separate the main interior from the outside. This includes walls, doors, windows, the roof, the cellar walls and the foundation. A garage or attic can also be part of the building shell, if your home has those features.

These structural control layers maintain temperature differences (your insulation), air pressure difference (the exterior sheathing, Tyvek, or self-adhering impermeable membranes), retard water vapor flow and rain water intrustion (again, Tyvek and roof shingling/roof felt).

As an energy auditor, one focus of my job is improvements to your home’s building shell, improving the insulation and reducing excess air leakage.

Flaws in the Building Envelope

And the job of improving your building envelope means finding the flaws in it. The thermal control layer (again, that’s your insulation) can be compromised by wind washing, thermal bridging or heck, just not being there. Many older balloon framed houses have uninsulated wall cavities and newer home’s basements are uninsulated (or insulated in the basement ceiling joists).

Houses always have air leakage through the air barrier. Often times the problem stems from builders thinking that fiberglass will stop air flow (it will not). Or a chimney chase that is open and unsealed. But these problems are easily solvable by some caulking and XPS foam blocks (or aluminum flashing if it is in contact with a potentially hot surface like a chimney). Knocking off the big leaks with simple solutions like chimney balloons can make a difference in your heating bills.

Open or Closed?

A building envelope can be classified as open or closed. An open building shell is loosely constructed, with plenty of space between the interior and exterior. Unfortunately, the more open the building shell is, the greater the risk for wind, moisture and even household pests. Many older homes and buildings have an open building shell. A closed building shell is more tightly constructed, with well-insulated walls and the space less prone to wind, moisture and pests. Many modern homes and buildings have a closed building shell.

Air Leakage

Air leakage is the number one factor affecting the building envelope that also has the most direct impact on the money you spend on your heating/cooling bills. Too much air leakage through the building shell results in money wasted and a house that isn’t comfortable. Too little natural air flow, and the house feels stuffy and the air is stale.

Common Energy Auditing Vocabulary

Whether you call it the building shell, building envelope or building enclosure, it is the essence of your home. I’m always trying to reduce excess air leakage through the building shell, so naturally it is of the utmost importance to me. It is for you, too, since this is the key area that is most likely to cost you money!

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