What Is a Home’s “Building Envelope”… and What’s It Got to Do with Plastics?

Professor Plastics

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house exterior, building envelope

Too cold out? Too hot out? Regardless, “out” is the best place to keep that weather. It belongs outdoors, not indoors.

How do you keep it there? That’s where the “building envelope” comes in.

Imagine an open envelope. (For you younger folks out there, an envelope is something used to mail a letter. You can look it up.) Its contents can move in and out, subject to theft and loss. Now imagine a sealed envelope. Much harder for anything to get in or out.

Now imagine a house with ill-fitting doors and windows, poorly sealed walls and roofs, leaky spaces around hose bibs and electrical outlets. Just like an open envelope, all that comfy air on the inside is subject to theft and loss. The weather steals your indoor air and replaces it with outdoor air.

And that can make you feel sort of under the weather. Ahem.

So just like an envelope, we should seal our house. Unfortunately, that can be really hard to do. Hot or cold weather has a way of insinuating itself through tiny, unseen spaces and even transferring through building materials. That causes you to spend lots of energy (and money) to keep replacing the air in your house.

That’s why sealing the building envelope is so important. But what exactly is that? Allow the Professor to explain. Briefly.

What is a building envelope?

The building envelope is the term for the physical barriers that separate the inside conditioned space from the outside unconditioned area. The building envelope provides barriers to heat/cold, air, water, light, and noise. From an energy efficiency standpoint, the barriers to temperature and air are the big ones—the idea is to “seal” the envelope from the outdoors just like that envelope carrying a letter in the mail.

This is important not only for homeowners but our nation, as well. Did you know it’s estimated that nearly 40 percent of our nation’s energy is consumed in our homes and buildings, and that heating and cooling account for most of the energy use in a typical U.S. home? Sadly, much of it is wasted due to outdated building practices. The building envelope in much of our nation’s housing stock is… well… leaky. Think porous roof, mold and mildew, drafty rooms, rotted wood…

OK, Professor Plastics, what’s this got to do with plastics?

Home energy efficiency.

It’s really hard to properly seal the building envelope without advanced, modern plastic building materials. To maximize a home’s energy-efficiency, multiple plastic materials such as foam insulation and sealants can be combined to help create a more continuous sealed barrier that is tight and resists air movement. Some of these products do not simply insulate well (check out my article on insulation and R-value), they also reduce leaks and air loss. Compared to outdated construction techniques, less of the home’s climate-controlled air escapes, and outside air is less likely to penetrate the home’s living spaces.

What kinds of plastics are used in many of these energy efficient home
Here are some examples:

  • Sheets of foam polystyrene under and around the foundation;
  • “Insulated concrete forms,” typically expanded polystyrene blocks that stack and are filled with concrete and rebar to create walls;
  • “Structural insulated panels” that sandwich large sheets of expanded polystyrene foam between wall boards;
  • Polyiso or polyurethane foam installed under the roof (instead of insulating the attic floor);
  • Plastic foam insulation on the outside  of the home, not just in the wall cavities;
  • Foam plastic insulated window frames clad in vinyl;
  • Plastic house wrap that significantly reduces the infiltration of outside air;
  • Vinyl siding and trim that provides an additional barrier between indoors and out, and
  • Plastic sealants (caulks, mastics, foams, tape) in remaining gaps that may exist between floors, walls, roofs, and windows, as well as around ductwork joints.

By combining many of these plastic building materials, the house’s building envelope behaves more like an efficient system, with all of the parts working together to create something much greater than what any building component would achieve by itself.

The result? Dramatically reduced air flow between indoors and out and dramatically reduced energy use and expense.

So… sealing the building envelope helps keep the weather where it belongs: outside.