Karim Allana Shares 7 Building Envelope Terms to Know for Your Next Home Improvement Project

Every building fit for long-term human occupancy has an “envelope” that keeps the elements out. 

You can probably name at least some building envelope components off the top of your head. The roof, the windows, the exterior walls, the foundation: these major features combine to keep the interior dry and comfortable year-round, with assistance from the building’s heating, cooling and ventilation systems.

The building envelope is more complicated than it first appears, however. That’s why building envelope experts like Karim Allana are in such high demand: there’s an amazing amount to keep straight in this business. We asked Allana to go over some common building envelope terms to help your next big home improvement project go a little more smoothly.

1. Insulation

This one most likely rings a bell. Insulation is a pretty basic building material, after all, being a crucial barrier against extremes of hot and cold. 

In short, insulation is what keeps your home’s furnace and air conditioner from working overtime to keep the interior comfortable, even on the coldest or hottest days. Some types of insulation are fire-resistant, as well.

Karim Allana explains that insulation comes in several different forms that depend on a variety of factors. Your home may have more than one of the following:

• Batt insulation: The familiar pink or yellow fiberglass mats, often found in attics, crawlspaces, and wall or joist cavities.
• Cellulose insulation: The grayish “newspaper” insulation that can be blown or sprayed in, also common in attics and crawlspaces.
• Spray foam insulation: A higher-tech material that needs to be installed professionally and is ideal for tight wall and roof spaces.
• Board insulation: These rigid styrofoam-like boards are often found on the insides of attic or crawlspace doors.

2. R-Value

R-value is a measure of how well a given material protects against heat and cold. It’s often discussed in the context of insulation but can also apply to windows, doors, and other building envelope components.

3. Vapor Barrier

A vapor barrier is an impermeable, usually thin-sheeted material that reduces moisture intrusion in the home. In residential settings, the vapor barrier is often a type of plastic called polyurethane. 

These sheets are attached — usually with construction adhesive rather than staples, to keep them intact — to more solid components of the building envelope, such as joists or posts. They’re usually between the insulation and the interior walls. 

4. Underlayment

This is a general term that can have several different meanings. However, in a building envelope context, “underlayment” usually refers to the thin, waterproof material between the outermost layer of the roof (such as the shingles or slats) and the solid base (usually wood) underneath.

5. Rim Joist

The rim joist is a little-known but crucial part of a wood-framed building’s exterior structure. It’s typically a rectangular wooden board that runs along the perimeter of the floor assembly and helps support it. In addition to its structural significance, the rim joist is a “thermal weak spot” in the building envelope and needs to be insulated accordingly.

6. Truss

Your home’s trusses quite literally hold up its roof. Most new-construction homes use factory-fabricated trusses, which are built to precise specifications and can be spaced a bit farther apart than “hand-framed” trusses. While the trusses themselves aren’t a solid barrier against the elements, a well-built truss system helps support an air- and water-tight roof.

7. Sheathing

Like “underlayment,” sheathing is a general building enclosure term that can apply to multiple components, including roofing materials, siding, and trim. A simple way to think of it: If it’s visible on the dwelling’s exterior, it’s part of the sheathing system.

Know Your Construction Lingo

As you can see, the building envelope is more complicated than it appears at first glance. It contains many other elements we haven’t discussed here, each with its own array of descriptive terms.

The best part? The building envelope isn’t the entire building. Not even close. Now that you’re more familiar with the components of the building envelope, you can probably think of a whole host of construction features independent of it. Basically anything within the building’s conditioned interior, in fact.

But we’ve covered a lot of ground already, so let’s save all that for another day.