The Thermal Boundary

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The thermal boundary (also called the thermal envelope) refers to the parts of a building that separate indoor space, which is heated or cooled with a furnace or an air conditioner, from the outdoors and other spaces that are not heated or cooled. The spaces in your home that are heated or cooled are called the conditioned space, and the purpose of your furnace or air conditioner is to maintain those spaces at a comfortable temperature.

The thermal boundary is not the same as the weather shell of the house. Although it may coincide in many places; for example, most exterior walls are part of both the thermal boundary and the weather shell—they have different purposes and are often not in the same place. The purpose of a building’s weather shell is to shed rain and snow and keep the building and its contents dry. It also is able to resist forces that try to break it down, such as sun and wind. The roof is usually part of the weather shell of a building, but most roofs are not part of the thermal boundary, unless they are built as an insulated cathedral ceiling. Usually, the thermal boundary is the flat ceiling, and there is some type of open attic space between it and the roof. A garage is another example; the exterior walls and roof of the garage form a weather shell, but the wall between the garage and the home is the thermal boundary (see ill. below).

Like the weather shell, the thermal boundary is not usually made of a single material, and some materials may play more than one role. Walls may consist of siding, building paper, sheathing, studs, thermal insulation, and drywall or plaster. All of those materials contribute in some way to the thermal properties; some of them are also structural, and some make the building weather resistant.

For the greatest comfort and energy efficiency, it is important that the thermal boundary of the home be clearly defined. Sometimes it is unclear whether or not a space is part of the conditioned area. For example, a basement may be comfortably warm in the winter due to heat loss from the furnace and ducts, even though there is no insulation on the foundation wall. One might think that the thermal boundary is the foundation walls. What if that same basement opens into a crawl space underneath the family room that was added on later? The crawl space may have louvered vents (which are often required by code), and it is definitely an unconditioned space. Perhaps the best place for the thermal boundary is at the foundation wall, between the full basement and the crawl space. On the other hand, if the inhabitants never go into the basement and don’t mind it being rather cold in the winter, it may make more sense to put the thermal boundary between the basement and the house. To do that, you must insulate and seal the heating ducts and the floor over the basement; then the entire basement and the crawl space are considered unconditioned.

Mean Radiant Temperature

The mean radiant temperature in a roam is the average temperature of all the surfaces in the room weighted by the percentage of the room that each surface occupies.

(NOTE: The temperatures around the edges refer to the surface temperatures of the wall, ceiling, floor, and windows)

Image: A. Even though the room temperature may be 70°F the person sitting in the middle experiences a mean radiant temperature of 67°F The window, because it is both large and cold, is the surface that has the most effect on lowering the mean radiant temperature. B. If the room has twice as much window area, the radiant temperature drops to 63°F -- too cold for comfort! This happens even though the room temperature is still at 70°F To keep this room comfortable, the thermostat will need to be set higher to compensate. 13-0-1.jpg

Image: The thermal boundary of a ceiling is typically made up of drywall or plaster, thermal insulation, and ceiling joists or truss members. If the attic space is vented to the outside, temperatures in the attic will be close to outdoor conditions, and the rafters, sheathing, and roofing materials play virtually no role in the thermal boundary.

Image: The Thermal Envelope: The boundary between the conditioned space and anything that’s not conditioned is called the thermal envelope (or thermal boundary). Like an envelope, it is supposed to surround its contents neatly and completely. The shape may be complex, and it usually does not include attics, garages, or crawl spaces.

Image: Even though this attic area behind a kneewall has insulation in the floor, wall, and rafters, it’s unclear whether the area is inside the conditioned space. In this example, the thermal boundary is ambiguous.

TIP: To maximize comfort and energy efficiency, it is important that the thermal boundary of the home is clearly defined

Caution: The thermal boundary should always be continuous, and you should be able to draw an imaginary, uninterrupted line around your house, in any direction, that represents the thermal boundary. Every part of that line should represent some type of insulation material—either that exists now or that you plan to install.

Image: Some Thermal Boundary Problem Areas

A. Where the ceiling height changes; B. At the band joist; C. At the kneewall; D. Warm air between the floor joists can easily move to the wrong side of the insulation in an unvented kneewall attic; E. Air from a tuck-under garage can get in through unblocked joist bays.

Discontinuous insulation at band joist; Discontinuous air barrier where wall cavity opens into attic; Discontinuous air barrier where kneewall floor has no blocking; The roof deck may be a better air barrier, but warm house air can easily reach this uninsulated surface; It’s common (though not good) to have gaps in the thermal boundary, either where the air barrier or the insulation is discontinuous (A, B, C) or where the air barrier isn’t aligned with the insulation (D, E).

Insulation does not stop air

Although all parts of the thermal boundary should include some type of insulation material, they must also incorporate some type of air barrier. Most insulation does not stop air. Let me repeat that: Insulation does not stop air. Although there are exceptions, the most common types of insulation materials—fiberglass and cellulose—do not stop air from moving under the driving force of a pressure difference. Fiberglass, in particular, does almost nothing to stop air movement under the forces of convection. People often think that if they put a lot of insulation in a house it will be airtight; this is simply not true.

When you define the thermal boundary of your home, it is critical to think about where the air barrier is located in relation to the insulation. Think of insulation as a sweater or fleece pullover:

On a cold, windy day, it doesn’t keep you warm because the air moves right through it. A wind breaker has very little insulating value but, by stopping the air, it makes the sweater much more effective. Similarly, stopping air leaks makes insulation much more effective.

Air barriers and insulation

Another thing to consider when defining the thermal boundary is whether the air barrier is close to the insulation. It is usually best to align them whenever you can; for example, it does not make sense to insulate the flat ceiling of a house and then create an air barrier at the roof sheath ing. If you did, warm air from the house would circulate through the insulation, warming the attic and increasing the heat loss through the roof.

If you’re not sure, knowing a place where it is easy to make a good air barrier can help you decide where to put insulation.

However, there may be places where the insulation and air barrier need not be aligned. The most common example is a basement, because it can be very difficult to provide a continuous air barrier between a basement and the first floor. Even if you wanted to insulate the floor over the basement and make it a cold space, if you have a solid concrete foundation wall, it may be much easier to air-seal the foundation walls and the sill area instead. If you can effectively stop cold air from leaking into the basement, that will prevent it from moving up into the house through the floor.

Image: Where Is the Thermal Boundary? In this common scenario, the thermal boundary is unclear. The crawl-space is clearly outside, but the rest of the basement could be conditioned (by insulating the foundation walls) or unconditioned (by insulating and sealing the furnace ducts and insulating the floor). Either way, it would be smart to seal off the opening between the basement and the crawl space.

Image: Unlike fiberglass batts, rigid foam insulation can provide a good air barrier, but all of the seams must be sealed carefully.

Prev.: Understanding Heat Transfer
Next: Energy, Moisture, and Building Durability

Other resources:

Interior Weatherization (from US DOE and Alaska Housing)

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