Heat Transfer And The Building Envelope

How much heat the building envelope—the construction that separates the interior spaces from the outside environment—gains or loses is influenced by the construction of the outside of the building envelope, along with the wind velocity outside the building. Each layer of material making up the building's exterior shell contributes some resistance to the flow of heat into or out of the building (Fig. 15-1). The amount of resistance depends on the properties and thickness of the materials making up the envelope. Heavy, compact materials usually have less resistance to heat flow than light ones. Each air space separating materials in the building envelope adds resistance as well. The surface inside the building also resists heat flow by holding a film of air

Wood (yo F

Siding

Plywood

Plywood

Figure 15-1 Insulated wall.

Insulation

Figure 15-1 Insulated wall.

along its surface. The rougher the surface is, the thicker the film and the higher the insulation value. Think of how a very thick fur coat creates a rough insulating surface that traps a lot of air around the person wearing it.

Warmer air moving around with cooler air creates a gentle motion in otherwise still air within a room. This results in the flow of room air in contact with the inside surface of the building envelope.

Walls and roofs don't usually have uniform thermal resistance across their surfaces. Some parts, such as framing members and structural ties in metal and masonry construction, transmit heat more rapidly than others. These elements that conduct heat quickly are called thermal bridges. Thermal bridges increase heat loss significantly in an otherwise well-insulated assembly. Metal studs can also create thermal bridges. When a thermal bridge exists in a ceiling or wall, the cooler area can attract condensation, and the water can stain the interior finish. Cooler areas of the interior surface collect a thicker layer of dust because of the higher electrostatic charge that they carry in dry air. In cold weather, condensation or frost can form on interior surfaces of thermal bridges. To avoid this, insulated masonry systems use ties made of fiber composite materials with less thermal conductivity than steel.

The easiest way to control the transfer of heat through a building envelope is to control heat transfer within the building envelope itself. You can increase thermal resistance by adding insulation or reflective sheets, or by creating more air spaces. The thickness of the air space is not usually critical, but the number of air spaces makes a difference. Highly efficient insulation materials, like fiberglass batt insulation, which hold multiple air spaces within their structure, are better than empty air spaces alone. High levels of insulation maintain comfortable interior temperatures, control condensation and moisture problems, and reduce heat transmission through the envelope.

Structural insulated panels (SIPs) are now available in a wide variety of structural surfaces and interior insulation type and thickness. A single factory-built panel replaces site-built framing, and thermal performance is considerably improved because no framing members penetrate the insulated core. The typical SIP consists of two structural surfaces, often oriented-strand board (OSB), enclosing a core of either expanded polystyrene or polyisocyanurate foam between 15 and 30 cm (6-12 in.) thick. Panels are connected with plywood splines or shiplap joints that don't break the insulating layer, and their uniform thickness and construction result in sound walls without the voids common in wood framing.

So-called super insulated buildings are designed to eliminate the need for a central heating system. By using state-of-the-art energy conservation practices, little heat is lost through the building envelope, and the heat generated by cooking, appliances, lighting, and the occupant's bodies is sufficient to heat the building. A building with enough insulation and the ability to store heat well can hold a comfortable temperature overnight without additional heating. Some additional heat is usually still required to warm the building up in the morning after a cold winter night.

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