As energy prices continue to escalate and random blackouts threaten much of the West Coast, the country is frantically searching for solutions in new oil fields, coal, nuclear and renewable sources, such as solar and wind. But there is another source of power that we can easily access now - the generation of "nega-watts," better known as energy conservation.

Retrofitting existing buildings and using appropriate conservation measures in new buildings can reduce energy usage in buildings by 50% or more, thereby reducing our need for new power sources. How effective the energy conservation measures are depends largely on the design and construction of the thermal envelope, or building shell. Insulation is just one component of the thermal envelope, and if the insulation or the other components are not carefully installed, the results are often dismal.

The selection of the insulation material(s), along with the installation, inspection and testing of the final assembly is essential to building performance. Just adding R's to the building does not make it energy efficient and may create hidden problems, such as condensation and mold growth within the outer walls and roof assemblies. Only through the complete understanding of the way a building works as a system can the appropriate changes take place in the industry.

Building Basics

Insulation, commonly referred to as the R-value of an assembly, is an essential component in the quest to reduce energy usage, and most think that more is better. While this is true to a point, the law of diminishing returns takes over when the "optimum level" - primarily dependent on energy costs and climate - is reached for the project (see Figure 1).

The factors that affect the rate at which a building loses or gains heat are best explained using the "systems approach." The systems approach is a method of design, construction, inspection and testing that accounts for the interactions of the various components, such as foundations, walls, windows, doors, roofs and mechanical systems, along with factors like site, climate and occupant behavior. If these interactions are not accounted for, the laws of physics will prevail.

An example would be the unhealthy results when a large capacity exhaust fan is turned on and the make-up air comes from the flue of the fireplace or water heater. This is known as backdrafting and is result of physics. The big fan wins, and indoor air quality and people lose.

Systems engineering for the thermal envelope involves three primary concerns: heat flow, air flow, and moisture flow. A variety of insulation materials, air barriers, vapor barriers and exterior weather barriers are available on the market. Determining which ones to use where and in which climate can be overwhelming, especially with all of the manufacturers' claims. It is up to the designer and contractor to decide how to put all the pieces together so that the thermal envelope works as it should. A clear understanding of the physics involved and the effects of the interactions between the various sub-systems have on the building is necessary for long-term sustainability.

The "HAM Sandwich"

In their simplest form, buildings are essentially boxes of air. Inside the box, a comfortable environment is maintained by keeping temperatures between 70-72°F and humidity levels between 30-50%. However, the air surrounding the building can vary in temperature from -30°F to 100°F+, with the humidity ranging from 20-100%. When different conditions exist between two spaces in nature, temperature and humidity levels constantly try to reach equilibrium, or balance. The task in buildings is to effectively separate the two environments, so that the desired interior conditions are continuously maintained, and the envelope is kept dry and durable.

To accomplish this goal, a carefully designed system of heat, air and moisture barriers is assembled and tested. We can think of this assembly as a HAM sandwich (Heat, Air, Moisture). All three components need to be carefully designed and constructed to optimize thermal performance.

Heat Flow. As a result of the energy crisis of the early 70s, we learned that the largest single heat loss of a building - accounting for 30-40% of total losses - is typically from air leakage, or random air infiltration. Air leakage is also responsible for drafts, comfort complaints and building condensation problems. The industry responded with weatherization programs, which added more insulation along with some caulking and sealing, and with better (tighter) components such as windows, doors and gaskets.

The primary purpose of the insulation system is to stop the movement of air within the cavity and the flow of air through the assembly. Traditional insulation systems typically allow convective heat loss within the cavity (due to installation voids) and also allow air to move readily through the system. We see the same material used for furnace filters. This air movement dramatically degrades the effectiveness of the insulation system. To solve this problem, weather barriers such as Tyvek and Typar were developed to reduce air movement through traditional fiberglass (air permeable) systems from wind action, yet allow water vapor from air leakage to escape - sort of like a Gore-Tex layer for the house.

Air and Moisture Flow. In a heating climate, continuous interior air barriers, such as 6 mil polyethylene, spray foam, foam board, and even sealed and gasketed sheetrock, are installed on the warm side of the insulation. This keeps the warm, moist air inside from passing through the insulation and condensing against the outer sheathing. Ninety-eight percent of the moisture that condenses into an assembly is from air leakage, which contains water vapor. Water condensation degrades the structure and the insulation value as the insulation gets wet. The goal is to keep the air from getting to a point in the assembly where the temperature is at or below the dew point or condensing temperature. Water is not a welcome guest in the walls and ceiling.

In a cooling climate, the concern is the high humidity in the outdoor air. The assembly is constructed to keep the outside air from entering the cavities and condensing against the backside of the sheetrock. Same physics; different flow direction. For the sake of brevity, the remaining information will describe heating climate assemblies.

R-Value

The R-value of a material is measured using a device known as a hot box. The material being tested is inserted between two surfaces that are 5 inches apart. A heat source is applied to one plate, and an instrument measures the rate that heat energy flows through the material to the other plate. The result is given as the R-value/inch. The test does not, however, represent the real world of buildings. It is conducted without any pressure difference across the sample (no air leakage or air intrusion), and the 5-inch sample is too thin to develop the convective currents that exist in the real world. These forces create air movement and significantly decrease the effectiveness of the insulation system.

In the systems approach, instead of just looking at the R-value of a given insulation material, the entire thermal envelope must be evaluated. The American Society for Testing and Materials (ASTM) has developed a whole-wall R-value test that accounts for losses due to improper installation of the insulation and the air barrier. Conductive heat losses are also accounted for due to thermal bridging across the framing members. A pressure difference is created across the assembly to simulate real world forces acting on the building shell. Using the ASTM method, the actual performance of the entire assembly can be tested. A 5% void in the insulation can account for up to 15% reduction in R-value effectiveness. The installed R-value is what counts in the real world.

Vapor Barriers

Vapor barriers are installed to reduce the flow of water vapor through the assembly by vapor diffusion. Diffusion occurs due to the difference in vapor pressures between the outside and inside air. A material with a permeability rating of <1 is installed on the interior to retard the flow by diffusion. A 4- to 6-mil polyethylene sheet is a commonly used for this purpose. Depending on the local code requirements, this sheet is usually installed against the interior face of the framed walls and ceiling. If the polyethylene sheet is taped, caulked and gasketed, it can also serve as an air barrier. However, the air barrier must be continuous for building durability and energy usage issues.

In many regions of the country, vapor barrier paints can be used in lieu of the polythethylene sheets. Installed to the coverage specifications, they provide a low permeability rating (<1 perm), which meets most North American codes. Vapor barrier paints are used on the first seal coat on the sheetrock and are covered by the final colored coats.

It is important to realize that moisture carried by vapor diffusion is a relatively small quantity and is measured in nanograms (one billionth of a gram). The big concern for moisture migration into the assemblies is from air leakage, which is quantified in pounds of water. For this reason, it is imperative that the continuous air barrier be selected carefully for any installation.

Insulation Types

Insulation materials can be classified in four primary groups: batts, loose-fill, spray foam and foam board.

Batts. The majority of the insulation that is installed is in the form of batts. Fiberglass is the primary material, but rockwool batts are common in commercial and industrial applications. The batts are usually offered in several densities, for example, low density fiberglass at R-11, or high density fiberglass at R-13. High density batts help reduce the convective heat loss within the cavity.

Careful installation of the batt with no voids is essential for thermal performance concerns. It is essential that all fibrous (air permeable) materials be protected from air leakage. An air-tight weather barrier on the outside, taped and caulked, along with a continuous taped and sealed interior air barrier, such as polyethylene sheets, are typically used.

Some manufactures are now offering fiberglass batts that are encapsulated with a plastic sleeve. This is primarily to reduce worker and occupant lung exposure to the fiberglass fibers. The wrap tends to help the air movement through the assembly, but a continuous air barrier is still critical for performance.

Loose Fill. Several loose fill options are available. For walls, an alternate system is blown-in-blanket. A nylon mesh is stapled to the face of the studs, and loose fill fiberglass with a glue mixture is blown in behind the mesh. The fibers are blown around pipes and wires to reduce the voids associated with batt installation. A similar technique is used with loose fill cellulose. No netting is used; instead, water is added to the cellulose to help it stick to the assembly. Care must be taken to let the cellulose fully dry before closing the system in. This has been a significant obstacle for this system, as it means slowing down the construction engine.

Loose fill insulation is a popular option for flat attics, as it is better able to fill in the spaces around the truss members than batts. Fiberglass, cellulose and rockwool are standard choices. Achieving the correct application density is very important for expected thermal performance. The unfortunate practice of "fluffing" loose fill has been the subject of many articles and is being addressed in the various insulation trade groups. When loose fill insulation is not applied to the required density, and wintertime attic temperatures reach around 20°F, convective loops within the loose fill degrade the R-value substantially, with some studies showing a 40% reduction in effectiveness.

A continuous air barrier installed on the warm side of the loose fill insulation is necessary to stop the moist air from ex-filtrating into the attic space and causing moisture problems. As with walls, some codes require vapor barriers on the ceiling while others do not. It is important to check the local codes.

Spray Foam. Spray foam has long been a logical choice for effectively insulating and air sealing in one step. Its air sealing ability makes the installation of a continuous air barrier a much easier task, and also makes it effective when used for sound attenuation. Spray foams are available in several densities and chemical compositions. The urethane foam that is commonly used for buildings has a density of 2 pounds/cubic foot and is known as rigid foam. The vast majority of urethane foam today is used for commercial roofing. However, these foams use HCFCs - which contribute to ozone depletion - as the foaming agent. HCFCs will be phased out in 2003. The urethane industry is searching for a substitute blowing agent with the same thermal properties.

Another spray foam that has been rapidly gaining acceptance is "soft" foam, which has a density of 0.5 pounds/cubic foot. Soft foams are water blown - they do not contain CFCs or HCFCs and do not emit any harmful chemical emissions. Soft foams are applied in wall, floor and ceiling assemblies, where traditional batt insulation was used. They remain flexible and are able to move with the building and maintain the continuous air seal through building settling along with seasonal temperature and moisture swings. These foams can also be injected into hollow assemblies due to their low expansion force.

Foam Board. Another popular insulation is rigid foam board, which has been extensively used with slabs and foundations. Foam board has also recently begun being used on walls to provide a thermal break at the stud, a function that is especially important with steel studs. It is also used in cathedral ceilings and in the rim joist area with sealants for air leakage control and insulation. Ensuring that interior air does not escape around the foam board joints is the key to stopping moisture damage. There are four primary types of foam board: extruded polystyrene, expanded polystyrene, polyisocyanurate and polyurethane. Each type has specific properties for a wide range of applications. Consult the manufacturer for correct application assistance.

Controlled Environments

Harold Orr, whom many consider to be the father of the Canadian R-2000 Energy Efficient Building Program, summed the overall concept of the systems approach using five words: "Build tight and ventilate right."

Building tight provides an effective separation between the inside and outside. It keeps the assemblies dry and allows the insulation system to remain effective. A sweater or pile jacket on a windy day is pretty ineffective in keeping a person warm and comfortable. However, if we add a wind-proof shell, the system will work just fine. The same goes for buildings. It's not enough to simply consider the R-value of the pile - we must evaluate the whole system, beginning at the early conceptual design stage and continuing through the final testing and commissioning of the building.

Once the heat, air and moisture barriers are selected and installed, the result will be an airtight environment. The next step is introducing controlled fresh air throughout the building, typically accomplished through mechanical ventilation systems. These systems range from simple, exhaust-only types to energy-recovery ventilation systems. With a leaky building, air exchange control is in the hands of Mother Nature, often resulting in too much on a windy day and not enough on a calm day. With an airtight building and the right mechanical ventilation system, the air exchange rate is in the building owners' control.