Fundamentals of Building Science: Thermal Insulation

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Whenever there is a temperature difference, heat flows naturally from a warmer space to a cooler space. Heat moves through the building envelope by radiation, conduction, and convection, but the main mode of heat transfer for solid, opaque building elements is by conduction and radiation.

When installed correctly, thermal insulation reduces heat transfer through the building envelope. It makes sense to use thermal insulation to reduce energy consumption while increasing com fort and saving money. Thermal insulation is an important component of a high-performance building.



Characteristics of Thermal Insulation

As introduced in Section 2, the "R-value" used to rate insulation products is thermal resistance (R). It is a measure of the ability of a material to resist heat transfer. In the United States, it is expressed in units of hr °F ft2/Btu (°C m2/W). The higher the R-value, the better the insulation will be at keeping the heat in (or out). In Canada and other countries, the metric equivalent referred to as RSI, is used. The RSI is about 1 6th the R-value. The exact relationships between R and RSI are:

[...] Insulation is a material, such as fiberglass, cellulose, mineral wool, foam, or fiberboard, that is installed in the building envelope to significantly reduce heat loss or heat gain. Insulation functions by trapping gases (e.g., air or another gas), which reduces conduction and convection heat transfer through the material. The thermal conductivity of most gases is very low, but gases tend to transfer heat well if natural convection cur rents are allowed to develop. By trapping gasses in extremely small pockets in the insulation, the formation of convective cur rents is curtailed.



Because air is a frequently used gas in insulation, most commercially available insulations approach the thermal performance of air. As a result, air-based insulations have similar theoretical thermal properties regardless of their basic composition-that is, thermal conductivities of no less than about 0.24 Btu-in/hr °F ft2 (an R of about 4.2 hr ft2 °F/Btu per inch of thickness or 0.03 m2 °C/W per mm). Because of the different materials used to trap air and limitations of manufacture, R-values will be somewhat lower for commercial grades of insulation, usually about 3.0 hr ft2 °F/Btu per inch of thickness (0.021 m2 °C/W per mm).

Other types of insulations, particularly closed-cell poly-isocyanurate and polyurethane foams that trap a gas with a lower thermal conductivity than air, offer slightly better insulating capability. The closed-cell structure of the foam retains the gas and yields a thermal conductivity of about 0.17 Btu-in/hr °F ft2 (an R of about 6.0 hr ft2 °F/Btu per inch of thickness or 0.042 m2 °C/W per mm). Manufacturers frequently produce rigid foam insulation boards with a reflective foil adhered to one or both surfaces, which improves the R-value of the insulation board by about 0.5 hr ft2 °F/Btu per reflective surface (0.0035 m2 °C/W per mm per reflective surface). Insulations constructed of multiple layers of reflective films with air voids in between the films are also very effective. These offer a thermal conductivity of about 0.13 Btu-in/hr °F ft2 (an R of about 8.0 hr ft2 °F/Btu per inch of thickness or 0.056 m2 °C/W per mm). A graphic comparison of average R-value per unit of thickness by insulation type is provided in TBL. 4.

Tbl. 4 -- a graphic comparison of average r-value per unit of thickness by insulation type.

Fiberglass (loose and blown fill) Cellulose (loose and blown fill) Fiberglass (blanket and batt) Rock wool (loose and blown fill) Polystyrene, expanded (rigid board) Polystyrene, extruded (rigid board) Polyurethane (rigid board) Polyisocyanurate (rigid board) Layered reflective films w/air voids Type of Insulation SI (metric) units | Type of Insulation Customary (U.S.) Units

The average temperature of the material affects the thermal properties of insulation. A slight increase in thermal conductivity and decrease in R-value occurs when the insulation temperature increases is typical for most types of insulation.

This increase is from the increased agitation of the gas molecules as temperature rises. Because building insulations are used within a small temperature range, heat transfer values are assumed to remain constant.

Density also affects the thermal properties of insulation.

The resistance to heat flow is dependent on gas trapped between the fibers of the insulation. As insulation is compressed, more fibrous materials fill the same thickness. This compression creates smaller air spaces and offers more resistance to heat transfer. However, at a specific density, part of the air spaces close and the fibrous material conducts heat more rap idly. Thus, stuffing R-19 insulation into a space only large enough to contain R-11 may actually reduce the effectiveness of the R-19 insulation to below R-11. The optimum density of "loose" insulation is typically achieved when the insulation can support its own weight without settling. Examples of insulation installations are provided in Ill. s 3.3 through 3.12.

Ill. 3 Loose-fill fiberglass being blown into an attic space.

Ill. 4 Loose-fill fiberglass insulation being sprayed into wall cavities.

Ill. 5 Loose-fill fiberglass insulation being scrapped to create a flat surface to accept gypsum wallboard finish. Excess insulation is vacuum collected for reuse.

Ill. 6 Loose-fill insulation is blown through a hose connected to a blower in a truck. Insulation is manually loaded into a hopper connected to the blower. This is one method of installing loose fill insulation.

Ill. 7 Blanket insulation installed between the cavities of wall studs or joists.

Ill. 8 Care must be exercised in placement of blanket insulation. When it is compressed significantly, such as at electrical outlets, it looses its thermal effectiveness.

Ill. 9 Icynene spray foam insulation and air-sealing system installed in ceiling and wall cavities.

Ill. 10 Close-up view of Icynene spray foam insulation.

Ill. 11 Foil-faced rigid foam insulation installed on basement foundation wall.

Ill. 12 Foil-faced sheathing improves the thermal performance of insulation by reducing radiant heat transfer in the winter and summer.

Types of Thermal Insulation

There are many different types of insulation. A description and comparison of types of insulation follows:

Blankets, Batts, or Rolls

Rolls and batts (or blankets) are flexible products made from mineral fibers (e.g., fiberglass, rock wool). They are available in widths suited to standard spacings of wall studs, attic or floor joists, and roof rafters. They are available with or without vapor retarder facings. Batts with a special flame-resistant facing are available in various widths for basement walls where the insulation will be left exposed. Continuous rolls can be hand cut and trimmed to fit, but this must be done precisely. Poor-fitting insulation diminishes expected thermal performance because of gaps in insulation coverage and random air movement within the insulation.

Loose-Fill/Spray Applied

Loose-fill insulation is made from a blend of virgin and recycled waste materials (e.g., fiberglass, rock wool, or cellulose) cut into shreds, granules, or nodules. These small particles are blown into spaces using special pneumatic equipment. Loose fill insulation is best suited for places where it is difficult to install other types of insulation because the blown-in material easily fills and conforms to building cavities in wall studs and attic or floor joists. Additional resistance to air infiltration (air leakage) can be provided if the insulation is sufficiently dense or thick. The performance of loose-fill insulation is strongly affected by its installation.

Materials used for loose-fill insulation (e.g., fiberglass, rock wool, or cellulose) can also be sprayed into building cavities. Water is generally added to activate an adhesive in the insulation, which causes the insulation to adhere within building cavities as it is sprayed. Sprayed insulation products completely fill voids in areas where it is difficult to get roll insulation types to fit properly. Because sprayed insulation tightly conforms and adheres to the building cavities, it reduces air infiltration (air leakage) and increases the insulation's effectiveness. It forms a uniform covering throughout the cavity and forms a good air seal between electrical wiring, pipes, framing members, and anything else inside the building cavity. Spayed insulation tends not to settle as the insulation fibers adhere to each other and the sides of the building cavity.

Liquid foam insulation can be sprayed into building cavities as a liquid or in larger quantities as a pressure-sprayed product (foamed-in-place). It will completely conform to a building cavity, sealing it thoroughly. Expanding liquid foams can seal small cavities but allowances for expansion must be made to prevent blowout of an assembly.

Rigid Insulation

Rigid insulation is typically made with foam, including molded expanded polystyrene (EPS or bead board), extruded expanded polystyrene (XPS), polyurethane, polyisocyanurate, or other related chemical mixtures. Rigid insulation is typically more expensive than fiber insulation (e.g., fiberglass, rock wool, or cellulose), but it is very effective in buildings with space limitations and where higher insulating values are needed. Rigid boards may be faced with a reflective foil that reduces heat flow when next to an air space. Rigid foam boards are lightweight and can provide structural support (i.e., sheathing) but must be covered with a fire barrier (i.e., gypsum wallboard).

Reflective Insulation and Reflective Barriers

A reflective insulation is formed with layers of aluminum foil (or another type of low emittance material) and enclosed air spaces that provide highly reflective or low emittance cavities.

Some reflective insulation systems also use other layers of materials such as paper or plastic to form additional enclosed air spaces. The performance of the system is based on the emittance (e.g., 0.1 or less) of the material(s). The smaller the air space, the less heat will transfer by convection. Therefore, to lessen heat flow by convection, a reflective insulation, with its multiple layers of aluminum and enclosed air space, is positioned in a building envelope cavity (e.g., stud wall, furred-out masonry wall, floor joist, ceiling joist) to divide the larger cavity into smaller air spaces. These smaller trapped air spaces reduce convective heat flow. Like other types of insulation, reflective insulation is labeled with R-values that provide a measure of thermal performance.

Radiant barriers (i.e., polished aluminum foils) can be installed directly under roof rafters or on the attic floor (above insulation) to reduce heat gain from the sun by reflecting energy back to the roof. They are very effective in reducing a building's cooling load but not at reducing the heating load. As a result, radiant barriers are more effective in hot climates.

Radiant barriers must have a low emittance (e.g., 0.1 or less) and high reflectance (e.g., 0.9 or more). Unlike other insulation products, there is no standard method for equating the performance of a radiant barrier to standard insulations. Manufacturers may use the term equivalent R-value but this really has no scientific meaning for a radiant barrier, and often reflects optimum conditions and not necessarily climate conditions. A radiant barrier used in the attic floor installation must allow water vapor to pass through it.

Insulating Techniques

The primary method used to reduce transmission heat transfer in a building is insulating or improving the insulation of the building envelope. Typically, insulation is situated as close to the heated space as possible. For example, in an open unheated attic above a sloped roof, it is best to lay the insulation on the attic floor (on the ceiling of floor below) rather than along the sloped roof rafters. ___ 3.1 provides examples of where to insulate a residence.

Typically, loose fill, blanket, or batt insulation made from fiberglass, cellulose, or mineral wool is used to fill cavities and voids in construction assemblies such as frame walls, floors, and attic spaces. Insulation is also installed against a foundation wall or below the perimeter area of a concrete slab.

Loose-fill insulation may be used to fill hollow voids in masonry, but this is less effective than covering the surface of the masonry wall.

Depth and rating (R-value) of insulation is dependent on construction technique, climate, and energy and construction costs. In an attic floor, insulation thickness can generally be increased without added cost (excluding cost of additional insulation). In a wall, however, the wall thickness limits maxi mum insulation thickness to the depth of the cavity. Additional insulation will require an increase in wall framing thickness and thus wall framing costs.

Fgr. 1 Examples of where to insulate a residence

1. Unfinished attic spaces, insulate between the ceiling joists to seal off living spaces below.*

2. In finished attic rooms with or without dormers, insulate:

a. Between the studs of "knee" walls

b. Between the studs and rafters of exterior walls

c. Ceiling with cold spaces above

3. All exterior walls, including:

a. Wall between living spaces and unheated garages or storage areas

b. Foundation walls above ground level

c. Foundation walls in heated basements (foundation can be insulated or inside or outside of wall

4. Floors above cold spaces, such as vented crawl spaces and unheated garages, and :

a. Any portion of the floor in a room that is cantilevered beyond the exterior wall below

b. Slab floors built directly on the ground**

c. Foundation walls of crawl spaces and perimeter plates

d. Storm windows as recommended

*Well-insulated attics, crawl spaces, storage areas, and other closed cavities should be adequately ventilated to prevent excessive moisture buildup.

**Slab on grade is almost always insulated, in accordance with building codes, when the house is constructed.

TBL. 5 approximate heat loss and heating cost per square foot of construction assembly for a typical heating season in Denver, Colorado, based on anticipated (2010) costs.

For a specific set of energy and insulation costs, it is no longer cost-effective to insulate beyond a specific insulation thickness. Simply, energy cost savings from added insulation becomes so small that it is no longer cost-effective to invest in the additional insulation. TBL. 5 shows a comparison of heat loss and heating cost per square foot of construction assembly for a typical heating season in Denver. The approximate heating cost per square foot of construction assembly with an R-value of 1.0 hr ft2 °F/Btu for a typical Denver heating season are $1.00 for natural gas heating, $4.00 for electric resistance heating, and $2.00 for heating with a geothermal heat pump. Costs vary with type of heating source. This means that one square foot of an R-1 assembly in a Denver building will contribute this amount to the annual heating costs. In reviewing this table, it is evident that an assembly rated at R-2 results in half the cost; an assembly rated at R-3, a third of the cost; and an assembly rated at R-10, one-tenth the cost. Increasing insulation decreases heating load and cooling load costs; however, this de crease is not linear.

TBL. 5 Can be used to analyze the cost-effectiveness of insulation levels. For example, natural gas cost per square foot of construction assembly for a typical Denver heating season is $0.15/ft2 -yr for an assembly rated at R-20 and $0.08/ft2 -yr for an assembly rated at R-40. An additional 6 in of insulation (from R-20 to R-40) in an attic space would result in cost avoidance (savings) of $0.07/ft2 per typical heating season with constant natural gas costs. If the home were heated with electricity, a $0.10/ft2 savings would result: $0.20/ft2 _ $0.10/ft2 _ $0.10/ft2 .

Depending on the costs of insulation and heating fuel used, it may be cost-effective to invest in the additional 6 in of attic insulation needed to go from R-20 to R-40. In the case of natural gas heating, adding an additional R-20 insulation (from R-40 to R-60) yields a savings of only $0.03/ft2 -yr: $0.08/ft2 _ $0.05/ft2 _ $0.03/ft2

This is less than half the savings achieved when going from R-20 to R-40. This means that the first inch of insulation saves more than the second inch, which saves more than the third inch, and so on. At some point it is no longer cost-effective to add insulation; this point is referred to as the point of diminishing return. It no longer makes economic sense when more is spent on the insulation than the savings it generates over its life.

Tbl 6 and 7 provide recommended total R-values for new house construction in the United States and Canada.

TBL. 8 indicates the thickness of insulation required to obtain commonly used R-values. R-values typically refer to the nominal cavity insulation R-value. Actually, thermal performance will yield lower R-values once the framing members are taken into account. TBL. 9 indicates the thickness of insulation required to obtain commonly used R-values.

Superinsulation

Superinsulation in buildings is the use of vast amounts of insulation, coupled with airtight construction, and controlled ventilation without sacrificing comfort, health, or aesthetics. The extra insulation in the building envelope is well beyond what is considered the current standard for insulation levels. Superinsulation is generally used in cold and moderate climates, but it is finding its way into warm climate design.

Superinsulation in cold climates is associated with design features with average U-values less than 0.033 Btu/ft2 -°F (0.2 W/m2 -°C) for chief opaque (nontransparent) structural assemblies. Superinsulation typically results in R-24 (RSI-4.3) or more insulation in exterior walls and over R-60 (RSI-10.6) insulation in the roof. As a result of these elevated insulation values, typical thicknesses of insulation materials are likely to be at least 8 in (200 mm) in walls and 20 in (500 mm) or more in roofs. Limits on insulation thicknesses used are often defined by the allowable cavity widths in cavity wall and attic construction-that is, of how much insulation fully fills the cavity.

Thermal Storage

Walls and floors with high heat capacities can store more energy, will have a larger thermal lag, and thus will generally be more effective for thermal storage and peak load shifting. The thermal diffusivity of wood is about 75 times less than steel, so steel conducts energy through it about 75 times faster than wood. Materials with low thermal diffusivities, such as concrete and masonry, have a slow rate of heat transfer relative to the amount of heat storage. These materials are effective for thermal storage and peak load shifting of heating and cooling loads.

In buildings, thermal diffusivity corresponds to the inter action between the effectiveness of thermal insulation (R-value) and the heat capacity of the materials. Both factors should be large to obtain the maximum thermal lag. Insulation is most effective for this purpose when it is installed on the exterior of concrete masonry walls. This keeps the mass in direct contact with the interior air for maximum heat transfer efficiency. Exterior insulation minimizes the effect of outdoor temperature swings on the temperature of the mass. For example, rigid board insulation is frequently used on the exterior of concrete masonry walls, and then covered with a mesh or fabric and a weather resistant finish. Interior finishes on exterior walls should be minimal to prevent insulating the mass from the interior air. See Fgr 2.

Fgr. 2 Daily interior temperature swings vary with thermal mass of the structure.

Tbl. 6 recommended r-values for new house construction in six insulation zones.

Tbl. 7 recommended r-values for new house construction in Canada based on heating degree-days (°c-days)

Tbl. 8 thickness of insulation required to obtain commonly used r-values.

Tbl. 9 range of differences in r-value estimations for center-of-wall and whole-wall r-values.

Wall Framing Material Wood framing Concrete framing Steel framing

Mass and Effective Thermal Resistance

In certain climates, construction of a heavyweight building envelope (e.g., concrete, earth, and insulating concrete forms) can be an effective way of reducing building heating and cooling loads. Several comparative studies have shown that heating and cooling energy demands in buildings containing massive walls can be lower than those in similar buildings constructed using lightweight wall technologies. This better performance results because the thermal mass integrated in the structure reduces temperature swings and absorbs energy excess both from solar gains and from heat produced by internal energy sources such as lighting, computers, and appliances.

Massive walls delay and moderate thermal fluctuations caused by daily exterior temperature swings. The steady state R-value introduced in Section 2 and traditionally used to measure the thermal performance of a wall does not accurately reflect the dynamic thermal performance of heavyweight building envelope systems. For example, manufacturers of insulating concrete form (ICF) walls suggest the effective R-value of an 8 in (200 mm) ICF wall is between 26 and 30 hr °F ft2/Btu (148 and 170°C m2/W).

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