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Insulation

All materials conduct heat, some at a relatively fast rate, others at a relatively slow rate. A material that conducts heat at a very slow rate is considered a thermal insulator. If a sufficiently thick layer of this material is installed in the outer shell of a house, it will reduce considerably the passage of heat into and out of the house.

The primary justification for insulating a house is economic. During the winter, thermal insulation prevents the loss of heat from within the house, which reduces the amount of fuel that must be burned to maintain the desired indoor temperature. This, of course, saves on fuel costs.

During the summer, thermal insulation prevents the transfer of solar heat into the house, which makes the house more comfortable to live in during hot weather. If the house, or part of the house, should also be air-conditioned, the insulation, by preventing the entrance of heat, increases the effectiveness of the air-conditioning system. This, in turn, reduces the amount of electricity required to operate the air-conditioning system.

One should realize, however, that the economic justification for installing insulation decreases as the thickness of the insulation increases. There is a limit to how much insulation can be installed. Consider, for example, a house in which the walls and roof are insulated with 1-in.-thick material. It is possible to calculate very closely the effect this 1 in. of insulation will have on the total amount of heat that's transmitted through any particular construction. This heat transmission is calculated as U, the coefficient of heat transmission. Further more, once the U value is known, one can calculate the amount of fuel that's saved because of this 1 in. of insulation. It is then a simple enough matter to put a dollar value on the fuel savings.

However, as the amount of insulation is increased, the amount of money saved decreases proportionately. Say, for example, that 1 in. of insulation results in an annual savings of $100 in fuel costs. If we add another inch of insulation to the house, the amount of heat transmitted from the house will be halved. Therefore, we will save an additional $50 in fuel Costs, for a total savings of $150. Suppose we add still another inch of insulation. The amount of heat lost from the house will now be one-third the original amount. Therefore, we will save an additional $33 in fuel costs, for a total savings of $183. Suppose we add a fourth inch of insulation. The amount of heat lost from the building will be one-quarter the original amount, for an additional savings in fuel costs of $25. Our total savings are now $208. But note how, as we continued to increase the thickness of the insulation by 1-in, increments, our additional savings amounted to only $50, $33, and $25. At the same time, the cost of the insulation will have been increasing proportionately. A point will be reached where it will cost more to install that one last additional inch of insulation than will be saved in fuel.

Where that point will be is difficult to say. It depends on the price of fuel and how cold and how long the winters are. Four inches of insulation may be economically justifiable in Minnesota and North Dakota, but in Georgia 1 in. of insulation may be excessive. In Georgia, however, it may be economically justifiable to install 2 in. of insulation because of the savings that will result in the cost of operating an air-conditioning system during the summer, whereas in Minnesota or North Dakota these air-conditioning savings may be ignored, since the house will be more than adequately insulated already.

Furthermore, the amount of insulation required depends on the shape of the house. A boxy, 2-story house with an attic is easier to heat and keep warm, with or without insulation, than a long, low, rambling 1-story house, especially when the ceiling is left exposed. The reason is that the boxy house, being more compact, has a smaller proportion of its volume exposed to the weather, which means that less of its heat will be transmitted to the outdoors, assuming the two houses have the same total interior volume. Eskimo are short and stocky for the same reason; their body shape helps them conserve body heat. In the tropics, however, a thin, lanky build is preferable, because such a body shape has a much larger surface area, which helps to dissipate body heat more efficiently.

In addition, in a 2-story house, the effect of having an uninsulated roof is felt less in the first-floor living quarters because the ceiling that separates the second floor from the attic has some insulating value. In a 1-story house, on the other hand, the insulating effect of an upper floor is absent, and , if the roof insulation should be inadequate, the entire house will feel colder and require more fuel to heat than a 2-story house.

An insulated house has other advantages over an uninsulated house, if we assume the design and construction of the two are the same. In an insulated house, the temperature throughout tends to be more equitable, especially the temperatures of the walls, which makes the entire house more comfort able to live in. The importance of having warm walls (and ceiling and floor) lies in the fact that most of one’s body heat is lost by radiation from the skin. The greater the difference in temperature between the skin and the surrounding walls, the greater the amount of heat that's radiated from the body and the colder one feels. When the walls are warm, therefore, there is less heat lost from the body by radiation and the more comfortable one feels—even though the air temperature in the room may not have been increased. For a more complete discussion see HEATING SYSTEMS AND HUMAN COMFORT. In an uninsulated house, however, not only are the walls colder but there also tend to be differences in the temperatures of the different rooms, and even in the temperature of different parts of the same room, which makes for discomfort. During the winter, because there is a lower rate of heat loss through the exterior walls, an insulated house also heats up much more quickly whenever the heating plant is started up, as on a cold winter morning after the thermostat changes from its nighttime setting of, say, 55°F to its daytime setting of 68°F or 70°F; this also helps to conserve fuel. During the summer, an air-conditioning unit installed in an insulated house cools the house down more quickly than in an uninsulated house, and this reduces the costs of operating the air-conditioning unit.

HOW INSULATION INSULATES

The three ways heat is transferred through the walls, roof, and floor of a house are by conduction, convection, and radiation.

Little can be done about reducing the amount of heat that's lost by conduction through the solid structure of a house. The house necessarily consists of solid materials fastened securely to each other, and it's only natural that these materials will conduct heat. Reducing the amount of heat lost from a house, therefore, means reducing the amount of heat radiated from the house or transferred from the house by convection currents.

A very large proportion of the heat lost from a building is lost through the doors and windows. It was found in one study, for example, that in the typical uninsulated, 2-story wood-frame house approximately 55 % of the heat lost through the shell of the house was lost through the doors and windows. This heat loss can be reduced considerably by double-glazing the windows and by installing weather stripping and storm windows and doors.

This leaves about 45 % of the total heat loss that can be reduced by installing insulation materials in the outer shell. In a wood-frame house, this heat is lost by a combination of convection currents and heat radiation. If this heat loss is to be prevented, or reduced significantly, the insulation materials installed within the wall cavities must be able either to trap the air—that is, prevent convection currents—or be able to reflect into the house the heat that's being radiated across the cavity.

A layer of perfectly still air is considered to be the best thermal insulation. Theoretically, such a layer of air 1 in. thick has an r value of 5.95, which indicates just how good an insulating material air is. (The insulating ability of a material is measured in r or R values, as described below. The higher a material’s r or R value, the better an insulator it's .) As may be seen in Table 1, only polyurethane-foam insulation, which has an r value of 6.25 at 50°F, exceeds air in thermal insulating ability. All the other materials listed are well behind air in their ability to prevent the transfer of heat.

In practice, it's extremely difficult, if not impossible, to provide a layer of absolutely still air in the outer shell of a house. The next best thing is to install a material that can trap air—and this defines the unique characteristic of most insulating materials. They may trap air within closed cells (as in the foamed plastics mentioned above, and in cork), or they may trap air because the material Consists of a dense network of fibers through which air finds it very difficult to move. Mineral wool is an example of this latter type of insulating material. In addition to preventing air currents, a microscopically thin film of air tends to cling to each fiber of a fibrous material, which further increases the material’s insulating ability.

The other basic type of thermal insulation—reflective insulation—insulates because it presents a barrier to radiant heat. The radiant heat is reflected off the surface of the insulation like light off a mirror. Reflective insulating materials include polished metal foils made of aluminum and copper, metallic paints such as aluminum paint, and anodized metallic finishes that are coated onto a backing such as paper or cardboard. The importance of reflective insulation within the walls and roofs of most buildings may be gauged by the fact that between 65 and 85 % of the heat that passes through these structures is radiated heat.

To sum up, therefore, the basic function of both fibrous and cellular insulation materials is to provide a layer of still air that enhances the ability of these cavity spaces to act as insulation in the frame of a house. In addition, the bulkiness of the insulation prevents the establishment of convection currents within the cavities that would otherwise transfer heat across them.

The purpose of reflective insulation is to prevent the transmission of radiant heat across these air cavities. The maximum degree of insulation is usually achieved when fibrous (or cellular) insulation material is used in combination with a reflective insulation material.

Table 1. Insulating Value of Common Insulating Materials and of Air Spaces

THERMAL RESISTANCE AND r, R, AND R(T) VALUES

Thermal conductivity, k, is defined as the amount of heat (in Btu per hr) that's transferred from one side of a material to the other when that material has an area of 1 sq ft, is I in. thick, and the temperature differential across the material is 1°F.

When, however, one is considering the ability of a material to resist the transfer of heat, it's more natural to think of the reciprocal, or inverse, of k, which is 1/k. The inverse of thermal conductivity is called thermal resistivity, r; and r = 1/ k. If, for example, the k value of a material is 1.25, then its r value is 1/1.25, or 0.80.

Also, when a material is other than 1 in. thick, its thermal conductance, C, is defined as the amount of heat (in Btu per hr) that's transferred from one side of a material to the other when that material has an area of 1 sq ft and the temperature differential across the material is 1°F. As in the previous paragraph, when one is discussing the ability of a material to resist the transfer of heat, it's more natural to think of the reciprocal, or inverse, of C, which is 1/C. This reciprocal of thermal conductance is thermal resistance, R, and R = 1/C.

Note that both r and R may refer to exactly the same material but that r measures the thermal resistance of that material when it's I in. thick, and R measures the thermal resistance of that material when it's some thickness other than 1 in. To convert from r to R, one need only multiply the r value by the actual thickness of the material. E.g., in Table 1, 1 -in.-thick mineral wool at a temperature of 30°F has an r value of 3.70. If one is considering installing a layer of mineral wool that's 3 in. thick, then its R value at 30°F would be 3.70 x 3m. = 11.10, and R= 11.10 would be the value inserted in one’s calculations when calculating what effect this insulation will have on the total heat transmitted through a wall.

Insulation R Values

The manufacturers of insulating materials assign R values to their products, rounding Out the R value to the nearest full odd number. These R values are commonly used when their effect on the heat loss of a construction is being calculated.

Glass-fiber insulation, for example, will have printed on its packing, or envelope, an R value that depends on its thickness, thus,

Thickness, in.

R value

6

7

11

19

22

If we examine Table 1, we see that the 1/k, or r, value of glass-fiber and other fibrous insulations depends on the outside air temperature, the r value decreasing as the air temperature increases. The reason for this decrease in r value is the increased thermal motion of the air molecules trapped within the fibers when the weather is hot. This means, of course, that the insulation is less effective in the summer when one wants to keep hot air outside the house than in winter when one wants to keep warm air in. We might also add that there is an optimum density to a fibrous insulating material (not only glass fiber) that maximizes its ability to insulate. Densely com pressed insulation isn't as effective as loosely compressed insulation—but only up to a certain point. The insulation can't be too loosely packed.

To return to R values and Table 1, glass-fiber insulation, for example, actually has not one but a range of R values that depends not only on its thickness but also on the outside air temperature. The range of these R values between 90°F and 30°F is as follows

Thickness, in.

Range of R values

6

9.6—13.2

13 .5—18.4

23.1—31.6

25.0—34.2

Comparing this table with the one above, one will note how conservative the manufacturers’ ratings are.

In any actual building, the construction consists of a variety of materials, each of which has its own r or R value. The total amount of heat transmitted through a wall or roof is summarized by the construction’s overall heat transmission coefficient, U. Thus, U equals the total amount of heat (in Btu per hr) that will be transmitted through an entire wall or roof section having an area of 1 sq ft when the temperature differential across that section is 1°F. The reciprocal of U is the total thermal resistance, R(T), and R(T) = 1/U. In practice, the R(T) value of any building construction is obtained by adding all the R values in that construction or, if the U value is known, simply by finding the reciprocal of U.

TYPES OF INSULATION

As we said, the basic division between insulating materials is between those that trap air and those that reflect radiant heat. Practically, however, insulating materials are separated into groups according to the way in which they are manufactured and used in building construction. Thus, the insulations used in most houses are classified as blankets, batts, fills, and reflective foils. There are, in addition, various slab and foam insulations that have as yet only a limited, though important, use in dwellings.

Blankets and Batts

Fibrous Insulating Materials

Blankets and batts are made of fibrous materials, of which mineral wools and wood fibers are the most common.

Mineral wool is a generic term that includes fibrous insulations made from limestones and shales (rock wools), blast- furnace slag (slag wools), and silica (glass wools).

To manufacture a rock wool, the raw material is heated with coke in a blast-furnace cupola until it melts. The melting temperature is somewhere between 2300 and 3400°F, depending on the raw material used. As the molten rock is discharged in a thin stream from the bottom of the cupola, a blast of steam blows it into long fibers that are from 0.0002 to 0.0004 in. in diam.

Slag wools are manufactured in the same way, the major difference being that slags melt between 2000° and 2850°F.

The silica from which glass wools are manufactured is melted and then forced through fine orifices into the path of a jet of air or steam that blows the molten glass into fibers. All three of these mineral wools can also be made into a granular form by altering the thickness of the molten stream and the velocity of the impinging jet of air or steam.

Mineral wools are inherently fungus-proof, vermin-proof, moisture-proof, and fire-resistant. They are usually treated with a chemical that enables them to shed surface water quickly, to prevent the buildup of moisture within a closed wall or roof construction.

Wood fibers and the other natural fibers have none of these inherent virtues, and they must be especially treated if they are to be resistant to damage from fungi, vermin, fire, and moisture.

Blankets

Blankets (which are sometimes called quilts) consist of long rolls of insulation that are usually 1, 1½, 2, and 3 in. thick (although they are available as thin as 1/2 in. and as thick as 6 in.), and in widths of 15, 19, or 23 in. These widths allow the blankets to be installed between studs, rafters, or joists that are spaced 16, 20, or 24 in. apart. The blankets are manufactured in 40- to 100-ft lengths, the actual length depending on the thickness of the material.

Although blankets are manufactured in which the fibers are simply matted together, most blankets are enclosed in a paper or vinyl envelope, or they may have a paper, cardboard, or wire-mesh backing cemented to one side. This backing, which may be reinforced for extra strength, extends 3/4 to 2 in. away from the sides of the blanket, forming two flanges that allows the blanket to be stapled or nailed to the adjacent framing members. The envelope enclosing a blanket has similar flanges made by folding and cementing the edges of the paper together.

To install a blanket, these flanges are stapled or nailed either to the faces or sides of the framing members, as shown in Fig. 1. If the flanges are attached to the sides of the framing, this will automatically cause the blanket to extend into the cavity by 3/4 to 2 in., which provides an air space between the blanket and the sheathing on the opposite side of the cavity. When the flanges are attached to the ends of the framing members, the blankets will form an effective vapor seal (see below). Care must be taken that the staples or nails used to hold the blankets in place are driven flush with the flanges and that the flanges themselves lie smoothly against the framing members; other wise there is a chance that the flanges will interfere with the installation of gypsum wallboard, if gypsum wallboard is to be used to finish the interior surface of the wall (see DRYWALL CONSTRUCTION).


Fig. 1. The flanges on blanket insulation can be attached either to the sides or ends of the studs within a wall cavity.

Very often one side of a paper envelope (usually the flange side) is coated with plastic or asphalt to make the paper impermeable to water vapor. When the flanges are correctly attached to the framing, this side of the envelope will form a vapor barrier that prevents the passage of water vapor into the wall or roof construction. Insulation having a backing on one side can be treated in the same way to make the backing vapor-resistant.

Although a combined blanket and vapor barrier appears to have advantages, especially to the builder, who is always intent on reducing costs, these advantages will prove illusionary unless the flanges are secured tightly against the framing. If a vapor barrier is to prevent the passage of water vapor, it must not have any gaps or openings through which water vapor can pass; for if water vapor does pass into the air cavity and condense against the construction, the consequences can be very serious (see CONDENSATION).

The warmer the climate, the less necessary a vapor barrier is in any case, since there is less likelihood that any vapor will condense within the air cavity. The colder the climate, how ever, the greater the probability that water vapor will make its way past an inadequately installed barrier and condense within the air cavity. When a vapor barrier is necessary, a sheet of polyethylene film installed against the inner side of the framing will make a far more effective vapor barrier than vapor-proofed blanket insulation (see Fig. 2), and the use of such a separate vapor barrier is always to be preferred in these cases.


Fig. 2. A 2-mill-thick sheet of polyethylene film makes an excellent vapor barrier. The film is unrolled and installed as a single sheet over the entire wall. After it has been nailed or stapled to the wall framing, any door and window openings are cut out of the film.

Balls

Batts are similar to blankets in every way in both their construction and use. They may be enclosed within envelopes or have backings attached to one side, and they may have a vapor barrier and /or reflective foil added, just as blankets have. The major differences between batts and blankets are that batts are usually thicker—3, 4, 5, or 6 in.—and they are only 24 to 48 in. long, although some manufacturers of 8-ft-long blankets refer to their products as batts.

Batts are usually manufactured to be slightly wider than the usual 16, 20, and 24 in. spacing that exists between framing members, which enables them to fit snugly between these structural members. They need only be pushed into place and there they remain. If, however, they are installed horizontally between the rafters of an unfinished roof, or between the first- floor joists over a basement or crawl space, and the builder is concerned lest they slip out one day, they can be held in place by paper flanges that are provided as part of the batts or they can be held in place by lengths of wire that are pressed into place under the batts. If batts are to be installed inside a finished ceiling, they can be pushed into place and kept from falling out until the finish ceiling has been installed by a polyethylene vapor barrier that's fastened in place under them.

There is a question whether blankets and batts should be installed in a wall in such a way that they press against the wall sheathing or whether an air gap should be left between the insulation and the sheathing. Certainly, whether the insulation is placed flat against the sheathing or whether a space is left between them will not affect the insulating value of the insulation at all. But insulation does lose its effectiveness when it gets wet, and one never knows whether a wall that's sound and watertight on the day it's finished will remain sound and watertight as the years pass, even when a sheet of waterproof building paper has been nailed against the sheathing to keep moisture out. It would seem more prudent, therefore, to allow an air gap between the insulation and the sheathing.

When a mineral-wool insulation is to be installed within or against a masonry wall, then under no circumstances should the insulation touch the exterior wythe of brick or concrete blocks since moisture will almost positively make its way through the wall during extremely wet weather, either by soaking through the masonry or by following cracks in the mortar joints (see BRICK-MASONRY CONSTRUCTION). The only way that blankets or batts should be installed against a masonry wall is to nail 1 X 2 in. or 2 X 2 in. furring strips to the masonry and then attach the insulation to the furring. The thickness of the insulation must be watched (or the size of the furring strips increased) to make sure that a gap remains between the masonry and the insulation after the flanges have been nailed or stapled to the furring.

Loose-Fill Insulation

Both mineral and vegetable wools are available as loose fill. The insulation material is packed loosely into bags that weigh 40 lb each and contain 4 cu ft of the material. The insulation is used in spaces too small for blankets or batts; or it may be blown into a cavity by machine.

Loose granules, or pellets, of insulation made of vermiculite and perlite are also available in 40-lb 4-cu-ft Capacity bags.

Vermiculite

Vermiculite is a mica-like mineral known chemically as hydrated magnesium-aluminum-iron silicate. Like all micas, it consists of very thin, flat layers of material pressed tightly together. Molecules of water are trapped between these layers. When the mineral is crushed into small pieces and the pieces are then heated to 2200° to 2400°F, the trapped water expands into steam, which forces the layers of mica apart. When the mica cools, the volume of each granule will have increased about 12 times, the spaces between the layers now being filled with trapped air.

Perlite

Perlite is a silica-like volcanic rock, the molecules of which hold combined water. When the rock is crushed into small pieces and the pieces are heated to about 1500°F, the rock softens. At the same time, the water content is liberated and converted into steam. As a result, the entire granule expands. When it cools again, it contains a considerable volume of trapped air.

Loose-Fill Installation

Both of these materials are usually poured or blown into wall cavities or between the floor joists of an unfinished attic. For use in a wall space, the exterior wall is, as usual, built first. But before the interior wall surface is constructed, a polyethylene vapor barrier is nailed or stapled to the bottom of the wall framing. The wall cavity is then gradually filled with the granules from the floor up as the interior wall is constructed.

The big problem with using loose granular insulation in a wall is that over a period of time the granules may pack down because of the small but continual vibrations any house is subjected to. Eventually an air gap may open up at the top of the wall across which heat may be transmitted by convection or radiation.

The walls of an old, uninsulated wood-frame house are often filled with insulation by blowing these granules between the wall studs. Holes are drilled in the top of the wall between each pair of studs (or the top strip of siding may be removed from the wall) and the insulation is then blown into the wall cavities through a hose, like a giant vacuum cleaner operating in reverse.

The same technique is used to blow the granules between the floor joists of an unfinished attic. Blowing the granules into place is quicker and cheaper than emptying bags by hand, and the insulation packs into place more evenly and more densely than if the work were done by hand.

The effectiveness of blown insulation, that's , its R value, depends on the density with which the granules are packed together as well as the overall depth of the material. Most manufacturers base the R values of their insulation on an assumed density of from 6 to 8 lb per cu ft. For a homeowner to specify only the depth to which attic insulation is to be laid isn't enough. It is possible, for example, for a builder or contractor to skimp on the amount of insulation laid down by failing to pack it in densely enough.

To ensure that the correct density is laid down, Federal Specification HH-l-1 030 requires that each bag of loose-fill insulation have a label that shows (1) the minimum thickness, (2) the maximum net coverage, and (3) the minimum weight of the insulation required per unit area to achieve a specified R value. Figure 3 is an example of such a label. The FHA requires that a label cut from one of the bags (or a card duplicating the label information) be signed by the builder and nailed some where in the attic as a guarantee that the insulation has been correctly installed.

Reflective Insulation

Bright metal surfaces reflect heat because of a property called emissivity. The emissivity of a material is a measure of how well it absorbs radiant heat energy. An ideal black object that absorbed all the radiant heat energy falling on it would have an emissivity of 1.0. The emissivities of ordinary construction materials such as wood, plaster, and concrete are around 0.80. Metals with brightly polished surfaces have extremely poor emissivities. Aluminum foil, for example, has an emissivity of only 0.05. That is, it will absorb only 5 % of the radiant heat energy striking it. The rest is reflected away. Aluminum foil, therefore, reflects 95 % of the radiant heat that strikes However, in order that a bright metal surface be capable of acting as reflective insulation, it must face an air gap that's at least 3/4 in. wide. The effectiveness of the reflective insulation decreases as the width of the air gap decreases from 3/4 in. If there is no air gap at all, the material loses its reflective- insulation ability altogether and becomes merely a conductor of heat, like any other metal. On the other hand, an air gap wider than 3/4 in. does not add significantly to the effectiveness of reflective insulation.

Aluminum foil, the most widely used reflective insulation, is too fragile a material to be used by itself. The foil is usually cemented to a paper or cardboard support. Two-sided reflective foils are made by cementing two of these single-sided sheets to both sides of a sheet of asbestos.

Aluminum foil is often cemented to one side of gypsum wallboards and insulation-board panels. These panels and wallboards must then be installed so that the foil side is facing the wall cavity. Foil cemented to a panel in this way has two functions: it serves as reflective insulation, and it also acts as a vapor barrier that prevents the passage of water vapor into the cavity from the interior of the house.



Fig. 3. A label cut from a bag of loose insulation must be placed in the attic where it can be inspected. The insulation must be installed in conformity with the label recommendations.


Paper- or cardboard-backed foil is made in rolls of standard widths to fit between framing members. The edges of the rolls can be bent to form flanges that are nailed or stapled to the framing members.

Double-sided foil is intended to be placed in the center of a wall cavity, thus dividing the cavity into equal halves. The inward-facing foil reflects heat into the interior of the house during the winter; the outward-facing foil reflects solar heat away from the interior of the house during the summer. Reducing the width of the air cavity by half in this way also helps to reduce the strength of convection currents within the wall space and thus helps to improve the insulating properties of the air itself.

The same general technique can be used with blanket insulation having foil on one side. If the blanket is installed in the center of a wall cavity, two air spaces are created where there had been only one. At the same time, the foil backing, which should be installed so that it's facing toward the interior of the house, reflects heat into the house. The fact that the foil is facing the interior of the house also enables it to act as a vapor barrier. In fact, if a foil is cemented to one side of a blanket or batt, the foil must face toward the warm side of the wall to prevent the condensation of water vapor on the insulation material.

Reflective insulation is particularly effective in preventing the transfer of heat through a roof during the summer, since so much of the solar heat that enters a house is radiated through the roof. Reflective insulation within a roof is much less important during the winter; instead, blanket insulation is required in the winter to prevent the setting up of convection currents within the roof construction that will transfer heat out of the house. The most effective insulation for roofs, therefore, is a combination of blanket or batt insulation to which is attached a layer of reflective foil facing upward, with an air gap between the roofing and the foil. But when this method of installing the insulation is followed, a vapor barrier must also be installed on the ceiling side of the insulation to prevent the condensation of water vapor in the insulation during the winter months.

Blanket or batt insulation placed between roof rafters must in any case be separated from the roofing itself to allow air to circulate between the insulation and the roofing (see Fig. 4). This will enable air currents to carry away a large proportion of the heated air that would otherwise be trapped within the roof space, which will allow the insulation to perform its function more effectively.


Fig. 4. When insulation is installed in a roof, an air gap must be left between the insulation and the bottom of the roof to allow air to circulate next to the roofing.

Slab Insulation

There is one place in a house where the flexible insulating materials described so far are unsuitable. This is around the perimeter of a poured-concrete slab-on-grade floor (see Fig. 5; and see also SLAB-ON-GRADE). A considerable proportion of the heat contained within such a floor is transmitted to the perimeter of the slab and thence into the surrounding soil. Insulation is required to prevent this loss of heat. But a flexible insulating material such as a mineral wool blanket has not the strength to support the weight of the slab nor is it resistant to moisture in the soil—and wet insulation loses almost all its insulating properties.


Fig. 5. The ways in which rigid insulation can he placed around the perimeter of a slab floor to prevent the loss of heat from the slab.

In order to be effective, any insulating material installed around the perimeter of a floor slab must be impermeable to water, or immune to water damage, and it must be strong enough to resist the crushing loads imposed by the slab, It must also be unaffected by fungi or termites. Materials having these qualities, more or less, include cellular glass, mineral wool that has been impregnated with a binder that stiffens the wool into a rigid mass, and foamed plastics, such as polystyrene and polyurethane.

Slab insulation materials are available as rigid panels that are anywhere from 12 x 32 to 20 x 96 in. in size, though the most common size is 24 x 48 in. The usual panel thicknesses are 1, 2, 3, and 4 in.

Cellular Glass

Cellular glass insulation is made in slabs 2, 3, 4, and 5 in. thick. This material has a crushing strength of about 150 lb per sq in. and can thus be used under a floor slab. It is possible, however, for moisture to soak into the insulation, and the slabs should, therefore, be dipped in hot coal tar or be completely coated with asphalt before they are installed. Otherwise, if the insulation should be subjected to wetting and freezing cycles during the winter, the surface of the insulation may spall away, exposing the interior to moisture. The insulation should also be surrounded by a vapor barrier to further protect it against moisture.

Rigid Mineral Wool

Rigid panels of mineral wool are available in thicknesses of 3/4, 1, 1 1/2, and 2 in. This insulation has a crushing strength of only 12 lb per sq in. and , therefore, is satisfactory for use as perimeter insulation only if it's installed vertically, so that it's not subjected to any crushing loads. The slabs of insulation should also be completely coated with asphalt to render them waterproof; otherwise if the binder should be exposed continuously to moisture, the slabs will disintegrate. The slabs should also be covered by a vapor barrier to further protect them from moisture.

Foamed Plastic Insulation

There are two foamed plastics in common use, polystyrene and polyurethane. Both these materials are widely used in the solid form. The only difference between these plastics as solid materials and as foams is that a gas is introduced into them during the manufacturing process that causes the plastic to expand about 40 times its original volume. Both of these foamed plastics are strong, lightweight, and easy to cut and shape with simple hand tools such as knives and saws. They are immune to attack from fungi or vermin and they are completely waterproof.

Although polystyrene insulation must be bought in slab form because the manufacturing process requires a source of steam heat as well as molds, the polyurethane insulation can be purchased either as solid slabs or in the form of two liquids that, when they are mixed together, foam in place.

Of the two, polystyrene costs about half as much as polyurethane, but the insulating value of polyurethane is about twice that of polystyrene, so that the price per R value is approximately the same.

Neither of these materials is widely used for home construction, however, since both cost much more than the more traditional insulating materials described in this article. Where they are widely used is around and under floor slabs. Here their insulating properties as well as their resistance to moisture and their ability to resist crushing loads makes them the preferred material, despite their high cost.

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