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There are three mechanisms by which this heat transport occurs: conduction, convection and radiation (Illustration 1). Conduction is the reason a frying-pan handle gets hot. The handle is not in the flame, but it is attached to the pan, which is in the flame, and the heat is transferred from molecule to molecule up the handle. Convection is the transport of heat by the molecules of a fluid.

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Information on Insulation

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There are three mechanisms by which this heat transport occurs: conduction, convection and radiation (Illustration 1).
Conduction is the reason a frying-pan handle gets hot. The handle is not in the flame, but it is attached to the pan, which is in the flame, and the heat is transferred from molecule to molecule up the handle.
Convection is the transport of heat by the molecules of a fluid. In the illustration, air molecules warmed by contact with the warm surface rise (warm air rises), flow across to the cooler surface, and warm the cooler surface, thus giving up their extra heat.
Radiation is the transport of heat by electromagnetic waves and is the method by which the sun warms your skin.


R-value
Recognizing these three processes helps to explain the seeming peculiarities of certain insulations but helps little in computing heat loss. To keep things simple, engineers pretend that all heat flow through building surfaces is conductive (Illustration 2) and so can be calculated using the formula for conduction. (See the sidebar on this page.)
Insulation Equals Dead Air
Ordinary unmoving air has a surprisingly high R-value - 5.7 per inch of thickness. Because of this, building insulation is a seeming paradox. In a real sense, you are paying for what you are not getting. Most insulations consist basically of the minimum amount of material required to stop air from moving. Look at most insulations under a magnifying glass, and you'll see thousands of tiny air pockets, between fibers, between particles or within cells.
But unmoving air isn't so ordinary. Warm air is buoyant and wants to rise, while cooler air wants to fall. To capitalize on the insulating property of dead air, we have to trap it within tiny spaces so that it doesn't travel very far. However, in stopping air movement with even minimal material, we add the conductivity of the material to the conductivity of the air. Building insulations are thus trade-offs between the amount of conductive material and the sizes of the air spaces created.
Commercial Insulations
There is a bewildering array of insulations on the market, each product promoted by its manufacturer as the best. Each may, in fact, be best for a particular application but certainly not for all. The smart homeowner or builder will use several different types of insulation in order to achieve optimum thermal performance. Price and R-value are just two of the characteristics with which we should be concerned.
The sidebar on the bottom of this page lists 11 insulation types, their R-values per inch, and the applications for which they are best suited. Material costs vary with season and sales promotions, but most are available at any home center. Most sprayed-in insulations can be installed only by certified contractors. Consult the yellow pages for contractors in your area.
Fiberglass Blanket & Batt
Blankets in long rolls and batts in 4-foot lengths are both available in 15- and 23-inch widths, the perfect sizes to fit between joists, studs and rafters with standard 16- and 24-inch on-center spacings. In this bound form, the fiberglass is less likely to settle and so is perfect for insulating regular framing cavities.
Fiberglass insulation manufacturers have apparently (and unfortunately, to my mind) made a marketing decision to promote the blankets and batts in low-density versions with a rather moderate R-value of 3.2 per inch. Compressing thicker batts into framing cavities is worthwhile: While the total batt R-value goes down, R-value per inch and R-value of the fixed-cavity thickness goes up.
Strong points include low cost, ease of installation and wide availability. A weak point is the difficulty of filling irregular spaces. The best applications are between studs and rafters.
Long-Fiber Blown Fiberglass
Fiberglass is essentially silica, or sand, that has been melted and spun into fibers. The fibers are coated after cooling with a colored resin (pink for Owens Corning, yellow for CartainTeed) and gathered into blankets of varying density, depending on the desired R-value and application. Blown fiberglass is chopped into small clumps with a density of 0.6 pounds per cubic foot. The glass fibers are extraordinarily resilient and resist settling once their settled density has been reached. Therefore, when installed at the proper density, blown fiberglass does not settle in walls and leave empty heat-convection channels. Although the binding resin is combustible, the amount of it present is small enough for the overall product to be considered noncombustible.
Strong points include cost per R-value, inertness and ability to fill irregular spaces. Its weakest point is low R-value per inch. The best application is on an open attic floor.
Sprayed Short-Fiber Fiberglass
This product differs from long-fiber fiberglass in size and length of fibers, and installed density. The fibers and air spaces are smaller, resulting in a higher R-value per inch than the long-fiber version. More than twice the amount of material is required, however, leading to a price that is also more than double. Since the cost of material is usually a small component in the life-cycle cost of insulating, I recommend short-fiber fiberglass over the long-fiber variety, when you can get it. Insul-Safe by CartainTeed is one such product.
Cellulose
Cellulosic fiber is a loose, fluffy, fibrous material intended for blowing into cavities. It is generally made by finely shredding old newspapers. (Some claim the finest to be derived from the New York Times, although I believe that's purely a case of elitism.)
Cellulose is not particularly resilient and must be installed at high density to avoid long-term settling. Minor settling is not very important in an open attic, where R-value is roughly proportional to thickness. Due to the thermal short-circuiting effect of air gaps, however, settling in wall cavities leads to significant loss of R-value.
Unlike fiberglass, cellulose is flammable, so it is treated with a flame retardant. It is important to keep cellulose dry, because saturating the fibers results in a permanent reduction in thickness and R-factor, and a possible loss of fire retardant.
Strong points are low cost, R-value per inch and ability to fill irregular spaces. Weak points are possible settling in walls, susceptibility to water damage, and possible corrosion of pipes and wiring by chemicals. The best application is, once again, on an open attic floor.
Perlite
Perlite is a naturally occurring volcanic mineral containing up to 5 percent water. When the mineral is crushed and then heated rapidly to its melting point, the trapped water turns to steam and blows minute glass bubbles. These highly insulating trapped air cells can be loose-poured, fused into a rigid board, or substituted for sand and gravel in insulating concrete.
Strong points include inertness, noncombustibility and ability to flow into irregular spaces. Weak points include high cost and settling in wall cavities. The best applications are in filling irregular voids between attic fiberglass batts and insulating concrete.
Vermiculite
Vermiculite is a naturally occurring mica with the same quirk as perlite. Between the mica layers are bound water molecules. When the mica is heated rapidly, the water turns to steam before having time to escape, thereby expanding the mica by a factor of 10 or more. Like perlite, vermiculite is noncombustible and completely inert. It must be treated with silicone to repel water, however.
Strong points are pourability and noncombustibility. Weak points are high cost and low R-value. The best application is in filling irregular voids and insulating between masonry and flue liner in a chimney.
Expanded Polystyrene
Expanded polystyrene - or beadboard, as it is appropriately nicknamed - is formed by heating polystyrene plastic beads in a mold until the beads expand and fuse together at their points of contact. In contrast with the extruded polystyrene (see below), beadboard is: 1) more easily permeated by water due to minute passages that remain between the beads; 2) physically weaker due to the type of bonding between cells; and 3) more heat-conductive due to larger cell size. All physical properties degrade with decreasing density, and the density of beadboard is exactly half that of extruded polystyrene.
With half the density (and amount of raw chemical) as the extruded version, beadboard sells for about 50 percent less. For this reason, contractors are sometimes tempted to substitute the lower-cost version when dealing with unsuspecting homeowners. In critical applications, such as below ground, where physical strength and resistance to moisture are critical, this is false economy.
The single strong point of molded polystyrene is the lowest cost per R-value of the foams. Weak points include fragility, low R-value per inch (compared with other foams), and the requirement to cover with a 15-minute fire-rated material in interior applications. The best application is as the core of large gypsum drywall/foam-sheathing panels, which are used to cover the roofs and walls of post-and-beam structures.
Extruded Polystyrene
The extruded version of polystyrene is formed by allowing a mixture of polystyrene, solvent and gas to escape from a pressurized container through a slot into air. As the pressure is released, the gas expands and creates a foam of tiny cells, the walls of which are shared. This shared-wall structure is what makes the foam so strong and so impermeable to moisture.
In areas other than building, extruded polystyrene is used for flotation of docks and for prevention of frost under highways. In residential construction, it has two primary uses: as a foundation insulation and as a nonstructural insulating sheathing for walls and roofs.
Strong points include high strength and near-perfect impermeability to moisture. Weak points are cost per R-value and the requirement to cover with a 15-minute fire-rated material inside the building. The best application is as foundation insulation.
Polyisocyanurate
This foam results from spreading a mixture of isocyanurate and alcohol in a thin sheet on a moving conveyor. The chemical reaction produces a very lightweight and fine-celled foam, filled with an inert, low-conductivity gas. The inert gas gives the foam a very high initial R-value, but the R-value ultimately falls to an "aged" R-value of 5.6 per inch, as the inert gas and ordinary air are slowly exchanged across the cell walls. Foil and asphalt paper facings are often applied to retard the aging process, but their long-range effectiveness is questionable.
Generally, polyisocyanurate is promoted for the same purposes as extruded polystyrene, but there are significant differences: The former has a higher R-value per inch, making it the prime candidate for applications where thickness is a constraint, and the latter retains its original R-value better when installed underground.
The single advantage of polyisocyanurate is high R-value per inch. Its weak point is high cost. The best applications are insulating flat and low-slope roofs under roofing membranes and as nonstructural exterior-wall sheathing under vinyl or aluminum siding.
Polyicynene (Icynene)
This is a modified open-cell urethane foam that uses water and carbon dioxide instead of ozone-depleting fluorocarbon blowing agents. It is applied either by spray gun to exposed framing cavities or by pouring a slightly different formulation into existing closed cavities. Its R-value of 3.6 per inch is about the same as those of blown cellulose and blown short-fiber fiberglass, but significantly less than that of the more expensive urethane foams.
Polyicynene expands to fill framing cavities and adheres to surfaces, so it virtually eliminates air leakage. It is not vapor-impermeable, but its air impermeability prevents significant moisture buildup in wall cavities.
The advantage of polyicynene is its combination of relatively high R-value and air sealing. Its weak point is that it can be installed only by a licensed contractor, with resulting high cost. The best application is in sealing and insulating difficult areas, such as box sills.
Reflective Foils
Reflective aluminum-foil insulations are chameleons, defying the convention of a simple R-value per inch. The polished aluminum foil in most commercial reflective insulations has a reflectivity of about 95 percent, meaning only 5 percent of the normal amount of radiation gets through. A Thermos bottle uses the same principle, coating the inside surfaces of an vacuum bottle, reducing all three mechanisms - conduction, convection and radiation - to near zero. The fly in the ointment is that, except when used in space, the cavities in foil insulations are not evacuated, so we still have conduction and convection to deal with.
Illustration 3 demonstrates how the R-values of building cavities lined with foil can depend on both cavity orientation and season.
The left side of the illustration represents the floor, walls and ceiling of a building in winter, when inside and outside temperatures are 72º F and 0º F, respectively. In all three 4-inch cavities, heat transfer by radiation has been virtually (95 percent) eliminated, while transfer by conduction proceeds normally. Convection is another story. Convective loops transfer heat across both ceiling and wall cavities, but the thermally stratified floor experiences no convection at all. The resulting disparity in total R-value is amazing: R-2.1 for the ceiling, versus R-8.9 for the floor.
The right side of Illustration 3 shows the identical cavities in summer, when the temperature gradients across the surfaces are reversed. We see that the R-values reverse, as well: R-8.9 for the ceiling and R-2.1 for the floor.
The upshot of this experiment is that one must consider the physics when applying reflective foil insulations. They can be very effective in floors over unheated areas in cold climates. They are even more effective in blocking heat from the roof in cooling regions.
But they are, by themselves, a poor choice for northern roofs.
Next issue: The U.S. Department of Energy and various energy-conservation groups publish recommended R-values for floors, walls, ceilings and roofs. How do they arrive at these numbers, and how seriously should we take them? We'll look at the math behind the numbers and see how to decide for ourselves.

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