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Thermal insulation

Encyclopedia : T : TH : THE : Thermal insulation

This page is about thermal insulation. For electrical insulation see electrical insulation, and for sound insulation see soundproofing. Often the same techniques used for thermal insulation can be used for sound insulation as well.
Insulation must not be confused with insolation (the latter word has an "o" where the former has a "u").
Insulation is any material used to reduce or “slow down” or “resist” the flow of energy. There are several different types of insulators: A material may insulate well in more than one way. Some materials, such as diamond, are superb insulators in one way (electrical), but extremely poor insulators in another way (thermal). A purified synthetic diamond conducts heat even better than copper, and has the highest thermal conductivity of any known solid at room temperature. Thus it is the worst thermal insulator known that's solid at room temperature.

Heat is the internal kinetic, vibrational energy that all materials contain (except at absolute zero). Heat spontaneously flows from a high temperature region to a low temperature region, and the greatest heat flow occurs through the path of least resistance.

The proximity of a high temperature region to a low temperature region constitutes a temperature gradient. Thermal insulation maintains a thermal gradient by reducing the flow of heat across the temperature gradient.

Insulation exists in most large appliances, for example, in ovens, refrigerators, freezers, and water heaters. In some cases, the insulation serves to prevent heat loss to the environment. In other cases, it serves to prevent heat gain from the environment.

Contents

Heat transfer

Materials transfer heat in three ways: Conduction, the most common means of heat transfer in solids, is the transfer of heat through atoms and molecules that are in direct contact with one another, when there is a temperature gradient between them. Materials with high thermal conductivity, such as metals, transfer nearly all of the heat via electrons, whereas materials with low thermal conductivity – insulators – transfer nearly all of the heat via phonons.

Convection, the most common means of heat transfer in liquids and gases, is the transfer of heat via a combination of conduction and fluid flow. For example, when you heat water on a stove, the entire pot of water warms up uniformly. The reason is that hot water at the bottom of the pan doesn’t just heat surrounding water via conduction, but also becomes less dense, rises, and heats the water in the middle and at the top of the pan. Once the water cools, it becomes more dense, sinks, picks up heat from the bottom, and continues the process.

Radiative heat transfer is the transfer of heat via photons (electromagnetic radiation). Radiation is the only form of heat transfer that can occur in the absence of any form of medium – through a vacuum. Movements of atoms and molecules cause the emission of photons into the surroundings constantly, carrying away some of the heat. Atoms and molecules are also constantly absorbing photons from other atoms and molecules. Since hotter materials emit more radiation than cooler materials, a net transfer of heat occurs from the hotter materials to the cooler materials. For room temperature objects, the majority of photons emitted are in the infrared spectrum. Hotter objects transfer heat through emission of photons in the visible spectrum or beyond

Insulators reduce the flow of heat by reducing one or more of these heat transfer mechanisms. For example, a layer of thick foam can reduce convection and conduction. A reflective metallic film or coat of white paint can reduce thermal radiation. Some materials are good insulators using one heat-transfer mechanism, but poor insulators using another. For example, metals tend to be excellent radiative insulators, but poor thermal (conductive) insulators.

For the remainder of this article, the term “insulation” will mean thermal building insulation.

Reasons for insulation

Americans spend large chunks of their money on heating and cooling. A house that is properly and sufficiently insulated:

Two indications that a house is poorly insulated and poorly ventilated: The phrase “thermal envelope” refers to the conditioned or living space in a house. Normally, people treat the attic and basement as unconditioned space, but you can make either the basement, or the attic, or both, part of the conditioned space, if you choose.

Materials used for thermal insulation

A wide variety of substances can serve as insulators. For example, an insulator can be organic, inorganic, fibrous, cellular, reflective, rigid, soft, or granular. Most organic insulators are made from petrochemicals and recycled plastic. Most inorganic insulators are made from recycled materials such as glass and furnace slag.

A new home might use half a dozen different insulating materials in different parts of the house. Each type of insulation is suited to specific tasks, and not suited to others.

Some types of insulation are difficult to find. Your local hardware store may only have fiberglass and polystyrene. Furthermore, some insulation is difficult to install yourself and you would be smart to hire a contractor.

While relatively poor insulators, adobe, earth, stone, and concrete can regulate indoor temperature by damping the daily swings in outdoor temperature. See thermal mass.

Commonly-used insulation materials in residential construction include batts and blankets, loose-fill, spray foam, rigid panels, and radiant barriers. Most residential insulation materials are primarily effective because they have low thermal conductivity; some are primarily effective at blocking convection or radiation. Some materials, for example, foil-faced rigid panels, reduce heat transfer effectively in more than one way.

Good thermal insulating materials can be styrofoam,aluminim foil/mylar,fiber glass batting,and fiber glass board.

Batts and blankets

These come in various thicknesses, depending on how much insulation capacity you need. Batts are precut, whereas blankets are available in continuous rolls. Blankets are ideal for covering joists and studs as well as the space between them. If you don't cover joists and studs with insulation, heat will tend to flow through them (known as "bridging" or "short-circuiting"), since they are the path of least resistance. Similarly, you can install batts in two layers across an unfinished attic floor, perpendicular to each other, for increased effectiveness.

Batts come in standard widths designed to fit snugly between framing (joists, rafters, studs) on 16-inch or 24-inch centers in ceilings, floors, and walls, and are generally easier to handle than blankets. However, batts can be challenging and unpleasant to install between joists under floors. You can run straps, or staple cloth or wire mesh across joists, to prevent insulation from sagging or falling down. Compressing batts reduces their effectiveness. Cut them to accommodate electrical boxes and other obstructions, and do not leave items resting on the batts.

Batts and blankets are susceptible to poor installation. Installers tend to leave bypasses (air gaps) that drastically reduce their effectiveness. Warm air and moisture always find their way to these bypasses. Even if you close all bypasses around batts and blankets, they remain poor barriers to air infiltration and are susceptible to convection loops, especially when there are large temperature differences on either side of the insulation (such as during cold weather). You can reduce air infiltration by adding a layer of cellulose loose-fill on top of the fiberglass. The only way to virtually eliminate air infiltration is with a near-perfect vapor barrier. An attic that only has fiberglass batts to keep out warm, moist air, but a poor or nonexistent vapor barrier, will probably not keep out the warm, moist air.

Commonly-used batts and blankets:

In May 1999, the North American Insulation Manufacturers Association began implementing a comprehensive voluntary work practice partnership with the U.S. Occupational Safety and Health Administration (OSHA). The program, known as the Health and Safety Partnership Program, or HSPP, promotes the safe handling and use of insulation materials and incorporates education and training for the manufacture, fabrication, installation and removal of fiber glass, rock wool and slag wool insulation products. (See health effects of fiberglass). (For authoritative and definitive information on fiber glass and rock and slag wool insulation, as well as the HSPP, consult the North American Insulation Manufacturers Association (NAIMA) website (www.naima.org).)

Natural fibre insulations (around 0.04 W/mK) all can be treated with low toxicity fire and insect retardents, available in Europe

Loose-fill and spray-in-place

Loose-fill materials can be blown into attics, finished wall cavities, and hard-to-reach areas. They are ideal for these tasks because they conform to spaces and fill in the nooks and crannies. They can also be sprayed in place, usually with water-based adhesives. They frequently consist of recycled materials, and therefore, are relatively inexpensive.

General procedure for retrofits:

Advantages The following regulatory standards are applicable to cellulose insulation. Whether individual cellulose insulation products comply with these standards must be determined on a case-by-case basis: Disadvantages of loose-fill: Commonly used loose-fill:

Loose-fill that is not often used but perfectly acceptable given the right thickness

You can expect to find one or more of these in an old house.

Spray foams (foam-in-place)

Normally, you would hire a contractor to install spray foam between wall studs. The contractor mixes the chemicals on site using special measuring and mixing equipment, and sprays or injects the expanding foam into the open wall cavity of an unfinished wall, or through holes drilled in the sheathing or drywall into the wall cavity of a finished wall.

Advantages of spray foams:

Disadvantages of spray foams: Advantages of closed-cell foams over open-cell foams: Commonly-used spray foams: Lingering question: If you install spray foam between rafters, and your attic is part of the conditioned space of your house (not kept cold in the winter), do you still need to leave an air baffle to allow air circulation against the underside of the sheathing?

Rigid panels

Rigid panel insulation is made from fibrous materials (fiberglass, rock and slag wool) or, more commonly, from plastic foam. They are sometimes sold in sections designed to fit tightly in standard wall cavities. When sold this way, they are called "batts", and they come in different thicknesses to match the depth of your wall cavity, for example, approx. 5½ inches to match a 2 x 6 wall cavity.

Where rigid panels are most-often used:

Important note #1: If you insulate the foundation with rigid panels, but you stop using rigid panels where the siding begins, then you should install flashing in between the bottom course of siding and the top edge of the rigid panels, to prevent water from seeping behind the panels.

Important note #2: When insulating the exterior foundation, you should install the rigid panels in two staggered layers, and fill the gaps at the seams with spray foam, to keep moisture from penetrating from the outside. However, when insulating between the sheathing and siding, you should leave slight gaps between the rigid panels to allow moisture to escape from the exterior side of the sheathing.

Advantages of rigid panel insulation:

Disadvantages of rigid panel insulation: Commonly-used rigid panels: Natural fibre insulations (around 0.04 W/mK) all can be treated with low toxicity fire and insect retardents, often used in Europe

Structural insulating panels

Structural insulating panels (SIP's), also called stressed-skin walls, use the same concept as in foam-core external doors, but extend the concept to the entire house. They can be used for ceilings, floors, walls, and roofs. The panels usually consist of plywood, oriented strandboard, or drywall glued and sandwiched around a core consisting of expanded polystyrene, polyurethane, polyisocyanurate, compressed wheat straw, or epoxy. Epoxy is too expensive to use as an insulator on its own, but it has high R-value per inch (7 to 9), high strength, and good chemical and moisture resistance.

SIP's come in various thicknesses. When building a house, they are glued together and secured with lumber. They provide the structural support, rather than the studs used in traditional framing.

Advantages of SIP's:

Disadvantages of SIP's:

Rigid cellular polystyrene panels

There are many types of rigid cellular polystyrene (RCPS). Styrofoam is simply Dow Chemical's brand name, and does not refer to any particular type of RCPS. Some polystyrene uses up to 50% recycled resin, including post-consumer plastic. Several states have banned polystyrene that uses CFC's as blowing agents.

Urea-formaldehyde foam (UFFI) and panels - no longer used

Description: Brittle, whitish gray or yellow foam produced through mixing of urea and formaldehyde, developed in Europe in the 1950's. Most states have outlawed urea-formaldehyde insulation since the late 1970's or early 1980's because of its tendency to release formaldehyde gas, causing air pollution and problems of indoor air quality. The entire foam insulation industry suffered a setback during this time.

Reasons for residential ban: The chemical bond between the urea and formaldehyde is weak, resulting in degradation of the foam cells and emission of toxic formaldehyde gas into the home over time. Furthermore, some manufacturers used excess formaldehyde to ensure chemical bonding of all of the urea. Any leftover formaldehyde would escape after the mixing. Since emissions are highest when the urea-formaldehyde is new and decrease over time, houses that have had urea-formaldehyde within their walls for years or decades do not require remediation.

Advantages:

Other disadvantages (besides release of formaldehyde): Links: [link] [link]

Radiant barriers

Main article: Radiant barrier

These materials reduce radiated heat, rather than conducted heat. For this reason, trying to associate R-values with radiant barriers is difficult and inappropriate.

Aerogels

Aerogels are high-performance, low-density materials used in special applications. They are mostly air, but they are not foam. They are still expensive and do not yet find much use in consumer applications.

The properties of silica aerogel complement those of carbon aerogel; the combination has the best insulating properties of any known material.

Straw bale construction

The use of highly-compressed straw bales as insulation, though uncommon, is gaining popularity in experimental building projects for the high R-value and low cost of a thick wall made of straw. Densely-packed straw, on a per-inch basis, only has an R-value of around 1.45, but the total R-value of a straw-bale wall can approach R-20. When using straw bales for construction, the bales must be tightly-packed and allowed to dry out sufficiently. Any air gaps or moisture can drastically reduce the insulating effectiveness.

Asbestos - no longer used

Asbestos once found common use as an insulation material in homes and buildings because it is fireproof, a good thermal and electrical insulator, and resistant to chemical attack and wear. We now know that asbestos can cause cancer when in friable form (that is, when likely to release fibers into the air - when broken, jagged, shredded, or scuffed). Only some people exposed to asbestos come down with cancer. The recommended course of action if you find asbestos in your house is to enclose (shield) and encapsulate (seal). Asbestos-cement shingles are harmless unless you saw into them.

When found in the home, asbestos often resembles grayish-white corrugated cardboard coated with cloth or canvas, usually held in place around pipes and ducts with metal straps. Things that typically might contain asbestos:

Health effects of fiberglass

Fiberglass is the most common residential insulating material, and is usually applied as batts of insulation, pressed between studs.

Fiber glass is now the most thoroughly evaluated insulation material in the market. The fiber glass insulation industry is committed to ensuring that fiber glass products can be safely manufactured, installed and used. This industry has funded tens of millions of dollars of research at leading independent laboratories and universities in the United States and abroad. The weight of the sicentific research shows no association between exposure to glass fibers and respiratory disease or cancer in humans.

In October 2001, an international expert review by the International Agency for Research on Cancer (IARC) re-evaluated the 1988 IARC assessment of glass fibers and removed glass wools from its list of possible carcinogens by downgrading the classification of these fibers from Group 2B (possible carcinogen) to Group 3 (not classifiable as to carcinogenicity in humans). All fiber glass wools that are commonly used for thermal and accoustical insulation are included in this classification. IARC noted specifically: "Epidemiologic studies published during the 15 years since the previous IARC Monographs review of these fibers in 1988 provide no evidence of increased risks of lung cancer or mesothelioma (cancer of the lining of the body cavities) from occupational exposures during manufacture of these materials, and inadequate evidence overall of any cancer risk."

The IARC downgrade is consistent with the conclusion reached by the U.S. National Academy of Sciences, which in 2000 found "no significant association between fiber exposure and lung cancer or nonmalignant respiratory disease in the MVF [man-made vitreous fiber] manufacturing environment."

The World Health Organization however, has declared fiber glass insulation as potentially carcinogenic. The product is still required to carry a cancer warning label in the USA.

Miraflex is a new type of fiberglass batt that has curly fibers that are less itchy and create less dust. You can also look for fiberglass products factory-wrapped in plastic or fabric.

Health effects of loose-fill cellulose

Although cellulose is 100% natural, and usually made from recycled material, loose-fill cellulose is not as environmentally-friendly as some people would have you believe:

Effectiveness of insulation

Thermal insulation usually works best on the outside of the structure, by allowing the walls, floor and ceiling to stay at room temperature, which prevents condensation in the living area of the house. Generally, spray foam and rigid panel insulation provide the most bang for the buck. They have the highest R-values per inch, and provide the best protection against air intrustion (and therefore, convective heat loss).

A well-insulated house requires a vapor barrier because of the risk of condensation on cold parts of the structure with resulting damage, such as mold and rot. The vapour barrier is usually a sealed plastic film inside the wall and should go on the warm side of the insulation:

When you are building a house or remodeling any rooms in the house, you should install expanding closed-cell spray foam against the interior side of the sheathing, and between the studs, before installing (fire-rated) drywall. (If fiberglass batts exist, remove them.) Advantages of closed-cell spray foam in wall cavities:

Insulation in construction

In the construction industry it is felt roofers who apply felt to a roof, usually in several layers bonded together, to form a watertight covering and keep the interior of the building dry and insulated. It is a skilled job that requires specific training.

Future felt roofers will need a head for heights as the occupation involves working on the top of buildings. They will need to enjoy working outdoors and have good people skills and have a head for heights. A good head for figures is also important, as a felt roofer will be calculating plenty of areas and volumes. Safety is important too, as working at heights can be dangerous.

There are no specific academic requirements to train as a built-up felt roofer, although GCSE passes (D-G) / Standard Grades (4-7) in Math, English and Technology will be helpful for the calculations, measurements and theory. A passion for working outdoors is also important.

R-values

Main article: R-value
The R (Resistance) value of a material indicates its resistance to heat flow. R-values are measured at 75 °F (22 °C) and are calculated from the thermal conductivity, k, and the thickness, d, of materials: R = d/k.

The United States calculates R-values using different units than the rest of the world, namely, feet, degrees Fahrenheit, and British thermal units. Metric R-values are expressed as “RSI”, where the units for R are kelvins times square meters divided by watts (K·m²/W).

Some materials lose or gain R-value during changes in temperatures. This must be taken into account [link] when comparing R-values [link] since some materials, especially blown fiberglass [link], suffer significant loss of insulating ability at lower temperatures. Other materials gain R-value at lower temperatures, such as polystyrene.

Old insulation usually loses its effectiveness to some degree. Furthermore, newer insulation sometimes uses different manufacturing techniques or materials than older versions of the same type of insulation, and can therefore have higher R-values.

Additionally, installation conditions [link] can significantly reduce R-value due to compression, wind washing [link] or temperature differences creating air convection bypassing the insulation through even very small air gaps. Sprayed in place foam and densely packed cellulose insulations [link] avoid air gaps.

The thickness of the materials must be considered when comparing R-values. For instance, the R-value of a 14-inch adobe brick is about R-4, while that of an equal thickness of cellulose, at R-3 per inch, is about R-42.

How much insulation your house should have depends on your building design, climate, energy costs, budget, and personal preference. Each state has different recommendations as to what R-values each component of a house should have.

The Federal Trade Commission (FTC) governs claims about R-values to protect consumers against deceptive and misleading advertising claims. "The Commission issued the R-Value Rule to prohibit, on an industry-wide basis, specific unfair or deceptive acts or practices." (70 Fed. Reg. at 31,259 (May 31 2005).)

The primary purpose of the Rule, therefore, is to correct the failure of the home insulation marketplace to provide this essential pre-purchase information to the consumer. The information will give consumers an opportunity to compare relative insulating efficiencies, to select the product with the greatest efficiency and potential for energy savings, to make a cost-effective purchase and to consider the main variables limiting insulation effectiveness and realization of claimed energy savings.

The Rule mandates that specific R-value information for home insulation products be disclosed in certain ads and at the point of sale. The purpose of the R-value disclosure requirement for advertising is to prevent consumers from being misled by certain claims which have a bearing on insulating value. At the point of transaction, some consumers will be able to get the requisite R-value information from the label on the insulation package. However, since the evidence shows that packages are often unavailable for inspection prior to purchase, no labeled information would be available to consumers in many instances. As a result, the Rule requires that a fact sheet be available to consumers for inspection before they make their purchase.

Typical R-values per inch

The FTC's R-value Rule generally prohibits calculating R-value per inch. (16 C.F.R. 460.20.) The FTC explained the reason for this prohibition: Since the record demonstrates that R-values are not linear, advertisements, labels, and other promotional materials that express a product's thermal resistance in terms of R-value per inch deceive customers. The FTC further explained that references to the R-value for a one-inch thickness of the material will encourage consumers to think that it is appropriate to multiply this figure by the desired number of inches, as though R-value per inch were constant. (44 Fed Reg. at 50,224 (27 August 1979).)

''All values are approximations, based on the average of the values listed on dozens of websites. If I saw wildly different values, then I took the lowest and highest values and expressed the R-value here as a range somewhere between them. For a more-official list, refer to one of these websites with duplicate R-value tables: [link] [link] [link]''

Materials such as natural rock, dirt, sod, adobe, and concrete have poor thermal conductivity (R-value typically less than 1), but work well for thermal mass applications because of their high specific heat.

Where to insulate

Where to insulate depends on where your living or conditioned space (the space that you heat and air-condition) ends and where your unconditioned space begins. Treat unconditioned space as if it were outdoors, minus the rain and snow. Insulate the living space as if you were insulating from the outdoors. For example, if your crawlspace is unheated, and you want it to stay that way, then make sure it has adequate ventilation, and insulate the floor above. If your attic is unheated, and you want it to stay that way, also make sure it has adequate ventilation, and insulate between and over the floor joists.

If you occasionally want to heat only some sections of the living space, you should insulate the walls between the sections you want to heat and the sections you don’t want to heat.

If the basement space is unheated, it may be best to insulate between floor joists (basement ceiling) instead of around the foundation (basement floor and walls). There is no harm done in insulating both the ceiling, and the floor and walls.

Generally, you should insulate:

If you are curious what kind of insulation already exists, here are some ways to inspect your walls for insulation:

Insulating ducts and pipes

Insulate all ducts and water supply pipes where they pass through unconditioned spaces, such as through an attic or crawlspace that is not heated or air-conditioned. This includes heating, ventilation, air-conditioning, and return ducts, and both cold and hot water supply pipes.

Before insulating ducts and pipes:

Why to insulate ducts and pipes: Insulating materials for ducts and pipes: You may want to wrap your water heater in a nonflammable thermal blanket, especially if you have an older, inefficient water heater that does not have much internal insulation.

Insulating around electrical fixtures

To prevent a house fire, keep insulation away from any electrical fixtures that generate great amounts of heat, such as ceiling fan motors, and older recessed lighting fixtures that are not IC-rated. Newer recessed lights contain a heat sensor to turn the lights off when they reach a threshold temperature. The newest recessed lights, known as IC-rated (Insulation Cover) fixtures, are designed so that they do not require any air clearance and will work safely buried deep in insulation. If you have non-IC-rated recessed lighting fixtures, you should install baffles around the fixtures to maintain at least 3 inches of clearance from insulation. This is a stop-gap measure to remedy the excessive heat. Ultimately, you should:

OR

Insulating exterior of foundation

Ideally, a home should have poured concrete walls, waterproofing, and 2-inch rigid foam panels. Complete retrofit foundation insulation may be prohibitively expensive. Since most of the heat loss from a foundation occurs where the foundation is above grade and exposed, you can partially insulate the foundation wall and still have good results:

The sheets of extruded polystyrene foam attached to exterior foundation walls before backfilling serve as insulation, but their main purpose is to protect the waterproof coatings or membranes applied to the foundation wall. You should protect them from backfilling, as well, since they will not function as effectively as possible if they are cracked or torn while backfilling.

Rigid foam panels applied to an external foundation wall:

How to apply insulation to an exterior foundation wall when building a house: Rigid foam panels do not have to stop where the siding begins. You can extend them up, underneath the siding, all the way up to the roof. When you secure them to the sheathing, make sure that you use galvanized nails or coated screws, and make sure they penetrate sufficiently into the studs, without cracking the panels. Unlike with the panels installed against the exterior foundation, with these panels, you should leave slight gaps between them to allow moisture to escape.

Self-insulating foundations

Exterior insulation and finish systems (EIFS) are a new type of home construction that uses “all-in-one” insulating and structural walls with a stucco-like finish. Face-sealed EIFS is susceptible to trapped moisture if it is not installed meticulously. Drainable EIFS allows moisture to escape. Some building experts think that EIFS should only be used in hot, dry climates because of its tendency to collect moisture. [link]

Some contractors pour insulation into concrete blocks while building the foundation. Turn this concept inside out, and you have an insulated concrete form. An insulated concrete form (ICF) is a rigid foam block, usually polystyrene, that homebuilders can stack so their centers are aligned, insert reinforcing bars into, and fill with concrete. These foundations are both structural and insulating – insulation is incorporated directly into the foundation walls, rather than added as an afterthought. The tricky part is making sure that concrete fills all of the voids in the foam blocks.

Here is an example of an ICF home: [Habitat for Humanity ICF Build Apr 2004]

Audits, help, and incentives

In a home energy audit, professionals evaluate the energy efficiency of the home, using blower doors, infrared cameras, and other air leakage measuring equipment. They identify the greatest leaks and recommend the best ways to improve the energy efficiency of your house. They tell you what you should do first for the best bang for the buck.

Who to call for a home energy audit:

Utility companies are usually eager to provide this service, as well as loans and other incentives to insulate. They also often provide incentives to switch, for example, if you are an oil customer considering switching to natural gas.

Where to look for insulation recommendations:

Remember, code tells you the bare minimum. You should always insulate beyond what code requires. There is no reason why you can't insulate far in excess of code.

Insulate as completely as you can while building a house. This is much easier than retrofitting. For example, if you don’t install a closed-cell foam gasket to the sill plate when your house is under construction, there is no way to install it afterwards. You can spray foam into the gap, but this is not nearly as effective as having the gasket installed from day one.

Even though it is much easier to insulate a home as it is being built, you can still add insulation afterwards. Some states provide loans to install insulation in a pre-existing house, or allow you to apply for a tax credit afterwards.

External links

References

See also

 

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