A building site for a row of riverside apartment blocks in Cambridge. The buildings are being constructed using a systems build with a steel frame and various prefabricated components. The blue plastic on the central building is the vapour barrier for the thermal wall insulation before the exterior cladding has been fixed.

Building insulation materials are the building materials that form the thermal envelope of a building or otherwise reduce heat transfer.

Insulation may be categorized by its composition (natural or synthetic materials), form (batts, blankets, loose-fill, spray foam, and panels), structural contribution (insulating concrete forms, structured panels, and straw bales), functional mode (conductive, radiative, convective), resistance to heat transfer, environmental impacts, and more. Sometimes a thermally reflective surface called a radiant barrier is added to a material to reduce the transfer of heat through radiation as well as conduction. The choice of which material or combination of materials is used depends on a wide variety of factors. Some insulation materials have health risks, some so significant the materials are no longer allowed to be used but remain in use in some older buildings such as asbestos fibers and urea.

Consideration of materials used

Factors affecting the type and amount of insulation to use in a building include:

Considerations regarding building and climate:

Often a combination of materials is used to achieve an optimum solution and there are products which combine different types of insulation into a single form.

Spray foam

See also: Spray foams (insulation)

Spray foam is a type of insulation that is sprayed in place through a gun. Polyurethane and isocyanate foams are applied as a two-component mixture that comes together at the tip of a gun, and forms an expanding foam. Cementitious foam is applied in a similar manner but does not expand. Spray foam insulation is sprayed onto concrete slabs, into wall cavities of an unfinished wall, against the interior side of sheathing, or through holes drilled in sheathing or drywall into the wall cavity of a finished wall.



Advantages of closed-cell over open-cell foams

Advantages of open-cell over closed-cell foams


Cementitious foam
One example is AirKrete,[5] at R-3.9 (RSI-0.69) per inch and no restriction on depth of application. Non-hazardous. Being fireproof, it will not smoke at all upon direct contact with flame, and is a two-hour firewall at a 3.5 in (89 mm) (or normal 2 in × 4 in (51 mm × 102 mm) stud wall) application, per ASTM E-814 testing (UL 1479). Great for sound deadening; does not echo like other foams. Environmentally friendly. Non-expansive (good for existing homes where interior sheathing is in place). Fully sustainable: Consists of magnesium oxide cement and air, which is made from magnesium oxide extracted from seawater. Blown with air (no CFCs, HCFCs or other harmful blowing agents). Nontoxic, even during application. Does not shrink or settle. Zero VOC emission. Chemically inert (no known symptoms of exposure per MSDS). Insect resistant. Mold Proof. Insoluble in water. Disadvantages: Fragile at the low densities needed to achieve the quoted R value[6] and, like all foams, it is more expensive than conventional fiber insulations. In 2010, the Ontario Building Code Commission ruled that AirKrete did not conform to requirements for a specific application in the building code. Their ruling states "As the proposed insulation is not impermeable, it could allow water or moisture to enter the wall assembly, which could then cause damage or deterioration of the building elements."[7] As of 2014-08-21, the domain airkretecanada.com appears to be abandoned.
Typically R-5.6 (RSI-0.99)[8] or slightly better after stabilization – higher values (at least R-7, or RSI-1.23) in stabilized boards.[9] Less flammable than polyurethane.
Phenolic injection foam
Such as Tripolymer R-5.1 per inch (ASTM-C-177). Known for its air sealing abilities. Tripolymer can be installed in wall cavities that have fiberglass and cellulose in them. Non-hazardous. Not restricted by depth of application. Fire resistant – flame spread 5, smoke spread 0 (ASTM-E-84) – will not smoke at all upon direct contact with flame and is a two-hour firewall at a 3.5 in (89 mm), or normal 2 in × 4 in (51 mm × 102 mm) stud wall, application per ASTM E-199. Great for sound deadening, STC 53 (ASTM E413-73); does not echo like other foams. Environmentally friendly. Non-expansive (good for existing homes where interior sheathing is in place). Fully sustainable: Consists of phenolic, a foaming agent, and air. Blown with air (no CFCs, HCFCs or other harmful blowing agents). Nontoxic, even during application. Does not shrink or settle. Zero VOC emission. Chemically inert (no known symptoms of exposure per MSDS). Insect resistant. Mold Proof. Insoluble in water. Disadvantages: Like all foams, it is more expensive than conventional fiber insulations when only comparing sq ft pricing. When you compare price to R value per sq ft the price is about the same.
Polystyrene (expanded polystyrene (EPS) and extruded polystyrene (XPS))
Closed-cell polyurethane
White or yellow. May use a variety of blowing agents. Resistant to water wicking and water vapor.: An example of a commercial closed-cell polyurethane product:

Ecomate ®

R-8 per inch. Ecomate ® is a trademarked foam blowing agent technology and family of polyurethanes which has a neutral impact on the environment [1](the worldwide patent was awarded to Foam Supplies Incorporated (FSI) in 2002. This is a new generation eco-friendly foam blowing agent that is free of Chlorofluorocarbons (CFCs), Hydrochlorofluorocarbons (HCFCs), and Hydrofluorocarbons (HFCs) based on naturally occurring methyl methanoate.

Open-cell (low density) polyurethane
White or yellow. Expands to fill and seal cavity, but expands slowly, preventing damage to the wall. Resistant to water wicking, but permeable to water vapor. Fire resistant. Some types of polyurethane insulation are pour-able.

Here are two commercial open-cell, low-density polyurethane products:

Icynene is a trademarked brand of isocyanate open-cell spray foam from Huntsman Building Solutions. The classic version has a thermal resistance (R value) of 3.7 per inch and other versions have even higher values.[10] The formula also includes a flame retardant. Icynene uses water for its spray application and the chemical expansion is caused by the carbon dioxide generated between the water and isocyanate material. Icynene will expand up to 100 times it original size within the first 6 seconds of being applied. Icynene contains no ozone-depleting substances such as CFCs, HFC's, HCFC's. Icynene contains volatile organic compounds (VOCs). Icynene will not emit any harmful gases once cured. Icynene has a Global warming potential of 1. Flammability is relatively low. Icynene maintains its efficiency with no loss of R-Value for the life of the install. Icynene is more expensive compared to traditional insulation methods. Any potential for harm is primarily during the installation phase and particularly for installers.[11] The manufacture of icynene involves many toxic petrochemicals.
Sealection 500 spray foam
R-3.8 (RSI-0.67) per inch.[12] a water-blown low density spray polyurethane foam that uses water in a chemical reaction to create carbon dioxide and steam which expands the foam. Flame spread is 21 and smoke developed is 217 which makes it a Class I material (best fire rating). Disadvantages: Is an Isocyanate.

Insulating concrete forms

Main article: Insulating concrete form

Insulating concrete forms (ICFs) are stay-in-place formwork made from insulating materials to build energy-efficient, cast-in-place, reinforced concrete walls.

Rigid panels

Main article: Rigid panel

Rigid panel insulation, also known as continuous insulation[13] can be made from foam plastics such as polyisocyanurate or polystyrene, or from fibrous materials such as fiberglass, rock and slag wool. Rigid panel continuous insulation is often used to provide a thermal break in the building envelope, thus reducing thermal bridging.

Structural insulated panels

Structural insulated panels (SIPs), 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 a high R-value (7 to 9), high strength, and good chemical and moisture resistance.

SIPs 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.



Fiberglass batts and blankets (glass wool)

Batts are precut, whereas blankets are available in continuous rolls. Compressing the material reduces its effectiveness. Cutting it to accommodate electrical boxes and other obstructions allows air a free path to cross through the wall cavity. One can install batts in two layers across an unfinished attic floor, perpendicular to each other, for increased effectiveness at preventing heat bridging. Blankets can cover joists and studs as well as the space between them. Batts can be challenging and unpleasant to hang under floors between joists; straps, or staple cloth or wire mesh across joists, can hold it up.

Gaps between batts (bypasses) can become sites of air infiltration or condensation (both of which reduce the effectiveness of the insulation) and requires strict attention during the installation. By the same token careful weatherization and installation of vapour barriers is required to ensure that the batts perform optimally. Air infiltration can be also reduced by adding a layer of cellulose loose-fill on top of the material.


Natural fiber

Thermographic image of a vulture in the winter. The vulture's feathers trap air, insulating the vulture from the cold.

Natural fiber insulations, treated as necessary with low toxicity fire and insect retardants, are available in Europe :[15] Natural fiber insulations can be used loose as granulats or formed into flexible or semi-rigid panels and rigid panels using a binder (mostly synthetic such as polyester, polyurethane or polyolefin). The binder material can be new or recycled.

Examples include cork,[16] cotton, recycled tissue/clothes, hemp, flax, coco, wool, lightweight wood fiber, cellulose, seaweed, etc. Similarly, many plant-based waste materials can be used as insulation such as nut shells, corncobs, most straws including lavender straw, recycled wine bottle corks (granulated), etc. They usually have significantly less thermal performance than industrial products; this can be compensated by increasing thickness of the insulation layer.[17] They may or may not require fire retardants or anti-insect/pest treatments. Clay coating is a nontoxic additive which often meets these requirements.

Traditional clay-impregnated light straw insulation has been used for centuries in the northern climates of Europe. The clay coating gives the insulation a half hour fire rating according to DIN (German) standards.

An additional source of insulation derived from hemp is hempcrete, which consists of hemp hurds (shives) mixed with a lime binder.[18] It has little structural strength but can provide racking strength and insulation with comparable or superior R-values depending on the ratio of hemp to binder.[19]

Cork insulation Board

Main article: Cork thermal insulation

During the 2nd century C.100 -C.200 it was the first time human civilisation was introduced to material of cork, and it was only until the 19th century when cork was widely used leading to major industrial production.[20] Cork, which is harvested from the Oak trees generally found in Portugal, Spain and other Mediterranean countries. When a tree reaches 20 to 35 years old, it can be harvested in 10-year intervals for more than 200 years. Oak bark has a lattice-like molecular structure filled with millions of air bubbles giving the bark resilience, elasticity, thermal insulating, acoustic dampening, and shock absorbing properties. The material is sustainable, reusable and recyclable.

There are two types of cork, the pure cork, which is preferable due to its natural bonding properties, and the agglomeration cork. The pure cork is made by processes of heating and steaming whereby cork granulates are molded into a block. The natural resin of the cork acts as a bonding agent. An artificial bonding agent is required for the production of agglomeration cork.

Cork is typically used for acoustic and thermal insulation within walls, floors, ceilings and facades. A natural fire retardant, thermal insulating cork board is also non-allergenic, simple-to-install and a considerably safer substitute to fiber and plastic based insulation. Notable challenges with cork include difficulty in maintenance and cleaning especially if the material is exposed to heavy use such as insulation for flooring. Minor damages to cork surface can make the material more prone to staining.[21]

Sheep's wool insulation

Main article: Sheep wool

Sheep's wool insulation is a very efficient thermal insulator with a similar performance to fiberglass, approximately R13-R16 for a 4-inch-thick layer.[22] Sheep's wool has no reduction in performance even when condensation is present, but its fire retarding treatment can deteriorate through repeated moisture.[23] It is made from the waste wool that the carpet and textile industries reject, and is available in both rolls and batts for both thermal and acoustic insulation of housing and commercial buildings. Wool is capable of absorbing as much as 40% of its own weight in condensation while remaining dry to the touch. As wool absorbs moisture it heats up and therefore reduces the risk of condensation. It has the unique ability to absorb VOC gases such as formaldehyde, nitrogen dioxide, sulphur dioxide and lock them up permanently. Sheep's wool insulation has a long lifetime due to the natural crimp in the fibre, endurance testing has shown it has a life expectancy of over 100 years.

Wood fiber

Main article: Wood fibre

Wood fiber insulation is available as loose fill, flexible batts and rigid panels for all thermal and sound insulation uses. It can be used as internal insulation : between studs, joists or ceiling rafters, under timber floors to reduce sound transmittance, against masonry walls or externally : using a rain screen cladding or roofing, or directly plastered/rendered,[24] over timber rafters or studs or masonry structures as external insulation to reduce thermal bridges. There are two manufacturing processes:

Cotton batts

Cotton insulation is increasing in popularity as an environmentally preferable option for insulation. It has an R-value of around 3.7 (RSI-0.65), equivalent to the median value for fiberglass batts. The cotton is primarily recycled industrial scrap, providing a sustainability benefit. The batts do not use the toxic formaldehyde backing found in fiberglass, and the manufacture is nowhere near as energy intensive as the mining and production process required for fiberglass. Boric acid is used as a flame retardant. A small quantity of polyolefin is melted as an adhesive to bind the product together (and is preferable to formaldehyde adhesives). Installation is similar to fiberglass, without the need for a respirator but requiring some additional time to cut the material. Cotton insulation costs about 10-20% more than fiberglass insulation.[23] As with any batt insulation, proper installation is important to ensure high energy efficiency.[25]



Loose-fill (including cellulose)

Main article: Cellulose insulation

See also: Cavity wall insulation

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.[26] They can also be sprayed in place, usually with water-based adhesives. Many types are made of recycled materials (a type of cellulose) and are relatively inexpensive.

General procedure for retrofits in walls:





US regulatory standards for cellulose insulation


Main article: Aerogel

Skylights, solariums and other special applications may use aerogels, a high-performance, low-density material. Silica aerogel has the lowest thermal conductivity of any known substance (short of a vacuum), and carbon aerogel absorbs infrared radiation (i.e., heat from sun rays) while still allowing daylight to enter. The combination of silica and carbon aerogel gives the best insulating properties of any known material, approximately twice the insulative protection of the next best insulative material, closed-cell foam.

Straw bales

Main article: 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. "Research by Joe McCabe at the Univ. of Arizona found R-value for both wheat and rice bales was about R-2.4 (RSI-0.42) per inch with the grain, and R-3 (RSI-0.53) per inch across the grain. A 23" wide 3 string bale laid flat = R-54.7 (RSI-9.64), laid on edge (16" wide) = R-42.8 (RSI-7.54). For 2 string bales laid flat (18" wide) = R-42.8 (RSI-7.54), and on edge (14" wide) = R-32.1 (RSI-5.66)" (Steen et al.: The Straw Bale House, 1994). Using a straw bale in-fill sandwich roof greatly increases the R value. This compares very favorably with the R-19 (RSI-3.35) of a conventional 2 x 6 insulated wall. 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.

Reflective insulation and radiant barriers

Main article: Radiant barrier

Reflective insulation and radiant barriers reduce the radiation of heat to or from the surface of a material. Radiant barriers will reflect radiant energy. A radiant barrier by itself will not affect heat conducted through the material by direct contact or heat transferred by moist air rising or convection. For this reason, trying to associate R-values with radiant barriers is difficult and inappropriate. The R-value test measures heat transfer through the material, not to or from its surface. There is no standard test designed to measure the reflection of radiated heat energy alone. Radiated heat is a significant means of heat transfer; the sun's heat arrives by radiating through space and not by conduction or convection. At night the absence of heat (i.e. cold) is the exact same phenomenon, with the heat radiating described mathematically as the linear opposite. Radiant barriers prevent radiant heat transfer equally in both directions. However, heat flow to and from surfaces also occurs via convection, which in some geometries is different in different directions.

Reflective aluminum foil is the most common material used as a radiant barrier. It has no significant mass to absorb and retain heat. It also has very low emittance values "E-values" (typically 0.03 compared to 0.90 for most bulk insulation) which significantly reduces heat transfer by radiation.

Types of radiant barriers

Radiant barriers can function as a vapor barriers and serve both purposes with one product.

Materials with one shiny side (such as foil-faced polystyrene) must be positioned with the shiny side facing an air space to be effective. An aluminum foil radiant barrier can be placed either way – the shiny side is created by the rolling mill during the manufacturing process and does not affect the reflective of the foil material. As radiant barriers work by reflecting infra-red energy, the aluminum foil would work just the same if both sides were dull.

Reflective Insulation

Aluminum panel facing an air space.

Insulation is a barrier material to resist/reduce substance (water, vapor, etc. ) /energy (sound, heat, electric, etc.) to transfer from one side to another.

Heat/ Thermal Insulation is a barrier material to resist / block / reflect the heat energy (either one or more of the Conduction, Convection or Radiation) to transfer from one side to another.

Reflective Insulation is one of the Heat/Thermal Insulation to reflect Radiation Heat (Radiant Heat) transfer from one side to another due to the reflective surface (or low emittance).

There are a lot of definitions about “Thermal/Heat Insulation” and the common misinterpretation of “Thermal/Heat Insulation” = “Bulk/Mass/Batt Insulation” which is actually uses to resist Conduction Heat Transfer with certain "R-Value".

As such Materials reflecting Radiant Heat with negligible “R-Value” should also be classified as “Thermal/ Heat Insulation”.

Thus Reflective Insulation = Radiant Barrier



Hazardous and discontinued insulation

Certain forms of insulation used in the past are now no longer used because of recognized health risks.

Urea-formaldehyde foam (UFFI) and panels

Urea-formaldehyde insulation releases poisonous formaldehyde gas, causing indoor air quality problems. 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. Most states outlawed it in the early 1980s after dangers to building occupants were discovered. However emissions are highest when the urea-formaldehyde is new and decrease over time, so houses that have had urea-formaldehyde within their walls for years or decades do not require remediation.

UFFI provides little mechanical strength, as the material is weak and brittle. Before its risks were recognized, it was used because it was a cheap, effective insulator with a high R-value and its open-cell structure was a good acoustic insulator. Though it absorbed moisture easily, it regained effectiveness as an insulator when dried.[31]


Asbestos is a mineral fiber that occurs in rock and soil[32] that has traditionally been used as an insulation material in many homes and buildings. It is fireproof, a good thermal and electrical insulator, and resistant to chemical attack and wear. It has also been found 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).

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:[33]

Health and safety issues

Spray polyurethane foam (SPF)

All polyurethane foams are composed of petrochemicals. Foam insulation often uses hazardous chemicals with high human toxicity, such as isocyanates, benzene and toluene. The foaming agents no longer use ozone-depleting substances. Personal Protective Equipment is required for all people in the area being sprayed to eliminate exposure to isocyanates which constitute about 50% of the foam raw material.[2]


Fiberglass is the most common residential insulating material, and is usually applied as batts of insulation, pressed between studs. Health and safety issues include potential cancer risk from exposure to glass fibers, formaldehyde off-gassing from the backing/resin, use of petrochemicals in the resin, and the environmental health aspects of the production process. Green building practices shun Fiberglass insulation.

The World Health Organization has declared fiber glass insulation as potentially carcinogenic (WHO, 1998[34]). 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 acoustical 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 US 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." However, manufacturers continue to provide cancer risk warning labels on their products, apparently as indeminfication against claims.

However, the literature should be considered carefully before determining that the risks should be disregarded. The OSHA chemical sampling page provides a summary of the risks, as does the NIOSH Pocket Guide.

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.

Fiberglass is energy intensive in manufacture. Fiberglass fibers are bound into batts using adhesive binders, which can contain adhesives that can slowly release formaldehyde over many years.[35] The industry is mitigating this issue by switching to binder materials not containing formaldehyde; some manufacturers offer agriculturally based binder resins made from soybean oil. Formaldehyde-free batts and batts made with varying amounts of recycled glass (some approaching 50% post-consumer recycled content) are available.

Loose-fill cellulose

Cellulose is 100% natural and 75–85% of it is made from recycled newsprint. Health issues (if any) appear to be minor, and most concerns around the flame retardants and mold potential seem to be misrepresentations.[36][original research?]

US Health and Safety Partnership Program

In May 1999, the North American Insulation Manufacturers Association began implementing a comprehensive voluntary work practice partnership with the US 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).

See also


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  16. ^ "Liège Spécial Façade / Cork ETICS external thermal insulation systems", Aliecor.com, Retrieved on 2009-03-26
  17. ^ "Insulation Materials". Energy.gov. Retrieved 24 February 2019.
  18. ^ Arehart, Jay H.; Nelson, William S.; Srubar, Wil V. (2020-09-01). "On the theoretical carbon storage and carbon sequestration potential of hempcrete". Journal of Cleaner Production. 266: 121846. doi:10.1016/j.jclepro.2020.121846. ISSN 0959-6526. S2CID 219024537.
  19. ^ "What is the R-value of Hempcrete? – Hempsteads". Hempsteads.info. Retrieved 2018-05-22.
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  29. ^ "Department of Energy – Cellulose Insulation Material guide". Eere.energy.gov. 2009-02-24. Retrieved 2009-05-08.
  30. ^ "Barndominium Insulation [Expert Guide] + [Proven Techniques]". 2024-01-13. Retrieved 2024-01-14.
  31. ^ "Urea-formaldehyde foam insulation (UFFI)". www.looksmarthomeinspections.com. Retrieved 2024-01-14.
  32. ^ "Learn About Asbestos". 5 March 2013.
  33. ^ "Learn About Asbestos". 5 March 2013.
  34. ^ "Man-made mineral fibres (EHC 77, 1988)". Inchem.org. Retrieved 24 February 2019.
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