Choosing the Best Insulation Delivers Energy Savings
In-depth reviews of Cellulose, Cotton, Fiberglass, Foam, and Mineral Wool insulations. Helps you choose the best insulation for the job so you can boost the energy performance of your home and lessen its environmental impact.
Alex Pennock wrote this article, with contributions from Bill Burke and Dietmar Lorenz.
Adding insulation is one of the most important steps you can take to improve the energy performance of your home and lessen its environmental impact. Space heating and cooling consume 44 percent of all energy used in the home, according to the Department of Energy. Adding a few hundred dollars worth of insulation can reduce your annual heating and cooling bill 10 to 30 percent. With the average U.S. household spending about $650 per year on heating and cooling, the investment in additional insulation can pay off quickly. Use this formula from the DOE to estimate the payback period for your home.
Quick links to materials reviewed in this article
Environmental Benefits and Impacts
Instant Energy Savings
According to the Rocky Mountain Institute (RMI), the attic is usually the top priority because installing insulation there is easy and provides immediate benefits. In most cases, homeowners can install attic insulation themselves.
Exterior walls, constituting the largest surface area to be insulated, are also important, as is your home’s foundation—RMI has found that uninsulated foundations can account for 20 percent of heat loss in a home.
You should insulate your home before installing new heating or cooling systems so that you can scale the systems to the efficiency of your house, saving more energy and money.
When selecting an insulation material, the most important environmental consideration is its performance and suitability for your application. Over the lifespan of a home, the energy saved with a well-insulated building envelope far outweighs the environmental impact of insulation’s manufacture. The North American Insulation Manufacturers Association states that the insulation produced annually in the United States saves 12 times the energy its manufacture consumes. While this calculation may ignore the energy required to extract and process materials used to make insulation, it gives you an idea of how much energy insulation can save.
When comparing two materials of equal performance, you should of course consider the environmental impact of each product's manufacture and disposal. Some insulation materials are made from almost entirely nontoxic, abundant or renewable materials, while others are made from limited petroleum resources and are difficult or impossible to recycle.
Insulation materials can affect indoor air quality, though most impacts are small when materials are installed properly. People with chemical sensitivities should check their response to a product before installing it. Concerns that may arise include irritation from airborne fibers and emissions from glues, flame retardants, or other additives, especially if they are bioaccumulative. (Bioaccumulative substances build up in your tissues over time, possibly causing long-term health problems.)
Insulation Pros and Cons in Depth
For a summary of the environmental pros and cons of the materials discussed here, see GreenHomeGuide's Buyer's Guide to Green Insulation.
Cellulose insulation is made primarily from recycled paper. About 75 percent of the material used to make cellulose insulation is post-consumer waste paper, giving it the highest average recycled content of all insulation types. The manufacture of cellulose insulation involves a fraction of the energy use and pollution required to make mineral wool and fiberglass insulation. Additionally, scrap cellulose generated during installation can be reused, cutting down on waste.
Cellulose insulation has no significant effect on indoor air quality. Offgassing of volatile organic compounds (VOCs) contained in ink on newspaper waste in cellulose insulation is not a health concern, since some ink is removed while recycling paper into pulp and much of the ink used is vegetable based. The amount of boron used as a flame retardant in cellulose is harmful only if ingested.
Cellulose insulation is blown into wall and ceiling cavities as well as onto attic surfaces, though to stick to attic ceilings it must be held in by netting or sprayed on wet with an acrylic binder to prevent settling. Like all sprayed or blown insulations, it can be installed into wall cavities through a series of small holes drilled in the wall, causing minimal disturbance during remodeling. It is not suitable for application below grade (below or at the perimeter of the foundation) or in other locations where it would be exposed to moisture.
Cellulose can absorb moisture, decreasing its R-value over time, and if it is exposed to moisture for long periods it will rot and grow mold. In some instances a vapor barrier should be installed once the insulation has fully dried (typically in two weeks) to prevent moisture from reaching the insulation. Good sources of information on construction practices that minimize moisture problems are the climate-specific Builder’s Guides available from the Energy and Environmental Building Association.
Loose-fill cellulose typically costs 25 percent less than fiberglass, though installation may be more expensive. Proper installation is important to prevent undue settling. Wet-spray or dense-pack installations are typically more expensive than fiberglass installation. However, installing cellulose installation with these methods is extremely airtight, so the nominal (rated) R-values are more commonly achieved than with fiberglass batts.
Cotton insulation is made mostly of cotton—a natural, renewable resource—with a small amount of boron as a flame retardant and some polyester. Cotton insulation has a similar R-value to cellulose for a given thickness of insulation.
The majority of cotton used in insulation is recovered from scrap generated in denim manufacturing; one company makes insulation with 85 percent recycled content. Cotton farming is very water- and pesticide-intensive, though manufacturing cotton insulation overall is not a very energy-intensive process. Cotton insulation contains no formaldehyde, and its fibers will not cause respiratory or skin problems unless you are specifically allergic.
Cotton batts can be used in the same places as fiberglass or mineral wool batts, rolled out between open roof rafters, ceiling joists, or wall studs. Loose fill is also suitable for attic floors and wall cavities. As with any cavity insulation and any natural material, an elevated moisture level is not acceptable. Cotton insulation can cost twice as much as fiberglass for similar insulation effectiveness, though you can save money by installing it yourself, since cotton can be handled without safety equipment or the health concerns of other types of insulation. Batts come in standard sizes.
Fiberglass insulation is made of silica sand and recycled glass, both abundant resources. The U.S. Environmental Protection Agency (EPA) requires that 20 percent of materials come from recycled sources, either post-consumer or post-industrial, and some products contain up to 40 percent.
Producing fiberglass insulation requires melting the materials in a fossil fuel–burning furnace, which consumes substantial amounts of energy and generates greater amounts of air pollution than the manufacture of other insulation types.
If installed properly, there is little danger of inhaling fibers, which are throat, eye, and skin irritants. Although the Occupational Safety and Health Administration still requires cancer warning labels on fiberglass insulation products, the American Lung Association states that glass fibers are not linked to increased cancer risk, even among glass fiber manufacturing workers.
Fibers can escape into the air during installation—or if ductwork is not sealed properly—becoming a problem for residents. Because of the concern that fiberglass emits phenol-formaldehyde, some manufacturers have switched to nontoxic acrylic binders or have had their products certified by Greenguard as low-emitting products. (A low-emitting product has half of what the EPA considers elevated formaldehyde levels.)
Loose-fill fiberglass seals air spaces best since it is blown in, preventing air movement and heat loss. Low-density batts are most commonly used, but can lose up to 50 percent of their R-value in cold climates due to moisture infiltration. High-density batts cost more, but have a higher R-value; they will pay back the difference in lower energy bills, especially in cold climates.
Foam insulations have greater environmental impacts than other types, due to extraction, refining and transport of raw materials such as natural gas and petroleum, and the use of ozone-depleting compounds in manufacture. However, they offer some significant benefits that can offset those drawbacks, including higher R-values for a given thickness and improved air sealing of surfaces. Over the lifespan of a home, foams will save more energy per inch of insulation because of these R-values and their durability.
Check our home foam insulation Q&A to see what homeowners and contractors are saying about the pros and cons of foam insulation, including cost.
Except in damp areas, such as below grade, polyisocyanurate is the environmentally preferable type of foam insulation. Polyiso is a rigid foam board, often backed with a foil layer serving as a vapor or radiant barrier, that can be used anywhere in the home. It is made with at least 9 percent recycled content, typically from mixed-color PET plastic bottles. Polyiso no longer uses ozone-depleting HCFCs (hydrochlorofluorocarbons) as a blowing agent to produce bubbles in foam. The flame retardants used in polyiso are also less toxic than those in other foam insulations. These facts, along with its high R-value, make polyiso preferable to extruded polystyrene and polyurethane foam insulation for all but below-grade or very damp applications. The Polyisocyanurate Insulation Manufacturers Association offers more in-depth information.
There are two types of polystyrene: extruded (XPS) and expanded (EPS), also called blueboard and beadboard, respectively. While there is no minimum recycled content limit for polystyrene, products may have some amount of recycled content because polystyrene itself can be recycled. Polystyrene foams contain brominated flame retardants that raise serious health and environmental concerns, since some brominated compounds are bioaccumulative. XPS is more expensive than EPS and has a slightly higher R-value. XPS is more moisture resistant and suitable for below-grade applications, but EPS can be used below grade if coated with a plastic or foil film.
Spray polyurethane foam (SPF) is an expanding foam made of polyiso and a polyol resin, which can be partially soy-based, used to fill cavities in walls. It contains a minimum of 5 percent recycled content.
SPF is installed into wall cavities through holes, like loose-fill insulation. SPF forms an airtight barrier and is water resistant, so you don’t need a vapor barrier. It does not sag or settle, and it's not attractive to insects. Once SPF has cured (which takes 24 hours at most) it is inert and does not affect indoor air quality. Filling cavities with foam may reduce the drying potential of a wall, so you should seek the advice of a building professional before remodeling.
HCFC blowing agents have been replaced by HFCs (hydrofluorocarbons, which do not harm the ozone layer), carbon dioxide or water. Formaldehyde was once common in polyurethane insulation but is no longer used.
When adding foam insulation to wall cavities is appropriate and practical, typically for remodels, alternatives to purely petroleum-based products are partly soy-based SPFs. These products are low-density SPFs, and can use up to 33 percent soy oil. High-density SPFs have higher R-values and are strong enough for structural applications, but are worse environmentally.
Mineral wool is made of strands of minerals, either from abundant rock or the recycled slag from iron-ore blast furnaces. The EPA requires that mineral wool contain at least 70 percent recycled content by weight, second only to cellulose. The proportion of recycled materials in mineral wool can surpass 90 percent; look for high recycled content and ask if you don’t see the information displayed.
Though more expensive than cellulose and fiberglass, mineral wool is more durable and moisture-resistant. It continues to insulate when wet, and it tends to maintain efficiency better under varied conditions. It also has superior flame retardant properties. Available as batts and loose fill, mineral wool can be used anywhere inside the home, though only specially made mineral wool products should be used below grade.
Like fiberglass, mineral wool is very energy intensive to produce in furnaces, though it requires less energy per R-value than fiberglass. Mineral wool fibers pose no health threat beyond possible short-term respiratory and skin irritation during installation. However, mineral wool may contain up to 5 percent phenol-formaldehyde by weight—more than most fiberglass insulations. This is potentially harmful to indoor air quality, so check a product’s material safety data sheet (MSDS) if you are concerned.
Most importantly, make sure that you use enough insulation and that you select the best type for your circumstances. Dietmar Lorenz, of Daniel Smith & Associates Architects, recommends that you "insulate as much as you can reasonably achieve and find the greenest material that works. In some instances a renewable or natural material might not work, like under a slab. On the other hand, a highly processed, petroleum-based material is often not needed in less demanding circumstances. Use the insulation that causes the least environmental impact to do the job, and use plenty of it."
Greenguard Environmental Institute
Most fiberglass insulation manufacturers have had their products certified as low-emitting by Greenguard, an independent, third-party testing program for products and materials.
Green Building Advisor
Visit this website for a list of insulation products published in the Greenspec directory of environmentally preferable building products.
Oak Ridge National Laboratory’s Insulation Fact Sheet
An in-depth look at insulation materials, when and how to insulate, how much to use based on your location, and much more.
National Insulation Association's Insulation Science Glossary
This glossary is a great resource for such a highly technical subject.
This page gives basic information on many types of alternative or developing forms of insulation not discussed here, such as phenolic foam, plastic fiber, and cementitious insulation.