Beyond R-Value: The Complex Calculus of Choosing Insulation
Which insulation is the greenest choice? Unfortunately there’s no simple answer.Posted Dec 24, 2015
When you’re trying to reduce energy use and “green” your home, insulation is an obvious step. What’s less obvious is how to choose what material to insulate with — finding the “greenest” insulation for your project can be a pretty complicated process. Some of the best insulators contain chemicals that may have unwanted impacts on human health or the climate, while more benign products made with recycled content won’t work in certain situations or can be hard to get. Here’s a rundown of currently available insulations and why you might choose one over the other.
There are a number of factors to consider when choosing insulation:
1) Effectiveness at insulating. There’s the well-known R-value (R is short for resistance to heat flow), but it’s also important to consider how well a material can block air movement. How a wall actually performs can be affected by its construction, air sealing, temperature swings, how the product is installed, and variations in different brands of insulation. R-value is not the only thing to base your choice on.
2) Cost of product vs. how much it will save in energy costs. A product that’s more expensive upfront may save money over the long run than a cheaper one if it’s a more effective insulator, but not always.
3) The environmental impact of the product itself. Some foam insulations contain compounds that emit greenhouse gases over 1000 times that of CO2, undoing the climate benefit of installing the insulation. Other insulations contain impressive amounts of recycled content and have a lower overall ecological impact.
4) Chemical safety concerns. To protect air quality, it’s wise to avoid products containing chemicals thought to adversely affect human health, like formaldehyde, a common binder being phased out of many types of insulation.
Insulation choices are complicated! If you choose to avoid spray foams because of chemical or environmental concerns, you may have to give up something else, like insulating ability or floor space if you build walls thicker to make up for the loss of thermal resistance. But because of all the other factors involved in real-world performance of actual walls, you might not actually give up much when you choose a product with a lesser R-value. A 2015 study by the Building Science Corporation found that different types of insulation performed comparably in controlled conditions, and were all significantly affected by construction choices, temperature, and air movement. Funded by manufacturers of several different types of insulation, the study concluded that when installed properly, whether you choose fiberglass, foam, or cellulose, you’ll likely have similarly-performing walls.
Deciding which kind of insulation to use will depend on the project you’re undertaking. If you’re building a new house or have an open wall cavity, you have different options than if you are trying to insulate an existing wall. Some materials are better suited to environments prone to moisture. Below are some explanations of the different types, their applications, and the pros and cons of each.
Types of Insulation
Perceived ease of use and low cost have made fiberglass a very popular choice, especially among do-it-yourselfers. But fiberglass doesn’t stop air flow and is commonly installed incorrectly, negatively impacting its performance. Recent audits in several countries found problems with installation in the majority of them. Fiberglass can work better when blown in, but dense-packed cellulose blocks airflow more effectively. The green building community generally advises against fiberglass batts, as there are better-performing products at comparable cost.
R-Value: 2.9-3.8 per inch (batts, when installed correctly), 2.2-2.9 (loose blown-in, though some brands get above 4)
Pros: Inexpensive and widely available, homeowners can install fiberglass themselves (though even experienced builders tend to install it incorrectly). In some cases, blown-in fiberglass may be recommended, especially when cellulose is not an option. Can contain up to 60% recycled content.
Cons: Not appropriate for moisture-prone areas, as mold can grow. Does not block air flow. In batt form, it generally does not perform well because installing it correctly is so challenging.
Cellulose is another inexpensive option. It can be sprayed into existing wall cavities, in crawl spaces and attics. DIYers can save money by doing the installation themselves. (It’s a little messy, but a relatively straightforward afternoon project involving a big machine usually loaned for free with the purchase of bales of insulation.)
We had professionals blow cellulose in through our siding, and the follow-up blower door test our energy auditor performed showed significantly reduced air flow through the house. The auditor estimated that the modest sum we paid for the insulation would be paid back in energy savings in under ten years. It certainly increased the comfort of our house noticeably.
To reduce flammability and deter pests, cellulose is typically treated with borates or sulfates, which are generally regarded as safe for human health.
R-Value: 3-4 per inch
Pros: Affordable, quick, up to 80% recycled content, significantly less embodied energy than fiberglass or mineral wool. It can be installed by professionals through holes in existing structures or by DIYer in open areas
Cons: Can settle over time, not as effective at blocking air flow as foam, not appropriate for moisture-prone areas such as basements.
Less common in the U.S. than fiberglass and cellulose, mineral wool comes in batts like fiberglass or extruded in boards (most often by the company Roxul). In batt form, it has the ease of use of fiberglass with lower risks of poor installation. Rather than being floppy and prone to folding, mineral wool is friction-fit between studs, which helps it block air flow better than fiberglass as well.
R-Value per inch: 3-4
Pros: Easier to install correctly than fiberglass batts, mineral wool has the one of the smallest environmental impacts of the options currently available (up to 93% recycled content, free of potent greenhouse gases). Naturally fire resistant, it requires no chemical retardants and is probably a good choice for the chemically sensitive. Mineral wool in board form may cost less than comparable rigid foam products.
Cons: Can be harder to find. Batt form may cost somewhat more than fiberglass. Mineral wool manufacturers use a small amount of formaldehyde as a binder, but executive editor of Environmental Building News Alex Wilson reports that “the high-temperature processing of the mineral wool during manufacture drives off any free formaldehyde, and test data I’ve reviewed… showed formaldehyde levels to be at or below background levels.” Not appropriate for basement interiors, though can be used around the outside of foundations.
Recycled denim insulation has gained popularity and is now readily available in many home centers. Because it comes in batt form, it’s another appealing option for DIYers who want to avoid the itchiness of fiberglass.
R-Value: 3.7 per inch
Pros: Recycled material (some post-consumer, some from industrial scraps), less embodied energy than some options.
Cons: More expensive and somewhat harder to find than cellulose or fiberglass. Some have found issues with the product leaving air spaces, effectively reducing its insulating ability. Post-consumer denim may contain residues from chemically-based laundry products, a potential problem for the chemically sensitive, to whom this product is marketed. Like cellulose, it’s treated with boric acid or sulfates as fire retardants and is not appropriate for moisture-prone areas.
Read the companion to this article, “Is Spray Foam Insulation Your Best Choice?” for more on why you might want to find alternatives to spray foam. Concerns about the health effects of improper installation make it worthwhile to consider other, less-risky options. Open-cell foams like Icynene have R-values pretty similar to cheaper and better-understood insulations, like fiberglass, denim, and cellulose, though these alternatives won’t airseal without an additional step.
Closed-cell foams have better R-value and also block moisture, but they have a very high global warming potential (GWP) that virtually undoes the benefit of insulating from a climate perspective.
R-Value: 3-4 per inch for open-cell and 6-8 per inch for closed-cell foam
Pros: Effective at completely filling cavities and creating an air seal. Closed-cell foam also blocks moisture. Appropriate for use in basements and other areas where moisture is a concern.
Cons: Chemicals in spray foams are not well understood and have caused serious reactions in some individuals. Certain ingredients have significant environmental impact.
There are several types of rigid insulation, and the type you choose can make a big difference in terms of GWP and insulating ability. Choose EPS (expanded polystyrene) or polyisocyanurate (polyiso) over XPS (extruded polystyrene) to lower the GWP of your project. XPS is a very effective insulator, but rates high for GWP. Polyiso has a higher R-value than EPS, but does not perform well in cold conditions. EPS is more suited to moisture-prone areas than polyiso.
R-Value: 4 (EPS) to 8 (polyiso) per inch
Pros: Effective insulator and air blocker, readily available.
Cons: Environmental impacts of ingredients, toxic fumes emitted when burned.
Foamglas is newer to the U. S. market, and though expensive and not widely available, it has some properties that may make it a good choice for some projects. Made of glass, this inert material is naturally fireproof, has low GWP, and can replace moisture barriers. Also, it holds up under weight, like beneath slabs.
R-Value: 3.4 per inch
Pros: Fire, mold, and pest resistant as well as chemical-free, it’s a good choice for areas where moisture is a concern, including beneath building slabs.
Cons: More expensive than other insulations, needs to be special ordered.
Air Krete is cement mixed with air and water to form a foamy insulation that’s fireproof and free of VOCs. The manufacturer claims it has similar insulating ability as polyurethane spray-foam, without the toxic chemicals.
R-Value: 3.7 per inch
Pros: Fire-proof, mold- and pest-resistant
Cons: Though the manufacturer claims Air Krete does not shrink, Martin Holladay, senior editor of Green Building Advisor, reports that “Most users of Air Krete are disappointed because the product shrinks and is quite crumbly.”
If you want a home built with natural materials and are starting from scratch, check out straw bale construction, which uses plastered — you guessed it — straw bales to make thick, highly-insulated walls.
R-value: 1.5 per inch
Pros: Natural, renewable materials, good insulating ability because of the walls’ thickness.
Cons: Time-consuming, likely more expensive than conventional construction (if you can find a builder to do it), and not particularly suited to wet climates.
There are a few other options, though they’re not easy to obtain, including cork, hemp, and lambswool. Choosing the right insulation will depend on your prevailing local conditions, budget, and goals.
If you’re building new, check out this informative article on the advantages of insulating outside your walls, which helps deal with the problem of heat loss from the uninsulated studs of typical home construction (thermal bridging).
To add insulation to an existing structure without ripping out your walls, blown-in cellulose is an affordable and reasonably effective option. Blown-in fiberglass is a good second choice if cellulose is not available in your area.
If your walls are open, you should consider mineral wool, which offers advantages in installation effectiveness and longevity over fiberglass and cellulose, but installed correctly any of these options should work.
In basement applications, you need something moisture resistant, and rigid foam is generally considered most effective. If it’s feasible, you might put the foam outside the foundation walls to eliminate some of the concerns about chemicals in foam and bring your foundation inside your thermal envelope, creating more thermal mass.
I highly recommend the incredibly helpful team of green building professionals at Green Building Advisor, who have an extensive database of articles on various types of insulation and building scenarios, as well as an active forum where people post and answer questions.
Green Building Advisor on building foam-free
Susannah Shmurak is an enthusiastic advocate for healthier, more sustainable lifestyles. She shares practical tips about gardening, food, and low-impact living at her new blog, HealthyGreenSavvy.com.