We hear a lot about our carbon footprints these days. Here’s one approach to defining the term from the Nature Conservancy: “A carbon footprint is the total amount of greenhouse gasses (including carbon dioxide and methane) that are generated by our actions. The average carbon footprint for a person in the United States is 16 tons, one of the highest rates in the world.” The Nature Conservancy also hosts a helpful tool to calculate your own carbon footprint.

We know that just living a normal life will have an impact and generate some carbon emissions, and we do usually have a lot of choices from day to day. Taking public transportation vs. driving your car will have a lower impact, for example. It’s not always that easy, though, to sort through the options and know how the choices stack up. If you’re building or remodeling a home and using insulation, for example, which we mention frequently on this blog, what are the lowest-carbon options?

Let’s look at some data from this article in Green Building Advisor. It’s important to know the term “global warming potential” or GWP. That’s the lifecycle “cost” of carbon associated with the manufacturing and use of each building component. A lower number is better. The GWP is important to consider because of how different materials rank and then perform in the real world. Spray foam insulation, for example, gets the highest rating for R-value, which is the insulating value, but it’s also higher for its GWP. However, there are several types of foam insulation, both spray foam and in boards, and their properties vary quite a bit.

It’s also important to understand how the “natural” materials perform. On this the author states that “carbon-containing insulation materials such as cellulose and wood fiber can have GWP less than zero, due to low energy intensity to manufacture plus credit for storing carbon in the product itself.” So that’s a strong recommendation for those types of insulation.

Types of Insulation

Wood fiber insulation

Though it’s not yet available everywhere, anytime, like fiberglass batts, wood fiber insulation has a lot going for it. It’s available in tongue-and-groove sheets for use on walls and roof decks, as batts (aka blanket insulation, the kind that comes in flat pieces), and as loose fill. It’s made from a mixture of post-industrial wood chips and shavings combined with sustainably grown timber. With an R-value of 3.47 per inch and -7.13 GWP, it is clearly sequestering a lot of carbon. Manufacturing is now taking place in the U.S., which will bring the price down for Americans.

Cellulose insulation

This type of insulation is often made from waste newspaper that has been treated with boric acid as a fire retardant and pesticide. Wet-sprayed cellulose, in particular, provides excellent results with very good air sealing of wall cavities. Its GWP is -2.16, with an R-value of 3.56 per inch. Wet cellulose insulation costs $0.60 to $2.00 per square foot with installation, according to HomeGuide, and dense-pack cellulose costs $1.60 to $4.20 per square foot.

Sheep’s wool insulation

This type of insulation is still a low-volume, high-cost “boutique” product. Still, it’s a natural material, though it must be treated with a pesticide. With an R-value of about 3.5 per inch, and a low GWP, this is a good product if you’re willing to pay more than $2 per square foot, which is twice the price of fiberglass insulation. I’m not sure of the GWP, as I kept seeing it listed as “0” and it’s not. Sheep emit CO2 during their lives, of course. But it is a low number.

Denim insulation

This is a recycled product, so it’s very environmentally friendly. It’s also available at Home Depot in my little Colorado town, competing with fiberglass insulation and doing fine, as it’s sold out today. At R 3.5 per inch, it’s about $1.26/square foot, which is competitive with fiberglass. Denim’s GWP is a bit less than 0, as far as I can tell. I couldn’t find an exact number, but this is a solid product for its environmental and performance benefits. And now it’s widely available. Great news!

Those four products are the best of the lot for GWP, and they’re all made from recycled materials. If I were building a home right now, my recommendation is to get ahold of wood fiber sheets for the sheathing and use wet-sprayed cellulose for its solid air-sealing qualities. Nothing against denim, but batts of all types require another air-sealing step.

The Best of the Rest

Fiberglass insulation

At 3.64 per inch R-value and 0.68 GWP, fiberglass insulation is an okay choice for a mass-market product. I’m seeing prices from $0.86 per square foot, so it’s definitely low cost, but denim isn’t that far off, nor is cellulose, and I’d rather use either of those.

The rest of these products don’t look good from a GWP standpoint, nor from a toxic material standpoint.

Mineral wool insulation

Mineral wool is a product made from waste materials, so that’s positive. It’s also fireproof and insect-resistant. With an R-value of 4.24 and a GWP of 3.25 for unfaced batts, I don’t see it as a good choice. I mention it to show that even a product made from waste can have too high a GWP to overcome, especially when you have better choices readily available.

Foam insulation

You’ll see a large variety of foam insulation, including XPS, EPS, and polyisocyanurate. You can get the stuff in 4x8-foot sheets and sprayed in. Foam needs a gas to expand a liquid chemical concoction into either a board, or to be sprayed in place. Those gasses contain the GWP demons, though the different materials can vary a lot in their GWP. XPS has a GWP of 46, for example, while Polyiso’s GWP is only 2.32. And here’s a caveat: if necessary, I would probably use Polyiso boards to insulate basement walls, if I were to build a house with a basement. I don’t see a better choice.

Finally, you do have other choices, but they’re very much niche choices. Using straw bales is an old-school building technique from the prairies that’s still in use. Those bales sequester carbon just as wood-fiber insulation does, and they require little processing. Plus, straw-bale houses often give you a stunning look with thick walls and great thermal mass. Straw can also be formed into boards, as well, but that’s still a niche product.

Hemp is another option that is not yet mass market but seems to perform well, similar to denim and wool. Maybe someday, it will be part of the mix of readily available, healthy building products.

You can learn more about insulation choices from this Insulation guide.