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When choosing insulation there are a few different attributes we can use to assess them. I always look at offgassing of volatile organic compounds (VOCs), as this is an immediate concern to many people. We also need to consider nonvolatile chemicals and at what stage in the product’s lifecycle they might be a concern. For example, low levels of metals, borates, and natural flame retardants like ammonium sulfate may be a concern only during production and installation, whereas more toxic flame retardants like those used in foam-board insulation can be a hazard at every stage of a product’s lifespan. Even if they are not a hazard when the insulation is in place, either because the flame retardant is well bonded to the polymer or because it’s on the exterior, we still have to think of its disposal at the end of its life.
Another aspect that comes into play—with foam boards in particular—is the product’s global warming potential (GWP). Fortunately, there have been moves of late to reduce the high GWP and toxicity levels of flame retardants. And, lastly, we should look at a product’s recycled content, and its ability to be recycled or composted at the end of its life. What follows is a detailed dive into 12 insulation types and the qualities that make them a good selection for people and planet.
There are lots of options for batt insulation, which affords us the ability to be picky.
Mineral wool is made of basalt stone and slag from steel and/or copper, a resin binder, and a little mineral oil. It is my top choice for batt insulation because it offgasses quickly and completely, it’s easy to source, and the benefits of its compression fit are not outweighed, in my opinion, by the benefits of other batt materials.
Rockwool is the most popular brand of mineral wool. Rockwool Comfortbatt, Safe ’n’ Sound, and ComfortBoard all contain <3% phenol-formaldehyde, <1% starch, and <0.2% mineral oil (source). The company says that “during manufacturing, the [formaldehyde] binder is cured at very high temperatures leaving only trace amounts in the product after it is produced” (FAQs). I do find the levels of formaldehyde in the product to be extremely low and what remains offgasses quickly (and completely) once the packages are opened. They also have a formaldehyde-free product, AFB Evo, which has a starch-based binder. Unfortunately, this product is harder to source than their standard batts.
The pre-consumer recycled slag used in Rockwool’s batts comes from the steel and copper industries and makes up 16% to 40% of the product. Steel slag can contain trace amounts of potentially toxic elements such as Ti, Al, Ca, and Cr. In an email, Rockwool said its slag doesn’t contain cadmium, chromium, lead, mercury, or silver (they didn’t mention aluminum or titanium). It’s not something I’m worried about once it’s behind an air barrier, but there could be a risk to installers who handle this every day.
Also, Rockwool’s Ontario location uses the highest amount of recycled material of any of its facilities. It can be recycled into new mineral wool insulation, where the facilities to do so exist.
Another common brand is Thermafiber from Owens Corning. They offer a regular phenol-formaldehyde-bonded line and a formaldehyde-free line (marked “FF”) that is made with a dextrose-based binder. In terms of its environmental profile, the use of up to 40% recycled material is a plus.
Fiberglass insulation can be bonded with phenol-formaldehyde, acrylic, or bio-based starch. The shift away from formaldehyde binders in fiberglass batt insulation began in 2002 and was complete by 2015 (source). Knauf lists their binder as starch, Owens Corning uses corn starch, and Certainteed’s binder is listed as sugars; Johns Manville is the only brand I know of to use an acrylic polymer binder.
Other ingredients Knauf lists include ammonium sulfate (fire retardant), mineral oil (dedusting), and silane. Certainteed lists citric acid, mineral oil, silane, and an undisclosed additive in their EPD. Owens Corning lists vegetable oils, sodium lignosulfonate, silane, and an undisclosed surfactant.
Fiberglass insulation has very low offgassing, and some of the most chemically sensitive folks have done well with the starch-based brands. Fiberglass also has recycled content in the form of both pre-and post-consumer recycled glass. Some areas do accept fiberglass insulation at a recycling facility, but this is not common.
As a side note, it is reassuring that the primary option used in lower-end homes isn’t too bad in terms of both short-term exposure to offgassing and long-term impacts.
Real sheep’s wool insulation is another eco-friendly option. This will appeal most to clients who want to go as natural and sustainable as possible with their build. It does have a natural odor and the main additive is boric acid to ward off pests.
Havelock Wool is the most established brand. They make batts and a loose fill. They don’t use any synthetic fibers or binders; it’s just wool and boric acid, which makes this the purest brand of batt insulation. Havelock Wool is biodegradable and compostable. That said, there are concerns with borax/boric acid if you inhale it, ingest it, or absorb it. But once it is behind an air barrier, I would not be concerned about it. Other manufacturers include Oregon Shepard, which uses recycled wool and adds borate, sodium carbonate, and casein; and Wool Life USA, which makes wool insulation treated with the chemical pesticide permethrin.
UltraTouch products used to be made from pre-consumer denim scraps, but they have moved to recycled denim and other cotton sources now. It’s about 80% post-consumer recycled fibers, and it is also recyclable. It contains boric acid, ammonium sulfate (fire retardant), and an olefin binding fiber, which is similar to rayon. It can be used between wood studs, though it does not hold up well in ceilings/between floor joists, especially compared to mineral wool.
I have used this insulation type and there is nothing objectively wrong with it, but it isn’t as good as other options, in my opinion, because it doesn’t fit tight in wall cavities. I worry about its ability to resist mold if it gets wet, and it does have a slight odor.
Hemp insulation is the newest alternative insulation in the North American market, but it has been used for much longer in Europe. Brands include Thermo-Hemp, a European manufacturer and one of the first hemp insulation brands. Their products are made of hemp and jute. In the U.S., there’s HempWool from Hempitecture, which is 90% hemp and 10% polyester fiber. There is also the MEM brand out of Canada, which is 88% hemp and 12% polyester.
The hemp insulation industry has been rocky, as evidenced by the fact that the following companies have gone out of business in the last few years: NatuHemp by Black Mountain Insulation, Sunstrand Batt, and Nature Fibers Inc.
Hemp has good specs when it comes to environmental sustainability, and as an insulation material it has promise. But the shaky market history means it hasn’t proven itself yet, and the polyester makes it nonbiodegradable.
My top picks for blown-in insulation are mineral wool and sheep’s wool even though these are less common choices. Both are two-ingredient products with no formaldehyde or flame retardants. Cellulose and fiberglass both have several chemical additives but low offgassing.
Blown-in cellulose insulation is made with post-consumer newspaper. It always contains boric acid as a flame retardant and pest deterrent, and sometimes has ammonium sulfate as a fire retardant, as well as mineral oil for a dedusting agent, plus other ingredients based on the brand. Newspapers are made with “soy-based” inks, but they contain many other chemicals apart from soy that can be bothersome to sensitive people.
The GreenFiber brand is 85% recycled newspaper. They have a Declare Label that lists boric acid, sodium pentaborate pentahydrate (another borate-based compound), ammonium sulfate (flame retardant), mineral oil (dedusting), monoammonium phosphate (flame retardant), calcium carbonate, and corn starch.
Ecocell makes cellulose insulation in batt form that is a mix of cellulose and cotton. It also contains PET as the binder, boric acid, sodium polyborate, and ammonium sulfate.
Cellulose production can use up to 10 times less embodied energy than fiberglass insulation, plus the use of recycled materials diverts waste away from landfills and adds to its sustainability credentials. However, it is not recyclable and though it is technically compostable, the additives may mean that’s not a good idea for residential composting.
Cellulose is not at the top of my list because of all the additives—especially if someone is very sensitive to offgassing—but I wouldn’t rule it out when a blown-in material is needed for walls. The additives are mainly a risk when in dust form.
Blown-in fiberglass brands that I have looked at contain borates, soda ash, lime, siloxanes/silicates/silane, mineral oil, and an unknown anti-static additive. Owens Corning carries an unbonded loose-fill product and Knauf Jetstream Ultra looks like a similar offering. Some sensitive clients have reported noticeable offgassing odors from blown-in fiberglass insulation compared to other options, though it is still a low-VOC option.
Other blown-in materials
Wool insulation, specifically the Havelock brand, comes in this form and is made of wool and borates. Mineral wool can also be blown in; brands include Thermafiber and Rockwool. Mineral wool blown-in formulas don’t have a formaldehyde binder, which makes the material the purest blown-in option with the least offgassing. Gutex Thermofibre is a blown-in insulation made of two ingredients, pinewood fiber and ammonium salts, which act as a flame retardant.
For board insulation, cork is my top material choice. It just can’t be beat in terms of its impact on humans and the environment. The downside is that it’s pricey. My next pick is mineral wool board, which is another great option for exterior applications. The plastic-based foam boards, on the other hand, all have concerning flame retardants, and some have a high GWP.
Polyiso is one of the foam boards that is used on the exterior side of sheathing and on roof decks. The blowing agents are CO2 and pentane or pentane blends. In closed-cell products, the blowing agents are retained within the cell structure. There is some slow thermal drift over time, but the offgassing is considered inconsequential to human health. Pentane has a GWP of less than 10.5, whereas some XPS products still have hydrofluorocarbon (HFC) blowing agents, which have a much higher GWP.
Apart from the GWP of the blowing agent, flame retardants are the major concern in foam board insulations of all types. The most common flame retardant in polyiso is TCPP, tris (2-chloro-1-methlyethyl) phosphate, a halogenated flame retardant with high toxicity for reproductive and developmental health, moderate toxicity for carcinogenicity and neurological health, moderate aquatic toxicity, and high persistence in the environment (source). Newer flame retardants include a non-halogenated phosphorus-based option.
GAF’s EnergyGuard NH Polyiso is the only brand I know of that uses a halogen-free flame retardant, which is phosphorus-based. GAF’s former director of sustainability, Martin Grohman, has said that this phosphorus-based flame retardant is bonded to the polymer, so that it can’t leach out, whereas TCPP is not chemically bonded to the polyiso foam and can become free (source). This makes EnergyGuard NH a much safer option, at least during its intended use.
Incidentally, Johns Manville Energy 3E was another brand that was made without TCPP but the product is no longer made. The manufacturer said the flame retardant was phosphorus-based and non-halogenate.
ZIP System R-Sheathing is the Huber Zip OSB sheathing, polyiso, and weather-resistant barrier all in one. In this case, the polyiso is on the interior side of the sheathing, and the flame retardant is TCPP.
I would still consider polyiso in a green home because the foil backing used on most polyiso boards does block the migration of flame retardants from two sides, and the use of the insulation on the exterior of the envelope also mitigates the risks. It has a fairly low GWP (at least compared to XPS). The only brand of polyiso I would go with is EnergyGuard NH due to the safer flame retardant.
Extruded polystyrene (XPS)
XPS is another exterior insulation used outbound of the sheathing. It is also used on exterior basement walls, under the slab, and on roofs. XPS, like polyiso, is a closed-cell foam that has some extremely slow offgassing of the blowing agents over time. Over the course of 50 to 75 years, the blowing agent diffuses through the foam and is replaced by air, according to this article by Martin Holladay, in which he discusses the R-values of closed-cell foam boards.
Typical blowing agents for XPS are CFC-12, HCFC-142b, and HFC-134a. There have been recent shifts away from the highest GWP HFCs. Eleven states have banned foams blown with HFC-134a (GWP of 1430) and other high-GWP blowing agents, including HFC-245fa, as of January 1st, 2022. Canada also created a maximum GWP for foam blowing agents.
Owens Corning doesn’t use HFC-134a anymore; they now use a hydrofluoroolefin (HFO) blowing agent with a GWP of less than 80. DuPont now offers a reduced GWP foam available in Canada and HFC-regulated U.S. states. The blowing agent is not disclosed. They say that the GWP of the blowing agent was reduced by at least 20%.
Flame retardants are also a concern. XPS insulation was treated with the flame retardant hexabromocyclododecane (HBCD) until recently. The two main brands, DuPont and Owens Corning, no longer contain HBCD. Most are now using a butadiene styrene brominated copolymer as the flame retardant, which has a better toxicity profile than HBCD.
While XPS still has a higher GWP than the other two foams, the new formula by Owens Corning is not that far off from the others. If a build requires foam board and the other two options can’t be used or sourced, the Owens Corning brand makes sense.
Expanded polystyrene (EPS)
EPS can be used as exterior insulation, on interior concrete basement walls, and sometimes under the slab. It’s also a common insulation in travel trailers, where it’s usually referred to as “block foam.” Typically, EPS is made with pentane, butane, and/or carbon dioxide as the blowing agents, which are replaced by air quickly after the product is manufactured.
EPS, despite the name polystyrene, does not offgas styrene under normal conditions. It is considered zero-VOC; and pentane and styrene are not exempt from the VOC ratings. The main concern here is the flame retardants. EPS used to be treated with HBCD, but like with XPS, manufacturers have transitioned to a butadiene styrene brominated copolymer. Some EPS is treated with borate because ants do love to make a home in it.
For green homes, EPS has been traditionally preferred over XPS due to the GWP. Now its environmental specs are comparable to EnergyGuard NH brand polysio and not too far off from Owens Corning’s XPS, which has a higher GWP blowing agent but the same flame retardant.
Thermacork insulation, unlike cork flooring planks, does not contain any adhesive or any other additives. It is held together by compressing it with heat, which releases a natural binder in the cork. The cork does give off a smoky smell, but that dissipates with time. It can be used as roofing insulation, exterior insulation outbound of the sheathing behind a rainscreen, or as the facade of a house, which serves the purpose of siding, thermal insulation, and noise reduction in one.
Trees are not cut down when harvesting cork, which is sourced from sustainably managed forests that are habitat for many species; these forests are also carbon sinks that capture CO2 and greenhouse gasses. And, of course, being only bark, it’s biodegradable.
This is the only one-ingredient product on this list, and in that sense, it is the purest. It’s my top choice for board insulation. It just doesn’t get any greener than this. The only downside is the high cost.
My second material choice for board-form insulation is mineral wool. The small amount of formaldehyde and the trace metals become even less of a concern when used on the exterior of the building. There are no persistent flame retardants or other chemicals of concern.
Wood-fiber board is often used in high-performance and Passive House designs for floors, walls, ceilings, and roofs. The most popular brand is Gutex. It is 95% wood, 4% polyurethane, and about 1% paraffin. The total VOC level is 187 μg/m3. I do not consider this low in VOCs, and it would not be the ideal insulation inside the building envelope but the benefit is there are no long-term persistent chemicals. Once the polyurethane offgasses, this product is a solid choice. It’s also recyclable.
The short list
In summary, my top picks are mineral wool batts because they are fast to offgas and fit snugly in between framing, which reduces air leakage. The fact that this is also the most popular insulation in mid- to high-ends homes makes it an easy choice.
Hemp insulation is a promising new insulation and I think we should support this development.
Blown-in mineral wool and sheep’s wool are the two options here with very few additives. There are no flame retardants and no offgassing. These are not the most common blown-in choices, but I do think they are preferable over fiberglass and cellulose.
Cork board insulation is an incredible product for human health and the environment. The cost will probably keep it out of most builds for now. Mineral wool boards are also a great choice, and I have no concerns when they are used on the exterior or under a slab.
All foam boards have their challenges. They contain toxic flame retardants to various degrees, and some have a high GWP. Sometimes they still are specified, so the newest improvements in GAF and Owens Corning brands are a good thing.
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Corinne Segura is a green building materials specifier and writer at mychemicalfreehouse.net. Photos courtesy of manufacturers.
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