Acelab Insights

Sustainable Continuous Insulation


Better than Good

Continuous Insulation is always a good idea, but some types are even better than others. The most sustainable CI options are not only good insulators, they also have fewer harmful ingredients, are made from renewable materials, or both. We find that the most sustainable options in continuous insulation are wood fiber insulation, cellular glass insulation, and mineral wool insulation, here's why:

Wood Fiber Insulation

Wood insulation may sound like a mistake, but it’s real, it's sustainable, and it’s showing up in more projects in the US. It’s the most sustainable CI option, made from a renewable material which stores carbon, and doesn’t contain toxic ingredients. Wood fiber insulation from TimberHP comes in batts, loose fill, and a board insulation which is suitable for continuous exterior insulation. Made in Maine, its pricing is comparable to other breathable board insulation such as mineral wool and EPS, though it’s easier to work with than mineral wool and more sustainable than EPS.

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Cellular Glass

Due to its combination of high compressive strength, high temperature performance, fire resistance, water resistance, AND chemical resistance, cellular glass has been used in industrial applications for many years. It is less common in architectural applications because it costs more than the XPS which has some similar characteristics and higher R value per inch. But cellular glass wins for lifespan and lower toxicity.

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Mineral Wool

When in doubt, use the Goldilocks insulation: mineral wool. It’s not the best at anything, but it is second best in everything. It’s made from stone and slag, so it’s not combustible. Undamaged, it will last the life of the building. It’s immune to water, pests, or mold, and it'll retain its R-value over time. It also lacks harmful chemical flame retardants or blowing agents. Some mineral wool uses formaldehyde in its manufacturing process, so look for products available in both low-VOC and formaldehyde-free options. This stuff is basically harmless.

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What to Look Out For

Water resistance:

All three of our favorite sustainable CI (Continuous Insulation) options are hydrophobic, meaning that they repel water, but only the mineral wool and cellular glass are inert and suitable for use below grade.

Wood fiber insulation is treated with wax, making it suitable for use as a continuous insulation. This makes it unique compared to other sustainable natural fiber products, which are not suitable as CI because they can’t handle getting wet. CI is installed outside the building's WRB and needs to be durable enough to handle the wetting.

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Wood fiber insulation is the most sustainable CI option, but due to its combustible nature, it's limited use to projects where combustible materials are allowed by code. It earns Class A fire performance from its char characteristics.

Cellular glass and mineral wool don't burn, so they can be used on any scale of project.

Where and how to use these products:

Wood fiber insulation is best in projects where combustible materials are allowed by code. Timber HP produces wood fiber insulation in batts, loose fill, and board insulation which is suitable for CI applications. 

Cellular glass, like any insulation, is mostly air except the air is trapped in glass bubbles. This provides all the benefits of glass as a super durable material. Use foam glass below grade, below slabs, on roofs, on walls, and as a load bearing insulation to eliminate thermal bridging. With options supporting over 300 psi, it can provide continuity of insulation between a masonry wall and the foundation supporting it. 

Mineral wool semi-rigid panels are safe to use anywhere. They can be used in open joint rainscreen assemblies with UV exposure, behind lime, cement or synthetic stucco and also for foundation walls below grade and roofs. Mineral wool is sustainable because it lacks harmful chemical flame retardants or blowing agents. Some mineral wool uses formaldehyde in its manufacturing process so look for the panels and batts available in both low VOC and formaldehyde free options. Harmless and sustainable, mineral wool doesn’t burn so it can be used on any scale of project.

Mind the Details

If you’re already looking for sustainable CI solutions, you are probably also looking for higher R-values. Our favorite sustainable solutions in CI don’t have the highest R-values per inch, but are highlighted here because the performance of the wall and the health of the building is more than just R-value.

Three more things to keep in mind are the effectiveness of continuous insulation vs traditional cavity insulation, the thermal bridging of supporting materials outside the insulation, and the diminishing returns of higher wall R-values relative to the impact of windows.

Continuous insulation is the most effective place to insulate a building. Always run the math for your project to find the sweet spot R-value, because the right number varies a lot based on location, size, fenestration and other considerations.

The effective R-value of continuous insulation is so drastically higher than cavity insulation in a metal stud wall that 2 inches of continuous has the same thermal performance as 6 inches of the same R-value material in a metal stud cavity.

Since adding cavity insulation moves the dew point inward in the wall assembly, it can sometimes be detrimental to the durability of the wall. Unless you really need the cavity insulation for acoustic performance, try to add more CI instead.

Add more insulation outside the sheathing and framing of your wall assembly, but not too much. You can easily attach 6 or 8 inches of CI to provide an R value over 30, but once you do, you complicate your cladding options. For example, standard brick ties are designed for 4 inch cavities with 2 inches of insulation. Similarly, the brick lintel supports need to be designed to handle the extra load imposed by moving the weight farther out. Each of these can be a significant thermal bridge just as compromising as the stud wall behind.

Committing to sustainable CI also requires smart solutions for the reset of the wall assembly. For example, Hohmann & Barnard makes brick lintel supports designed to support CI while minimizing the thermal bridge, and Cladinator has a rainscreen support which can accommodate up to 14 inches of CI.

Higher R-values have a direct impact on lowering energy consumption for the life of the building. They also contribute to comfort inside the building and the durability of the envelope. The effective R-value of CI is higher than cavity insulation, but the effective R-value of the whole wall is limited by the weakest link: the windows.

Insulate your wall as much as you can afford, but remember that the windows, glass, and other openings you specify have much more impact, especially if you have a lot of them. The average R-value of the wall is the inverse of the percentage of the wall which is opaque, divided by its R-value, plus the percentage of the wall which is window divided by its R-value.

The equation used to calculate the average R value of a wall: R (average) = 1 / ((%Area/R value wall)+(%Area/R value window))

If you need a refresher, here's a link to a short video we have on why windows matter so much more than walls. It also talks through the calculations you use.

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