Cork Insulation Header

Cork Insulation Guide

By Tom Saxton Rise Writer
Jan 30, 2021

As we push forward with new and innovative building designs, sometimes looking to the past can give us great insight into long-lasting and sustainable building materials. Cork is a versatile material that has been used by humans for thousands of years. Cork flooring is well known, but what about other uses? Did you know cork is also very useful as wall and roof insulation?

Cork Oak Rainforest Alliance
Cork Oak Tree. Photo Credit: Rainforest Alliance

What is Cork?

Cork is the bark of an oak tree, Quercus Suber, native to the western Mediterranean and Northern Africa. The trees live between 150-250 years, and the bark can be harvested every nine years without damaging the tree. Each tree can be harvested 15-20 times during its life. Harvesting the cork tree's bark has proven to increase the tree's vigor and improve overall health. 

What Is Continuous Wall Insulation and Why Is It Important?

On a building's exterior, continuous wall insulation is a practical step to improving its performance and energy efficiency, particularly in climate zones 3 to 8. Adding a layer of continuous insulation on wall assemblies and underneath the siding reduces thermal bridging and condensation in the wall assembly. As a result, the overall thermal performance of the wall increases substantially. In a typical structure, framing can account for 20-30 percent of its surface areaEvery framing member acts as a hole in the insulated wall assembly, allowing heat to pass through it. 

What Types of Insulation Are Used For Continuous Wall Insulation? 

Foam board insulation (XPS or polyisocyanurate) has been the standard insulation used as continuous exterior insulation. However, corkboard is becoming an increasingly popular and more sustainable option.  

How Is Cork Insulation Made?

Corkboard insulation typically uses the cork granule by-product of the stopper industry. The granules are steam-heated and pressed into a board, causing the cork to expand, which activates a natural binder known as suberin, which fuses the granules. No other binders or chemicals are used in the process. No cork goes to waste during the manufacturing process; any waste gets fed back into the production cycle. 

Cork was used as insulation for cold storage until the rise of plastic foam insulation materials. The wine stopper industry is experiencing an increase in foam stoppers' utilization. As a result, the cork industry is looking for new ways to remain viable, bringing back old uses for the material, particularly its use as an insulation material. Cork has been used extensively in Europe as insulation for decades and is becoming more popular in North America.

Expanded Cork Boards Cork Link
Expanded Cork Boards. Photo Credit: Cork Link

What Types of Cork Insulation Exist and Where Can it Be Used?

For exterior insulation, cork insulation comes in the form of a semi-rigid board. For wall and roof insulation, boards with a density of 7lbs per cubic foot are most common. A rigid cork board is generally used as continuous wall insulation on the exterior of the wall or roof assembly over the top of the exterior sheathing and underneath the siding or roofing material. 

What is the R-Value of Cork Insulation? 

Cork's R-value is typically listed at R-3.6 per inch to R-4 per inch.

How Thick Should Cork Insulation Be?

Cork comes in all different thicknesses. Rigid cork boards for wall insulation can range from 1" to 12" in thickness. Check your local building codes to determine what R-value you need for continuous exterior insulation in your climate zone.

How Long Does Cork Insulation Last?

Cork is said to maintain the maximum thermal performance for at least 50 years without a decrease. It will not lose R-value over its service life, unlike XPS and polyisocyanurate foam, which gradually degrades thermal performance over time due to off-gassing. Also, since cork has a higher vapor permeability than foam, there is a reduced risk of moisture issues in the wall assembly, potentially extending the structure's lifespan. 

Amorim
Photo Credit: Amorim

Is Cork Insulation Environmentally Friendly?

Cork is tough to beat on this front for a variety of reasons. It is a rapidly renewable resource produced through sustainable forestry practices that protect cork forests, a necessary habitat type that is considered biological hotspots. If the cork industry vanishes, these habitats will risk being destroyed for other economic pursuits. Cork forests stabilize soil, protecting from wind erosion in a region of the world where desertification is a real threat. Cork forests also serve an essential part of hydrologic cycles by filtering pollutants, thus improving water quality. Cork forests act as a carbon sink by absorbing CO2. Because cork bark has such a low combustion rate, cork forests act as fire barriers and reduce forest fires' risk to nearby communities. 

Much of the cork used for insulation is a by-product of waste from the cork stopper industry. No cork waste is generated in the manufacturing process. Cork is biodegradable and recyclable at the end of service life. 

Is Cork Insulation Healthy?

Absolutely! Cork insulation has no added chemicals; the materials to manufacture are just cork and water. Cork will not off-gas, and its surface doesn't attract dust and pollutants. It is naturally anti-fungal and anti-microbial. At the end of its life, it will decompose. It will not release toxic microplastics or other synthetic compounds into the environment.

Jelinek
Photo Credit: Jelinek

How Much Does Cork Insulation Cost? 

Cork is considerably more expensive to install than foam alternatives. Cost varies by geographic region and can be reduced by purchasing large amounts. As cork becomes more prevalent in North America, it is expected that its price will decrease.

Cork is not widely available in North American stores. Your contractor might be able to source cork for less than one can purchase online. A few online vendors with prices (excluding shipping) are listed below:

  • Jelinek Cork Group: $6.5USD per square foot for 2" thickness (R7.2)
  • Thermacork: $3.57USD per square foot for 2" Thick (R8), $6.23USD per square foot for 3" Thick w/ Shiplap Cut (R12)
  • Amorim Cork: Contact with your specifics for a quote

Costs of rigid foam board insulation:

With the large difference in cost, take a holistic approach of all the pros and cons to determine if cork benefits are worth the cost long-term. The cost difference won't be as substantial for a small structure, but the cost could be out of reach for cork in large installations. 

Thermacork
Photo Credit: Thermacork

How Can You Install Cork Insulation?

Installation of continuous exterior cork insulation will differ depending on your climate zone and building codes. A general description would be to install the cork boards on the outer side of the wall/roof sheathing so that seams run horizontally. Screw 1" x3" wood furring vertically over the cork and through to the sheathing (if 5/8" sheathing or greater) or through to the framing if ½" sheathing is used. Install the desired siding/roofing materials overtop of the cork boards and nail to the furring strips or through to the sheathing. 

If installing 3" or thicker cork, you can order boards with shiplap joints to reduce through gaps. If using ship laps, start installing panels at the base of the wall and work up, so the shiplap keeps moisture on the outside of the boards. Screw the board to the sheathing or framing. You can save on labor time by installing a thicker cork layer with shiplap joints compared to 2 thinner layers with staggered seams. 

A fine-toothed hand saw works well to cut corkboard.

How Can You Remove and Dispose of Cork Insulation?

Removal is easy; remove the fasteners holding the cork to the exterior sheathing or wall stud. Removing cork insulation will not expose you to fibers, toxic dust, or harmful chemicals.  

Cork is a biodegradable product. At the end of its life cycle, it can be composted or recycled. Cork is frequently recycled into new products.

Jelinek
Photo Credit: Jelinek

What Are The Benefits of Cork Insulation? 

Cork insulation has many benefits. It has is a respectable R-Value at 3.4 to 4 per inch depending on density (typically listed at R-3.6). On top of that, its R-value is stable over time, unlike XPS and polyisocyanurate foam that reduce in thermal performance as they off-gas over several years. Cork is hypoallergenic, anti-fungal, anti-microbial, AND has excellent sound-absorbing properties!

Cork's high vapor permeability means it can breathe, an ideal attribute for wall insulation allowing the wall assembly to dry out. Cork is inherently fire-resistant, and there are no added toxic flame retardants like foam insulation and is stable at a broader range of temperatures than XPS foam. 

Cork is a sustainable material that will maintain its performance functions overtime for a longer service life than synthetic foams.

What Are The Drawbacks of Cork Insulation?

As with all things, there are some drawbacks to cork insulation. It is heavier than foam, so weight-sensitive applications make it less feasible to use. Corks density is 7lb/ft3 versus 2lb/ft3 for XPS and polyisocyanurate foam insulation. 

Being sourced from the western Mediterranean, cork can have a higher carbon footprint regarding its shipping. This footprint is still much lower than using synthetic foams. While availability in North America is increasing, it remains tough to source, outside of online vendors, and contractors are not familiar with it. As a result, its installation cost can be more expensive than synthetic foams. 

Can Cork Insulation Be Used in any Climate? 

Continuous exterior insulation should be installed at different thicknesses depending on your climate zone and is optional in climate zones 3 to 5 but often required in zones 6 to 8. Always check your local buildings codes before proceeding with any building project. 

Thermacork
Photo Credit: Thermacork

Cork is an intriguing insulation material gaining popularity in North America after experiencing long-term success in Europe. With a greater interest in bio-based materials among consumers, there will likely be an increased demand for cork insulation in the future. Still, it will likely remain a small niche in the insulation realm. The material's initial cost is the largest barrier for consumers, but it is predicted to drop in price as it becomes more popular. Cork is a high quality and resilient material for continuous exterior insulation on a building. It doesn't stop there; cork as a building material has a large number of applications. Check out our in-depth article on cork flooring.  

Disclaimer: This article does not constitute a product endorsement however Rise does reserve the right to recommend relevant products based on the articles content to provide a more comprehensive experience for the reader.Last Modified: 2021-06-15T05:30:49+0000