Cork is an eco-friendly insulation material made from the bark of cork oak trees. Its sustainability, renewability, and natural properties make it a versatile choice for insulation and soundproofing.
Where Cork Insulation Comes From
Raw cork is harvested from cork oak trees that grow in the Mediterranean basin–approximately 300,000 tons every year. Sixty percent is used for wine bottle corks. Most of the by-products are used to make cork board insulation and products like floor and wall tiles and bulletin boards.
The leftover cork chips are steam heated and pressed into boards of various thicknesses. The process activates a natural binder in the cork called suberin. Suberin binds the cork. Cork insulation production uses no chemicals. It is a wholly natural product and biodegradable. Rigid cork board insulation ranges in thickness from one inch to twelve inches.
Cork insulation has many undeniable advantages. Some of them include:
- R-value. R-3.6 – R-4.2 per inch. R-value does not degrade over time.
- Waterproof. Resists moisture and rot.
- Breathable. Suitable for walls that need breathable insulation that does not trap moisture.
- Pest Resistant. Does not attract rodents, termites, or other insects.
- Fireproof. Extremely fireproof class B2 rating. Emits no flames or toxic gasses when subjected to fire.
- Sound Suppression. Excellent sound insulation for street noise or interior entertainment centers.
- Eco-Friendly. Manufactured from cork byproducts of the wine industry. Biodegradable.
- Anti-Fungal. Does not provide a growing medium for mold and mildew growth.
The biggest disadvantage of cork insulation is cost. Cork insulation can cost two or three times more than traditional insulation products like fiberglass and cellulose.
Fastening siding and even stucco wire when the insulation is over 2” thick requires strapping. Strapping directly to the framing causes thermal bridging. A thinner layer of cork over the strapping solves the problem.
Applying thick layers of cork to exterior walls requires special treatment around windows, doors, and other wall penetrations like vents. Small roof overhangs and gutters can also be compromised. Take these into consideration when insulating with thick cork.
Cork Insulation R-value
Cork insulation boasts an R-value of R-3.6 – R-4.2 per inch. Better than fiberglass batts, sheep wool insulation, cotton insulation, denim insulation, and even cellulose insulation. Cork board r-value does not degrade over time. Rigid cork panels also provide excellent acoustics and soundproofing.
Types of Cork Insulation
Cork insulation is available in many forms. All of them have the same properties.
Cork Insulation Spray
Spray cork is a relatively new product that can be applied to almost any surface. It is 80% cork mixed with water-based resins and paint–natural and non-toxic. Sprayed on the outside of a house, it completely seals the walls from moisture. It also protects against fire, and pests, and is mold and mildew resistant.
Spray cork can be used on any type of roofing to prevent leaks or seal existing leaks. It is available in multiple colors. Most applications are done by contractors, but cork spray can be a DIY project for someone familiar with spray equipment if the product is available.
Spraying cork insulation inside stud cavities before installing fiberglass batts or mineral wool acts as an air seal and provides soundproofing. Cork spray does not expand like other spray foams.
Expanded Cork Insulation
Expanded cork insulation–also called semi-rigid cork insulation–has an R-value of R-3.6 per inch. It is manufactured using superheated steam to activate the natural binder. The resulting blocks are cut into 1’ x 3’ or 2’ x 3’ sheets up to 12” thick. It is a natural fire retardant material and provides superb soundproofing.
Unlike most cork products, expanded cork looks almost black in color. It also gives off a “burnt cork” odor that dissipates over time. It was designed and is used almost exclusively as an exterior insulation product for walls and roofs.
This type of cork insulation has been used in Europe for decades. Ten inch thick layers on exterior walls and 12” layers on roofs are not uncommon. North American distributors are importing it from Portugal–the world’s largest manufacturer.
Expanded cork insulation sells for approximately $1.05 per board foot. (A board foot is one square foot of material one inch thick.) To achieve R-19, expanded cork costs about $5.50 per square foot of covered area. As a comparison, R-19 extruded polystyrene rigid board insulation costs about $2.25 per square foot of covered area.
Rolled Cork Insulation
Cork is available in rolls that are used as insulation underlay for carpets, laminate, and hardwood flooring. It is especially effective over concrete to preserve a room’s heat. When used between living areas, it reduces noise transfer. Thinner rigid cork boards can also be used as flooring underlayment. Either type can easily be glued down.
Cork Granule Insulation
Cork granules are about a quarter inch in size–between perlite insulation and vermiculite insulation. They are used the same way–to fill wall cavities, as loose-fill attic insulation, and in small irregular spaces. Cork granules are more expensive than other options.