Product Guide
Board Insulation
Rigid mineral wool was installed on the exterior of the Design Building at the University of Massachusetts–Amherst.
Photo: Paula MeltonThe best products...
- Are mineral wool, cellular glass, cork, or wood fiber board insulation that have no blowing agents and require no flame retardants
- May be vacuum insulation with superior thermal performance
- May be halogen-free polyisocyanurate insulation with Greenguard Gold or Indoor Advantage Gold certification
- May be low-global-warming-potential XPS with Cradle to Cradle Silver or higher
But watch out for...
- Extruded polystyrene (XPS) with high-global-warming-potential blowing agents
Products we like
Note: Manufacturers don’t pay to be listed here, and our publications are not sponsored or supported by ads. BuildingGreen experts curate this list of products that best reflect the guidance listed above.
- Owens Corning Foamglas
- Rockwool Comfortboard
- Gutex Wood Fiberboard
- Kingspan Optim-R
Behind the recommendations
Board insulation has good thermal performance ranging from about R-4 to R-8 and is used as an integral part of a building envelope’s continuous insulation system.
Because board insulation is used on the building exterior, it is not always subject to the same material health and emissions requirements as batt insulation used inside the building envelope.
BuildingGreen bases its recommendations on a combination of performance, global warming potential, environmental impacts, and material health. For instance, mineral wool’s combined thermal performance, durability, recycled content, pest resistance, and lack of flame retardants and blowing agents make it a solid choice, even though it uses formaldehyde-based resins. Plastic foam boards, such as polyiso, XPS, and EPS, all require chemical flame retardants (and still are not as flame resistant as mineral wool).
The embodied carbon in insulation is significant. The rock or slag for mineral wool has to be melted at high temperatures, and the blowing agents used in most XPS have extremely high global warming potentials (GWP). Over a lifetime of use, mineral wool has a lower overall GW than these products. Note, however, that low-GWP XPS is now available, making these board products reasonable alternatives. But mineral wool still has better flame resistance than XPS without requiring chemical flame retardants. Wood insulation has low embodied carbon and is a good alternative in the right project.
Though BuildingGreen prefers the use of products not made solely from petrochemicals, energy performance is a high priority with any building, so the use of low-GWP XPS or EPS may be appropriate when those are the only options available to meet functional performance goals.
Cautions
- Extruded and expanded polystyrene (XPS and EPS) are petroleum based, require flame retardants, and are manufactured using a number of potentially hazardous chemicals, including benzene and styrene.
- Standard XPS is still made with high-global-warming-potential blowing agents, though low-GWP versions are available in select markets.
- Some foam board insulations once used persistent, bioaccumulative, and toxic halogenated flame retardants. Though these compounds have been replaced with less hazardous chemistries, the long-term environmental impacts of their replacements are not fully understood.
Additional context
Mineral wool
Mineral wool board is a versatile insulation made from molten slag (a waste product of steel production) or natural rock (such as basalt or diabase), held together with a phenol-formaldehyde binder.
Mineral wool has a higher density than fiberglass, is more resistant to fire, and is better at blocking sound. It is appropriate for foundation wall and general exterior insulation.
Vacuum Insulation
Panels that incorporate vacuum insulation have the highest R-value of any board insulation product. Typically sandwiched between sheets of reinforced high-density polyiso, these products can have R-values of R-50 for a 1.5"-thick panel. Though it has a high first cost and has to be used where it will not be penetrated (such as roofing or behind some cladding systems), they can be the best option in applications where space is a premium or maximum R-value is required.
Expanded Cork
Cork is natural, it sequesters carbon, and it is produced through a sustainable silviculture process with a 2,000-year tradition. The material regenerates itself and can be harvested every nine years. In buildings, cork is best known as a floor-tile product and a sound-control underlayment, but it offers good thermal performance as well.
Cork insulates well, absorbs sound, and is durable in use yet ultimately biodegradable. Care has to be taken with air infiltration and insects, but cork can be an excellent choice in the right application and has been used in Europe for several decades.
Wood
Wood insulation is made from wood fiber, along with wax and small amounts of polyurethane binder. These products are often tongue-and-groove and can double as exterior sheathing, providing a more natural insulation alternative that has good thermal performance along with moisture resistance and breathability.
Polyisocyanurate
Of the rigid plastic foam insulations on the market, polyisocyanurate, or polyiso, may have the smallest footprint—though its impacts are still significant.
Polyiso manufacturers were among the first to move away from ozone-depleting blowing agents, and they are the first to have removed halogenated flame retardants.
Cellular Glass
Although expensive, cellular glass is naturally flame retardant, and its high compressive strength and moisture resistance make it appropriate for below-grade applications, especially sub-slab applications—where XPS currently dominates the market.
Extruded Polystyrene
Extruded polystyrene (XPS) is thermoplastic, closed-cell foam insulation derived from petrochemicals (some of which are potentially hazardous). But XPS’s excellent moisture resistance, high compressive strength, and low cost make it a very popular insulation material—particularly for below-grade applications, including foundation walls and concrete slabs. XPS is now available with low-global-warming-potential blowing agents.
Expanded Polystyrene
Expanded polystyrene (EPS) is a closed-cell, thermoplastic foam made by heat-expanding polystyrene beads using low-gwp pentane blowing agents. EPS is produced in smaller batches and, though used in board form, it can also be found in structural insulated panels and other custom forms.
Fiberglass
Rigid fiberglass is similar to fiberglass batts but formed into denser boardstock. It is less common as an exterior insulation. Some formaldehyde-free versions are available for use in interior applications.
Complexities of selection
Board insulation products vary widely not only in R-value but also in permeability, moisture resistance, insect resistance, fire resistance, need for flame-retardant additives, chemicals of concern, and end-use applications.
