System and material comparisons
Rigid foam vs mineral wool insulation
Rigid foam boards and mineral wool boards both serve as continuous insulation, but behave differently in fire, vapor permeability, moisture and cost. Compare them in a full assembly.
On the outside of a steel-framed wall, continuous insulation reduces heat flow through the studs. Two common choices are rigid foam boards and mineral wool boards. They look similar on a drawing but differ in vapor permeability, fire behavior, compressive strength, water handling and fixing. The decision belongs to the wall assembly the designer draws for your climate. This page lists the comparisons to make. See also the rigid foam buying guide and mineral wool buying guide.
Key properties side by side
| Property | Rigid foam | Mineral wool board |
|---|---|---|
| Thermal resistance per thickness | Often higher on the datasheet | Moderate; thicker to match |
| Vapor permeability | Often low (varies by foam type and facing) | Generally high |
| Fire behavior | Organic polymer; check combustibility, flame spread and local limits | Mineral based; check product data and local code |
| Water handling | Closed cell; some absorb little water | Water-repellent grades drain, but not designed to sit wet |
| Compressive strength | Product grade dependent | Product grade dependent |
| Sound | Little absorption | Absorbs more sound in many products |
| Fixing | Long screws and washers or cladding clips | Same approach, heavier boards |
Where vapor behavior matters
Whether a wall can dry, and in which direction, depends on the permeability of each layer. A low-permeability foam outside the studs changes where condensation risk sits compared to a vapor-open mineral wool board. The right choice depends on climate, indoor humidity and the other layers. Ask the designer to run or show a moisture analysis for your site. See vapor control layers and vapor retarder.
Fire and code limits
Foam plastics are subject to specific code requirements in many jurisdictions, including thermal barriers, exposure and cladding combinations. Mineral wool is often used where non-combustibility is required. Do not rely on a general statement: request the product evaluation report and the tested wall assembly, and confirm acceptance with the local authority. See fire-rated assemblies documentation.
Cladding support and fixing
Thick exterior insulation means long fasteners or clips through the insulation to the framing. The cladding load, wind load and insulation compressive strength all matter. Ask the designer and the cladding supplier to confirm the attachment system. See exterior cladding guide and rainscreen.
Cost and supply
Prices vary by region and market. Compare installed cost per unit of thermal resistance, include fixings, tapes and extra labor, and use local quotes rather than rules of thumb. Both materials have lead times that vary by supply; plan with the lead time tracker.
Checklist
- Name the climate zone and wall assembly layers.
- Collect datasheets with test standards for both options.
- Request the fire assembly evidence.
- Have the designer confirm moisture performance and cladding fixing.
Questions people ask
What counts as rigid foam?
Boards such as polyisocyanurate, extruded polystyrene and expanded polystyrene. Properties differ between them.
Which allows water vapor to pass?
Mineral wool is generally more vapor-permeable than most foam boards, but check product data and assembly design.
Which has the higher R-value per inch?
Many foam boards state higher values per inch, but values depend on product and aging. Compare datasheets.
Can I use either on the outside of steel framing?
Both are commonly used as continuous insulation, with fixing and cladding support designed for the system.
Sources and further reading
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