Mineral Wool Vs. Fiberglass Insulation: Which is Better?
When choosing insulation for your home, consider mineral wool vs. fiberglass insulation. These products work for many of the same applications, like insulating interior walls, attics, and basements.
Both types of insulation keep temperatures regulated, preventing heat loss or heat gain depending on the climate. A few distinct differences, like ease of installation, weight, and cost, can help you make a final decision.
The Difference Between Mineral Wool vs. Fiberglass Insulation
The following chart provides a quick comparison of mineral wool batt insulation and fiberglass batt insulation products.
| Mineral Wool | Fiberglass | |
|---|---|---|
| Thermal Performance | R-15@ 3.5 Inches | R-13@ 35 Inches |
| Insulation Weight | Heavier/denser | Lighter/less dense |
| Sound Control | Comparable | Comparable |
| Fire Resistance | Non-combustible 0 Flame Spread /0 Smoke Developed | Non-combustible < 25 Flame Spread/< 50 Smoke Developed |
| Insulation Handleability | Rigid | Flexibily |
| Insulation Facing & Moisture Control | Unfaced Only | Unfaced, FSK Facing, Kraft-Facing |
| Typically Used In | Exterior Walls, Basements, & Heated Crawl Spaces | Interior/Exterior Walls, Basement, attics |
Thermal Performance
Winner: Mineral Wool
R-value measures the thermal resistance of an insulation product. Higher R-values mean lower thermal conductivity and warmer or cooler buildings–depending on local climate conditions. In a standard 2 x 4 wall, mineral wool batts are R-15, and fiberglass batts are R-13. Both types of batts are 3 ½” thick.
Installation Comparison
Winner: Fiberglass Insulation
Mineral wool batts are more rigid than fiberglass and fit snugly between studs. The rigidity makes them difficult to fit around services inside the wall cavity–like electrical wires, plumbing pipes, ducting, and framing braces. When cut and fit tightly, they provide excellent insulation value. Mineral wool is rigid enough to accept the use of spray foam to fill any gaps left by over-large cuts.
Fiberglass batts are soft and pliable. They are easy to separate to fit around pipes and wires. You can compress fiberglass batts, if required, around pipes. But, since the insulation value is in the dead air pockets of the fiberglass, the more compression, the lower the R-value. Fiberglass does not accept spray foam well, but it’s easy to cut and add extra batting where required.
Despite its softness and flexibility, fiberglass will not slump in the stud cavities. Once you install a vapor barrier and drywall over properly fiberglass, it won’t shift.
Most insulation installers have personal preferences. Some like the flexibility of fiberglass, while others prefer the solid feel of mineral wool.
Insulation Weights
Winner: Fiberglass Insulation
The density of mineral wool makes it much heavier than fiberglass. When used as wall insulation, this doesn’t present a problem, but using mineral wool in an attic may cause a weight problem if it’s resting on drywall screwed to the trusses or joists.
R-40 mineral wool insulation in an attic adds over 2lb. per square foot, meaning a 2000-square-foot attic would carry over two tons of extra weight. Fiberglass is much lighter and may be a better option in an attic, even if it requires an extra couple of inches to achieve R-40.
Fire Resistance
Winner: Mineral Wool
Unfaced fiberglass and unfaced mineral wool are non-combustible. They both help delay the spread of fire, although fiberglass will begin to melt at about 1000 degrees F. Mineral wool begins melting at higher temperatures–around 1100 degrees F. Mineral wool is more fire-resistant than fiberglass.
Moisture Control
Winner: Fiberglass Insulation
Both types of unfaced insulation – mineral wool and fiberglass batts – require a vapor barrier to prevent condensation build-up. Fiberglass insulation offers the option of a product faced with kraft paper or foil scrim kraft and is superior to 6 mil poly installed over mineral wool insulation.
Sound Suppression
Winner: Mineral Wool
Mineral wool has a sizable soundproofing advantage over fiberglass. It’s a denser material that provides more mass and, therefore, better soundproofing.
Sound Transmission Class (STC) is a system that measures noise. Each one-point increase in an STC rating represents a one-decibel noise reduction.
Typically, a 2 x 4 wall with drywall on both sides has an STC value of 34. Filling the stud cavities with fiberglass raises the STC rating to 39. Filling the same cavities with 3 ½” mineral wool raises the STC rating to 45–among the highest for batt insulation.
Architects, engineers, and designers regularly specify mineral wool insulation between units of multi-family buildings and noisy locations. It’s also a popular product when soundproofing home theaters and music rooms.
| STC | What can be heard at this level |
|---|---|
| 25 | Soft speech can be heard and understood |
| 30 | Normal speech can be heard and understood |
| 35 | Loud speech can be heard and understood |
| 40 | Loud speech can be heard, but not understood |
| 45 | The threshold at which privacy begins |
| 50 | Loud sounds can be heard, but are very faint |
| 60+ | At this level, good soundproofing begins. Neighbors generally are not disturbed by very loud speech from inside. |
Insulation Costs
Winner: Fiberglass Insulation
Three-and-a-half-inch fiberglass batt insulation costs around $0.50 per square foot. Mineral wool is approximately 25% more expensive–around $0.62 per square foot. In certain circumstances, mineral wool can be as much as 50% more expensive. Availability, special orders, and shipping distances all affect cost.
Twelve cents per square foot does not sound like much, but insulating 8000 square feet of new house walls, basement, and attic adds at least $1000.00 for material. Also, it’s more difficult to find professional installers that use mineral wool.
Typical Uses
Mineral wool and fiberglass batt insulation are suitable in many of the same locations, such as exterior walls, interior walls, attics, basements, and garage walls. Mineral wool has an advantage, though – it’s water resistant, so contractors can install it in crawl spaces and as a continuous exterior application. You can’t use fiberglass batts in crawl spaces, they absorb water and become worthless as insulation.
Conclusion
As we have shown, mineral wool insulation provides slight advantages in some areas–such as R-value and soundproofing. But fiberglass insulation is less expensive and easier to work with–making it an obvious choice for DIY projects.
Both products provide excellent insulation value when installed properly. Insulation does not all have to be the same. Use fiberglass batts for exterior walls to save money and insulate the home theater with mineral wool for a quiet entertainment area and to keep the sound inside.