Murat
Most people are familiar with fiberglass blanket or batt insulation. It is the fluffy pink or yellow stuff used in construction to keep houses warm or cool. Mineral wool batt insulation is also installed between wall studs and performs the same function.
Both products are very good at keeping homes warm and/or cool. Each has advantages and disadvantages. This article aims to provide the necessary information to help make informed decisions.
Mineral Wool and Fiberglass Comparisons
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 |
| 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
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. fiberglass batts are R-13. Both types of batts are 3 ½” thick.
Warmer air will always flow towards cooler areas. Good insulation will not only keep heat inside the house, it will prevent hot humid air from leaking into the building.
Installation Comparison
Mineral wool batts are much more rigid than fiberglass. They fit snugly between studs. The rigidity makes them more 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. Batts can be compressed if required around pipes. Remember that the insulation value is in the dead air pockets of the fiberglass. The more fiberglass is compressed, the lower the R-value. Fiberglass does not accept spray foam well but it is easy to cut and add extra batting where required.
Despite its softness and flexibility, fiberglass will not slump in the stud cavities. Once the vapor barrier and drywall are installed over properly fitted fiberglass it will go nowhere.
Most insulation installers have personal preferences. Some like the flexibility of fiberglass while others prefer the solid feel of mineral wool.
Insulation Weights
The density of mineral wool makes it much heavier than fiberglass. When used as wall insulation this does not present a problem. Used in attics where it is likely resting on drywall screwed to the trusses or joists may cause weight problems.
R-40 mineral wool insulation in an attic adds over 2lb. per square foot. A 2000 square foot attic is carrying over two tons of extra weight. Fiberglass is much lighter and may be a better option even if it requires an extra couple of inches to achieve R-40.
Fire Resistance
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. It is usually considered more fire-resistant than fiberglass.
Moisture Control
Both types of unfaced insulation 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. When installed properly these products provide the vapor barrier. They are often considered superior to 6 mil poly installed over mineral wool insulation.
Sound Suppression
Mineral wool has a sizable soundproofing advantage over fiberglass. Mineral wool is a much denser material that provides more mass and better soundproofing. Sound Transmission Class (STC) is a system used to measure noise. Each one point increase in 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 is 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. |
Typical Uses
Mineral wool and fiberglass batt insulation are used in many of the same locations. Exterior walls, interior walls, attics, basements, and garage walls. Mineral wool is water resistant. It can be used in crawl spaces and as a continuous exterior application. Fiberglass batts absorb water and become worthless as insulation.
Insulation Costs
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. Finding an insulation installer who is comfortable using mineral wool may also be a problem.
Conclusion
As we have shown, mineral wool insulation provides slight advantages in some areas–such as R-value and soundproofing. But fiberglass insulation is significantly 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.
