Rockwool vs Fiberglass: Which Insulation to Choose
Rockwool (also called mineral wool or stone wool) is denser, handles water better and usually does more for sound. Fiberglass is lighter, more flexible and cheaper. Neither is “best” everywhere. The right pick depends on the cavity you are filling, the moisture risk, how much noise you want to block and your budget. Below, we compare the two on the points that decide most projects, then place cellulose and spray foam next to them.
TL;DR
Mineral wool is denser and semi-rigid, and it is reported at about R-3.8 to R-4.3 per inch, against R-3.0 to R-3.7 for fiberglass batts.
Mineral wool sheds water. Fiberglass can hold moisture between its fibers and lose R-value while wet, so basements and exterior walls favor mineral wool.
Fiberglass batts cost less. The U.S. Department of Energy’s guide put them at $0.20 to $0.40 per square foot in its 2010 figures.
For sound, a well-fitted batt of either material helps. Mineral wool tends to edge ahead in absorption ratings, but wall design matters as much as the material.
Check your local energy code and climate zone before you buy. In NYC, Long Island, Westchester and northern New Jersey, that decision is best made before demolition starts.
Table of Contents
What is the difference between Rockwool and fiberglass?
Both are fibrous batt-type insulations, but they are made from different materials. Fiberglass is made of fine glass fibers and comes as batts, blankets, rigid boards, and blown or sprayed fill. Mineral wool is made from spun stone or slag.
The physical difference you notice on site is density. Because stone wool fibers are shorter than long glass fibers, stone wool batts must be packed to a higher density. That makes them firmer and heavier. A mineral wool batt holds its shape between studs. A fiberglass batt is soft and can be compressed or bunched if it is forced into a cavity.
The same material goes by several names, which causes confusion when you shop:
Rockwool and Roxul are brand names for stone wool products.
Mineral wool, stone wool and rock wool are the generic terms for the same family.
Glass wool is the term used outside the U.S. for what American contractors call fiberglass.
Rockwool vs fiberglass at a glance
Factor | Mineral wool (Rockwool) | Fiberglass batts |
Typical R-value per inch | R-3.0 to R-3.7 | |
Feel and fit | Dense, semi-rigid, holds its shape | Light, flexible, can sag or compress |
Water behavior | Hydrophobic, water beads off | Can trap moisture between fibers |
Sound absorption | Reported NRC 1.00 to 1.05 | Reported NRC 0.85 to 0.95 |
Installed cost | Higher | Lower: $0.20 to $0.40 per sq ft (DOE, 2010) |
Best suited to | Basements, exterior walls, sound-sensitive rooms, assemblies where water may appear | Standard dry stud and attic cavities on a tighter budget |
The sections below explain each row.
R-value: how much warmth per inch
R-value measures resistance to heat flow. Higher means better. Mineral wool generally provides R-3.8 to R-4.3 per inch, while fiberglass batts typically range from R-3.0 to R-3.7.
In practice that gap matters most where depth is limited. A standard 2x4 wall cavity cannot be made deeper, so a denser product gets you a little more resistance in the same space. In a deep attic or a 2x6 wall, you can often make up the difference by installing more fiberglass.
One point applies to both: the label R-value only counts if the product is installed well. The U.S. EPA notes that properly installed insulation and air sealing reduce gaps and help indoor air quality. Gaps, compression and voids cost you more performance than the difference between the two materials.
Moisture: where mineral wool pulls ahead
This is the deciding factor for many New York area projects, because basements, older masonry buildings and exterior walls all see moisture at some point.
Mineral wool is hydrophobic: water beads off it. Fiberglass traps moisture in the air spaces between fibers, which can reduce its R-value while wet. Other published figures describe both as absorbing little water by weight: fiberglass and rock wool absorb about 1% of their weight in moisture, while cellulose absorbs 5 to 20%. Absorbing little water and holding up when wet are not the same thing, which is why the hydrophobic behavior of mineral wool is the reason many builders choose it for damp-prone locations.
No insulation fixes a leak. If a basement wall is wet, the source needs to be found and repaired first. Our exterior envelope repair and waterproofing work exists for exactly that reason: insulation goes in after the water problem is solved, not instead of solving it.
Fire resistance
Mineral wool is made from stone and is generally not combustible, which is why it is common in fire-rated assemblies and between dwelling units. Fiberglass is also widely used in fire-rated walls, so both can meet code in the right assembly. Foam is different: the Insulation Institute notes that spray foam insulation will ignite at 700°F. In multi-unit buildings, commercial spaces and mechanical rooms, the fire rating of the full wall or ceiling assembly is what the inspector checks, so confirm the listed assembly before choosing a product.
Sound control: is Rockwool better for soundproofing?
Often yes, but less dramatically than the marketing suggests.
Published comparisons report mineral wool with NRC 1.00 to 1.05 and STC 43 to 52, while fiberglass typically falls at NRC 0.85 to 0.95 and STC 36 to 39. NRC describes how much sound a material absorbs. STC describes how much sound a whole wall or floor blocks.
An engineering-focused article offers an important correction. Both stone wool and fiberglass can increase a partition’s STC by 4 to 10 points, depending on wall design and cavity size. Full-wall assemblies perform comparably, and a snug fit prevents sound leaks.
What this means for your project:
Home theaters, bedrooms next to living areas, home offices: mineral wool in the cavity is a sensible upgrade, particularly with staggered studs or resilient channel.
Party walls and apartment partitions: the assembly design (double studs, extra drywall layers, sealed edges) does most of the work. Mineral wool is a strong fill choice.
Standard interior walls with no noise concern: fiberglass is usually adequate.
Sealing matters in every case. Gaps around outlets, pipes and the top and bottom plates let sound through regardless of what is in the cavity. If your noise concern is outdoors, such as a patio or yard, it is also worth reading about how awnings affect outdoor acoustics.
Do you need an air gap with Rockwool?
In a normal framed wall cavity, no. Mineral wool is meant to fit snugly between studs and joists, filling the cavity fully without being crushed. A gap or a loose fit is a weakness, because it allows air to move around the insulation and cuts performance. The same rule applies to fiberglass.
Where a gap does belong is in the wall or roof design itself, not in the insulation layer. Examples are a ventilated space behind some cladding systems, or the ventilation channel above insulation in certain roof assemblies. Those details depend on the assembly, the climate and the code. Building science guidance warns that unvented roofs need careful moisture management to prevent condensation on roof sheathing. Have the contractor and the building department confirm the design before the insulation goes in.
What about cellulose and spray foam?
Many homeowners comparing batts also ask about these two, so here is where they sit.
Cellulose is often made from recycled newspapers, with additives to limit pest and fire risks. The same federally supported guide notes it loses effectiveness when wet and is generally not recommended for basement walls in direct contact with concrete. It is commonly blown into attics and closed cavities.
Spray foam comes in two types:
Open-cell foam is low-density and vapor-permeable, and it can absorb and hold water.
Closed-cell foam is high-density, less vapor-permeable and stops liquid water from passing through.
Building Science Corporation’s Joseph Lstiburek notes that model codes limit open-cell spray foam to IECC Climate Zones 1 to 4, while closed-cell can be used in all climate zones. Check which zone your county falls in with your local building department.
Another practical difference is downtime. Both stone wool and fiberglass avoid the “several days to set up” or “24-hour evacuation” periods associated with cellulose and spray polyurethane foam. In an occupied apartment building or an open business, that can decide the question.
Cost: what to budget
The most useful published benchmark is the Department of Energy’s home insulation guide. Its 2010 figures, which are old and should be treated as relative comparisons rather than today’s quotes, list average costs per square foot of:
Insulation type | Average cost per sq ft (DOE, 2010) |
Fiberglass batts | |
Loose-fill (fiberglass, cellulose, mineral wool) | $0.45 to $1.35 |
Closed-cell spray foam | $1.30 to $2.00 |
Open-cell spray foam | $1.70 to $2.50 |
The guide also says costs depend on R-value and material type, and that loose-fill and spray foam prices are based on professional installation, including materials and labor. Mineral wool batts are not broken out in that table, but the ranking is clear: fiberglass batts are the low-cost baseline, and denser or specialty products cost more.
Current labor rates in the New York metro area are higher than the national averages of 2010, so ask for a written, itemized quote. We do not publish fixed prices because each job differs in access, wall condition and code requirements.
Which one should you choose?
Start with the location, not the product.
Finished basement: mineral wool is a strong choice where moisture is a possibility, once any water entry is fixed. Avoid cellulose against concrete walls.
Exterior walls in a gut renovation: fiberglass works well in dry, well-detailed walls. Mineral wool adds sound control and water tolerance.
Interior partition walls and ceilings between floors: mineral wool for noise control, fiberglass if noise is not a concern.
Attics: blown fiberglass or cellulose is common for open attic floors. Batts of either material suit rafter bays.
Commercial build-outs and multi-unit buildings: choose from the listed fire and acoustic assemblies required for the occupancy.
Contractors at a manufacturer of fiberglass and mineral wool make a similar point: select insulation based on project-specific requirements, such as rigidity for continuous exterior insulation versus lightweight flexibility for standard cavities, rather than comparing properties in isolation.
Code requirements in the New York area
Building codes set minimum insulation levels for each part of the building envelope, and the EPA explains that these vary by state according to climate. Homeowners should check local requirements before building or renovating. New York City, Long Island, Westchester and northern New Jersey fall under different state codes and local amendments, so a spec that passes in one place may need changes in another. If you are a manager of a multifamily property pursuing certification, note that ENERGY STAR’s 2025 multifamily new construction checklist requires Grade I insulation installation per ANSI/RESNET/ICC 301, subject to specified alternatives.
A licensed general contractor handles this coordination: choosing the assembly, pulling the permit where one is needed and passing the inspection.
Our perspective
In our work on basements, additions and full interior renovations, the insulation choice rarely stands alone. It sits inside a wall, a ceiling or a roof, and it has to work with the vapor control, the air sealing and the finishes around it. That is why we do not push one material for every job. Where moisture, fire separation or noise is a concern, we lean toward mineral wool. Where the cavity is dry and the budget is tight, a well-installed fiberglass batt is a sound answer. In both cases, a careful installation matters more than the brand on the bag. We handle the work in accordance with applicable local laws, licensing requirements and building codes, so the insulation, the framing, the drywall and the inspection are planned together instead of left for separate trades to sort out. DJ
Planning an insulation upgrade with DJ Custom Contracting
If insulation is part of a larger project, such as a basement finishing and conversion, a bathroom gut renovation or an addition, we can specify and install it with the rest of the work. We serve homeowners, business owners, property managers and facility managers in NYC, Long Island, Westchester and northern New Jersey.
For older buildings with water or envelope problems, our exterior envelope repair and waterproofing work addresses the source before new insulation goes in. You can see everything we do on our services page and ask us for a quote.
FAQ
Is Rockwool better than fiberglass?
It depends on where it goes. Mineral wool offers a higher reported R-value per inch, sheds water and generally absorbs more sound. Fiberglass costs less and is fine for dry, standard cavities. For basements and sound-sensitive rooms, mineral wool is usually the stronger choice.
Is Rockwool good for sound deadening?
Yes, as part of a well-built wall. Both stone wool and fiberglass can raise a partition’s STC by 4 to 10 points, depending on the design. A snug fit and sealed gaps do as much as the material itself.
What is the difference between stone wool and fiberglass insulation?
Stone wool is made from spun rock and has shorter fibers, so batts are packed denser and feel firmer. Fiberglass is made from fine glass fibers and is lighter and more flexible. Stone wool is hydrophobic, while fiberglass can trap moisture between its fibers.
Is cellulose or spray foam better than fiberglass in an attic?
Neither is better in every case. Blown cellulose and fiberglass are both used for open attic floors and cost less than spray foam in the DOE’s 2010 figures. Spray foam suits sloped or unvented roof assemblies, but code limits apply to open-cell foam by climate zone. Ask your contractor which assembly your roof needs.
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