Building thermal insulation · Glass wool
Microfiber MicroRoof FLS
- Thickness
- 25–75 mm
- Density
- 12–48 kg/m³
- Thermal conductivity (λ)
- 0.032–0.042 W/m·K
What does insulation density tell you, and how should you choose?
In short
Density is the mass of insulation per unit volume, in kg/m³. It tells you how tightly the material is packed and affects stiffness, compression resistance, weight and, in some uses, acoustic behaviour. But higher density does not always mean better insulation. Compare products using the property that matches the job — for heat, the R-value at the thickness you will actually install.
How is density measured?
Density is the weight of a sample divided by its volume. For flexible fibrous blankets and batts there are dedicated test methods for thickness and density, such as ASTM C167, because the thickness of a soft material changes with the pressure applied while measuring it. Manufacturers use density for production quality control and to separate product grades.
What does density affect?
- Stiffness and shape retention — denser boards are usually stiffer, stand better in wall cavities and are easier to cut cleanly
- Compression resistance — where the insulation carries a load, such as under floors or on roofs with foot traffic, check the compressive strength the manufacturer tested (e.g. to ASTM C165 or EN 826), not density alone
- Weight — matters for ceiling framing or other supports, and for transport
- Sound — in sound-absorbing panels, density together with thickness affects absorption at each frequency, but what you should compare is the measured NRC or sound absorption coefficients
- Thermal conductivity (λ) — can change with density, but the direction depends on the material and the density range (see below)
Density and λ: a tendency, not a rule
In fibrous insulation, heat passes through the fibres, through the air between them and by radiation. As fibres pack more tightly, less heat radiates through, but more is conducted along the fibres, so the net effect does not always go one way. Examples from published documents:
| Source | Density range | Reported λ |
|---|---|---|
| --- | --- | --- |
| Microfiber MicroRoof (FLS), glass wool | 12 / 16 / 24 / 32 / 48 kg/m³ | 0.042 / 0.038 / 0.035 / 0.033 / 0.032 W/m·K (24 °C mean, ASTM C518) |
| ROCKWOOL Safe 'n' Silent Pro, stone wool | 40 / 60 / 80 / 100 kg/m³ | 0.036 / 0.035 / 0.034 / 0.033 W/m·K (20 °C, ASTM C518) |
| NIST reference material SRM 1450 (fibrous glass board) | about 60–180 kg/m³ | λ rises slightly as density increases |
In the first two examples λ falls as density rises, and falls less and less at the higher densities. The NIST data, covering a higher density range, show λ rising slightly instead. So do not conclude that "denser always insulates better" — check the λ of the actual product in its documents.
Thickness usually matters more
R-value = thickness ÷ λ. From the MicroRoof FLS table at 24 °C mean:
- 12 kg/m³, 75 mm thick — R = 1.786 m²·K/W
- 48 kg/m³, 25 mm thick — R = 0.781 m²·K/W
In this example the lighter but thicker roll gives the higher R-value. For thermal work, compare R-values at the thickness you can actually install. See Insulation thickness.
Sound insulation in wall cavities
Sound insulation ratings STC / Rw belong to the whole wall system. Research on gypsum board walls by the National Research Council Canada found that the type of sound-absorbing material in the cavity has a relatively minor effect compared with isolating the two wall faces, the mass of the boards and the cavity depth. A denser infill therefore does not automatically make a wall block more sound. Use the density and thickness specified in the manufacturer's tested wall system. See How cavity insulation helps sound insulation.
How to choose
- Define the function first: heat, absorption, sound insulation, load bearing or temperature
- Compare the matching value: R at the real thickness, NRC, system STC/Rw, compressive strength or service temperature
- Use density as supporting information — stiffness, weight, or the grade a tested system calls for
- Do not compare density across material types, such as glass wool vs stone wool vs foam, because their structures differ
Caution
The stated density is that of the product before installation. If fibrous insulation is squeezed thinner during installation, its density rises but its total R-value falls. See Gaps and compression.
Sources
- ASTM C167 — Standard Test Methods for Thickness and Density of Blanket or Batt Thermal Insulations
- ASTM C518 — Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus
- ASTM C165 — Standard Test Method for Measuring Compressive Properties of Thermal Insulations; EN 826 — Thermal insulating products for building applications: determination of compression behaviour
- NIST — Retrospective Analysis of NIST Standard Reference Material 1450, Fibrous Glass Board, for Thermal Insulation Measurements (J. Res. NIST)
- Microfiber Industries — MicroRoof brochure (EX-BR-001, 2023)
- ROCKWOOL — Safe 'n' Silent Pro data sheet
- National Research Council Canada (IRC) — Control of Sound Transmission through Gypsum Board Walls
- North American Insulation Manufacturers Association (NAIMA) — guidance on compressed glass fibre insulation
Related products
Categories and example products for this topic
Building thermal insulationSound insulationSound absorptionIndustrial and high-temperature insulation
- Microfiber MicroRoof FLS — Glass wool 12–48 kg/m³: λ at 24 °C goes from 0.042 to 0.032 W/m·K with density
- ROCKWOOL Safe 'n' Silent Pro — Stone wool 40–100 kg/m³ (Pro 330–380): λ 0.036–0.033 W/m·K at 20 °C by grade
- SCG Cylence ZoundBlock — Glass wool for sound-insulating wall cavities, made at a density of 32 kg/m³
- ROCKWOOL Thermalrock S — Stone wool S40–S150: NRC 0.90–1.05 by grade - the densest grades are not the highest
Sound insulation · Stone wool
ROCKWOOL Safe 'n' Silent Pro
- Thickness
- 25–100 mm
- Density
- 40–100 kg/m³
- Sound absorption (NRC)
- 0.90–1.00
Sound insulation · Glass wool
SCG Cylence ZoundBlock
- Thickness
- 50–100 mm
- Density
- 32 kg/m³
- Thermal conductivity (λ)
- ≤ 0.033 W/m·K
Building thermal insulation · Stone wool
ROCKWOOL Thermalrock S
- Thermal conductivity (λ)
- 0.034–0.036 W/m·K
- Sound absorption (NRC)
- 0.90–1.05