Insulating against temperature extremes is a sensible move whether you live in the desert, the cold north or somewhere in-between, but it has other benefits too, including sound proofing potential.
The sound deadening qualities of some insulation products make them a great product in areas where you might be disturbed by road noise or industrial activity, at home or work, adding to the criteria of which insulation material to choose.
Some insulation products, while technically doing a terrific job of maintaining a comfortable environmental temperature inside a building, perform less well at excluding unwanted noise nuisance, either internal or external.
Here’s our guide to help you get the balance right.
What Does Sound Insulation Need to Do Differently?
Thermal and acoustic insulation both do similar jobs, and have plenty in common, but the main difference is that sound insulation needs to manage vibrational energy, which is mechanical in nature. For example, lower frequency, bass sounds, are transferred through the timber framework in a partition or external wall, but timber does not conduct heat so easily.
Sound insulation usually needs more mass than thermal insulation, and is often blocking, absorbing, sometimes reflecting material. However, more porous materials are required to dampen or block out medium to high frequency sounds that travel at speed through the air.
The Science Bit
The Sound Transmission Class
STC ratings give an indication of how well insulation, and other products, fare in laboratory conditions when tested for sound mitigation. They are a reasonable benchmark against which different products can be measured, and while real world conditions can differ considerably, it is good to have somewhere to start.
The rating numbers relate to the decibel grade that the material is expected to absorb or block and bears little relation to the R-value that denotes an insulator’s efficiency at preventing thermal energy transfer, which means it is possible for a product that keeps out the cold well, could also let sound through.
The Noise Reduction Coefficient
NRC ratings measure how much sound a material can soak up, like a sponge with water. The effects are measured on a scale from zero to 1, the higher the rating, the more sound the material can soak up. Some materials can be rated at more than one, but this is simply because the original material used to signify the top level of sound absorption (lamb’s wool) has since been surpassed.
Ultimate Sound Proofing
If you need silence, the complete absence of noise, perhaps you are thinking of putting a home studio or cinema into a basement, garage, or attic room, then additional structures will need to be installed. Decoupling partition walls, by building a secondary framework within a space, will help to stop that vibrational transfer through solid objects.
If you can, box in heat and ventilation ducts as they are fantastic at transferring noise from one space to another. Seal around electrical outlets, and consider double sheet-rocking walls, with thicker 5/8 ” boards and acoustic green glue between the sheets to deliver ultimate sound proofing.
How Insulation Materials Line Up
Blown Cellulose Insulation
Blown cellulose insulation, whether loose fill in the attic or dense pack in the wall, will deliver a decent level of sound mitigation. In an external wall with a 4” cavity dense packed cellulose will tie with fiberglass with an STC rating of 41, loose fill is not really suitable for wall installations but in the attic at 17” it should be perfectly adequate in most situations.
Damp spray cellulose does better with an STC rating of 49, which will cut out most noise transmission between internal spaces and its high absorbency makes rooms insulated with it feel very quiet. Like other sprayed products it will find its way into every gap around pipes, electrical outlets and other obstacles.
Fiberglass and Mineral Wool Insulation
Fiberglass
Fiberglass insulation scores well in sound insulation tests with a standard 4” wall cavity installation delivering an STC rating of 39. It is also the least expensive insulation product on the market with the lowest cost per square foot, which makes it an ideal candidate when doubling up and decoupling wall structures.
Mineral Wool
Mineral wool scores even higher in STC tests than fiberglass at 45, so delivers a better level of sound deadening, albeit at a somewhat higher cost, however it is less vulnerable to moisture and is, overall, a much more resilient product.
What neither do however, is stop air transmission through ducts or electrical outlets, so these will need to be sealed in order to let the fiberglass or mineral wool do its job. Over time fiberglass may sag, opening up gaps in the insulation.
Spray foam Insulation
Spray foam insulation comes in two formats, open cell, and closed cell. Both deliver different performance in both sound and thermal insulation. Closed cell spray foam is the denser of the two and so it has a better R-value than its open cell cousin, but its STC rating is lower. That said, in a real-world scenario, both spray foams secure a big upvote due to their air sealing qualities.
Because foam expands into every tiny space, each crevasse and gap in a wall structure, it is an effective barrier against medium and high frequency airborne sound. In a 4” wall cavity, closed cell spray foam insulation can bring a wall up to an STC rating of 41, which is the minimum level at which privacy can be achieved.
Open cell will deliver an STC rating in a partition wall of 50, which should suppress almost all sound transference, and will be sufficient for most domestic requirements.
Foam Board Insulation
It is hard to find reliable STC ratings for foam boards such as Styrofoam or Polyiso as their prime use is for thermal insulation, however, as part of a soundproofing wall assembly, perhaps coupled with spray foam to seal the edges a specialist foam board insulation could be used to deaden transmission or reflect sound.
Perhaps using a ¼” inch bead of acoustic latex caulk, or Green Glue, to fix the foam board would also help as it would also trap an air gap, thereby adding to the sound insulation.
Blue Jean Denim and Cotton Insulation
Denim
Denim insulation is one of the most eco-friendly recycled insulation products that money can buy, and it has a fantastic NRC sound absorption rating, which makes it a great choice when considering making sound panels for use in a recording studio setting for musicians or podcasters.
Cotton
Cotton is a very dense natural material, which is cheap, sturdy, hygienic and resistant to mould, and can be a very efficient sound insulator as well as being attractive when hung as drapes or a wall hanging. When hung in multiple layers, trapping air between surfaces, its sound deadening performance increases.
Due to their density both denim and cotton are excellent at absorbing and deadening all frequencies, low, medium and high. Cotton acoustic panels are aerated in order to make them more porous while retaining density, however, these are not a low budget item to purchase.
Other Options
Clay-slip Straw and Straw Bale Construction
Straw bale construction is typically very good at dampening lower frequency sounds due to its elasticity, coupled with its denser overall mass. Clearly this is not commonly used in traditional build settings, however it is worth noting that in laboratory testing straw bales consistently score around 53 in STC ratings on thicknesses of 19”, including an inch and a half clay plaster layer.
If you were considering constructing a garden room for quiet contemplation, then straw bale construction may be the inexpensive, eco-friendly route to follow.
Mass Loaded Vinyl
Mass loaded vinyl is a rubberised sound insulator specifically designed for use in soundproofing spaces, for use beneath drywall, under floors and in ceiling assemblies. It delivers an STC rating of 27 inside 1/8” thickness for $2 per square foot and comes on rolls up 25” x 4”.
Medium Density Fiberboard (MDF)
If you are considering building a decoupled wall to insulate a home cinema, home office, or other dedicated space, then it would be worth considering using MDF as part of the wall assembly. It is an engineered timber-based board, the fragmented, composite nature of the fiberboard gives it excellent sound absorption properties.
Alternatively, MDF is ideal for cutting into strips to make sound treatment panels that scatter and absorb sound waves.
