Blown In Insulation: Ultimate Guide

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Blown in insulation is often a wonderful solution for owners of older properties who may find that their thermal efficiency is somewhat lacking. Essentially, it is made up of small bits of material – such as wood or paper based products – which is literally ‘blown in’ to the gaps between internal or external walls and in your loft space as a form of loft insulation. This increases your home’s thermal efficiency and reduces your energy bill costs.

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What Is Blown In Insulation?

Blown in insulation is a type of insulation that’s designed for homes and businesses who require additional support in terms of thermal efficiency. It can be used as loft insulation, cavity wall insulation, or external wall insulation. Often, it’s used in loft spaces and in the gaps between walls in older properties to boost a property’s energy performance certificate (EPC).

It can be made up of a variety of things, and there are 3 main types:

The Three Main Types Of Blown In Insulation

Loose-fill fibreglass insulation is made of glass that is then heated until it becomes liquid. At this point, the liquid is spun to create thin fibres of highly efficient loose fill insulation. On average, loose-fill fibreglass has an R-value (used to measure the efficiency of insulation) of around 2.5 per inch of insulation installed.

For exterior walls, an R-value of 13 to 23 is recommended, so around 5.5 inches to 9.5 inches of loose-fill fibreglass will need to be installed here. For attic spaces, an R-value of 30 to 49 is recommended, meaning you’ll need between 12 and 20 inches of this insulation type to reach recommended levels.

Cellulose is a more eco-friendly loose fill insulation option, because it’s made up of shredded paper, cardboard, and wood – basically any cellular plant source. The recycled cellulose insulation is then treated with chemicals to help it resist mould and make it more fire retardant. On average, it has an R-value of around 3.7 per inch of insulation installed.

For external walls, this means around 3.5 inches to 6.5 inches of cellulose insulation will be necessary, and as loft insulation around 8 to 13.5 inches will be required.

Rock wool insulation, which is also known as mineral wool, is insulation created from something known as blast furnace slag, a byproduct of firing iron and iron ore. When heated, the blast furnace slag is able to be spun with other minerals to create a lightweight material similar in appearance to wool, hence the name. Rock or mineral wool has an average R-value of 3.3 per inch of insulation installed.

For exterior walls, that means around 4 to 7 inches of rock wool insulation will be required, and as loft insulation around 9 to 15 inches.

How Is Blown In Insulation Installed?

Whilst each type is different, they’re all installed in much the same way.

In loft spaces, the insulation is ‘blown in’ using a blower to insulate uninsulated joist cavities that form the timber frame of your house and in enclosed areas surrounded by pipes and ductwork that might otherwise be difficult to insulate.

As cavity wall insulation, it is again ‘blown in’ using a blower, but first holes need to be drilled into the masonry outer leaf of the wall to create a space for the blower’s nozzle to fit through. This then fills the uninsulated gaps between the masonry outer leaf and inner leaf, improving the property’s thermal efficiency. It can also be blown into the gap between timber studs in external walls, too.

Who Needs Blown In Insulation?

This type of insulation is appropriate for a range of properties when thermal efficiency is a concern, and it’s generally much more cost effective than other insulation options whilst still improving your home’s comfort and reducing carbon emissions and the cost of energy bills in the property.

Basically everybody with a poor EPC rating could benefit from insulation. If you have uninsulated gaps between your walls (older properties are especially prone to this) or an uninsulated loft space, then you ought to consider this type of insulation for your home.

Blown In Insulation: Benefits

Still not convinced? There’s a range of benefits to installing this insulation type:

Green Option

This is especially true of cellulose insulation because it’s made from recycled materials, but it’s also true of rock wool because it uses a waste product, and of loose-fill fibreglass because of the green benefits it brings to the home.

Insulation means better thermal efficiency, and better thermal efficiency means less heat wasted in your home. The longer your home can hold on to its warmth, the less you’ll use your heating (no doubt powered by carbon emitting fossil fuel powered boilers), reducing your property’s carbon emissions, and thus your own carbon footprint.

Cheap To Install

Compared to other insulation types, blown in insulation is considerably cheaper. Rock wool and cellulose insulation are even made from waste or recycled products, making them cheaper still. This is especially true when compared to spray foam insulation which is considerably more expensive.

But don’t think buying cheaper insulation means you’ll be scrimping on potential savings. With average R-values ranging from 2.5 to 3.7, they offer significant energy efficiency boosts, and will therefore save you money on your energy costs, too.

Flexible Installation

The key benefit of ALL blown in insulation types is its flexibility when being installed. Tight gaps which would usually struggle to be filled with insulation, and areas surrounded by pipes and ductwork which would usually be left uninsulated, can all be filled with this type of insulation.

The level of flexibility this offers makes it a popular insulation option for this reason alone, improving the thermal performance of your entire home.

Fire Protection

All three types of insulation discussed today have some level of fire protection. Cellulose insulation is specifically chemically treated in order to make it fire resistant. Rock wool is made of a byproduct of blast furnacing iron and iron ore, so it’s naturally resistant to fire anyway. And fibreglass is made from heating glass, meaning it is naturally fire resistant too.


Besides the obvious thermal benefits, insulation also offers certain soundproofing benefits, too. That means by using blown in insulation, you’ll be able to enjoy a quieter home when you reduce nuisance noise transfer from outdoors, and between rooms in your home.

Less Condensation

Because this type of insulation is so flexible in its application, it’s able to plug small gaps and spaces that might be missed by other insulation types. By filling these spaces, condensation, moisture, mould, and mildew are greatly reduced.

Energy Efficiency

And, of course, it goes without saying that new insulation increases your home’s energy efficiency, preventing heat loss, and making it a more comfortable place to live and saving you money on your heating and cooling bills.

Blown In Insulation: Drawbacks

There are some drawbacks that should be considered before going all in on this type of insulation.

Could Leave Gaps

Although one of its main benefits is how flexible it is in filling gaps, it’s true that gaps could be left. When filling spaces between walls, for example, where the insulation goes specifically can’t be controlled all that well through the hole that’s been drilled, meaning small gaps might be missed.

Over time, this insulation type can flatten, too. When this happens, it’s possible that thermal bridges can be created, meaning the heat or cold from outside can be transferred more easily in the home, making the insulation ineffective. Should this happen, new insulation will need to be installed to plug the thermal bridge and restore your effective insulation.

Moisture Problems

Although this insulation type prevents condensation thanks to its flexible installation, it also is especially susceptible to moisture, which can affect its performance, bring down its R-value, and even lead to mould issues.

Can You Install Blown In Insulation Yourself?

It’s possible, if you have the right equipment for the job – but we imagine most of you don’t have blowers designed for insulation in your home? That’s why we always recommend leaving it to the professionals.

Not least because improper installation could leave your insulation susceptible to moisture, or with thermal bridges, making it ineffective.

How Long Does Blown In Insulation Last?

It depends on the insulation type and the conditions the insulation is kept in. If in optimal conditions – where the insulation isn’t exposed to moisture, doesn’t settle down over time, and is professionally installed – then the different types of insulation can last for:

  • Loose-fill fibreglass: 80 to 100 years (will need an inspection after 15 years)
  • Cellulose: 20 to 30 years
  • Rock wool: 100 years

How Insulation Advisor Can Help

No matter which type of insulation you’re hoping to install – blown in, spray foam, or otherwise – you’ll need to find the perfect insulation installer for the job. That’s where Insulation Advisor comes in.

We’re your go to comparison tool when looking for a trusted insulation installer near you. Use our insulation installer finder tool to compare installers who serve your postcode.

Then collect some quotes, and choose the best installer for the job. It couldn’t be simpler!

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