Fibreglass and Mineral Wool Insulation are two commonly used insulation materials in Irish households, but which is the better choice for anyone considering home improvement works?

Fibreglass and Mineral Wool are often used as interchangeable terms, with products described as glass mineral wool or glass wool. The difference between older fibreglass and modern mineral wool is typically found in its density, fire resistance, and moisture resistance.

Mineral Wool is more often used to describe products such as Rockwool and Earthwool which are more popular in newer builds and retrofit works.

Fibreglass Insulation is made from molten glass which has been spun into fibres to create a material which traps air in tiny gaps between the fibres. Fibreglass Insulation was once one of the most common forms of insulation used in houses, and is likely present in some quantity if a house has an attic space.

Fibreglass is flexible, which allows it to fit around irregular shapes and fill gaps. However, its thermal performance suffers if it is compacted, as fibreglass relies on its air pockets to trap heat.

Fibreglass Insulation also comes in the form of loose fill which can be stuffed or blown into small spaces and cavities to provide additional thermal protection in hard to reach areas.

Mineral Wool insulation is made from stone, silicate, or other materials, and encompasses terms such as Rockwool and Earthwool which are also often seen on insulation products.

The raw materials are heated into molten slag and then spun into fibres, much like fibreglass. However, the end product is much denser than fibreglass, making mineral wool perform better in many key areas.

Stone wool, or Rockwool, is made from volcanic rock that is melted down and spun into fibres. 

Earthwool is similar to fibreglass in that it is made out of silicate (glass) but is different from most fibreglass in that it is denser, with improved thermal resistance and the ability to block moisture. 

Earthwool also has less harmful chemicals used in its manufacture and is not as irritating to the airways when handling.

Mineral Wool can come in rolls, batts or slabs, which can be easily cut to fit any shape and provide a tight seal in a variety of applications. The denser layer of mineral wool insulation also provides good acoustic protection.

Both fibreglass and mineral wool insulation are relatively easy to apply, suitable for multiple surface types and easily able to be cut to fit spaces.

Fibreglass, with its lower density and looser structure, is harder to cut but easy to lay down in rolls. Fibreglass insulation is easy to apply for loft attic insulation, where it only has to be laid down in the gaps between the joists.

The lower weight of fibreglass insulation also means there is no risk of causing damage to their ceiling with attic insulation, regardless of how thick the applied layer is.

Mineral Wool’s denser structure, especially in semi-rigid batts, also makes it far easier to use on vertical surfaces such as when insulating walls, whether being applied directly to the wall with mechanical fixings, or placed between wall batts.

Both fibreglass and mineral wool are made in a similar fashion, and rolls of the two will often look the same. But how does their performance compare?

Mineral wool carries a clear advantage here, offering higher R-values at the same thickness level as fibreglass. Fibreglass can also suffer if poorly installed as its performance suffers if compacted.

Both fibreglass and mineral wool are highly fire resistant materials, with very high melting points. However, mineral wool performs better overall at blocking flame. Rockwool in particular, has a melting point several hundred degrees higher than glass-based insulations and is completely flame resistant.

Mineral wool has a clear advantage here, with all varieties of mineral wool excellent at repelling water. Fibreglass in contrast can absorb water, which can lead to problems with damp and mould over a prolonged period of time.

Mineral wool insulation in all its forms will cost considerably more than fibreglass. More is needed to reach the same level of performance as mineral wool, but that may not be needed depending on the climate and property in question.

Whether fibreglass or mineral wool is a better choice for a project will depend on its requirements and the cost. Mineral wool does provide an overall better performance than fibreglass in both its thermal resistance and acoustic properties.

However, this is countered by its higher cost compared to fibreglass. Fibreglass insulation is also a high-performing insulation for homes, and in climates which aren’t too cold it may be more than sufficient.

Ultimately the question of which insulation is better will vary from project to project, though mineral wool carries more advantages than fibreglass overall.