A Complete Guide to Fibreglass Insulation – Pros, Cons & Cost

fibre glass insulation

Fibreglass insulation is commonly found in the majority of properties that already have some form of home insulation. This type of insulation – often found in lofts, ceilings, and walls – consists of rolls of (usually pink) batts that resemble candy floss. This type of fibreglass insulation is usually a pretty straightforward and cost-effective means of insulating your home.

While it has good insulating capabilities, fibreglass insulation isn’t always the best option for insulating your home in every situation. Here, we discuss the benefits and drawbacks of fibreglass insulation and suggest an alternative insulation solution.

The UK government has established a target of reducing carbon emissions by 80% by 2050, and as part of this policy, they are keen to encourage home insulation, including offering grants for households to improve their home insulation.

With more than a third of UK homes (with a loft) estimated to have less than the recommended level of insulating material, the existing insulation in many homes is either non-existent or inefficient, making it critical that households upgrade their insulation for the government targets to be reached.

Fibreglass insulation plays a role in this but does have some pros and cons when it comes to home insulation.

What Is Fibreglass Insulation?

Fibreglass being Installed
Image: Fibreglass being Installed

Fibreglass insulation is a manufactured product made of fragile glass fibres initially used in homes circa 1930. It gained popularity, in particular once it was revealed that asbestos, another good insulator, was dangerous and a risk to human life.

In spite of its structure and the high-energy manufacturing process, fibreglass insulation was considered an excellent alternative.

Fibreglass insulation, whether in the form of batts, rolls or blown insulation, works similarly to other types of insulation in that it traps escaping heat within its structure. Air pockets surrounding the fibreglass material prevent the exit of warm air and also help with noise insulation. These air pockets must remain dry for the insulation to be effective.

In most cases, fibreglass is simple to install and may be completed as a simple do-it-yourself project, as long as there are no issues with damp or access. This is why for many people it is a highly popular home insulation product.

The Pros of Fibreglass Insulation

Fibreglass insulation benefits are many, making it one of the most used type of home insulation. It comes in the form of blankets, sheets or loose fill and is commonly used as an acoustic and thermal insulation material indoors. It is usually used beneath wooden floors, interior walls and pitched roofs. and offers many advantages over other types of insulation…

  • Frequently produced from recycled glass, making it more beneficial for the environment. In addition, because glass is manufactured from sand, including the portion that is not made from recycled materials, it is an excellent renewable alternative.
  • Has non-combustible or fire resistant properties, making it an excellent choice for walls, ceiling insulation, and around light fixtures and other electrical devices.
  • Does not shrink or collapse easily and has many air pockets that help it retain its shape and retain its insulation value over time.
  • An excellent choice for blown insulation because it settles in a more uniform layer with plenty of air pockets.
  • In most cases it is usually easy to install and may be completed by the DIYer.
  • Provides good thermal and acoustic insulation and might help to reduce noise in the home.
  • Is one of the least expensive insulation options yet still a good insulator and will improve energy efficiency and potentially bring energy bills savings of up to £445 per year.


The Energy Saving Trust estimates that the following energy bill savings could be enjoyed by properties by insulating a previously uninsulated loft…

Property typeEnergy Bill Savings (£/year)
Detached house£445
Semi-detached house£270
Mid-terrace house£240
Detached bungalow£440
Energy bill savings with loft insulation

Estimates based on insulating a gas-heated home with a totally uninsulated loft (0mm) with 270mm of loft insulation. Figures are based on fuel prices as of January 2024.

The Cons of Fibreglass Insulation

Mineral fibres are released during the installation of fibreglass insulation. These can cause eye, skin, and airway irritation and inflammation. Wearing goggles, a mask, and gloves is therefore advised when installing fibreglass insulation.

  • Tiny slivers of glass used in this product are potentially hazardous to the skin and lungs if inhaled. As a result, it is frequently coated in a vapour barrier. If laid in the loft, it should be protected by boards and not be touched.
  • Loses its insulating properties if it becomes wet, either through a leak or if it is affected by condensation. Once soaked, fibreglass condensate cannot dry out properly and must be replaced. Damp insulation can become mouldy and encourage mould growth in other areas of the room.
  • Fitting batts into tiny gasps and unique or awkward spaces can be problematic. While it is cut to size relatively easily, there are usually gaps between each batt and along the borders which are hard to avoid.
  • Can be rather too welcoming for pests looking to take up residence! They will not consume the insulation, but will be rather too keen to make it their home.
  • Can start to sag over time, especially when used inside wall cavities or on rafters, leaving spaces for air to enter.
  • Must be installed with a vapour barrier or ventilated to prevent moisture buildup.
  • A sufficient insulation R-value requires at least 270mm of thickness in the ordinary loft, creating difficulties if wanting to construct a floor in the attic or maintain head height.


How Much Does Fibreglass Insulation Cost?

One of the most common roof insulation materials is glass wool or fibreglass.

It is lightweight, offers good insulation, and is reasonably priced. Fibreglass insulation would only be possible if a roofing underlay is present. Additionally, the interior should always be coated with a vapour barrier. As a result, the insulation will be unable to absorb moisture from the room.

Some typical costs of insulating different types of property are suggested by the Energy Saving Trust…

Property typeTypical insulation cost
Detached house£1,200
Semi-detached house£930
Mid-terrace house£880
Detached bungalow£1,200

Fibreglass insulation comes in a variety of forms and thicknesses. Always double-check the type you require to avoid excessive costs and poor insulation ratings. An overview of the many possibilities is provided below:

Fibreglass insulation thicknesses available

Fibreglass insulation thickness has a significant impact on acoustic and thermal insulation value. Fibreglass insulation thicknesses range from 60mm to 180mm (80mm, 120mm, 140mm, 160mm, 180mm). To insulate a pitched roof, a minimum thickness of 160mm is suggested. You could as well work with two alternating layers to avoid seams.

Fibreglass wool finishes and shapes are available.

Glass wool can be purchased in the form of pliable sheets, blankets, or in bulk. On the outside, a finish layer is sometimes already present. This can either make it easier to deal with (in the case of paper) or act as a vapour barrier (in the case of aluminium). The overlapping finish can be used to seal seams or to staple it to timber projects.

Install glass wool insulation and make sure to include a vapour barrier!

You must always install a vapour barrier on the interior of every room to prevent the fibreglass insulation from absorbing moisture from the rooms. Use insulating blankets that come with a special aluminium coating, for example, and seal the seams with tape.

What Other Insulation Options Are There?

Spray foam insulation is an excellent option for insulating a loft or roof space, eliminating many of the issues associated with fibreglass insulation while retaining many benefits. Click here for more info about foam v fibreglass insulation.

While it requires professional installation and is slightly more expensive, spray foam insulation provides higher insulating properties based on the application depth. In addition, it can get into all the little tight areas, which is something that cannot be achieved with fibreglass.

Spray foam insulation expands rapidly, allowing it to fit into even the tightest of spaces, making it a good solution for areas such as lofts, where uniform-sized fibreglass batts can cause problems with the roofline.

Wool insulation (including sheep’s wool and mineral wool insulation) has qualities similar to fibreglass insulation, except that sheep’s wool is better able to cope with moisture without losing its effectiveness. However, it is one of the more expensive insulation materials.

Cellulose insulation is a loose fill insulation option and provides an eco-friendly alternative as it is usually made from recycled wood and paper products.

Fibreglass Insulation Summary

Fibreglass insulation has great insulation qualities and is a fantastic way to boost your home’s insulation and energy savings. However, when selecting the correct insulation for your home’s needs, you need to evaluate both the advantages and disadvantages of each option.

If you want a cost effective product for your loft or roof space that you can install yourself, then fibreglass insulation may be ideal, but remember that it can in some instances cause damp problems and may not provide the insulation properties you anticipate. Spray foam insulation can on occasion be a more suitable option and provide the longer lasting durable solution you are seeking.

Either way, it’s always best to speak to a professional regarding insulation for your home; they will be able to recommend the best options available to you and provide you with comparable quotations.

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