How Thick Should Loft Insulation Be?

For loft insulation, there are recommended levels for both the U-value of the roof of your house and for the depth of insulation in a loft. There are many benefits to installing a high depth of insulation within your loft, including reductions in heat loss and lowered energy bills. There are also recommended levels of insulation to install within a pitched roof if you are looking to create a warm loft.

What U-value is Needed for Loft and Roof Insulation?

Proper insulation within a roof or loft is vital to prevent heat loss, and the loft is the easiest part of the thermal envelope to add insulation to, simply laying it between joists. A U-value of 0.2 W/m²k is recommended for the roof of a building. Increasing the thickness of your loft insulation can help your house approach the ideal U-value.

What is the Recommended Loft Insulation Thickness?

In new-build UK properties, there is an enforced minimum of 270mm, regardless of the insulation materials used. This rule came into place in 2003, meaning that if your home was constructed after that year it will contain the recommended thickness of insulation already. If it was constructed in earlier years the amount of insulation installed is likely to be lower, as shown in the table below.

These numbers differ drastically, meaning your house may very well have a considerably lower level of insulation than the modern standard. Providing the insulation within your loft is of a good quality you can simply add additional insulation on top of it. Older insulation can also be topped up with loose-fill options like vermiculite. In addition to your loft floor, Loft hatches should also be insulated to the same height of 270mm.

Insulation Rolls and Batts

For insulation rolls and batts, the recommended thickness level is 270mm. This is what you will have to insulate to in a new property, and what you should be aiming to insulate to in an older one. If starting from nothing, you will probably have to buy and install two rolls or batts. There are 200mm depth insulation rolls available, so installing a roll of 200mm insulation topped with 70mm one will give you a fully insulated loft.

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Solid Insulation Boards

Were you to install solid insulation boards within a new build you would need to reach a height of 270mm. But if you are looking to insulate your loft, then a lower depth would offer the same level of insulation as rolls. 150mm of PIR insulation board would offer a U-value of 0.2 W/m²k. This can be achieved through one board or two. You’ll have to cut the insulation to size, ensuring a snug fit within your joists.

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Foam Insulation

The target U-value of 0.2 W/m²k can be achieved through the installation of closed-cell spray foam to a depth of 150mm. Open-cell spray foam will require a higher amount due to its lowered insulation value.

How Much Will I Benefit From Installing Extra Loft Insulation?

The most important level of loft insulation is the first 75mm. This is because it will bring tremendous energy savings to any property. Once you begin to reach higher levels of pre-installed insulating the savings drop dramatically. For instance, if you have 150mm of conventional mineral wool insulation in your property, adding 120mm to reach the recommended thickness would save you an estimated £7 a year. This may not be worth it for a lot of homeowners, considering the costs of products and installation. Below is a chart showing estimates of the savings that can come from adding insulation to an average house.

If your home currently has a low level of insulation then there will be clear savings from topping it up, making your house far more energy-efficient. But if you have 150mm or more insulation installed then the savings will not be especially large. This will depend on the amount of energy used within a house, and rising energy costs could make it a more prudent investment in the years to come.

Is it Possible to Install Too Much Insulation?

It is not possible to install too much insulation in the sense of causing issues to your property, such as dampness or lack of airflow. However, there is a point of insulation thickness where installing any more will have such minor benefits to your energy savings. Once you have reached the recommended depth of 270mm, adding additional insulation will save you at most several pounds a year. As a result, this can take decades to pay for itself. 

What Thickness of Insulation Is Needed For a Pitched Roof?

An alternative to insulating your loft floor is to insulate between the joists in a pitched roof. This will mean that your loft space is also insulated. If you want to use your loft for storage or living you should be looking to insulate the roof instead. You will still be looking to hit the desired insulation level of 0.2 W/m²k within the roof of your building. This can be achieved through 120mm of rigid foam boards, such as PIR boards. These will be installed either within the joists of the roof or over the top of the joists. If needed, you can use two boards: one between the joists and one over them to hit the desired levels of thermal insulation.

If looking to use mineral wool batts between joists in a pitched roof, you will need to install around 200mm of rock wool or fibreglass to reach the desired U-value. If opting for sprayed foam insulation in your roof, a recommended depth of 120mm will help your building adhere to the recommended U-value of 0.2 W/m²k.

What Thickness of Insulation Is Needed For a Flat Roof?

To fully insulate your flat roof to the desired U-value you’ll need to install insulation both within the flat roof and in the joists below it. The same level of insulation is needed as a pitched roof. Therefore, you can install an insulation board of up to 100mm within the flat roof construction and another layer of insulation between the joists below. You will need to use specialist insulation boards for a flat roof, and will not be able to use mineral wool batts.