How thick should loft insulation be?

The minimum recommended loft insulation thickness is 270mm for mineral or glass wool. This spans across two layers, a base layer which is typically 100mm and a top layer that’s often 170mm. The recommended thickness of loft insulation isn’t the same for other materials, though, and spray foam and rigid PIR or insulation boards have different ideal thicknesses.

Is 100mm of insulation enough in a loft?

No, 100mm of insulation is not enough for any kind of insulation material.

Is 200mm loft insulation enough?

It depends on what kind of insulation material you’re using. 200mm is more than enough for rigid PIR boards and spray foam insulation, which each require 150mm of thickness, but is not enough for mineral or glass wool, or loose-fill wool, which each require 270mm of thickness.

Is 300mm loft insulation too much?

No, it is only slightly above the recommended minimum thickness of 270mm for standard household mineral/glass wool insulation.

Do I need to remove old loft insulation before topping up?

No, you don’t need to remove old insulation as long as it is clean, dry and undamaged.

Does a room-in-roof loft conversion need the same thickness?

No, a room-in-loft conversion will use different insulation buildup comprised of different thicknesses and insulation materials.

Loft Insulation Thickness: How Thick Should It Be

Contents:

How thick should loft insulation be? Find recommended loft insulation thicknesses by material, how to measure your current depth and when to top it up.

The recommended minimum loft insulation thickness in the UK is 270mm. This consists of a 100mm base layer laid between the joists and 170mm top layer over them.

This guide will cover how to assess loft insulation thickness by material, how to measure your current loft insulation depth, the benefits of loft insulation and what UK Building Regulations have to say about it.

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Key takeaways

Recommended Depth – 270mm is the current UK minimum for mineral or glass wool loft insulation.
Material Differences – The recommended thickness varies by material, with mineral or glass wool and loose-fill wool requiring at least 270mm, and spray foam and rigid PIR boards requiring 150mm.
UK Building Regulations – These state that loft insulation should be a minimum of 270mm thick, and this requirement applies to all insulation, not only that in new builds. In some new builds and projects the minimum is even higher at 300mm.
Thicker ≠ Better – 270mm is the minimum, so a little more insulation can be added. Too much, however, and the insulation will block airflow in the loft and trap moisture, both of which can prove detrimental to your home.

Recommended loft insulation thickness by material

How thick loft insulation should be depends largely on which material the insulation is made up of, as different materials benefit most from different thicknesses.

Material	Recommended Thickness	Notes
Mineral/Glass Wool	270mm	Standard, budget-friendly option. Easy to install but requires significant depth.
Loose-Fill Wool	270mm	This can be blown into awkward spaces and gaps, so it works well for irregular lofts.
Rigid PIR Boards	150mm	Highly efficient, space-saving sheets, though more expensive than some alternatives.
Spray Foam	150mm	Liquid expands to seal gaps tightly, requires professional installation.

The reason why the recommended insulation depth changes depending on the material is because different materials have different thermal conductivity properties. Materials like rigid PIR board and spray foam are dense and trap heat far more effectively than mineral or glass wool, for example, so aren’t needed in such high thicknesses.

How to measure your current loft insulation depth

If you’re looking to measure your own loft insulation depth to see how it compares to the recommended loft insulation depth, there are three important steps to undertake.

Step 1: Measure at several points across the loft

Depth can vary in different parts of the loft, so measuring in several points is the best way to get a clear picture of the true loft insulation depth. You can measure this using a standard tape measure by pushing it through the insulation until it hits a solid foundation.

Step 2: Check for compression or uneven depth

Keep an eye out for any areas that have uneven depth or points in the loft where the insulation has been compressed. Crushed or compressed insulation isn’t as able to retain heat or perform effectively, so it will need to be topped up.

Step 3: Calculate how much more insulation to add

Simply subtract the current insulation depth from the target measurement of 270mm, and add the difference in insulation onto the uneven or compressed area.

Loft insulation thickness and UK building regulations

UK Buidling Regulations (Approved Document Part L) suggest a minimum thickness of 270mm for mineral wool loft insulation, which is the most common kind of insulation in UK homes. This applies to all insulation work, not just in new builds.

In fact, some new-build and Scotland projects even carry a higher minimum loft insulation thickness of 300-350mm.

Benefits of loft insulation

Properly installed, adequately thick loft insulation carries many benefits for homeowners, with some being:

Improving the heat retention capacity of your home.
Reducing heat loss through the roof.
Improving overall comfort by retaining heat.
Helping to maintain a consistent indoor temperature.
Potentially lowering energy use and bills.

Having loft insulation of the right thickness will both save you money and keep you comfortable in the cooler months.

Take a look here for a full step-by-step guide to installing insulation.

Common loft insulation thickness mistakes

Now that you know how thick loft insulation should be and how it varies by material, there are just a few more considerations you should be aware of. These are some of the most common mistakes people make with regard to loft insulation thickness:

Compressing Insulation – Squashing or crushing insulation will harm its ability to retain heat and perform well.
Uneven Depth – Always check the depth of multiple points in the loft to ensure no areas are left uneven.
Blocking Ventilation – Overfilling gaps can block eaves ventilation, so always make sure ventilation routes remain open.
Thicker isn’t Better – Beyond 270mm, thicker insulation does not mean better insulation. Too much insulation will trap moisture and block airflow. Always aim for the recommended loft insulation thickness only.
Not Wearing PPE – Working in a loft requires appropriate PPE to prevent irritation from fibreglass dust. This includes goggles and a dust mask.

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Loft Insulation Thickness: How Thick Should It Be?