What is the recommended thickness of loft insulation?

Over the years, the recommended thickness of loft insulation has increased. The reason for the increase is pretty simple – the thicker the layer of loft insulation, the bigger the energy savings!

The Government are very keen for every house in the UK to be have loft insulation where possible, since it helps lower heating demand in these properties. The insulation benefits households through lower energy bills, as well as having a home that stays at a more comfortable temperature for longer.

For the Government, energy efficiency is very important since it means that peak demand (i.e. the time of day when gas/electricity use is at its highest) decreases – since each building requires slightly less energy. This means new power plants don’t need to be built, so some nice cost savings for the Government!

So over the years, the amount of loft insulation recommended by the Government has changed – the table below details how it has changed over the years:

Year Recommended thickness of loft insulation
1965 25mm
1975 60mm
1985 100mm
1990 150mm
1995 200mm
2002 250mm
2003 270mm
2017 350mm?

You can see from the table that the recommended thickness of loft insulation is currently 270mm. The amount of insulation has not changed over the last 14 years though, so it seems an increase in the recommended amount will most likely come shortly.

The Government try to ensure the correct levels of insulation are used though building regulations (specifically Part L, which you can access by clicking here). This also details the thickness of insulation required for other building elements, like walls and floors. Building regulations are really only relevant when doing new building works though (in the case of loft insulation), so it goes without saying there are going to be homes that don’t have the recommended thickness of loft insulation in their lofts.

How to increase the thickness of loft insulation in your home

The good news is that even if you fall into the camp where you have very little loft insulation, adding additional insulation is very cheap. If you go to a local DIY store, you can normally get loft insulation for about £20 a roll. Obviously the cost is going to add up if you don’t have any loft insulation in place at the moment; but even then it works out at about £6 per m2 to insulate a virgin loft (one with zero existing loft insulation) with 270mm of loft insulation.

The rolls of loft insulation you can buy come in lots of seemingly random thicknesses, for example 100mm, 120mm, 150mm and 170mm. The reason for this is that the insulation manufacturers are trying to make it easier to hit the 270mm thickness sweet spot. If you are insulation a virgin loft, the easiest way is to roll 100mm thick insulation between the joists. Once this layer has been laid, then lay 170mm insulation across the top of the joists at 90 degrees to the joists.

In order to insulate the loft, always start as far away from the loft hatch as possible and then work back towards the hatch. There should be no gaps between the top layer of insulation; once this layer has been laid, you should no longer be able to see the joists! You are trying to achieve a seamless ‘blanket’ of insulation across the entire loft.

The rule of diminishing return – loft insulation

So at what point does it become pointless to add more insulation to the existing insulation in the loft? Well obviously extra insulation will always help to lower energy bills, but there is a concept known as the rule of diminishing return that comes into play. Basically this means that the first centimetre of insulation will do more than the next and so on and so on.

The graph below shows this phenomenon – the scale on the left shows the rate of heat loss, so a lower number is better. The reason for mentioning this though is that if you have 150mm of insulation or more already in place, then the energy savings you will generate from adding additional insulation won’t be massive.

The first 100mm of insulation is where the energy savings are really significant. If it were my home, then if I had less than 150mm of insulation then I probably would look to top up to meet the recommended thickness of loft insulation. If I had more, then I would probably only do this if my energy bills were really high!

Obviously if it is a new build then you have to lay 270mm of insulation in order to meet building regulations, but basically the answer to the question is this: 270mm is the recommended thickness of loft insulation!

Installing loft insulation

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