Ask anyone how thick loft insulation should be and you will hear the same number: 270mm. It appears on government advice, on the side of insulation rolls and in every installer's quote. But it is worth knowing where that figure comes from, what you lose if your loft has less, and why simply piling more on top is not always the answer.
This guide walks through the numbers in plain terms, shows what each depth does to the heat leaving your home, and explains how to check what you have got.
The Short Answer: 270mm of Mineral Wool
For the most common type of loft insulation in UK homes, glass or rock mineral wool laid in rolls, the usual recommendation is a total depth of 270mm. In practice that means one layer laid between the ceiling joists, usually around 100mm deep, and a second layer of about 170mm laid at right angles across the top of the joists.
Laying the second layer across the first matters as much as the depth. Timber joists conduct heat far better than insulation, so a ceiling insulated only between the joists has a stripe of poorly insulated timber every 400mm or so. The cross layer covers those stripes and closes the gaps where the rolls meet.
When a loft is insulated at ceiling level, the building regulations for England set a target U-value of 0.16 W/m²K for the upgraded roof. With standard mineral wool, roughly 270mm is the depth that gets a typical ceiling to that level. Our Part L building regulations guide covers the rules in more detail.
What a U-Value Actually Means
A U-value tells you how much heat passes through one square metre of a building element for every degree of temperature difference between inside and outside. It is measured in watts per square metre per kelvin (W/m²K). The lower the number, the less heat escapes.
The chart below shows an approximate U-value for a loft ceiling at different depths of mineral wool. It is a simplified calculation, but it shows the shape of the curve clearly.
Two things stand out. First, the first few centimetres do an enormous amount of work: going from nothing to 100mm cuts the heat flowing through the ceiling by around 85%. Second, the curve flattens. Going from 270mm to 400mm only moves the U-value from about 0.15 to about 0.11, a far smaller step than the earlier ones.
How Much Heat Each Depth Lets Through
U-values are abstract, so here is the same calculation turned into watts. Take a loft ceiling of 50m², which is about the size of a three-bedroom semi, on a day when it is 20°C inside and freezing outside.
At 100mm, around 370 watts is leaking up through the ceiling continuously, roughly a third of a small electric heater running all day and night. At 270mm it is about 150 watts. That difference, sustained through every cold day of a heating season, is why topping up a thin loft is one of the cheapest energy improvements you can make.
Depth by Depth: What You Have and What to Do
| Depth you measure | What it usually means | What we would suggest |
|---|---|---|
| Under 50mm | Very old or patchy insulation, often from the 1970s or earlier | Check its condition; if it is flattened or dirty, remove it and lay new to 270mm |
| 50mm to 100mm | An original layer between the joists, never topped up | If clean and dry, top up across the joists to 270mm |
| 100mm to 200mm | Topped up once, or one thick layer | Add a cross layer to reach 270mm, as long as the existing layer is in good condition |
| 270mm and over | Meets the usual recommendation | Leave it, unless it is damp, soiled or squashed |
| Deep but squashed | Boxes, boards or foot traffic have flattened it | Its real performance is far below its original depth; see our guide to boarding without squashing insulation |
How to Check the Depth of Your Loft Insulation
You do not need any special equipment, just a torch, a ruler or tape measure and some care about where you put your feet. Only stand or kneel on the joists or on boarding; the plasterboard between the joists will not take your weight.
- Measure in several places. Insulation near the hatch is often the most disturbed. Check the middle of the loft and towards the eaves as well.
- Push down to the ceiling. Slide the ruler down through the insulation until it touches the plasterboard, then read the depth at the top surface.
- Look at the condition, not just the depth. Grey, dusty insulation, dark staining from leaks, droppings or a musty smell all change the answer.
- Note what has been laid on top. Boxes, boards and old carpet compress the insulation underneath them.
Is Thicker Always Better?
In heat-loss terms, more depth always helps a little. In practice there are reasons to stop around 270mm to 300mm in most homes.
Diminishing returns
As the charts show, every extra 50mm past 270mm saves less than the one before. The money is usually better spent elsewhere, on draught-proofing, a hatch seal or getting the existing insulation right.
Ventilation at the eaves
Most UK lofts are cold roofs that rely on air moving in at the eaves and out higher up to carry moisture away. Pushing very deep insulation hard into the eaves can block that airflow and lead to condensation on the underside of the roof. We explain how to get this right in should loft insulation go into the eaves.
Cables, lights and tanks
Cables buried in insulation run warmer than cables in open air, and some recessed downlights need clear space around them. The cold water tank needs the opposite: no insulation directly underneath it. Our guide to insulating around downlights, cables and water tanks covers each one.
Different Materials, Different Depths
The 270mm figure applies to mineral wool. Other materials reach the same performance at different depths because they resist heat flow differently. Rigid PIR boards achieve the same U-value with considerably less thickness, which is why they are used where space is tight, such as between rafters in a loft conversion. Sheep's wool and loose-fill materials vary by product. If you are weighing up options, our insulation materials comparison sets them side by side.
When Topping Up Is the Wrong Move
Topping up only works if what is already there is worth keeping. If the existing insulation is flattened to a fraction of its depth, grey with decades of dust, damp from a roof leak or soiled by rats or mice, laying a new roll over it traps the problem underneath. The new layer gets damp or dirty too, smells can come back on warm days, and you have paid for insulation that is not performing.
In those cases the old material needs to come out first. We cover how to decide in should you remove old loft insulation before adding new, and our loft insulation removal service explains how we clear a loft back to the joists before laying fresh insulation to 270mm.
Frequently Asked Questions
Is 270mm loft insulation enough?
For mineral wool laid between and over the joists of a typical UK loft, yes. 270mm gets the ceiling to roughly the level the building regulations ask for when a loft is upgraded, and going further brings only small extra savings.
Is 200mm of loft insulation enough?
200mm is a lot better than 100mm, but it is short of the usual 270mm target. If the existing 200mm is clean, dry and evenly laid, topping it up with a 100mm layer across the joists is normally the cheapest way to reach 270mm or a little more.
Can you have too much loft insulation?
Not in terms of heat loss, but there are practical limits. Very deep insulation can block the eaves ventilation the roof needs, bury cables that then run hot, and make the loft hard to walk on. Past about 300mm the extra savings are small.
How do I measure the depth of my loft insulation?
Push a ruler or tape down through the insulation until it touches the ceiling board, in several places away from the hatch. Measure the depth of the layer itself, not how high it sits above the joists, and look for areas where it has been squashed or pushed aside.
Does the type of insulation change the thickness I need?
Yes. 270mm is the figure for mineral wool. Rigid boards such as PIR insulate better per millimetre, so they reach the same performance with less depth, while loose-fill or older, less efficient materials need more.
Related Reading
Need a Hand With Your Loft?
Not sure how deep yours is or whether it needs to come out? Send a couple of photos from the loft hatch and we will tell you what we see and give you a written price.
Or email info@loftrestore.co.uk. Based in Cheshire, working across the UK.