How Much Loft Insulation for a 5m x 4m Loft?

A 5 × 4 m loft is 20 m². Topping up from 100 mm to the recommended 270 mm needs 5 rolls of 100 mm mineral wool (about 8.2 m² each, with 5% for cuts). From bare joists it is 7 rolls. Enter the depth you have already, in the calculator below.

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Your project

How do you want to enter the area?

Length

Width

Insulation already in the loft

0 if there is none. Measure the depth with a ruler.

Depth to aim for

Thickness of the rolls you will buy

You can lay several layers, for example 100 mm between the joists and 170 mm across them.

Coverage of one roll (0 = typical)

m²

Printed on the pack. Leave at 0 to use a typical value: about 8.2 m² for a 100 mm roll, and proportionally less for thicker ones.

Waste allowance

Cuts around the joists and pipes. 5 to 10% is typical.

Results

Example job shown. Change the numbers to match yours.

Rolls to buy

5 rolls

100 mm rolls of 8.2 m² each

Depth to addTarget 270 mm less the 100 mm already there: 170 mm
Insulation to lay, as m² of the roll thickness: 35.7 m²
Loft floor area: 20.0 m²
Coverage of one roll: 8.2 m²

Rolls are compressed in the pack, so a thicker roll covers proportionally less: the 100 mm and 170 mm rolls of one brand cover about 8.2 and 4.8 m². Use the figure printed on your pack. If you already have 270 mm or more, there is nothing to add.

Before you buy: round up to whole packs, check the size printed on the label, and add a little spare. Your supplier has the final say.

Loft insulation for a 5 × 4 m loft, by what is already there

The loft floor is 20 m². Rolls to top up to 270 mm, using 100 mm rolls that cover about 8.2 m² each, with 5% for cuts:

Insulation already there	Depth to add	Rolls to buy
None	270 mm	7
50 mm	220 mm	6
100 mm	170 mm	5
150 mm	120 mm	4
200 mm	70 mm	2

About a 5 × 4 m loft

This size is typical of a small terraced house or a flat-roofed extension. Measure the actual depth of what is there with a ruler, and check that the old insulation is dry and not squashed. Keep the eaves clear so air can flow, keep insulation back from recessed downlights unless they are rated for it, and do not insulate under the cold water tank. Wear gloves, a mask and long sleeves, and step only on the joists. The insulation thickness calculator compares materials for the same target, and the loft boards calculator helps if you want to board part of it for storage.

Want to change anything else? The calculator above is the full Loft Insulation Calculator (UK): edit any number and the results update instantly.

Tools & materials for this project

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Loft insulation roll (100 mm)Loft insulation roll (170 mm)Loft boards Dust mask (FFP2)Protective gloves and overalls

Frequently asked questions

How much loft insulation do I need for a 5m x 4m loft?

A 5 × 4 m loft is 20 m². To reach 270 mm from nothing you need 7 rolls (100 mm thick, about 8.2 m² each), including 5% for cuts. With 100 mm already there it is 5 rolls.

How many rolls of loft insulation for 20 m²?

Topping up from 100 mm to 270 mm needs 5 rolls, and starting from 150 mm needs 4. A thicker roll covers proportionally less, so the number of rolls changes little with the thickness you buy.

What depth of loft insulation do I need?

The usual recommendation for mineral wool is about 270 mm. Lay the first layer between the joists and the second across them.

Related sizes and guides

Related sizes

Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.