What is the recommended loft insulation thickness in the UK?

The Energy Saving Trust recommends a minimum of 270 mm of mineral wool. In our model, 100 mm of λ 0.044 loft roll between the joists plus 170 mm across them gives U-value 0.16 W/m²K, the Part L standard for an upgraded loft.

Is 200 mm of loft insulation enough?

Not quite. In our model, 200 mm of mineral wool gives a loft U-value of about 0.20 W/m²K, short of the 0.16 Part L value. One 100 mm layer laid across the joists takes it to 0.14.

How thick is R 5 insulation?

Thickness = R × λ. R 5 is 220 mm of λ 0.044 mineral wool roll, 110 mm of λ 0.022 PIR board, 190 mm of λ 0.038 EPS board, or 225 mm of blown mineral wool at λ 0.045.

Why does PIR need less thickness than mineral wool?

Because its thermal conductivity is lower. PIR board has λ 0.022 and Loft Roll 44 has λ 0.044, so PIR reaches the same R-value in half the thickness. It costs more per square metre for the same R-value, which is why rolls are the usual choice in an open loft.

Does squashing loft insulation reduce its R-value?

Yes. Mineral wool works by trapping air. Squash it under boards or boxes and it gets thinner and insulates less. Raise storage on loft legs so the full depth fits underneath.

Loft Insulation Thickness: How Many mm Do You Need?

R	Mineral woolλ 0.044	PIR boardλ 0.022	EPS boardλ 0.038	Blown (installer)λ 0.040	Blown (installer)λ 0.045
R 1.00 m²K/WU ≈ 1.00 W/m²K	44 mm	22 mm	38 mm	40 mm	45 mm
R 2.00 m²K/WU ≈ 0.50 W/m²K	88 mm	44 mm	76 mm	80 mm	90 mm
R 3.00 m²K/WU ≈ 0.33 W/m²K	132 mm	66 mm	114 mm	120 mm	135 mm
R 4.00 m²K/WU ≈ 0.25 W/m²K	176 mm	88 mm	152 mm	160 mm	180 mm
R 5.00 m²K/WU ≈ 0.20 W/m²K	220 mm	110 mm	190 mm	200 mm	225 mm
R 6.00 m²K/WU ≈ 0.17 W/m²K	264 mm	132 mm	228 mm	240 mm	270 mm
R 7.00 m²K/WU ≈ 0.14 W/m²K	308 mm	154 mm	266 mm	280 mm	315 mm
R 8.00 m²K/WU ≈ 0.13 W/m²K	352 mm	176 mm	304 mm	320 mm	360 mm

How to read the table

Each row of the table above is an R-value in m²K/W, from R 1 to R 8. Each column is a type of insulation, with its thermal conductivity, λ (lambda), in W/m·K. The cell shows how thick that product has to be to reach that R-value:

Thickness (mm) = R × λ × 1000

The lower the λ, the less thickness you need. Under each R-value you'll also see U ≈ 1 ÷ R. That's the U-value of the insulation layer on its own, not of the whole loft or wall, which also includes surface resistances, the ceiling or masonry, and any timber joists. The U-value calculator explains the difference.

The columns are:

Column	λ (W/m·K)	Used for
Mineral wool loft roll	0.044	Lofts and suspended floors
PIR board	0.022	Solid walls, floors, rooms in the roof
EPS board	0.038	Floors and solid walls
Blown (installer), cavity fill	0.040	Cavity walls
Blown (installer), loft	0.045	Lofts

Two columns are labelled Blown (installer): the λ 0.040 one is the cavity wall fill, and the λ 0.045 one is blown loft insulation.

Why 270 mm?

The Energy Saving Trust recommends at least 270 mm of mineral wool in a loft. Here's how that lines up with the Building Regulations.

The target. Approved Document L asks for U-value 0.16 W/m²K when you upgrade a loft.
The R the insulation must supply. A U-value of 0.16 means a total R of 1 ÷ 0.16 = 6.25 m²K/W. The surfaces and the ceiling provide about 0.57 of that, so the insulation needs R 5.68.
The thickness on paper. R 5.68 × λ 0.044 = 250 mm of loft roll.
The joists. The first layer sits between timber joists, which conduct heat about three times better than mineral wool. That costs about 20 mm, so in practice you need 270 mm: 100 mm between the joists and 170 mm across them.

Knauf and the RdSAP 10 specification both give U-value 0.16 for that 100 + 170 mm build-up, and the loft calculator is calibrated to it.

Worked example: how much to add

Most lofts already have some insulation. For a 45 m² loft with old mineral wool, the loft insulation calculator returns these top-ups to reach U-value 0.16, with 10% waste:

Depth there now	U-value now	Add	Rolls of Loft Roll 44	U-value after
None	1.75	100 + 170 mm	4 + 7	0.16
50 mm	0.65	150 + 100 mm	6 + 4	0.14
100 mm	0.40	170 mm	7	0.16
120 mm	0.33	150 mm	6	0.16
200 mm	0.20	100 mm	4	0.14
270 mm	0.15	Nothing needed	–	0.15

Rolls only come in 100, 150, 170 and 200 mm, so the total doesn't always land on exactly 270 mm. The calculator picks the fewest layers, then the thinnest stack, that reaches the target. Existing mineral wool is valued at λ 0.040, the figure RdSAP 10 uses.

If you'd go further, the New-build standard of U-value 0.15 takes 300 mm in an empty loft: 200 mm plus 100 mm of roll, reaching 0.14.

λ 0.044 or λ 0.040 roll?

Loft rolls are sold with different λ values. Our data uses Knauf Loft Roll 44, at λ 0.044. A λ 0.040 roll reaches the same R-value in about 9% less thickness, which can help where space is tight. What counts is the R-value of the layer, so compare products by the R-value of the thickness you'll lay, not by thickness alone. The same goes for boards: PIR at λ 0.022 needs half the thickness of λ 0.044 roll, and EPS at λ 0.038 sits in between.

Things that cut real thickness

Compression. Boards, boxes and walkways laid straight on the joists squash the insulation. Use loft legs.
Thin spots. Insulation is often thinner at the eaves, around the hatch and near the water tank. Measure in several places, not just one.
Gaps. Rolls that don't meet, or that stop short of the joists, leave paths for heat. Butt the edges together.
Blocking the eaves. Don't solve a thin spot by stuffing insulation into the eaves. The loft needs that ventilation.

For other areas of the house, see Part L U-values for the targets, or size a job with the solid wall insulation calculator.

Loft insulation thickness

How thick does it need to be?

How to read the table

Why 270 mm?

Worked example: how much to add

λ 0.044 or λ 0.040 roll?

Things that cut real thickness

Sources

Frequently asked questions