Loft Insulation Calculator: Rolls Needed to Reach 270mm

The usual advice for a cold loft is 270mm of mineral wool, laid as two layers: one between the ceiling joists and one across them. Most lofts already have some insulation, so the real question is how much to add and which roll thicknesses get you there without buying far more than you need. Enter your loft size and what is already down, and the calculator works out the layers, the square metres of each and, if you type in the coverage from the pack, the number of rolls.

How much loft insulation do you need?

The figure you will see everywhere, from the Energy Saving Trust to installers' quotes, is 270mm of mineral wool for a loft insulated at ceiling level. It is the depth most loft top-ups aim for, and the one quoted for a ceiling-level loft in a cold pitched roof. Anything much above it still helps, but each extra 50mm saves less than the last, which is why the calculator offers 300mm and 350mm as options rather than defaults.

Many older lofts have 100mm or less, often laid decades ago and flattened since. That is the common case the calculator starts from: 100mm down, 170mm to add.

How the layers are worked out

Mineral wool loft roll is sold in a handful of standard thicknesses, most commonly 100mm, 150mm, 170mm and 200mm. The calculator subtracts what you already have from your target, then finds the combination of one, two or three of those thicknesses that reaches the target with the least overshoot. Where two combinations tie, it picks the one with fewer layers, because every layer is another pass across the loft on your knees.

On a bare loft the result is normally a thin layer to go between the joists and a thicker layer to lay across them at right angles. Crossing the second layer over the joists matters: timber conducts heat far better than wool, so leaving the joist tops exposed leaves a cold stripe every 400mm or so across the ceiling. Where you are topping up, the new layer goes across the old one in the same way.

Check the thickness range at your supplier before you buy. If a thickness the calculator suggests is out of stock, two thinner rolls that add up to the same depth do the same job.

Turning square metres into rolls

Every pack prints its coverage in square metres, and it varies between brands and thicknesses: thicker rolls cover less area because the pack holds a similar volume of compressed wool. Rather than guess a figure for your brand, the calculator asks you to type the coverage from the pack into each layer's row, then rounds up to whole rolls. Some rolls are also perforated to split into narrower widths for joists at 400mm or 600mm centres; that changes the width, not the area covered.

Before you lay it

• Keep the eaves clear. In a cold loft the ventilation usually comes in at the eaves. Stuffing insulation hard into the eaves blocks it and invites condensation on the underside of the roof. Our roof ventilation calculator and the guide to loft condensation cover why.
• Do not squash it under boards. Mineral wool works because of the air trapped in it. Boarding straight over 270mm crushes it and throws most of the benefit away. If you need storage, fit raised loft legs so the boards sit above the insulation.
• Insulate the hatch too. A bare loft hatch is a large uninsulated hole in the ceiling. Fix a slab of insulation to the top of it and fit a draught seal around the frame.
• Leave the cold water tank and pipes to one side. Do not insulate under the tank, because the warmth coming up from below is what stops it freezing. Lag the tank and the pipes separately instead.
• Keep clear of downlighters and cables. Recessed lights need covers or a clear gap as per the fitting's instructions, and cables buried under deep insulation can run hotter than they were rated for. If in doubt, ask an electrician.

If you are converting the loft or insulating at rafter level instead, this calculator is the wrong tool: the insulation moves into the roof slope and the build-up changes. See warm roof vs cold roof and the loft and roof insulation guide for costs and options.

Common questions

Is 100mm of loft insulation enough?

No, not by current standards. It was a common depth when many older lofts were insulated, but the usual recommendation now is 270mm. Topping up 100mm with a 170mm layer across the joists brings it to 270mm.

Can I lay new insulation on top of old?

Yes, as long as the old insulation is dry and not contaminated. Lay the new layer at right angles to the joists over the top. If the existing material is damp, mouldy or shows signs of vermin, clear it out and start again.

Why does the calculator suggest more than my target?

Because rolls only come in set thicknesses. If you have 50mm down and want 270mm, no standard pair adds up to exactly 220mm, so it suggests the closest combination that gets you to at least the target. A little extra depth does no harm.

Does the loft size mean the roof area?

No. Loft insulation sits on the ceiling, so you want the floor area of the loft, the same as the footprint of the rooms below it. The sloping roof area is bigger and is what you need for tiles or underlay, which the roof area calculator works out.