Insulation Calculator UK · metric

Depth, U-value and cost of loft insulation by area and material. Metric units and £ costs, updated for 2026.

Last reviewed July 2026 · Free · Nothing you enter is stored

How this calculator works

Required depth = target thermal resistance × the material's thermal conductivity (λ), in millimetres. Mineral wool (λ 0.034) needs about 270 mm to reach R-7, the UK loft standard; PIR board (λ 0.022) reaches it in far less depth. Element U-value ≈ 1 ÷ R.

The waste allowance

Insulation is not ordered with a percentage waste factor the way plasterboard or tile is, and adding one is usually the wrong instinct. Mineral wool rolls are cut to fit and the offcuts get used in short bays and around obstructions, so the practical allowance is small. What you should build in instead is depth. Joists that are not level with one another, wool that gets compressed as you crawl over it, a hatch that never gets covered and the strip at the eaves where the roll has to stop short all pull the finished average below the figure you aimed for. Mark the target depth on a rafter or a batten and lay to the mark rather than to the roll count, and treat any area you cannot reach as uninsulated when you work out what the loft is actually achieving.

The UK works in U-values, and Part L sets them

Thermal resistance and U-value are two views of the same thing. Resistance is what a layer adds: its thickness in metres divided by its thermal conductivity, the lambda value the manufacturer declares. The U-value is the rate the whole element loses heat, roughly one divided by the total resistance of every layer plus the surface resistances. Building Regulations are written in U-values, so that is the number that decides whether the work passes.

Approved Document L Volume 1, the 2021 edition with 2023 amendments, is the governing document in England. For a new dwelling the limiting values are 0.16 W/m²K for a roof, 0.26 for an external wall and 0.18 for a ground floor, with the notional dwelling that SAP compares your design against sitting much tighter at 0.11, 0.18 and 0.13. For an extension, Table 4.2 asks for 0.15 on a roof, 0.18 on a wall and 0.18 on a floor, with 1.4 for windows and glazed doors.

Renovation of an existing element is treated separately again, in Table 4.3, and the targets are more forgiving because you are working with what the house already has. Filling an existing cavity is expected to reach 0.55 W/m²K. Internal or external insulation on a solid wall is 0.30. A floor is 0.25 and a roof of any type is 0.16. Wales, Scotland and Northern Ireland set their own standards, so if you are not in England, check the equivalent document rather than assuming these figures carry across.

Where 270 mm of loft insulation comes from

The Energy Saving Trust recommends a minimum depth of 270 mm for mineral wool at ceiling level, and that figure is not arbitrary. Take Knauf Loft Roll 44, one of the common glass mineral wool products, with a declared thermal conductivity of 0.044 W/mK. Knauf publish a thermal resistance of 2.25 m²K/W for the 100 mm roll and 3.85 for the 170 mm roll, which is 6.10 m²K/W for the usual pairing of 100 mm between the joists and 170 mm laid across them.

Add the surface resistances and the plasterboard ceiling to that 6.10 and the assembly lands at roughly 0.15 to 0.16 W/m²K, which is precisely the roof figure in Table 4.3. That is the whole reason 270 mm is the number everyone repeats. It is the depth at which an ordinary mineral wool loft meets the renovation standard for a roof, and it stops there because going deeper produces steadily smaller savings.

The lambda value is what actually drives the depth, and it varies more than people expect. Knauf's Loft Roll 40 has a conductivity of 0.040 W/mK rather than 0.044, so 200 mm of it gives 5.00 m²K/W where 200 mm of the 44 grade gives 4.50. At the other end, a denser slab quoted at 0.034 W/mK would give close to 7.9 m²K/W over the same 270 mm. So the resistance you actually get from a 270 mm loft is anywhere between roughly 6 and 8 m²K/W depending on which product you bought. A calculator that assumes one lambda for all mineral wool will be wrong for most rolls on the shelf, which is why this one asks you to select the conductivity. Take the figure off the packaging rather than from memory.

PIR board where depth is short

Rigid polyisocyanurate board is the answer where you cannot give up the headroom. Soprema, who now make the Celotex GA4000 range, declare a thermal conductivity of 0.022 W/mK on 2400 x 1200 mm boards, half the conductivity of mineral wool. Resistance is again thickness divided by lambda, so 100 mm of it gives about 4.5 m²K/W, roughly what 200 mm of a good mineral wool roll delivers. In a room in the roof, a dormer cheek or under a floor being lifted anyway, that halving of depth is worth paying for.

It costs a great deal more per square metre, and it is far less tolerant of a sloppy fit. Mineral wool springs back into an irregular bay and closes the gap itself. A rigid board cut two millimetres shy leaves an air path around its edge, and air moving through that path bypasses the insulation entirely. Boards between rafters need cutting tight and the perimeters sealing with expanding foam or tape, or the installed performance falls well below the calculated figure.

The other thing a calculation hides is the timber. Joists and rafters conduct far better than the insulation between them, so an assembly insulated only between the members performs worse than the material figures suggest. A continuous layer over the top, whether that is wool laid across the joists or an insulated plasterboard lining, is what deals with that, and it is why the standard loft detail crosses the second layer over the joists rather than packing it all between them.

Ventilation, boarding out, and the things that undo it

A cold loft has to stay ventilated or the moisture rising from the house condenses on the underside of the roof. BS 5250 sets the requirement, and it depends on the underlay. Under an older non-breathable felt, the guidance is eaves ventilation equivalent to a continuous 25 mm gap for pitches of 15 degrees or less and 10 mm for pitches above that, with a further 5 mm at high level where the pitch exceeds 35 degrees, where the span is over 10 m, or on a lean-to. Modern breather membranes reduce what is needed but do not remove it. Pushing wool hard into the eaves is the single most common way a loft upgrade causes damp.

Boarding out over the insulation is the next problem. Laying boards straight onto joists that are 100 mm deep squashes 270 mm of wool down to 100 mm and throws away most of what you paid for. Raised loft boarding systems, the plastic legs that stand the boards clear of the wool, exist for exactly this reason. If you want storage and insulation you have to raise the deck, and it is worth doing that before the wool goes down rather than after.

The rest is small and easily missed. Do not insulate underneath a cold water tank, which needs the warmth from below to stop it freezing, but do insulate its sides and lid. Insulate and draught-seal the loft hatch, since an uninsulated hatch is a hole in an otherwise good ceiling. Keep wool clear of recessed downlighters unless they are rated to be covered, and do not block vents, grilles or airbricks.

Part L U-value standards, England

Maximum U-values in W/m²K from Approved Document L Volume 1, 2021 edition incorporating 2023 amendments. The notional column is the target the SAP calculation for a new dwelling is compared against, not a limit.

U-values from Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments (England). The 270 mm loft depth is the Energy Saving Trust recommendation; the supporting thermal resistances are Knauf's published figures for Loft Roll 44 and 40, and the 0.022 W/mK conductivity is Soprema's declared value for the GA4000 PIR board. Wales, Scotland and Northern Ireland set their own standards.

What goes wrong insulating

Compressing the wool. Mineral wool works by holding still air at its manufactured loft. Squashing 270 mm under loft boards or stuffing it into a shallow bay gives you less than the labelled resistance, not the labelled resistance in less depth.

Blocking the eaves. BS 5250 asks for eaves ventilation equivalent to a continuous 10 mm or 25 mm gap depending on pitch and underlay. Wool pushed tight into the eaves stops that airflow and the condensation appears on the roof timbers.

Assuming all mineral wool has the same lambda. Knauf's Loft Roll 44 is 0.044 W/mK and Loft Roll 40 is 0.040, and the depth needed for a given resistance moves with it. Read the conductivity off the packaging rather than using a remembered figure.

Insulating under the cold water tank. A tank in a cold loft relies on warmth rising from the house below it. Insulate the sides and the lid, leave the ceiling beneath it clear, and lag the pipes that run to it.

Insulating only between the joists or rafters. Timber conducts far better than the insulation beside it, so an assembly filled only between the members underperforms its material figures. A continuous layer crossing the timbers is what deals with the bridging.

Frequently asked questions