What U-value do I need for a loft?

U-value is the single number that tells you how well a roof keeps heat in. Here is what target to ask for, why 0.16 is the everyday figure, and where the rules differ for a warm loft.

What a U-value actually measures

A U-value is the rate at which heat flows through one square metre of a building element for each degree of temperature difference between inside and outside, measured in watts per square metre per kelvin (W/m²K). A bare, uninsulated loft might have a U-value well above 2.0 — heat pours out of it. Adding insulation drives that number down: the more (and the better) the insulation, the lower the U-value and the less heat escapes. Because the scale runs the opposite way to most people's intuition, the key point is simple — a smaller U-value is better. The 0.16 W/m²K figure used for lofts represents a well-insulated roof that loses very little heat through the ceiling plane.

U-value is not the same as thickness or as the material's own R-value. R-value describes a material's resistance to heat flow per unit thickness; the U-value is the whole build-up combined — insulation, joists, any boarding and air layers. That is why cross-laying insulation over the joists matters: the timber joists conduct more heat than the wool around them, so leaving them exposed worsens the overall U-value even where the depth between joists looks correct.

One more idea is worth holding onto, because it explains why chasing an ever-lower U-value above the ceiling eventually stops paying: the relationship between depth and heat loss is not linear. Halving the U-value of a bare loft takes only a modest first layer, but halving it again takes proportionally much more material, and a third time more still. In numbers, the jump from an uninsulated roof above 2.0 down to around 0.16 captures the overwhelming majority of the available saving, while pushing on from 0.16 to, say, 0.13 shaves only a sliver more heat loss for a noticeable extra cost. This is the same diminishing-returns curve that makes 270mm the everyday recommendation rather than “as low a U-value as possible”: the target sits near the point where each added millimetre stops earning its keep. So the U-value to aim for is genuinely a balance point, not a floor to beat — 0.16 at ceiling level is where cost and benefit meet for most homes, and the regulations are pitched at roughly that sensible economic optimum rather than at the theoretical lowest the material could reach.

How 270mm reaches the 0.16 target

The reason 270mm of mineral wool is quoted so often is that it is roughly the depth needed to hit a ceiling-level U-value of about 0.16 W/m²K with common loft-roll products. It is normally laid in two layers — around 100mm between the joists, then a thicker layer across them — so the timber is covered and the cold bridge through the joists is reduced. Exact depth varies a little with the specific product's thermal conductivity (lambda): a higher-performance wool reaches the target in slightly less depth, a cheaper one needs a touch more.

Indicative U-values for guidance; exact figures depend on the product's thermal conductivity and the build-up. Sources: Approved Document L, Energy Saving Trust.

When the figure is tighter — warm lofts and conversions

The 0.16 W/m²K figure applies to insulating the flat ceiling plane in a cold-roof arrangement, where the loft itself stays cold. If you want a usable, heated loft room, the insulation moves up to the rafters and the regulations treat it as a heated roof. Here the U-value target is similar, but it must be reached with much thinner rigid boards (such as PIR or PUR), often combined between and under the rafters, because there is far less room than the open space above a ceiling. A full loft conversion is building work in its own right and triggers the relevant Building Regulations approval, including the insulation standard for the new room.

One practical caution: do not assume that simply piling more wool above the ceiling guarantees compliance for a converted room. A warm loft and a cold loft are different specifications. If you are converting, the U-value should be designed as part of the conversion, with ventilation and any vapour control considered together rather than added as an afterthought.

It is also worth knowing that the U-value the Building Regulations expect depends on whether the work is new build or a retrofit upgrade, and the rules treat the two slightly differently. Approved Document L sets a tighter standard for a roof in a new dwelling than the “reasonable provision” threshold applied when you improve an existing one, where the practical limits of an old structure — shallow rafters, fixed headroom, awkward junctions — are taken into account. In practice this is why the same 0.16 W/m²K figure is quoted as the everyday target for a cold-loft top-up: it is comfortably achievable above an open ceiling and sits at or below what the regulations look for. At rafter level the figure may be allowed to relax a little where it is genuinely impractical to fit enough board without losing unreasonable headroom, which is one more reason that detail belongs with a designer who can document why a given build-up is reasonable for that roof. The honest takeaway is that 0.16 is the number to aim for and to ask about, but the precise duty that applies to a specific job is set by which part of Document L governs it — new work, a material change of use, or an upgrade — not by a single universal threshold.

How thermal conductivity changes the depth you need

Two lofts can both be aiming at the same 0.16 W/m²K target yet need different depths of insulation, because not all mineral wool performs identically. The property that decides this is thermal conductivity, written as a lambda (λ) value in watts per metre per kelvin (W/mK) and printed on the product. A lower lambda means the material resists heat flow better, so less of it is needed to reach a given U-value. A standard loft-roll glass wool sits around 0.044 W/mK, while a higher-performance wool might be nearer 0.034 W/mK. The better-performing product reaches the 0.16 target in slightly less depth, which can matter where headroom or eaves clearance is tight.

This is why quoting a depth alone is imprecise. “270mm” is shorthand for “whatever depth of typical mineral wool reaches about 0.16 W/m²K” — with a poorer-lambda product you might need a touch more, and with a premium one slightly less. For a normal open loft the difference rarely matters and 270mm of standard wool is the sensible default. Where space is constrained, choosing a lower-lambda wool or a rigid board is how you hit the same U-value in a thinner build-up. The figure that ultimately counts on your EPC and for the Building Regulations is the U-value of the finished build-up, not the headline thickness, so it is always worth asking what U-value a quoted specification actually achieves rather than reading depth as a guarantee of performance.

Frequently asked questions

Sources & further reading

• gov.uk — Approved Document L (conservation of fuel and power)
• Energy Saving Trust — roof and loft insulation

Figures on this page are typical UK ranges drawn from published sources and depend on your specific loft. They are guidance, not a quotation.