Loft Conversion Insulation: PIR Between Rafters to Reach 0.16 W/m²K
Quick answer
Converting a loft usually means bringing the sloping roof up to 0.16 W/m²K (Table 4.3, Approved Document L, England). With Kingspan Thermapitch TP10 (PIR, λ 0.022) between the rafters and insulated plasterboard below, 150mm-deep rafters can reach 0.16 with 120mm of PIR plus 37.5mm of board, or 0.15 with 100mm plus 57.5mm, in an unventilated roof. 125mm rafters need 90mm plus 57.5mm. Try your own depth in the PIR U-value calculator.
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What Part L asks for
A loft conversion changes the energy status of the building, so the existing roof elements being retained should meet Table 4.3: threshold 0.35, improved 0.16. If the target would limit headroom, Note 3 allows a lesser standard, provided the insulation plus any air gap is at least the depth of the rafters and you choose the lowest practicable U-value. A newly built roof element, such as a new dormer roof, is judged against Table 4.2, which is 0.15. See our Part L targets guide for the full picture.
Option 1: unventilated roof, 600mm centres
TP10 fills part of the rafter depth, leaving at least 13mm of air space, and Kingspan Kooltherm K118 insulated plasterboard of 37.5, 57.5 or 72.5mm overall thickness (including the 12.5mm board) goes under. Typical U-values in W/m²K (Kingspan TP10 Figure 1c):
| TP10 between rafters | 125mm rafters: with 37.5 | with 57.5 | with 72.5 | 150mm rafters: with 37.5 | with 57.5 | with 72.5 |
|---|---|---|---|---|---|---|
| 75mm | 0.21 | 0.18 | 0.16 | 0.21 | 0.18 | 0.16 |
| 90mm | 0.19 | 0.16 | 0.15 | 0.19 | 0.16 | 0.15 |
| 100mm | 0.18 | 0.16 | 0.14 | 0.18 | 0.15 | 0.14 |
| 120mm | – | – | – | 0.16 | 0.14 | 0.13 |
Anything at 0.16 or below meets Table 4.3 (and 0.15 is the figure for a newly built roof). The 90mm and 57.5mm combination does it in a 125mm rafter, and 100mm with 57.5mm gives the same 0.16. The dashes mean the board would leave no room for the air gap, so a 120mm board does not suit a 125mm rafter.
Option 2: ventilated roof, 400mm centres
Where a ventilated gap of at least 50mm is kept above the insulation, the usable depth is smaller:
| TP10 between rafters | 125mm rafters: with 57.5 | with 72.5 | 150mm rafters: with 57.5 | with 72.5 |
|---|---|---|---|---|
| 75mm | 0.20 | 0.18 | 0.20 | 0.18 |
| 90mm | – | – | 0.18 | 0.16 |
| 100mm | – | – | 0.17 | 0.15 |
A ventilated 125mm rafter does not reach 0.16 in these tables. This is where Note 3 on headroom matters. Ask Building Control whether a lesser standard is acceptable for your roof, or consider an unventilated build-up. Kingspan advises that condensation risk should be checked for the roof as built.
Option 3: between and over the rafters
If the roof covering is coming off, a second layer of TP10 over the rafters (unventilated, 600mm centres) gives lower U-values:
| Between + over | 100mm rafters | 125mm rafters | 150mm rafters |
|---|---|---|---|
| 60 + 60mm | 0.18 | 0.18 | 0.18 |
| 70 + 70mm | 0.16 | 0.15 | 0.15 |
| 75 + 75mm | 0.15 | 0.15 | 0.14 |
| 90 + 90mm | 0.13 | 0.12 | 0.12 |
The over-rafter layer's R-value must be at least that of the layer between the rafters. This suits a re-roof; it also raises the roof line, which may affect planning, so check first.
Dwarf walls
Low walls beside the sloping ceiling can be insulated with TP10 between the studs: 100mm between 47mm-wide studs (deep enough to take the board) with 57.5mm Kooltherm K118 insulated plasterboard gives 0.17 W/m²K in Kingspan's ventilated construction, with studs at 400 or 600 centres. With 37.5mm plasterboard it is 0.21 or 0.20. Dwarf walls and sloping sections are often a larger share of a loft room than people expect, so do not skip them.
Practical points for a loft room
- Insulation that is fixed tightly between the rafters, with no gaps at the sides or ridge, performs closer to the table figures.
- Kingspan states that a detailed U-value calculation and a condensation risk analysis should be completed for each project, so ask your designer to confirm the build-up.
- Keep the specified air space above the board. Pushing the insulation into it to gain thickness can cause condensation problems.
Choosing your build-up
- Measure the rafter depth and centres.
- Decide whether the roof is ventilated or unventilated and keep the air gap the manufacturer's detail shows.
- Check the U-value in the table or the PIR U-value calculator.
- Check headroom and your target with Building Control.
Frequently asked questions
Is 100mm of PIR enough for a loft conversion?
Only just: with 57.5mm insulated plasterboard it reaches 0.16 in a 125mm unventilated rafter, but it does not in a ventilated one. Deeper rafters do better.
Can I use insulated plasterboard alone?
In the tables above the PIR between the rafters and the insulated plasterboard work together, and the U-value is worse if you leave out either. See how to install insulated plasterboard for fitting.
What about a flat roof dormer?
Flat roofs are built differently and have their own tables. Use the manufacturer's data and your designer's calculation.
Order PIR insulation
See the roof insulation range and thermal insulated plasterboard, including Kingspan Thermapitch TP10 100mm, 90mm and 120mm. Most orders over £400 + VAT are delivered free across England, with delivery to mainland Wales and Scotland too. Mainland Wales and mainland Scotland: Celotex insulation and Knauf loft roll have £40 ex VAT delivery on every order; ceilings have a £400 ex VAT minimum plus £40 ex VAT delivery; British Gypsum plaster and plasterboard have a £400 ex VAT minimum plus £40 ex VAT delivery. We do not deliver to Northern Ireland or the Scottish islands. Compare plasterboards in our K118 vs PL4000 guide.
Sources: Kingspan Thermapitch TP10 brochure; Approved Document L (GOV.UK); Approved Document L Volume 1, 2026 edition (PDF). Figures are indicative and correct at the time of writing; they are not a substitute for a calculation by your designer or the manufacturer's technical team, and Building Control has the final say.