Partial-Fill Cavity Wall PIR Thickness: Thermawall TW50 and Eurowall Cavity
Quick answer
For a partial-fill cavity wall, the PIR board thickness you need depends on your target and your inner leaf. With a brick outer leaf and plasterboard on dabs, Recticel Eurowall Cavity gives 0.25 W/m²K at 60mm on a dense block inner leaf, and 0.18 at 80mm on aerated block; 100mm reaches 0.17 on dense block. Your residual cavity stays at 50mm, so a 100mm board needs a 150mm cavity. Check your own wall in the PIR U-value calculator.
How partial fill works
A partial-fill board is fixed to the outer face of the inner leaf with retaining clips, leaving a clear residual cavity between board and outer leaf. Both manufacturers set the standard residual cavity at 50mm. Kingspan's brochure adds that it can be 25mm in isolated areas (it cites its BBA certificate 14/5133 for this) and that NHBC normally requires 50mm in exposure zone 3 and a minimum of 75mm in zone 4. Total cavity width = board thickness + residual cavity. That is why the thicker boards need a wider cavity and a longer tie. See our guide to cavity wall insulation for the alternatives.
Targets for new walls
In England, Approved Document L requires 0.26 W/m²K for walls in new dwellings (Table 4.1) and 0.18 for new walls in extensions (Table 4.2); the extension wall figure can be relaxed if meeting it would cut the room's floor area by more than 5%. See our Part L targets guide.
Thickness by inner leaf: two manufacturers
Typical U-values (W/m²K), 102.5mm brick outer leaf, 100mm inner block, plasterboard on dabs, with the inner block's conductivity shown in W/mK:
| Board thickness | Kingspan TW50: dense block (1.13) | TW50: aerated block (0.11) | Recticel Eurowall Cavity: dense (1.13) | Eurowall Cavity: aerated (0.11) |
|---|---|---|---|---|
| 50mm | 0.28 | 0.24 | 0.29 | 0.24 |
| 60mm | 0.25 | 0.22 | 0.25 | 0.22 |
| 70mm | 0.22 | 0.20 | 0.23 | 0.20 |
| 75mm | 0.21 | 0.19 | 0.22 | 0.19 |
| 80mm | 0.20 | 0.18 | 0.21 | 0.18 |
| 100mm | 0.17 | 0.15 | 0.17 | 0.16 |
A few things stand out:
- For 0.26, 60mm is enough on dense block in both tables, with 50mm on aerated block.
- For 0.18, only a 100mm board gets there on dense block, while 80mm does it on aerated block (Eurowall) or TW50 aerated.
- Swapping dense block for aerated saves roughly 20mm of PIR. Mortar bridging and tie assumptions explain the small differences between the two columns.
Another check: the BBA figures for Eurowall Cavity
Table 7 of the Eurowall Cavity BBA certificate gives the thickness needed, with a 50mm residual cavity and a 102mm brick outer leaf (using stainless steel ties at 2.5 per m²):
| Target U-value | 13mm dense plaster on dense block (mm) | Plasterboard on dabs on 100mm aerated block (mm) |
|---|---|---|
| 0.17 | – | 90 |
| 0.18 | 100 | 80 |
| 0.21 | 80 | 65 |
| 0.26 | 60 | 45 |
| 0.28 | 55 | 40 |
| 0.30 | 50 | 35 |
The certificate does not give thicknesses for 0.13 or 0.15, so deeper-cavity walls on tighter targets need a designer's calculation.
Cavity width and wall ties
Kingspan's TW50 notes say its tables assume a stainless steel tie with a 12.5mm² cross-section for cavities up to 125mm, 23.40mm² above 125mm and 80mm² above 175mm. A 100mm board plus a 50mm residual cavity is 150mm, so the 100mm results use the larger tie, while the Eurowall BBA table uses 12.5mm². Do not mix manufacturers' tie data. Lightweight and aerated blocks carry a 6.6% mortar bridging correction in the TW50 tables.
Points to check before you order
- Allow for the extra wall thickness. A 100mm board with a 50mm residual cavity gives a 150mm cavity, which affects foundations, cavity trays, lintels and window reveals.
- Check the tie length and type with the board manufacturer's installation guide, since the wall tie must reach across the full width.
- Plan to close the cavity at openings and corners so that the insulation line is continuous. Use the manufacturer's detail drawings.
Choosing a board
- Pick your target (0.26 or 0.18, say).
- Choose your inner leaf: its conductivity is worth up to roughly 20mm of insulation.
- Read off the thickness above and the cavity width it needs.
- Check the width suits the ties, foundations and window reveals.
Frequently asked questions
Can I fill the whole cavity with PIR?
Full-fill boards are a different product, with different certification and a different design. See our cavity wall insulation range.
Does the residual cavity matter?
Yes. It keeps a clear gap between the outer leaf and the board, which is why 50mm is the standard. Exposure zone changes the minimum, so follow NHBC and the manufacturer's certificate.
Are these walls the same in Wales or Scotland?
The construction is similar but the targets and rules are not. Check Approved Document L (Wales), the Scottish Technical Handbooks and your Building Control.
Order PIR insulation
Browse cavity wall insulation and wall insulation boards, including Kingspan Thermawall TW50 100mm, 75mm, Recticel Eurowall Cavity 100mm and 50mm. We have a £400 ex VAT minimum, with free delivery in England. British Gypsum (including Gypframe), Celotex and Knauf can also go to mainland Wales and mainland Scotland (£400 ex VAT minimum plus £40 + VAT delivery). Access panels can also go to mainland Wales and mainland Scotland, with their own charges (£15 + VAT under £300 ex VAT, free from £300). Other ranges, including suspended ceiling tiles and grid, are delivered in England only. We do not deliver to Northern Ireland or the Scottish islands.
Sources: Kingspan Thermawall TW50 brochure; Recticel Eurowall Cavity product page; Recticel Eurowall Cavity datasheet; Approved Document L (GOV.UK). 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.