How the solid wall calculation works
Solid walls, usually found in older brick or stone homes, have no cavity to fill. In our model an uninsulated solid brick wall loses about four times as much heat as a filled 65 mm cavity wall. Instead of filling, you add insulation to the inside (internal wall insulation, IWI) or the outside (external wall insulation, EWI).
The target
- Building Regs (Part L): U-value 0.30 W/m²K. Approved Document L, Table 4.3, gives this for a wall upgraded with internal or external insulation.
- New-build standard: U-value 0.18 W/m²K. The Table 4.2 value for a new wall in an existing home.
The formula
U = 1 ÷ (Rsi + Σ thickness/λ + Rse)
For a wall, Rsi is 0.13 and Rse is 0.04 m²K/W. The calculator gives an uninsulated solid brick wall a further 0.42, calibrated so that a solid 225 mm brick wall comes out at about U-value 1.7, the RdSAP 10 value for that wall as built. To reach 0.30, the total R must be 1 ÷ 0.30 = 3.33, so the insulation has to supply about R 2.74 m²K/W.
The boards
The default product is a PIR board (λ 0.022), such as SOPRATHERM GA4000 (formerly Celotex) or Kingspan TP10, in 2400 × 1200 mm sheets of 2.88 m², from 25 to 120 mm thick. You can also choose EPS 70 board (λ 0.038). The calculator treats the boards as a continuous layer and picks the fewest layers, then the thinnest stack, that reaches the target.
Real systems have more to them: battens or fixings, plasterboard, a vapour control layer, or a render system on the outside. If your boards go between timber battens, the timber bridges part of the insulation and you'll need a little more. Use the result as a guide to the insulation thickness, not a full shopping list.
Worked example: 90 m² of solid brick wall
Say you're insulating 90 m² of solid wall. With 10% for waste and cutting, that's 99 m² of board, or 35 sheets per layer.
| Option | Boards | To buy | Material cost | U-value after |
|---|---|---|---|---|
| Uninsulated wall | – | – | – | 1.69 |
| Part L, PIR | 75 mm | 35 boards | £1,610–£2,251 | 0.25 |
| New-build standard, PIR | 120 mm | 35 boards | £2,100 | 0.17 |
| Part L, EPS 70 | 75 mm + 50 mm | 35 + 35 boards | £1,645 | 0.26 |
The exact thickness of PIR needed for 0.30 is about 60 mm, but boards come in set sizes, so the calculator picks 75 mm and you end up better than the target. EPS insulates less well per millimetre, so it takes 125 mm, in two layers, to do the same job.
These are materials only. The insulation is a small part of the total cost of a solid wall job: the Energy Saving Trust puts internal wall insulation at about £12,000 and external wall insulation at about £15,000 for a whole house.
Internal or external?
Internal wall insulation (IWI)
- You lose room space. In the example, 75 mm of board plus plasterboard and fixings comes off every insulated wall.
- It can be done room by room, for example when you're redecorating, but it's disruptive: skirting boards, sockets, radiators and window boards have to come off and go back.
- Cold bridges. Where floors and internal walls meet the outside wall, heat bypasses the insulation. Good detailing reduces this.
External wall insulation (EWI)
- No loss of room space, and the whole wall is wrapped, including the edges of floors.
- It changes the outside of the house. Window sills, downpipes and the roof edge usually need extending.
- Planning permission may be needed in some cases, such as conservation areas, listed buildings or where it changes the look of the street. Check with your local planning authority.
- It's a job for a specialist installer with scaffolding.
Condensation and moisture
Insulating a solid wall makes the masonry behind the insulation colder. If warm, moist indoor air reaches it, the moisture can condense inside the wall.
- With IWI, a vapour control layer on the warm side, with taped joints, keeps moist air out. Foil-faced PIR boards help, but the joints and edges still need sealing.
- Older walls built with lime mortar, and stone walls, may need a breathable system rather than foil-faced boards. Get advice from someone who knows that kind of building.
- The calculator doesn't assess condensation risk. For anything beyond a simple wall, ask for a condensation risk analysis.
- The Energy Saving Trust has warned about poor-quality solid wall insulation installed under past government schemes. Choose a reputable, registered installer and check the work.
When Part L applies
Approved Document L counts dry-lining a wall or adding a new layer of render or cladding as renovation. If you renovate more than half of a wall, or more than a quarter of the whole envelope, the guidance expects the wall to be brought up to 0.30. If that isn't technically possible or wouldn't pay back within 15 years, you upgrade as far as is feasible within 15 years, and an upgraded wall generally shouldn't be worse than 0.70. Note 6 adds that if meeting 0.30 would reduce a room's floor area by more than 5%, a lesser standard may be acceptable.
If your walls turn out to have a cavity, use the cavity wall insulation calculator instead. See all the targets on Part L U-values, check board thicknesses on loft insulation thickness, or work through the sums with the U-value calculator.
Sources
- Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments)
- Energy Saving Trust: Solid wall insulation
- RdSAP10 Specification (BRE, February 2024)
- BRE BR 443: Conventions for U-value calculations (2019)
- SOPREMA UK: SOPRATHERM GA4000 (previously Celotex GA4000)
- Wickes: insulation boards (prices)
- Toolstation: insulation (prices)