Internal wall insulation (IWI)
For solid-wall homes that can't take cavity fill, insulating the inside face of the external walls is often the most cost-effective route to a warmer, compliant property.
What it is
Insulated boards or a stud-and-mineral-wool system fixed to the inside of your external walls, then plastered and finished ready to decorate. It is the alternative to external wall insulation for solid-wall homes — chosen when you don't want to change the outside appearance, in a conservation area, or where only some rooms need doing.
Why it matters for 2030
Roughly a third of the UK's rented stock is solid-walled and these are the homes that score worst on EPC and are hardest to get to band C. Wall insulation is usually the decisive measure for them. IWI delivers a large heat-loss reduction and the points to match, and it can be done room by room around a tenancy.
Which homes suit it
Pre-1930 solid brick or stone homes, or non-traditional builds without a fillable cavity. IWI slightly reduces room size and must be designed to manage moisture — cold-bridging at floors, reveals and party walls, and adequate ventilation — or it can cause condensation and mould. We design every IWI job to PAS 2035 with a retrofit assessor precisely to avoid that. Where the outside can be altered, compare with external wall insulation.
Grants that apply
Solid-wall insulation attracts the largest grant support: the Warm Homes: Local Grant for eligible tenants, and ECO4 until 31 December 2026, both of which prioritise solid-wall homes. There is also a dedicated solid-wall exemption route under MEES if costs exceed the cap. VAT is zero-rated until 31 March 2027. See the grants page.
How we do it
Assessed and designed to PAS 2035, installed to PAS 2030 by our accredited partner LAMTANS Engineering, lodged with TrustMark and guarantee-backed. Fixed price from our own book, produced as part of your free assessment.
Related: external wall insulation · ventilation · loft insulation