DIY Internal Wall Insulation: Dew Points, Hidden Damp and When You Need a Professional

Quick answer: DIY internal wall insulation goes wrong for one main reason: it moves the dew point — the temperature at which water vapour condenses — inside the wall, where any damp it causes stays hidden behind the boards. On solid-walled homes especially, badly planned IWI can rot joist ends and grow hidden mould. On pre-1919 walls, use a professional.

Internal wall insulation (IWI) looks temptingly DIY-able: insulated plasterboard is sold in every builders’ merchant, and sticking it to a wall seems no harder than dot-and-dab. The materials are the easy part. The risk sits in building physics — and in what you cannot see once the boards are up.

The dew point problem, in plain English

Warm indoor air carries water vapour, and that vapour constantly migrates through walls towards the colder, drier outside. In an uninsulated wall, the masonry is kept warm-ish by the room, so vapour mostly passes through without condensing. Add insulation on the inside and you deliberately stop heating the wall — which is the point — but the masonry behind the insulation now runs cold all winter. If moist air reaches it, it condenses there, inside the construction. Builders call this interstitial condensation; in practice it means damp masonry, saturated adhesive dabs, mouldy cavities behind the boards and, over time, decay in any timber built into the wall. Our guide to condensation after insulation covers the surface version of the problem; IWI adds this hidden version.

The five DIY mistakes that cause it

• 1. No continuous vapour control layer. Insulated plasterboard relies on a VCL on the warm side, sealed at every joint, socket, and perimeter edge. Gaps of a few millimetres channel moist air straight to the cold masonry.
• 2. Insulating over existing damp. Rising damp, penetrating damp or a leaking gutter must be fixed and the wall dried before insulating. Boarding over damp locks it in.
• 3. Wrong materials on old walls. Solid brick and stone walls manage moisture by breathing. Sealing them with foil-faced PIR can trap moisture; breathable systems (wood fibre, calcium silicate, lime finishes) are usually the right family — our EWI vs IWI guide compares the options.
• 4. Ignoring cold bridges. Window reveals, floor junctions and party-wall junctions stay cold where the insulation stops. Condensation and mould then concentrate along those lines — the classic black stripe at the ceiling edge.
• 5. Forgetting joist ends. In many homes the floor joists are built into the external walls. After IWI, those pockets sit in colder, damper masonry. Competent designs assess and detail them; DIY jobs almost never do.

What a competent installation involves

A professional IWI job is mostly assessment and detailing rather than boarding. Expect a moisture survey of the existing wall, a condensation-risk analysis of the proposed build-up, insulation returned into window reveals, a ventilation plan for the affected rooms, and either a meticulously sealed vapour control layer or a deliberately vapour-open, breathable build-up — not a mixture of the two. On funded work, the whole-house retrofit approach requires exactly this kind of design stage, for good reason.

Building regulations also apply: in England and Wales, insulating a wall internally counts as renovating a “thermal element”, which normally requires the wall to be brought up to a set U-value standard and the work to be notified to building control. Check with your local authority before starting — retrofitting compliance afterwards is painful.

When DIY is reasonable — and when pros are essential

Cost perspective

IWI sits among the pricier insulation measures per room, which is exactly why cutting corners tempts people. For context, our modelled installed cost for home insulation work is £974–£2,322 (mid-point £1,498; how we calculate costs) — solid-wall measures sit above simpler jobs, and insulation currently attracts 0% VAT until March 2027. Whole-home insulation improvements could save up to £381 a year (our modelled estimate). Before committing either way, compare internal against external wall insulation in our EWI vs IWI comparison, check the basics in our solid wall insulation glossary entry and insulation guide, and run your own numbers through the calculator before deciding which wall treatment fits your home and budget.

FAQs

Can I just stick insulated plasterboard straight over my walls?

On a dry, modern cavity wall, possibly — with a sealed vapour control layer, insulated reveals and building control notified. On solid walls or anywhere with a damp history, no: that shortcut is precisely how hidden interstitial condensation starts.

Does internal wall insulation always cause damp?

No. Properly designed IWI — right materials for the wall type, moisture sources fixed first, continuous vapour control or a fully breathable build-up, and ventilation addressed — performs well. The damp horror stories overwhelmingly come from unassessed, badly sealed installations.

Do I need building regulations approval for IWI?

Normally yes in England and Wales: insulating a wall counts as renovating a thermal element, which sets a performance standard and requires notification to building control. Your installer can handle this; if you DIY, contact building control yourself before starting.