Wall Insulation Cost and U-Value Calculator

Age is the first clue: in Northern Ireland, houses built from roughly the 1930s onwards are usually cavity, and anything older than that is more likely to be solid brick or stone. Then measure the wall thickness at an open window or door reveal. A cavity wall is normally about 270 to 300mm through; a solid brick wall is nearer 220mm. If the brickwork is exposed, look at the bond: a wall laid entirely in stretchers, every brick showing its long face, is almost certainly a cavity wall, while a pattern that includes short header faces every few courses is solid. Finally, look for a regular grid of filled 22mm holes in the mortar joints, roughly a metre apart. That is the signature of a cavity that has already been drilled and injected. Your EPC will also record what the assessor concluded, though they were working from the same clues you are.

Correctly installed, in a wall that was suitable for it, it does not. The problems that make the headlines come from walls that should never have been filled in the first place. The usual causes are severe exposure to wind-driven rain, cracked render or failed pointing letting water into the outer leaf, mortar droppings bridging the cavity, a cavity narrower than about 50mm, or damp that was already there before anyone drilled a hole. Exposure matters a great deal here: the north coast and the west of Northern Ireland are among the wettest and windiest places in the UK, and an exposed gable facing the prevailing south-westerly gets a hammering. That is why a pre-installation survey, including a borescope look into the cavity, is mandatory rather than a formality, and why the installer's guarantee is worth reading before you sign. If a contractor is willing to quote without inspecting the cavity, use someone else.

External insulation is the better building physics answer nearly every time. It leaves the masonry inside the insulated envelope where it stays warm and dry, it deals with cold bridging at floor and party wall junctions, you keep every square metre of your floor area, and the work happens outside rather than through your living room. The costs against it are real though: it needs scaffolding, it needs sills, eaves, vents and downpipes reworked, and it changes the way the house looks, which can be a planning problem and is often a personal one. Internal insulation is cheaper per square metre, can be done one room at a time, and leaves the outside untouched. In exchange you lose floor area, you disturb skirting, sockets, radiators and anything fitted against the wall, you are left with cold bridges where floors and partitions meet the external wall, and you have to think carefully about where moisture goes. On a mid-terrace with no rear access, internal is often the only option. On a detached house with a plain render finish, external usually wins.

Technical Booklet F1 of the Building Regulations (Northern Ireland) sets elemental targets for work to an existing dwelling. If you are insulating a wall internally or externally, the target is 0.30 W/m²K. If you are filling an existing cavity, the target is 0.55 W/m²K, which reflects the fact that you can only work with the cavity you have. Those are the numbers building control will look for, and the calculator tells you plainly whether the build-up you have chosen reaches them. Getting a solid brick wall to 0.30 takes roughly 115mm of EPS externally, or around 70 to 75mm of PIR or phenolic board internally, which is a good illustration of why the thin-board options are so popular indoors.

Yes, usually noticeably. Walls are typically the largest single heat loss element in a house, so the wall U-value is one of the heaviest inputs in the SAP calculation behind an EPC. Filling an empty cavity is one of the best-value measures there is in points per pound spent; solid wall insulation moves the rating further but costs a great deal more. One practical warning that catches people out: RdSAP records what the assessor can actually evidence. If you have had a cavity filled, keep the installation certificate and guarantee and show it to the assessor, otherwise they may have no choice but to record the wall as unfilled and you will have paid for an improvement that never reaches your certificate.

There usually are, but the schemes change often enough that naming figures here would do you a disservice. Support in Northern Ireland has historically come through the Northern Ireland Sustainable Energy Programme, delivered by the energy suppliers, through Housing Executive schemes, and through council-run initiatives, with eligibility normally tied to income, certain benefits, or the property itself. The reliable route is to check NI Direct and your local council before you commit to anything, and to speak to your energy supplier. Two rules apply to almost every scheme: the work has to be arranged through an approved installer, and it has to be approved before it starts. Paying for the job yourself and applying afterwards is the most common way people lose the funding.