Ask three installers which insulation to use and you'll get four answers, usually shouted. The honest answer is that it depends entirely on the application. Below is the framework we use on every OMEGA survey — built on 12,000+ homes insulated across our service area, and backed by the BBA, KIWA and ETA certificates that building control actually signs off.
Multifoil, mineral wool and PIR — at a glance
| Criterion | Mineral wool | PIR board | Multifoil |
|---|---|---|---|
| Depth for 0.18 U-value | ~270mm | ~110mm | ~30–50mm (with air gaps) |
| Typical installed cost | £600–£900 | £1,400–£1,800 | £1,100–£1,500 |
| Ease of install | High — rolls out | Medium — cuts to fit | Medium — needs counter-battens |
| Moisture performance | Excellent (breathable) | Good (foil faced) | Excellent (built-in VCL) |
| Acoustic performance | Excellent | Moderate | Moderate |
| Fire rating (typical) | A1 non-combustible | Class E–B | Class E–B |
| Product guarantee | 50 years | 25 years | 50 years (BBA) |
Mineral wool — the default when depth is on your side
If your joists or rafters can take 270mm of insulation, mineral wool almost always wins on pounds-per-U-value. It's the cheapest per square metre, it's fire-rated A1 non-combustible, it breathes beautifully, and it doubles as acoustic dampening for upstairs rooms. The downsides are depth (you need a lot of it) and compression — anywhere it gets squashed under board, floor or skirt, the performance drops off sharply.
We specify mineral wool on open lofts, stud-work cavities where depth isn't the constraint, underfloor suspended-timber voids, and the flat ceiling plane above a vaulted room. When it fits, it wins.
PIR board — the workhorse for floors and flat roofs
PIR (polyisocyanurate) is a rigid foil-faced board. Per millimetre, it's the best-performing mainstream insulation on the market — which is why almost every flat-roof build-up in the last 20 years has been PIR-heavy. It cuts cleanly, it doesn't slump, and it plays nicely with every common wall and roof assembly.
The trade-offs: PIR is expensive, its foil facing is vapour-closed, and the foam itself is combustible (Class E in most formulations). Building control applies tight restrictions on PIR in external walls above 11m — not an issue for domestic work, but worth knowing if you're in a flat.
We specify PIR on solid concrete floor overlays, flat-roof warm-roof build-ups, internal wall insulation on period homes where depth is limited, and as the core layer in hybrid rafter-plane build-ups paired with multifoil.
Multifoil — the answer when depth is the enemy
Multifoil earns its place exactly where mineral wool can't fit and PIR can't be cut cleanly. Room-in-roof conversions from the 1990s and 2000s almost all have sub-50mm insulation between the rafters. You can't drop the ceiling another 200mm without losing the conversion that was the whole point — and multifoil solves that in a 30mm build-up.
Because the reflective mechanism only works with properly detailed air gaps, multifoil rewards careful installation more than either other product. Rates-per-metre fitters routinely skip the counter-battens. We refuse to — every OMEGA multifoil job has both gaps detailed on the spec sheet and signed off by the site supervisor before plasterboard goes up.
How we pick the right insulation on survey
- Measure the available depth. Under 100mm is multifoil territory. 100–200mm is PIR or a hybrid. 200mm and above is mineral wool unless there's a specific reason not to.
- Check the moisture profile. Solid wall interiors need breathable systems; flat roofs need vapour-tight ones. That narrows the choice long before cost enters the picture.
- Check grant eligibility. ECO4 and GBIS both name specific product categories — we design within the eligible set so the install stays fully funded.
- Check fire-rating requirements. Most domestic work is unconstrained, but any measure within 1m of a boundary or in a shared space pushes A1-rated mineral wool back to the top of the list.
What to ask any installer before you sign
- 'What U-value does the specified build-up achieve — and what's the certificate number?' BBA for multifoil, ETA or KIWA for PIR, BBA or KIWA for mineral wool.
- 'Where is the vapour control layer?' If the answer is vague or hand-wavy, walk away.
- 'How are the air gaps detailed?' Only relevant for multifoil — but completely critical to performance.
- 'Is this measure ECO4 or GBIS eligible as specified?' The answer should come with the scheme name and the official measure code, not a shrug.
If you'd like a fabric-first survey that honestly prices all three systems for your home, book online or ring 0800 229 4094 — Monday to Saturday, freephone from any UK number. Every quote names the certified product, the U-value, and the grant eligibility. No cost. No obligation. No pressure.
Want to find out how much you could save?
Book a free 45-minute survey. We'll check every active grant, measure your home, and give you a written quote — with no obligation to proceed.
No cost. No obligation. No pressure.