Best Insulation for Loft Conversions
So you're sorting out insulation for a loft conversion. Honestly, it's one of those decisions that can make or break the whole project — mess it up and you're freezing, or burning cash on heating. The right stuff depends on your roof, your wallet, and how much headroom you're willing to sacrifice. I'll walk you through what actually works, what doesn't, and how to not screw it up.
What is the best insulation material for a loft conversion?
Look, if you want my honest opinion, go with PIR rigid foam boards. They're basically the gold standard for lofts. Why? Because they give you the best thermal performance without eating up all your space — and in a loft conversion, every inch of headroom matters. A decent PIR board can hit a U-value around 0.18 with just 100-150mm thickness. That's way thinner than other options. Now, if your roof has weird angles or awkward nooks — spray foam fills everything like magic, seals airtight. But it's pricier and you can't DIY it. Professionals only, mate.
PIR vs. Mineral Wool vs. Spray Foam: Which is best for a loft conversion?
Each one's got its thing. PIR boards? Super efficient — thermal conductivity as low as 0.022 W/mK. They're rigid, easy to cut with a sharp knife, fit between rafters nicely. Mineral wool? Cheaper, sure, and it's decent for soundproofing — but you'll need double the thickness (like 200-250mm) to match PIR, which seriously cuts into your headroom. And you absolutely need a vapour control layer with it, otherwise condensation becomes a nightmare. Spray foam gives you the best airtight seal, works on any shape roof, but good luck removing it later — it's basically permanent. Plus if not applied right, moisture gets trapped. For most people doing a standard loft conversion, PIR is the sweet spot. Not too expensive, works great, easy enough to install yourself if you're handy.
Building regs in England and Wales (Part L) want a U-value of 0.18 or lower. With PIR, that's usually 100-150mm. Mineral wool? 200-250mm. Spray foam, similar to PIR at 100-150mm. But here's the thing — it depends on the exact product's lambda value, and your existing roof structure. Don't just guess. Check the manufacturer's data sheet. Better yet, talk to a building control officer or a pro installer. A common trick is doing 100mm PIR between the rafters and another 50mm underneath — that kills thermal bridging through the timber, which is a big deal.
Should you insulate between rafters or under rafters in a loft conversion?
Honestly? Both. Doing just between rafters leaves the rafters themselves acting like thermal bridges — cold spots where heat escapes and condensation forms. Not good. The smart way is a two-layer system: PIR boards between the rafters, then a vapour control layer, then another layer of PIR underneath fixed to the rafters, then plasterboard on top. This creates a continuous insulation barrier. No gaps, no bridging. Yeah, it eats a bit more headroom, but it's worth it. You'll meet regs easily and your loft will be properly warm.
Expert insights: Common mistakes to avoid
People screw up the vapour control layer all the time. Seriously. If you skip it or install it wrong, warm moist air from your living space seeps into the insulation, hits the cold roof deck, condenses, and bam — rot and mould. Absolute nightmare. Another one? Compressing mineral wool. Don't. It ruins the thermal performance. Cut your boards accurately so they fit snug but not jammed in. And for god's sake, don't block your eaves vents. That ventilation is crucial, especially in warm roof constructions. Trust me, you don't want to learn this the hard way.
Comparison table: Top insulation materials for loft conversions
| Material | Thermal Conductivity (W/mK) | Typical Thickness for 0.18 U-value | Cost per m² (approx) | Key Advantages | Key Disadvantages |
|---|---|---|---|---|---|
| PIR Rigid Foam Board | 0.022 - 0.025 | 100 - 150mm | £20 - £35 | High performance, space-saving, easy to cut | More expensive than mineral wool |
| Min Wool (Glass/Rock) | 0.032 - 0.044 | 200 - 250mm | £10 - £20 | Cheaper, good soundproofing, fire resistant | Bulkier, needs VCL, can sag over time |
| Spray Foam (Closed Cell) | 0.028 - 0.030 | 100 - 150mm | £40 - £60 | Airtight, fills gaps, good for irregular shapes | Expensive, hard to remove, professional install only |
| Sheep’s Wool | 0.038 - 0.042 | 200 - 250mm | £25 - £40 | Natural, breathable, good acoustic properties | More expensive than mineral wool, can settle |
Checklist: Insulating your loft conversion correctly
- Measure your rafter depth and spacing to figure out the cavity you're working with.
- Pick a material with a lambda value of 0.025 or lower if you want to save space.
- Calculate how thick you need to hit that U-value of 0.18 or better.
- Install a vapour control layer on the warm side — no excuses.
- Make sure your insulation layer is continuous to stop thermal bridging.
- Seal every joint and gap with proper tape or foam.
- If you're doing a cold roof, keep a 50mm air gap above the insulation.
- Talk to a building control officer or approved inspector before you start — cover your arse.
Frequently asked questions
Can I use loft roll insulation for a loft conversion?
Technically yes, but I wouldn't. Loft roll is bulky — you'll need way more thickness to meet regs, which kills headroom. Plus it compresses and sags over time, performance drops. And you still need a separate vapour control layer. PIR boards are just better for this job, plain and simple.
What is the cheapest insulation for a loft conversion?
Mineral wool is cheapest upfront — like £10-20 per m². But you'll lose valuable headroom because it's thicker, and you've gotta buy a vapour control layer on top. So the total cost might be lower, but the space penalty? That's the real price you pay.
Do I need planning permission for loft conversion insulation?
Usually no — it's considered permitted development. But if your house is listed or in a conservation area, you might need consent. Always check with your local planning authority before starting. Better safe than sorry.
How long does loft insulation last in a conversion?
Most stuff lasts 30 to 50 years if installed right and kept dry. PIR and spray foam are tough. Mineral wool can settle or degrade if moisture gets in. That vapour control layer and good ventilation? That's what keeps it going long-term.
Short Summary
- Best material: PIR rigid foam boards offer the highest thermal performance per thickness, making them ideal for space-constrained loft conversions.
- Thickness requirement: Typically 100-150mm of PIR is needed to meet building regulations (U-value of 0.18 W/m²K or lower).
- Installation method: A two-layer system (between and under rafters) eliminates thermal bridging and ensures optimal performance.
- Critical detail: Always install a vapour control layer on the warm side to prevent condensation and mould.
