Insulation advice is often written for ideal conditions. Perfect cavities, straight timbers, no damp history, and unlimited depth everywhere you need it. Real buildings rarely cooperate. Renovations come with odd junctions, shallow voids, bridged lintels, legacy materials, and the kind of surprises that don’t show up until you lift a floorboard. If you spec insulation as if you’re working on a clean, new-build shell, you can end up spending more and achieving less.
That’s why the best starting point is not “what insulation is best”, but “what can this building realistically take”. A quick conversation with experienced trade building merchants can save hours, because they’ll often flag availability, thickness constraints, compatible tapes and membranes, and what tends to work on typical UK housing stock. The aim is to choose a build-up that meets performance targets without creating moisture problems or workmanship traps.
This piece is a practical framework you can apply whether you’re topping up a loft, insulating a suspended floor, or upgrading walls in an older property.
Start With the Building, Not the Product
Before you pick PIR, mineral wool, wood fibre, EPS, or anything else, define four things:
The construction type
The moisture and ventilation reality
The space you have to work with
The performance target you’re trying to hit
If one of those is misunderstood, the insulation choice becomes guesswork.
Construction type changes everything
Loft insulation is not the same problem as internal wall insulation. A solid wall terrace behaves differently to a cavity wall semi. A suspended timber floor has different risks to a solid slab. Get the construction type clear first, then work out what the insulation needs to do.
Loft Insulation: Simple, But Don’t Ignore Airtightness
Lofts are usually the easiest win. But even here, the “just add more” approach has limits.
Mineral wool vs PIR in lofts
Mineral wool is forgiving. It’s easy to fit around irregularities, it’s non-combustible, and it’s typically cost effective. PIR can be useful where depth is limited, but it demands tighter workmanship and careful cutting to avoid gaps.
The real performance gain often comes from airtightness at ceiling level. If the loft hatch leaks, downlights aren’t sealed, or service penetrations are left open, you can add insulation and still feel draughts.
Practical spec notes:
Maintain loft ventilation at eaves
Avoid blocking airflow paths
Seal obvious penetrations before insulating
Consider raising boarding rather than compressing insulation
Cavity Walls: Check What’s There Before You Add Anything
If the building has cavity walls, confirm:
Whether they’re already filled
Whether the cavity is suitable for fill
Whether there’s exposure or driving rain risk
Whether there are damp issues that need solving first
Filling a cavity that isn’t suitable can make moisture problems worse. In many cases, the right move is to address water ingress, pointing, gutters, and ventilation before you touch insulation.
Trade building merchants can often point you toward local installers and the common red flags they see in your area, but the decision still needs to be based on the building’s condition, not just the product claim.
Solid Walls: Breathability and Condensation Risk
Solid walls are where brochure thinking really fails. Old properties often manage moisture by being able to dry out. If you add an internal system that traps moisture, you can create condensation and mould even if the room looks warm.
Internal wall insulation choices
PIR backed plasterboard systems can deliver good thermal performance in limited thickness, but they require careful detailing at junctions and a sound vapour strategy. Wood fibre and other hygroscopic systems can be more forgiving in certain traditional buildings, but they often need more thickness to achieve the same U-value.
Key risks to manage:
Thermal bridging at floors and internal walls
Cold spots behind furniture and in corners
Interrupted vapour layers at sockets and services
Moisture trapped against cold masonry
If you’re not confident on the moisture behaviour, you can still move forward by choosing systems with clear guidance, compatible components, and proven detailing. This is another area where trade building merchants are useful, because they’ll often stock the tapes, seals, and ancillary bits that make the difference between “installed” and “working”.
Floors: Suspended Timber vs Solid Slab
Suspended timber floors
With suspended floors, you’re insulating a ventilated void. The job is to reduce heat loss while preserving the underfloor ventilation that keeps timbers healthy.
Common options:
Mineral wool supported between joists
PIR between joists where depth is tight
Insulated boards from below where access allows
Practical spec notes:
Keep air bricks clear
Use netting or battens to support wool
Avoid compressing insulation
Consider an airtight layer above or below to reduce draughts
A lot of comfort issues in older homes come from air movement, not just lack of insulation. If the floor is draughty, improving airtightness can be as important as adding insulation.
Solid floors
With slabs, you’re often working upward. That can affect door thresholds, skirting heights, and stairs. Insulated overlay systems exist, but the detail at edges matters to avoid cold bridging.
Roofs and Rafters: Don’t Create a Condensation Trap
Warm roof build-ups and rafter insulation require a clear approach to vapour control and ventilation. If you add insulation without respecting the roof’s drying path, you risk rot.
Broadly:
Ventilated cold roof: keep ventilation paths open
Warm roof or sealed build-up: vapour control and airtightness become critical
This is where “swap one insulation for another” can quietly break the design. Different products have different vapour resistance and installation tolerances. Make sure the build-up is still coherent, not just thicker.
How to Compare Insulation Products Without Getting Lost
You don’t need to compare everything. Focus on:
Lambda value (thermal conductivity)
Reaction to fire classification
Compressive strength where load matters
Moisture behaviour and vapour resistance
Thickness needed for target U-value
Workmanship tolerance and gap sensitivity
A slightly “worse” lambda product that installs cleanly and consistently often outperforms a “better” one that ends up with gaps, compression, or poor sealing.
A Spec Starter Pack That Works in the Real World
When you’re putting together a workable spec, aim to include:
The insulation product and thickness
How it’s fixed or supported
The airtightness layer approach (if relevant)
The vapour control approach (if relevant)
Compatible tapes and sealants
Key junction details (edges, penetrations, transitions)
This is the bit people forget. Insulation is not just boards or rolls. It’s the system around it that stops air leaks and moisture problems.
If you keep buying from the same trade building merchants, ask them to help you build repeatable “packs” for common jobs: loft top-up, suspended floor, internal wall upgrade. Consistency reduces mistakes, and mistakes in insulation are hard to see until the building is lived in.
The Bottom Line
The best insulation spec is the one that suits the building you actually have, the space you can realistically insulate, and the moisture and ventilation conditions you’re working with. Brochure specs assume perfect geometry and perfect installation. Real jobs need forgiving details, compatible components, and a clear plan for air and vapour control.
Choose the build-up first, then the product. That’s how you get predictable performance without creating new problems you’ll be asked to fix later.
