When exterior access is awkward or you’re working with an older building, internal flat roof insulation can be a pretty sensible move. It’s often one of the few ways to boost energy efficiency without disturbing the exterior. The thing is, there isn’t just one way to go about it. From rigid boards and spray foam to multifoils and mineral wool, each method brings something a little different to the table.
This guide walks you through five flat roofing insulation methods, so you’re better equipped to plan things out, whether you’re doing it yourself or calling in the pros.
Use of Rigid Insulation Boards Between Joists
For those starting out, rigid boards are probably the most straightforward way to insulate between joists. You’ll often see PIR (polyisocyanurate) or phenolic foam boards being used, mostly because they offer solid thermal resistance without taking up too much space.
The boards are cut down to fit tightly between the rafters, and for a bit of extra snugness, gaps are sealed with expanding foam or insulation tape. This helps to prevent air leaks, which matters a lot more than it seems at first. To stop moisture creeping in and causing issues down the line, a vapour control layer usually goes on the warm side of the insulation, just beneath the boards.
It’s worth adding that the airtightness strategy needs to be thought through. A little care here means fewer heat losses and better performance overall. As internal flat roof insulation goes, this is one of the more DIY-friendly options if you’ve got the right tools and a steady hand.
Pros:
- High thermal efficiency per mm
- Easy to source and cut
- Compatible with most ceiling types
Cons:
- Needs precise fitting
- Gaps can reduce effectiveness
- May not suit irregular joist spacing
Applying Multifoil Insulation with Vapour Barriers
Multifoil insulation is, in a way, the flexible option in the internal insulation toolkit. It’s made up of layers of reflective foil, separated by wadding or foam, and it’s usually fixed below the rafters rather than in between. That makes it a decent choice when joist depths are limited or a bit uneven.
What it does well is reflect radiant heat back into the room, which can help keep things warmer with less material depth. Still, thermal bridging can be an issue if it’s not installed properly, especially where the insulation meets the timber.
Here’s a basic overview of how it’s fitted:
- Measure and cut the multifoil to length
- Staple it across the underside of the rafters
- Tape joints with foil tape to maintain continuity
- Fix battens underneath to create an air gap
- Finish with plasterboard
For UK homes and commercial spaces, performance can vary depending on climate and roof build-up, so it’s always worth checking compatibility. It’s fairly lightweight, and when paired with a good vapour barrier, it handles condensation pretty well.
Installing Spray Foam Insulation
Spray foam might be the closest you’ll get to a “fit-and-forget” insulation option—but it’s definitely not a DIY task. When done right, it expands to fill every nook between the rafters, creating a more or less seamless barrier. You’ve got two main types to consider: open-cell, which is softer and allows some breathability, and closed-cell, which is much denser and packs a higher R-value per inch.
A lot of people like spray foam flat roof insulation for its air-tight seal. It’s particularly helpful in properties where draughts or uneven cavity shapes make traditional materials a bit awkward. That said, it’s more expensive than most other internal insulation methods and must be installed by licensed professionals.
Things to check before choosing spray foam:
- Does your flat roof need extra ventilation?
- Is the product compliant with fire safety ratings?
- Have you checked with your local authority or mortgage provider?
Because of how it bonds to the surface, reversing the process isn’t easy. So if you’re leaning this way, a chat with a roofing specialist is more or less essential.
Adding Mineral Wool Between Rafters
Mineral wool insulation—whether it’s glass or stone-based—is a bit of an all-rounder. It’s soft, easy to work with, and pretty forgiving if your joist spacing isn’t exact. For older buildings with less-than-perfect cavities, it often makes life a little simpler. Plus, there’s the added bonus of sound dampening, which you might appreciate in busier commercial settings.
Fitting it involves pushing semi-rigid slabs or rolls snugly between the rafters. Unlike rigid boards, mineral wool can be gently compressed to fill corners and edges, so it helps cut down on air leaks, too. Still, it doesn’t act as a vapour barrier on its own, so you’ll want to add one to prevent condensation issues.
From a safety point of view, this method has one of the best fire resistance ratings of any flat roof insulation method. That’s definitely worth keeping in mind for properties with strict compliance needs.
Compared to spray foam or PIR, the U-value might not be quite as strong for the same depth, but it’s a fairly cost-effective and eco-friendly choice.
Using Insulated Plasterboard on the Ceiling Underside
If you’re after a method that finishes things off in one go, insulated plasterboard for flat roofs might be the way to go. It’s basically plasterboard backed with a layer of insulation—usually PIR or similar. Rather than insulating between joists, you’re fixing it straight to the ceiling’s underside.
It’s often used in retrofits where time and disruption need to be kept low. Once installed, you’re pretty much ready to skim and decorate. That said, you’ll want to ensure you’ve chosen the right thickness to hit the target U-value. Fixing methods vary—some boards go up with adhesive, others are mechanically fixed.
Here’s a quick comparison to help weigh things up:
| Material Type | Approx Depth (mm) | U-value (W/m²K) | Relative Weight |
| PIR-backed plasterboard | 72.5 | 0.24 | Medium |
| EPS-backed board | 93 | 0.30 | Light |
| Phenolic-backed board | 63.5 | 0.18 | Medium-Heavy |
It’s tidy, quick, and more or less fuss-free, as long as the ceiling can take the load.
Figuring out how to insulate a flat roof from the inside isn’t always simple, but it’s far from impossible. Whether you’re looking at insulating between joists or trying something like spray foam flat roof solutions, planning early with an experienced roofer can save a lot of guesswork later on. If you’re weighing up your options or need a second opinion, feel free to contact us for straightforward advice.
