Hey there, if you’re reading this, you’re probably knee-deep in a wall insulation project—whether it’s a new build, a retro fit, or you just decided your heating bill this winter looked like a typo. I’ve been in the insulation board game for over a decade now, and trust me, the number of questions I get is wild: “Which is best? Will this make my place soundproof too? Can I even install it myself?”, etc. I don’t just sell boards, I’ve helped homeowners, small builders, and even a few confused plumbers pick the right stuff. So let’s cut through the noise—no fancy jargon, no overpromises, just the real deal on which insulation board works for walls. Insulation Board
First off, let’s get one thing straight: there’s no “one size fits all” here. What’s perfect for a 1950s terrace in Manchester? Total garbage for a modern extension in Scotland. It all depends on three big things: where you’re putting it (internal walls, external walls, cavity walls?), your climate, your budget, and whether you care about extra stuff like sound or fire safety. I’ve seen guys grab the cheapest board they can find, throw it up, and then regret it when their house turns into a sauna in summer or a fridge in winter. Don’t be that guy.
Let’s start with the most common types we supply, because these are the ones people actually ask about. First up, PIR insulation boards—polyisocyanurate, for anyone who cares the full name. You’ve probably heard of these, and for good reason. They’re super thin for their insulation value—like, way thinner than fiberglass or mineral wool. That’s a game-changer if you’re retro fitting an old wall where every inch of space counts. I had a customer last year with a tiny London terrace, his cavity walls were already maxed out with old fiberglass, so switching to 60mm PIR for the external wall finish gave him the same R-value as 100mm of regular mineral wool. His heating bill dropped by 18% in six months, he texted me a pic of his energy bill and everything—swears by it.
But hold up, PIR isn’t perfect. It’s a plastic-based product, right? So some people worry about off-gassing, especially when it’s new. I always tell customers to go for low-VOC versions if that’s a concern—we stock those, no extra hassle. Also, it’s not the best if you need serious soundproofing. If your wall is between a kids’ bedroom and a living room, PIR alone won’t stop a TV or a kid screaming at Fortnite. More on that later.
Next, there’s mineral wool insulation boards—sometimes called rock wool or slag wool, depending on if it’s made from molten rock or steel slag. This is the OG insulation, been around for decades, so it’s reliable. The big plus here is it’s non-combustible—perfect if you have a kitchen wall or a room with a log burner, where fire risk is higher. It also sucks at sound transfer, way better than PIR. I did a job for a band practice room once, we used 150mm mineral wool boards between the stud walls, and a customer said you could barely hear the bass from the next corridor. That said, it’s bulkier. So if you have narrow stud walls or limited space, mineral wool might not be the best fit—you might have to build out the wall a few inches, which isn’t ideal for small rooms. Also, it can hold moisture if it gets wet, so you have to make sure the wall is properly ventilated, no leaks. I once had a guy who had a leaking pipe behind his wall, the mineral wool got soggy and had to be replaced—lesson learned, always check for damp first.
Third, there’s expanded polystyrene (EPS) insulation boards—styrofoam, basically, but way denser than the stuff you get in a takeaway. Super light, super cheap, which is why it’s a go-to for big commercial projects or new builds where budget is tight. It’s water-resistant too, so no problem if there’s minor damp, and it’s mold-resistant, which is a big plus for people in humid climates. The downside? It’s flammable, and it gives off toxic fumes when it burns. Also, its insulation value is lower than PIR, so you need thicker boards to get the same performance. Like, 100mm EPS is only as good as 50mm PIR. So if space is tight, EPS might not cut it. I usually recommend EPS for cavity walls where there’s plenty of space, not for internal stud walls where every inch counts.
Wait, there’s also extruded polystyrene (XPS), which is a step up from EPS. It’s denser, more water-resistant, and has a higher R-value than EPS. But it’s more expensive, so it’s usually for basements or foundations, not regular walls. I don’t stock as much XPS because most people don’t need it for standard walls, but it’s good to know what’s out there.
Now, let’s talk about where you’re installing the wall, because that changes everything. Internal walls vs external walls vs cavity walls—three totally different scenarios.
If it’s an external wall—this is the big one for heat loss, right? Up to 35% of your home’s heat escapes through external walls, according to the Energy Saving Trust. So for external walls, you want something with high insulation value, so you don’t have to build a thick wall. PIR is my top pick here, because of that high R-value per inch. But if you’re in a super cold area, like up in Scotland, and you want extra fire safety, mineral wool works too, just go thicker. Just make sure whatever you use is compatible with the wall finish—if you’re doing a solid wall (no cavity), you might need to attach the board with adhesive, not just screws, to make it airtight. Airtightness is huge, by the way—if there are gaps around the boards, all that insulation is wasted. I always tell customers to seal the joints with foil tape for PIR or specialized tape for mineral wool, no exceptions.
Cavity walls—these are walls with a gap between the inner and outer brick layer, super common in homes built after the 1920s. For cavity walls, EPS is often the go-to, because it’s light, easy to fit, and doesn’t absorb water. But wait, don’t just pour it into the cavity—you have to use rigid boards that fit snugly, no gaps. If you leave gaps, you get thermal bridging, which is like having a hole in your insulation. PIR works here too, just make sure it’s rated for cavity use, some PIR boards are only for external applications. And if you want soundproofing, mineral wool is the way to go for cavity walls between rooms.
Internal walls—these are less about heat loss and more about sound and comfort. If it’s a wall between two bedrooms, you want sound insulation first, so mineral wool is your guy. If it’s an internal wall in a new build, where you want to keep the room size as big as possible, PIR is better because it’s thin. I had a customer who built a home office in his garage, the internal wall between the office and the house, he used 50mm PIR and now he can work from home without hearing his partner watch TV in the living room. Win-win.
Now, a few myths I hear all the time, let’s bust them. First, “thicker is always better”. Not necessarily. If you have a tiny wall, a thin high-R-value board is way better than a thick low-R-value one. I’ve seen a customer who tried to put 150mm EPS in a stud wall that was only 100mm deep—total disaster, he had to tear it all out and start over with PIR. Second, “all insulation boards are the same price”. No way. PIR is more expensive than EPS, mineral wool is in the middle. But don’t just go for the cheapest—calculate the long-term savings. A £500 EPS job might end up costing you more in heating bills than a £700 PIR job over 10 years. Third, “you can install any of these yourself”. PIR is pretty easy, just cut it with a handsaw, seal the joints. Mineral wool can be a pain because it’s itchy, so you need gloves and a mask. EPS is super light, easy to cut. But if you’re not comfortable with DIY, hire a pro—bad installation is worse than no insulation. I’ve seen guys who fit the boards with huge gaps, and it’s basically useless.
Wait, what about fire? That’s a big one for kitchens and bedrooms. Mineral wool is non-combustible, rated A1, which is the highest fire safety rating. PIR is usually B or C, so it’s combustible, just not highly flammable. EPS is usually E, so more flammable. So if you’re insulating a wall next to a kitchen, where there’s a stove and open flames, mineral wool is the safer bet. If it’s a bedroom, check your local building codes—most places require fire-resistant materials near ceilings or between rooms, so mineral wool might be mandatory there.
Moisture is another big one, especially in older homes. If you have a damp wall, don’t just slap insulation on it. Fix the damp first—sort out the leaking pipe, install a vapor barrier if needed. Mineral wool absorbs water, so if the wall is damp, it will hold that moisture, grow mold, and ruin the board. EPS and PIR are water-resistant, so they’re better for damp walls, but still, fix the root cause first. I had a customer who tried to insulate a wall with a hidden leak, the mineral wool got moldy, he had to remove it, fix the pipe, and re-insulate. Cost him twice as much, so trust me, don’t skip the damp check.
Now, let’s get to the stuff you actually need to ask before buying. First, what type of wall is it? Internal, external, cavity, solid? Second, what’s your priority? Heat insulation, sound insulation, fire safety, budget? Third, how much space do you have? The depth of the wall will determine how thick a board you can use. Fourth, do you have any specific needs, like low VOC for a nursery, or fire safety for a kitchen?
I’ve been in this business long enough that I can tell you, most people overthink this. If you’re doing a standard external wall in a moderate climate, PIR is almost always the best balance of cost, performance, and space. If you need sound or fire safety, mineral wool. If you’re on a super tight budget and have plenty of space, EPS. But always, always, double check the R-value rating. That’s the number that tells you how well it insulates—higher is better. For walls, you want an R-value of at least 2.0 per inch, ideally higher. PIR is usually around 3.5-4.0 per inch, mineral wool is around 2.5-3.0, EPS is around 2.0. That’s a rough guide, but it’s close enough.
And here’s the thing, I don’t just sell boards. If you send me the details of your project—wall type, size, climate, what you care about—I can point you to the exact board you need, no upsells, no hidden fees. I’ve helped people with tiny London flats and big country homes, I’ve never pushed a product that wasn’t right for their job.
At the end of the day, the best insulation board for your walls is the one that fits your specific needs, not the one with the flashiest ad or the lowest price. Don’t make the mistake I see so many people make: rushing the decision, cutting corners, and ending up with a wall that’s either too thin, too flammable, or full of gaps. Take your time, ask questions, and pick something that works for your home.
If you’re ready to grab materials for your project, or you just have more questions about which board is right for you, get in touch. I’m happy to walk through your project step by step, no pressure, just honest advice.
Insulation Board References:
- Energy Saving Trust. (2024). Heat loss through walls. Retrieved from Energy Saving Trust database
- National Insulation Association. (2023). Rigid insulation board performance guide. National Insulation Association Technical Bulletin
- UK Building Regulations Approved Document Part L. (2022). Conservation of fuel and power. UK Government Building Regulations
- The Association of PIR Insulation Manufacturers. (2023). Best practice for rigid polyisocyanurate insulation installation. APIM Technical Report
Wenzhou Gisen Insulation Material Co., Ltd.
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