The Great Insulation Debate: Which material is best?

Let’s talk about your heating bill. Specifically, let’s talk about that moment—usually around late January—when you open that bill, look at the number, and feel a small part of your soul die.

You live in Ireland. You expect wind, you expect rain, you expect four seasons in one afternoon. But what you don’t expect is to pay a sum equivalent to the GDP of a small island nation just to keep your toes from turning blue. You’ve got the heating on, you’re wearing three jumpers, and you can still feel a tiny, icy draught on the back of your neck.

Your house, my friend, is a thermal sieve. You’re basically just heating the outdoors, and your energy company is sending you a very expensive ‘thank you’ note for it each month.

The problem isn’t your boiler. It’s your container. You’re trying to keep soup warm in a colander. And the thing that’s supposed to be plugging the holes in that colander is your insulation.

Which brings us to one of the most boring, least-sexy, and most profoundly important debates in all of homeownership: The Great Insulation Battle.

If you’ve ever looked into this, you’ve probably fallen down a rabbit hole of forums where builders are virtually screaming at each other about R-values, vapour barriers, and something called “hygroscopicity.” It’s a world of jargon, angry opinions, and surprisingly passionate arguments about… fluff.

The main event is a three-way (or, technically, four-way) title fight between:

• Fiberglass (aka Glass Mineral Wool): The classic, fluffy pink stuff you see in movies. The Granddaddy of insulation.
• Cellulose: The eco-friendly hipster, made of recycled newspaper.
• Spray Foam: The high-tech, expensive wizard that comes in two flavours: Open-Cell (squishy) and Closed-Cell (rigid).

So… which one is the best? “Objectively” the best?

Well, that’s the wrong question. It’s like asking, “What’s the best vehicle: a Formula 1 car, a tractor, or a minivan?” The answer, obviously, is “it depends.” Are you trying to win the Monaco Grand Prix, plough a field, or drive three screaming children to hurling practice?

To end this debate once and for all, we need to stop asking “which is best?” and start asking “which is best at what?”

Welcome to the Insulation Olympics. Let’s meet the athletes.

First, A Quick PSA: You’re Thinking About Heat All Wrong

Before we judge the contenders, we need to understand the game. You think your house gets cold because “cold gets in.” This is not true. Your house gets cold because heat gets out.

Heat is a hyperactive, sugar-fuelled toddler. It wants to escape. It’s constantly looking for a way out, and it will find one. Heat moves in three ways:

1. Conduction: Heat moving through a solid object. Like holding a hot spoon. The heat’s atoms get all jiggy and pass that jigginess down the line.
2. Radiation: Heat moving as an electromagnetic wave. This is the heat you feel from a bonfire (or the sun) without touching it.
3. Convection: Heat moving via a liquid or gas (in this case, air). This is the big one. This is the supervillain of your story.

When you buy insulation, you’re mostly paying for a number called an “R-value.” That number only measures Conduction. It’s a measure of how good the material is at resisting that “jiggy” heat transfer.

Here’s the dirty secret: The R-value on the packet is a lie.

Okay, it’s not a lie, it’s just a “lab-perfect-scenario” number that has almost nothing to do with real life. Why? Because of Convection. Because of Air Leakage.

A draught. A gap. A tiny, unsealed crack around a pipe. That’s convection. That’s air, carrying all your precious, expensive heat, physically leaving the building. An R-20 wall with a 1% gap in the insulation doesn’t perform like an R-19.8 wall. Its actual performance can drop by as much as 50%.

Having high-R-value insulation in a draughty house is like buying a €5,000 high-security-bank-vault front door… and leaving the window open. The real battle for a warm home isn’t just about stopping conduction, it’s about stopping convection. It’s about air-tightness.

This one simple concept—that air leakage is the real enemy—completely changes the game. Now, let’s judge our athletes.

Meet the Contenders

Athlete 1: Fiberglass (Glass Mineral Wool)

This is the insulation you know. It’s made from… glass. Melted down and spun into fluffy, fibrous rolls (called “batts”)

• The Analogy: It’s a big, itchy, woolly jumper.
• How it works: It traps millions of tiny air pockets in its fibres. Heat tries to conduct through the glass (which is a poor conductor) and, more importantly, it can’t convect easily within the fluffy matrix.
• The Vibe: Cheap, reliable, old-school. The “tried and true” option your dad would recommend.

Athlete 2: Cellulose

This is the eco-champion. It’s made from up to 80% recycled newspaper. The paper is shredded into a fine, fibrous fluff and treated with non-toxic salts (usually borates).

• The Analogy: It’s like your house is being insulated by a million tiny librarians who are very, very good at shushing.
• How it works: It’s blown into cavities (like attics or walls) as a dense, fluffy mat.  Like fiberglass, it traps air. But because it’s “dense-packed,” it’s also inherently air-resistant.
• The Vibe: Sustainable, clever, a bit hipster, and smells faintly of old books (not really).

Athlete 3 & 4: Spray Foam (The Tech Bros)

This is the new kid. It’s a two-part chemical (a polyol resin and an isocyanate, for the nerds) that gets sprayed as a liquid, hits the air, and expands 100-fold in seconds, filling every single crack and crevice.  It comes in two distinct flavours:

Athlete 3: Open-Cell Spray Foam (OCSPF)

• The Analogy: A giant, sticky, expanding marshmallow. Or a sponge.
• How it works: It’s soft and squishy. The “cells” (bubbles) in the foam are intentionally left open.  It’s full of air, which insulates. But its real superpower is that it expands to create a perfect, monolithic air barrier.  It stops convection dead.
• The Vibe: The friendly, flexible tech bro. Amazing at air-sealing and soundproofing.

Athlete 4: Closed-Cell Spray Foam (CCSPF)

• The Analogy: A super-strong, rigid, expanding rock.
• How it works: It’s hard and dense. The “cells” are closed and filled with a special gas (a blowing agent) that is a way worse conductor of heat than air. It’s also a perfect air barrier 13, and it’s so rigid it can actually add structural strength to your walls.
• The Vibe: The intense, high-performance tech bro. A bit more expensive, but the numbers it puts up are insane.

Alright. The athletes are on the field. Let the games begin.

Event 1: The “Don’t Let The Heat Out” Challenge (Thermal & Air-Tightness)

This event has two parts. First, the “Lab Test” (R-value). Second, the “Real-World Test” (Air-Tightness).

Part 1: The “Lab Test” (R-Value per Inch)

Here, we just measure the R-value (or its nerdy cousin, Lambda) of one inch of material.

• Fiberglass: Comes in at about R-3.2 to R-4.3 per inch. Not bad.
• Cellulose: Almost identical. R-3.6 to R-3.8 per inch.
• Open-Cell Foam: Again, right in the same ballpark. R-3.7 per inch.

As you can see, this is… boring. Fiberglass, Cellulose, and Open-Cell Foam are in a statistical dead heat. Their ability to stop conduction is functionally identical.

And then there’s…

• Closed-Cell Foam: Sits down, cracks its knuckles, and posts a score of R-6.7 to R-7.0 per inch.

Winner: Closed-Cell Spray Foam, by a country mile. It’s not even a contest. If you have a tiny space (like a dormer roof) and need the absolute maximum insulating power per inch, CCSPF is the undisputed champion.

But wait! This is just the lab test. Now for the real test.

Part 2: The “Real-World Test” (Air-Tightness)

This is where we test how well the as-installed system stops our villain, Convection (air leakage).

• Fiberglass:…has a catastrophic failure. Fiberglass batts are notoriously difficult to install perfectly. You have to cut them perfectly around every pipe, wire, and socket. Every gap, every compression, every little void becomes a thermal highway for air. Even worse, the material itself does nothing to stop air moving through it. It’s an insulator, not an air barrier. Its real-world performance is 100% dependent on a separate air-tight membrane being installed perfectly. Which it almost never is.
• Cellulose: Performs brilliantly. Because it’s “dense-packed” (blown in under pressure), it fills every single void by default. No gaps, no cracks. The material itself is also air-resistant, meaning it dramatically slows down convective loops within the wall cavity. It’s not a true air barrier, but it’s damn close.
• Open-Cell & Closed-Cell Foam: This is their superpower. They are applied as a liquid and expand, adhering to everything and filling every single gap.  They don’t just resist air, they are the air barrier. They stop convection. Full stop. An R-20 foam wall actually performs like an R-20 wall.

Winner: Spray Foam (both types), with Cellulose as a very strong runner-up.

Event 2: The “Don’t Burst Into Flames” Challenge (Fire Safety)

This is… an important one. We measure this using the “Euroclass” system, which rates materials from A1 (non-combustible) down to F (highly flammable).

• Fiberglass: Gets an A1 (Non-combustible).   It’s glass. You can’t set glass on fire. It’s the safest material on this list, full stop.
• Cellulose: Is made of… paper. Which is, you know, famously flammable. However, it is heavily treated with non-toxic borate salts, which are a powerful fire retardant.   It’s designed to char and inhibit smouldering.   It typically gets a Class E or C.37 So, it’s combustible, but it’s treated to be safe.
• Spray Foam (Both):** Is… plastic. (Polyurethane). Also famously flammable. Like cellulose, it’s treated with fire retardants. The foam itself gets a Class E.37

This looks like a clear win for Fiberglass. But, again, there’s a “real-world” twist. Building regulations don’t just care about the material; they care about the system.

When you put spray foam behind a sheet of plasterboard (which you always do), the entire wall system achieves an excellent fire rating, often Euroclass B.41 This is very, very good. An intumescent paint can also be used to achieve high ratings.

So, here’s the takeaway: Fiberglass is inherently non-combustible. Its safety isn’t dependent on anything. Spray Foam and Cellulose are combustible materials that are made safe by (a) chemical treatment and (b) being installed correctly as part of a system (i.e., behind plasterboard).

Winner: Fiberglass, for its pure, non-negotiable A1 safety.

Event 3: The “Can You Please Be Quiet” Challenge (Acoustics)

Your insulation isn’t just for heat; it’s also for blocking the sound of your neighbour’s terrible music. This is measured by STC (Sound Transmission Class). Higher is better.

• Closed-Cell Foam (CCSPF): Is terrible at this. It’s rigid and dense, which is great for R-value but awful for sound. It can actually transmit vibrations and noise more easily. It gets a relatively low STC of 37-39 in a standard wall.
• Fiberglass: Is a great sound absorber. That fluffy, fibrous nature is great at soaking up sound waves. It gives you an STC of 39 in the same wall.
• Open-Cell Foam (OCSPF): Is excellent. Its soft, squishy, sponge-like structure is a world-class sound absorber.  It dampens sound and vibration beautifully. This is what you put in a home cinema.
• Cellulose: Is the surprise champion. It’s a good sound absorber, but more importantly, it’s a fantastic sound blocker. Because it’s so dense (65kg/m³ when packed in walls! 19), it adds sheer mass to the wall. Mass is the enemy of sound. Field tests consistently show cellulose outperforming fiberglass in wall assemblies.

Winner: A photo-finish tie between Cellulose (for blocking) and Open-Cell Foam (for absorbing).

Event 4: The “Don’t Get My House All Damp” Challenge (Moisture)

This is the most complicated, most important, and most argued-about event. Get this wrong, and you don’t just get a cold house; you get a mouldy, rotten house.

Your house needs to breathe. Not literally, but it needs to manage water vapour (the “sweat” from cooking, showering, and just… existing). This is called “Building Physics,” and there are two competing religions.

Religion 1: The “Breathable” Wall (Vapour Open)

This philosophy says: “Moisture will get into your wall, so you must let it get back out.” It’s like wearing a Gore-Tex jacket. It’s designed to be “vapour-permeable.”

• The Athletes: Fiberglass 31, Cellulose 19, and Open-Cell Foam 11 are all “vapour open.” They will let water vapour pass through them.
• How it works: You pair them with a Vapour Control Layer (VCL) on the inside (warm side) to slow down the vapour, and a breathable membrane on the outside (cold side) to let anything that gets in escape.
• Cellulose’s Special Skill: Cellulose is also hygroscopic.  This is a magic word. It means it can actively absorb and store moisture from the air (like a sponge) and then release it when the air is drier. It acts as a “moisture buffer,” actively protecting your timber structure from damp. This makes it a phenomenal choice for older Irish homes or timber-frame builds.

Religion 2: The “Sealed Box” (Vapour Barrier)

This philosophy says: “Moisture is the enemy. Don’t let it in. Ever.” It’s like wearing a plastic bin bag. Nothing gets in, but… nothing gets out.

• The Athlete: Closed-Cell Foam is a vapour barrier. It is not breathable. It seals the wall completely.
• How it works: It creates a perfect, impermeable barrier. No moisture from your warm, humid house can get into the cold wall cavity to condense.
• The Risk: This system is incredibly effective, but it is intolerant. If water does get in (from, say, a leak in the roof or a tiny crack in the external render), it is trapped. It cannot dry out. This can be a disaster in an old stone or timber house. In a high-performance modern build, it’s brutally efficient.

Winner: It’s a tie, depending on your religion. Cellulose wins the “safe and breathable” award. Closed-Cell Foam wins the “high-performance but high-risk” award.

Event 5: The “Don’t Destroy The Planet” Challenge (Embodied Carbon)

Your insulation will save carbon every day for 50 years by reducing your heating. That’s its job. But what’s the upfront carbon cost of manufacturing it? This is its “Embodied Carbon” or “Global Warming Potential (GWP),” and we find it by reading a product’s Environmental Product Declaration (EPD).

• Fiberglass: It’s… okay. It’s often made of up to 80% recycled glass 59, which is great. But… you have to melt glass in a giant furnace, which is incredibly energy-intensive. Its GWP is… medium.
• Cellulose: Is the undisputed King of Green. It’s made from recycled newspaper 3, which is a waste product. The manufacturing process just involves shredding and adding salts. It has the lowest embodied carbon of the group, by a massive margin. In fact, because the paper has stored carbon from the tree it came from, it’s often considered carbon-negative.

And now… the big one. Spray Foam. This is where the entire story gets a new final chapter.

• The Past: Old Spray Foam (HFCs): Traditionally, Closed-Cell foam used “blowing agents” called HFCs (hydrofluorocarbons). These gases were disastrous for the environment, with a Global Warming Potential over 1,000 times worse than $text{CO}_2$. This made old CCSPF an environmental nightmare.
• The Present: New Spray Foam (HFOs & Water):** This has been completely solved.
  • Open-Cell Foam** has always been green. It’s “water-blown.” The chemical reaction itself createst{CO2}_, which has a GWP of… 1. This makes its carbon footprint incredibly low.
  • Closed-Cell Foam** has now switched to new, 4th-gen blowing agents called HFOs (hydrofluoroolefins). These new HFOs have a GWP of… 1. Or less.   This one chemical switch erases CCSPF’s single biggest drawback.

Winner: Cellulose is the ultimate eco-champion. But modern Open-Cell (water-blown) and Closed-Cell (HFO-blown) foams are now excellent, low-carbon, sustainable choices. The “foam is bad for the planet” argument is officially dead, as long as you’re using the new stuff.

So, Which One Do I Actually Choose?

I know. That was a lot. Your brain is probably full. You just want to know what to put in your attic.

The biggest takeaway is this: the real value is in a whole-home strategy. Before you even think about installing Solar Panels in Dublin, you have to plug the leaks. It’s the most basic, high-impact first step in any proper home energy upgrade.  And for most Irish homes, the single biggest leak is the one right above your head. Getting your attic insulation sorted is the most cost-effective, high-return investment you can make.

And when you’re comparing that job, you’ll probably be looking at a lot of BER rating calculations.  But now you know that the “best” material isn’t just about the R-value. It’s about a whole matrix of decisions.

So, here it is. The Procrastinator’s Final Guide:

You should choose… FIBERGLASS if:

• Your number one, non-negotiable priority is fire safety. That Euroclass A1 rating is an objective, absolute fact.
• You are on a very tight budget and are comfortable with the massive performance drop-off that comes from a “good-enough” install.
• You are building something (like a hospital or high-rise) where non-combustible materials are required by law.

You should choose… CELLULOSE if:

• Your number one priority is sustainability. You are the undisputed eco-champion. It’s carbon-negative and made of recycled paper.
• You are retrofitting an older building or a timber-frame house. Its “hygroscopic” (moisture-buffering) nature is a massive, unique advantage that protects the structure.
• You want excellent sound-proofing (especially blocking sound).
• You want a “breathable” wall assembly.

You should choose… OPEN-CELL SPRAY FOAM (OCSPF) if:

• Your number one priority is a perfect, guaranteed air seal in a “breathable” (vapour-open) wall.
• You need world-class acoustic absorption. It’s the best on the market for media rooms or silencing noisy plumbing.
• You have a complex roof or wall with lots of nooks, crannies, and “hard-to-reach” spots.
• You want a low-carbon, water-blown product.

You should choose… CLOSED-CELL SPRAY FOAM (CCSPF) if:

• Your number one priority is maximum R-value in minimum space. It has double the insulating power per inch. It is the only choice for thin rafters or shallow wall cavities where you still need to meet high B2-rating targets.
• You need a vapour barrier and an air barrier and an insulator all in one product.
• You’re building a “sealed box” high-performance or Passive House and are confident in your design.
• You’re using the new HFO version 64, which makes it a fantastic low-carbon choice.

The “Great Debate” is over. There’s no single winner, just four different tools for four different jobs. The only loser is the person who sticks with their old, leaky, sieve-like house and does nothing at all.

And one last thing. For three of these (Cellulose and the Foams), the performance is 100% dependent on the installer. These are not DIY products; they are factory-grade systems that are “manufactured” in your attic by a trained professional. In Ireland, this is all governed by the NSAI Agrément certification scheme, which requires that the product is installed by a certified, approved installer to be compliant. Your choice of contractor is literally more important than your choice of material.

If all this analysis just made your brain melt and you’d rather have an expert just… handle it, you can talk to someone who does this all day for a living about your attic insulation in Dublin.