The Ultimate Guide to Eco-Friendly Attic Insulation

Here is the thing about houses: they are essentially just large, expensive boxes that we trap ourselves inside to avoid the weather.

For most of human history, the “box” strategy was pretty basic. You built walls thick enough that the wind wouldn’t knock them over, and you lit a fire in the middle. If you were cold, you moved closer to the fire. If you were hot, you moved away. It was a simpler time. It was also a time when people regularly froze to death or died of smoke inhalation, but we don’t need to dwell on that.

Today, we have decided that the Box Strategy needs an upgrade. We want the box to be exactly 21°C, all year round, regardless of whether the outside world is a blistering heatwave or a damp, miserable Tuesday in November. To achieve this, we pump energy into the box (heating) or pump energy out of it (cooling).

But there is a problem. The universe hates your 21°C box. The Second Law of Thermodynamics is basically a cosmic bully that wants to steal your heat and spread it around the universe until everything is a uniform, lukewarm soup. This is where your home energy upgrades come into play.

This brings us to the unsung hero of the modern world. It’s not the solar panel (the celebrity). It’s not the heat pump (the expensive new gadget). It is the fluff. The stuff in your walls and attic. Insulation.

The Physics of the Fluff

Before we decide what to put in your attic, we need to understand why we are putting anything there at all. If you look at a house through a thermal camera, an uninsulated home looks like it’s bleeding heat. It’s a crime scene of energy waste.

Heat travels in three ways, and insulation is the bouncer that stops two of them:

1. Conduction: This is heat moving through solid stuff. Like when you touch a hot pan and regret it immediately. In a house, heat conducts through the wood and plaster of your ceiling.

2. Convection: This is heat moving through air. Hot air rises. In your attic, the warm air from your living room tries to escape up through the roof.

3. Radiation: This is heat travelling as waves. It’s how the sun warms your face.

Insulation works by trapping billions of tiny pockets of air. Still air is a terrible conductor of heat. By trapping the air in a matrix of fibres, the insulation prevents the air from moving (stopping convection) and forces the heat to fight its way through a difficult obstacle course (slowing conduction).

Now, usually, when people talk about “going green,” they immediately want to slap photovoltaic panels on the roof. And don’t get me wrong, generating your own power is fantastic. But if your house is leaking heat like a sieve, adding solar panels is like trying to fill a colander with water by turning the tap on harder. It is inefficient. This is why addressing your attic insulation is often the single most cost-effective first step in a whole-home energy strategy. You need to stop the bleeding before you start the transfusion.

The Contenders: A Battle of Garbage

So, we agree we need fluff. But what kind of fluff? For decades, the answer was “Fiberglass”—that pink, itchy stuff that looks like candy floss but feels like a thousand tiny needles stabbing your skin. But we are in the future now, and we are trying to be more responsible about our carbon footprint. We want materials that don’t just save carbon when they are installed, but also have a low “embodied carbon”—meaning they didn’t require a volcano’s worth of energy to create.

Enter the two heavyweights of the “Circular Economy” insulation world:

Contender 1: Recycled PET (rPET). Also known as “Polyester.” This is made from the plastic bottles you threw in the recycling bin hoping they wouldn’t end up in the ocean. It is essentially a fleece jacket for your house.

Contender 2: Recycled Cotton (Denim). This is made from post-consumer textiles. Old blue jeans. T-shirts. The stuff you donated to charity shops that was too torn to be sold. It is literally clothing for your house.

Both claim to be the eco-friendly saviour. Both claim to keep you warm. But which one is actually better? To find out, we have to dive deep into the chemistry, the carbon math, and the harsh reality of Irish weather.

Round 1: How It Is Made (The Origin Story)

The Plastic Journey

Plastic is a miracle material with a terrible reputation. Polyethylene Terephthalate (PET) is the stuff water bottles are made of. To make rPET insulation, we take those bottles, wash them (because nobody rinses their Coke bottles properly), and shred them into flakes.

These flakes are then melted down. This requires heat—about 260°C. The molten plastic is extruded through tiny holes, kind of like making spaghetti, to create long, thin fibres. These fibres are then stuck together (bonded) usually using heat, not nasty chemical glues.

The result is a soft, non-itchy white fluff. According to technical data from studies on PET recycling, this process uses significantly less energy than making virgin plastic from oil. You are taking a waste product that would take 450 years to decompose in a landfill and turning it into something useful.

The Denim Journey

Denim insulation starts in the “rag trade.” It takes scraps from jean factories and old clothes, and puts them through a terrifying machine called a “Garnett.” This machine basically tears the fabric apart until it returns to its raw, fluffy cotton state.

The beauty here is that this process is mostly mechanical. There are no giant furnaces melting rocks or plastic. It’s just shredding. This gives denim a very low energy footprint in the manufacturing stage. However, cotton is flammable. Very flammable. If you filled your attic with untreated shredded jeans and had an electrical spark, your roof would be on fire in seconds.

To stop this, the cotton must be treated with fire retardants. Usually, this is a borate solution (boric acid). This accounts for about 10-15% of the weight of the product. While borates are generally considered safe for humans, they are a chemical additive that complicates the “all-natural” narrative.

Round 2: The Carbon Footprint (The Maths Bit)

Here is where it gets tricky. We need to talk about “Embodied Carbon.” This is the total greenhouse gas emissions released to make the product.

The Case for Denim

Cotton is a plant. When it grows, it sucks CO2 out of the atmosphere to build its fibres. This is called “biogenic carbon.” If you take that cotton and put it in your attic for 50 years, you are essentially sequestering that carbon. You are locking it up so it can’t heat the planet.

Because the manufacturing process (shredding) is low-energy, and the material itself contains stored carbon, some Life Cycle Assessments (LCAs) suggest that denim insulation can be “carbon negative” at the point of production. This sounds amazing. It’s like the insulation is paying you a carbon tax credit just for existing.

The Case for Plastic

Plastic is made from oil. It is fossil carbon. When we recycle it, we aren’t sucking new CO2 out of the air; we are just preventing the old carbon (the oil) from being burned or wasted. The melting process requires energy. Therefore, rPET insulation usually has a positive carbon footprint. It “costs” carbon to make.

However, we have to look at the SEAI data regarding energy ratings. The amount of carbon the insulation saves by lowering your heating bill is massive compared to the carbon cost of making it. Both materials will “pay back” their carbon debt in a few months of winter usage.

Winner of Round 2: Denim. If you strictly care about the embodied carbon of the material itself, the plant-based option generally wins because of photosynthesis. Thanks, nature.

Round 3: The Moisture Monster (Why Ireland Changes Everything)

If we were building a house in the Arizona desert, I would tell you to use denim and never look back. But we are not in Arizona. We are in Dublin. And in Dublin, the air is essentially soup.

Moisture is the enemy of insulation. If insulation gets wet, two bad things happen:

1. It stops insulating. Water conducts heat 25 times faster than air. Wet insulation is just a cold, damp blanket.
2. Mould. The biological hunger of fungi.

The Soggy Jean Effect

Have you ever worn jeans in the rain? It is miserable. Cotton is “hygroscopic” and “hydrophilic.” It loves water. It absorbs it into the very structure of the fibre. If your roof leaks, or if you have condensation issues (which is common in Irish attics), denim insulation will soak up that water like a sponge.

Even though it is treated with borates to resist mould, if it gets saturated, it gets heavy. It can slump, creating gaps in your insulation layer. And drying out a thick mat of wet cotton in a cold attic is incredibly difficult. The Environmental Protection Agency monitors various environmental hazards, but good old-fashioned damp is the silent killer of building materials.

The Plastic Raincoat

Polyester (rPET) is hydrophobic. It hates water. If you spill water on a fleece jacket, it beads up. In an attic, rPET is breathable—it lets water vapour pass through it—but it doesn’t absorb the water into the fibre itself.

If rPET gets wet from a roof leak, it will drain and dry out much faster than cotton. Crucially, because it is plastic, it is not a food source for mould. Even without chemical treatments, mould struggles to eat a plastic bottle. It might grow on the dust on the bottle, but the insulation itself won’t rot.

Winner of Round 3: rPET. In a damp climate, the resilience of plastic is a massive safety factor. It is the pragmatic choice for longevity.

Round 4: The Itch Factor and Installation

Let’s take a moment to appreciate that neither of these is fiberglass.

Fiberglass is awful to install. It gets in your skin, your eyes, and your lungs. You have to dress like you are handling nuclear waste just to roll it out.

Both rPET and Denim are “soft touch.” You can literally sleep on them. You don’t need a hazmat suit to install them (though a dust mask is always a good idea in an attic). This makes them both excellent for retrofitting projects where you might be doing some of the work yourself, or where you just don’t want toxic dust floating around your house.

However, denim can be harder to cut. The fibres are tough (that’s why jeans last so long). You often need special shears. rPET tears and cuts a bit more easily.

Furthermore, because denim is made from recycled clothes, the quality can sometimes vary. A batch made mostly of old heavy denim might behave differently than a batch made of thinner cotton blends. rPET is an engineered product; the fibres are extruded to exact specifications every time. Articles in The Irish Times have highlighted how consistency in building materials is key to achieving the expected BER rating.

The Circular Economy: End of Life

What happens in 50 years when your house is being renovated again?

rPET: It is plastic. It can be melted down and recycled again. In fact, some manufacturers operate “take-back” schemes where they will take your old insulation and turn it into new insulation. It is a closed loop.

Denim: This is trickier. Because of the borate fire retardants and the polyester binders often mixed in to hold the batts together, you can’t just compost it. You can’t easily turn it back into jeans. It often ends up in landfill. And while cotton is biodegradable, in the anaerobic environment of a landfill, it can break down to produce methane, which is a potent greenhouse gas.

There is immense research being done into this. For example, The Ellen MacArthur Foundation does extensive work on the circular economy of textiles, trying to solve exactly this problem. But right now, rPET has a slightly clearer path to being recycled a second (or third) time.

Conclusion: The Verdict

So, who wins the Battle of the Garbage?

If we are looking at pure, theoretical sustainability in a perfect world, Denim is the romantic winner. It stores carbon, it uses natural fibres, and it keeps your old Levi’s out of the bin.

But if we look at the practical reality of keeping a home warm in a wet, temperate climate like Ireland, rPET (Recycled Plastic) takes the gold medal.

Why? Because it is fail-safe. It handles moisture better. It won’t rot. It maintains its loft (and therefore its insulation value) for decades, even if conditions aren’t perfect. And frankly, we are producing so many plastic bottles that finding a permanent home for them in our attics is a noble cause. It turns a piece of single-use trash into a permanent climate-fighting asset.

By using rPET, you are effectively taking a waste product that plagues our oceans and hiring it to lower your energy bills. That is a solid deal.

But remember, the “best” insulation is the one that is actually installed. Leaving your attic empty while you agonise over the perfect eco-choice is the worst option of all. Every day your attic is uninsulated, you are paying to heat the sky. Met Éireann’s climate data confirms we have plenty of cold days left in the year, so delaying is costly.

Furthermore, the effectiveness of any insulation depends heavily on how airtight your home is. You can have the best fluff in the world, but if wind is blowing right through it, it won’t work. This is why The Irish Green Building Council emphasizes holistic retrofits, not just single measures.

So, whether you choose the blue jeans or the plastic bottles, the goal is the same: Stop the heat escaping. Seal the box. Defeat the Second Law of Thermodynamics (at least temporarily).

If you’re ready to stop heating the neighbourhood and start keeping that warmth for yourself, it is time to get the professionals involved. Get a quote for your attic insulation today.