How to Insulate a Roof: A Practical Guide for Homeowners

If your home feels like an oven in summer and an icebox in winter—because it can feel like an icebox, even in the South West—poor roof insulation could be the culprit.

Now, we don’t mean to play down the importance of comfort, but insulating your roof is more than just that. It also helps you pay less in energy bills, muffles the noise from outside, and deals with moisture before it goes through and sets in.

But with so many insulation options out there, what’s best for an average home? And does it matter whether you have a flat or pitched roof? Let’s take a detailed look.

What Does Roof Insulation Actually Do?

Roof insulation works by slowing down the transfer of heat, which is a 2-way process: it keeps the warmth inside during winter and blocks the heat out in summer.

What with the wild temperature swings nowadays, good insulation is that one simple(ish) tweak that can make a huge difference in your home’s energy efficiency.

Then there’s the soundproofing. While it won’t be much of an issue in most homes, you’ll appreciate it if you live under a flight path.

Types of Roof Insulation (+ What Works Best in Practice)

The best insulation depends on your roof type, budget, and whether you’re insulating for heat/cold or sound.

1. Batt Insulation (Fibreglass or Polyester)

It’s a classic choice, arriving in handy pre-cut slabs. Slip them between your roof rafters or ceiling joists, and you’re good to go. Fibreglass batts lead the pack in popularity, but don’t overlook polyester batts. Made from recycled plastic, they’re an eco-friendly option. Plus, you won’t have to contend with itchy fibers!

Best for: Pitched roofs, retrofitting existing homes, thermal insulation.

2. Foam Board Insulation

Rigid foam boards are a great choice for insulating flat roofs or spaces where room is limited. They offer excellent insulation without taking up much space, which makes them ideal for tight spots.

These boards come in various materials like polyiso, EPS, and XPS, each with its own advantages. For instance, polyiso boards have a high R-value, meaning they’re great at resisting heat flow. EPS and XPS boards, on the other hand, are known for their moisture resistance.

So, if you’re looking to insulate a flat roof or a cozy corner, rigid foam boards are definitely worth considering.

Best for: Flat roofs, metal roofs, homes needing high thermal resistance.

3. Reflective Foil Insulation

When it comes to battling the relentless Aussie sun, reflective foil insulation is a standout choice. This clever insulation works by bouncing radiant heat away from your roof. Picture it like a shield, deflecting the harsh rays.

It’s particularly great for metal or tin roofs, which tend to soak up a lot of heat. So, if you’re looking to keep your home at least manageably cool, reflective foil insulation might just be your thing.

Best for: Metal and tin roofs, hot climates, homes needing extra summer heat protection.

4. Spray Foam Insulation

This type is applied directly to the underside of your roof, that is in the attic, and then it expands to fill every nook and cranny. This makes it fantastic for sealing air leaks.

While it might be a bit pricier than other options, it delivers top-notch thermal and sound insulation. Think of it as an investment in both comfort and quiet. Worth considering if you’re looking for a high-performance solution!

Best for: Homes with irregular roof shapes, high-performance insulation needs, sealing draughts.

5. Loose-Fill Insulation

Ever considered blown-in insulation? Made from materials like cellulose or rock wool, it’s literally blown into roof cavities. This makes it perfect for any tricky, hard-to-reach spaces.

There’s a catch, though. Over time, it might shift or compact, reducing its effectiveness. But don’t worry, we’re talking decades here. It could be your grandkids or even great-grandkids who’ll need to think about replacing it.

Best for: Ceilings with existing insulation gaps, soundproofing, irregular roof spaces.

Does Roof Insulation Differ for Pitched vs. Flat Roofs?

Yes, and here’s why.

Pitched roofs offer plenty of flexibility when it comes to insulation. Why? Because they usually come with an attic or ceiling cavity. This means you can tuck insulation between rafters, above the ceiling, or even on top of the roof structure. Options galore!

Flat roofs, however, present a different challenge.

With little to no space for insulation, your best bet is often rigid foam boards or spray foam. These can be applied directly to the roof surface, ensuring you still get great insulation without the need for extra space.

Insulating Tiled vs. Metal Roofs: What’s the Difference?

Tiled roofs come with a built-in bonus: natural air gaps. These gaps help with ventilation, keeping your home breezy and comfortable. When it comes to insulation, batts or loose-fill insulation are usually installed in the ceiling cavity below the tiles. Easy peasy!

But what about condensation? If it’s a concern, consider adding a vapour barrier. This extra layer can help keep moisture at bay, ensuring your insulation stays dry and effective.

Metal and tin roofs absorb a lot of heat, so they benefit most from reflective insulation (foil-backed blankets) and foam board insulation.

Because metal roofs expand and contract more than tiled roofs, it’s important to use insulation that can handle these movements without deteriorating.

How Is Roof Insulation Installed?

The process depends on whether you’re building a new home or retrofitting an existing one.

For New Builds:

• Insulation can be installed under the roof before the final roofing material is laid down.
• Reflective foil or foam boards are often used directly beneath metal roofing.
• Bulk insulation like batts is placed between rafters before the ceiling is added.

For Existing Homes:

• Insulation is typically added inside the roof cavity, either by laying batts over the ceiling or blowing in loose-fill insulation.
• If access is difficult, spray foam may be used to fill gaps and improve insulation without tearing up the ceiling.

How Much Does Roof Insulation Cost in Australia?

Roof insulation costs depend on the type of material used and the overall size of your home. Here’s a rough estimate to give you an idea:

• Batt insulation: Expect to pay around $8–$16 per square metre. This is a popular and cost-effective option for many homeowners.
• Foam board insulation: This can range from $20–$40 per square metre. It’s great for flat roofs or spaces with limited room.
• Reflective foil insulation: This type typically costs between $7–$16 per square metre. As we said above, it’s perfect for reflecting radiant heat away from your home.
• Spray foam insulation: This is one of the pricier options, ranging from $35–$50 per square metre. It’s ideal for hard-to-reach areas and provides excellent insulation. Note, however, that the labour cost for this type is also higher than with other types such as batts.
• Loose-fill insulation: This usually costs around $10–$20 per square metre. It’s a good choice for filling irregular spaces.

Keep in mind, labour costs for installation can add another $30 to as much as $50 per hour. Not all of these types are equally tricky or time consuming, so the cost varies immensely.