Types of Underfloor Insulation Exposed EcoMaster

When we’re asked about all of the different types of underfloor insulation available, people are always shocked to see our scrapbook of underfloor horror stories.

If you’re researching all of the different types of underfloor insulation, this article is likely to confirm all of your worst fears. But don’t worry, we’ll also show you what to look for too so that you can bet on a winner and get the best possible result.


For over a decade, we’ve been inspecting the underfloor areas of homes and we’ve seen all of the types of underfloor insulation you can imagine. Materials like foam board, fibreglass batts, foil sheets and polystyrene held up with string, chicken wire, packing straps and nails!

So why do these solutions let us down? Here’s some of the most common types of underfloor insulation and installation disasters from our collection and why they should be avoided.

Insulation works by trapping air pockets in its fibres. This slows down air movement to keep your winter warmth in and the summer heat out. This means that the thicker and more secure the insulation the better! So you want to choose  types of underfloor insulation that stays thick and secure despite moisture, rodents and gravity.

You can choose the best insulation on the market, but if you don’t secure it well enough, you can never expect the best result.

Here’s a few homes that learned the hard way:

1. Builder’s Blanket Held Under the Floor with Wire

Underfloor areas are exposed to the elements, particularly wind and water, and your insulation should be able to ‘weather the storm,’ so to speak. As you can see, this insulation has not stayed in place long-term due to water damage and the wire has not held the material firmly against the floorboards.

Types of Underfloor Insulation: Builder's Blanket and Wire

As the material has become wet, the weight of the water has squashed the insulation, making the fibres permanently change shape. Parts of the materials have broken away, exposing the floorboards that are meant to be protected.

This means that the insulation that remains is not nearly as effective as intended and the effort that it took to nail that wire under the floor was a waste of time and money.

There’s a video that shows water being poured through polyester insulation (click here), which shows you what you should be looking out for when comparing different types of underfloor insulation and how they react to water.

2. Insulation Batts Tied Up with Packing Strap (Well, Almost)

Packing straps are a very popular way to secure underfloor insulation batts these days. The idea is that the batts are pressed between the joists and held up by their own friction while the straps are slung across the joists to support them.

You walk away feeling proud of your hard work but in a few short days you can probably expect something like this:

Types of Underfloor Insulation: Insulation Batts with Packing Strap

Gravity always wins and in less than a week the owner of the home above was devastated to discover the insulation sagging and hanging down everywhere! Even though he’d invested in premium polyester (which is proven to be the most robust and highest performing insulation there is), his securing method was a dismal failure.

3. Rigid Polystyrene Boards Used to Insulate Under the Floor

Polystyrene boards are sometimes held up with small brackets or even just balanced on nails, which are pinned into the joists.

While ideally, we’d like to think our floor joists are evenly spaced and that neat strips of polystyrene will fit perfectly in between, this is rarely the case.

As you can see below, the wobbly edge doesn’t come close to covering between the joists and the nails have not been enough to hold the sheets up.

Using a rigid material to insulate under your floor is likely to be a case of ‘square-peg-round-hole’, or, more specifically ‘large-boards-tiny-access-door’. And, even if you can comfortably feed the board under the house, it can be very clumsy to maneuver around, making it even harder to avoid gaps.

Types of Underfloor Insulation: Polystyrene Boards Under the Floor

It’s such a shame because you can see how much effort would have gone in to installing these polystyrene boards, especially with all of that aluminium duct getting in the way!

Here’s another example of rigid reflective polystyrene insulation boards falling down over time, leaving the home unprotected:

types of underfloor insulation foilboard polystyrene boards under floor EcoMaster

4. Fibreglass Bulk Insulation Held Up with Chicken Wire

It’s not uncommon for rats to live underneath our homes, doing what they can to build themselves a cosy nest to breed it. Fibreglass bulk insulation contains loose fibres that can come apart over time, which makes it very appealing to rodents.

As you can see below, this insulation has been torn apart and, even though the wire has stayed in place, the insulation has sagged and been pulled down by rats.

Types of Underfloor Insulation: Fibreglass Batts Disturbed by Rats

There are two things to look out for when it comes to rats and how they behave around different types of underfloor insulation.

Number one is that if the rats can get into the pocket between your floorboards and the insulation batt, they’ll nest there, so make sure you secure your insulation tight to stop any access. And remember, what’s secure today will need to still be secure in ten years’ time.

Number two is that if rats can break the fibres apart with their claws, they’ll carry it away to use for bedding. Looking for an insulation that is heat-bonded is the best way to avoid this.

This means that there are no glues that can fail over time and the heat-bonded fibres are too strong for rats to disturb (in fact, they avoid it to save themselves from getting their claws caught!)

5. Reflective Zig-Zag Foil Sheets

Insulation is all about still air space. The better you can trap air and stop it from moving around, the better thermal protection you’ll get. No matter how good your fine motor skills are, preventing gaps is virtually impossible when you’re trying to match a zig-zag sheet like this end-to-end.

types of underfloor insulation reflective foil sheets EcoMaster

Remember, having enough space to trap the insulating still air layer is essential to thermal performance. The foil sheet pictured above is fitted up close to the floorboards, making a very slim space between the material and the underside of the floor.

This makes the material very ineffective and the household living up above won’t be feeling much benefit at all.

So, where does that leave us?

Well, having compared all of the different types of underfloor insulation on the market, there’s only one that meets our strict criteria – given all of these insulation disasters we’ve seen over the years.

Polyester underfloor insulation is thick, strong, and the only bulk insulation with fibres strong enough to secure with staples. It makes a huge improvement to your comfort and lasts the lifetime of your home.

It’s safe to handle without gloves and won’t make you itch, in fact, think about this – a lot of the soft furnishings in our homes are actually made out of the same material, so that shows just how safe it is! This is why we believe is the best underfloor insulation for timber floors.

You can learn more about why we only recommend polyester underfloor insulation and watch a demo of how we install it in the video on this page (click here).

But before you go …

Got an underfloor insulation horror story to add to our collection? Let us know in the comments below.

If you’re looking for insulation, get In touch via our Contact Form. We supply high-quality polyester insulation for all types of Australian homes and commercial establishments.

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