Types of Underfloor Insulation Exposed

Types of Underfloor Insulation

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 an 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:

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.

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 installers in Melbourne or Sydney, call ecoMaster at 1300 326 627. We supply and install high-quality polyester insulation for all types of Australian homes and commercial establishments.

Contact ecoMaster To Get The Best Underfloor Insulation

Boost your thermal comfort and lower your energy bills FOREVER! Stuff your floors with the best underfloor insulation in Australia.

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20 comments… add one
  • Ann Leonard May 3, 2019, 12:03 pm

    Are these images more about the failure of the installation process rather than the material used?

    • ecoMaster May 27, 2019, 9:43 am

      Thanks for your question Ann.
      The results shown are a consequence of both an unsuitable product being chosen as well as poor installation practices.
      It is important that both are chosen with a long term outcome in mind.

  • Susan Hickey July 11, 2019, 11:50 am

    Hi, I am interested in underfloor insulation. But, our home floor is steel sheeting. We have carpet and lino as cover. It is a ‘mobile home’ but is a house! By Glendale in Qld. Is there a suitable underfloor insulation for us? As staples/clips etc will not work with a steel sheeting? No timber joists but steel joists and up on steel pylons in concrete. Can get underneath, bit tight at front, but can crawl.
    So, anything? Very interested in a solution – have double glazed. Cheers, Susan

    • ecoMaster December 19, 2019, 2:03 pm

      Hi Susan, We do have a method of insulating steel floors but unfortunately it’s not DIY and we don’t service Qld. Sorry we can’t help you.

  • Donna O'Reilly July 28, 2019, 7:46 pm

    Hi we’re looking for an installer near Murwillumbah nsw. 2484.

    • ecoMaster December 19, 2019, 1:54 pm

      Hi Donna, We have now implemented an updated freight quoting system on the ecoMasterStore so you could re-check the freight options by putting some Underfloor Insulation in the cart. This is new this week. I hope we can now help you.

  • Ange July 29, 2019, 8:01 pm

    Is this polyester insulation you recommend unappealing to rodents? Our house is on brick piers, on a slope so the range of distance from the ground is about 20cm to 120cm. We live in the country so often have rats and mice. Our 1930’s house has hardwood flooring with gaps so the cold air comes through. How is this product best for my house? Thanks

    • ecoMaster December 19, 2019, 1:49 pm

      Hi Ange, in 16 years the only rodent attack was a resident wombat that ripped down some of the insulation. Mice and rats do not eat, remove of live in the polyester insulation. The insulation is fitted hard onto the underside of the floor boards so there is no opportunity for mice to get in there. I hope that helps.

  • Bernie July 30, 2019, 4:55 pm

    Is the polyester underfloor insulation fire resistant and if so to what extent?

    • ecoMaster December 19, 2019, 2:00 pm

      The underfloor insulation does comply with building regulations. Having said that, assuming you floor is on fire, the polyester insulation will melt and fall to the ground where it will not burn of itself.

  • Chris February 10, 2020, 2:51 am

    Good morning,
    I live in the USA, so you have installers here? If not, do you ship to USA?

    • ecoMaster February 27, 2020, 12:44 pm

      Hi Chris,
      No we don’t supply or install to the USA. We are unaware of the insulation available so cannot be of assistance to you.
      Warm regards
      Team ecoMaster

  • Steve February 21, 2020, 1:21 pm

    Have you guys heard of or used Reflecta Thermofloor, any comments.

    • ecoMaster February 28, 2020, 10:01 am

      Whilst we have not used that particular product, we do have experience with other reflective sheet material.

      These have a number of issues:
      1. In a retrofit application, any kind of sheet material is difficult to handle under a floor, particularly in tight spaces.
      2. Reflective insulation only works with a still air space, and while that is easy to draw in an installation sketch, in practice it is really difficult to achieve. Therefore you don’t really know what insulation value you are achieving.
      3. Electrical safety. It doesn’t matter how carefully you memorise where the wires are, as soon as you place the reflective sheet material above you onto the floor joists, you have immediately forgotten where the electrical wires are. The risk of stapling a wire is heightened. Of course the aluminium is a great electrical conductor and may raise serious safety issues with installers and any other trade working in that space afterwards.
      4. The “Still air spaces” make excellent hidey holes for rodents.
      5. One particular foam filled foil material was so insubstantial that the staples went straight through it into the timber, tearing a hole in the material and not holding it up.

      So while Relecta Thermafloor may or may not be a good floor insulation material for new build, I can promise you that the underfloor polyester rolls we have tested and use are in every way superior for a retrofit application.

      Finally, now that you have brought this product to our attention, we would point out that the R ratings in the Product Spotlight Sheet states that the “Summer downwards R value” is R2.0 in total (including the floor, underlay and carpet) and “Winter Upwards” is R3.8.

      This table appears to be configured for Ceiling Insulation – not underfloor – and may be very confusing to a lay person wanting a good quality result.

      If the downward insulation value (which is a winter value for underfloor) is R2.0 in total, it is clearly inferior to adding R2.5 polyester rolls to the underfloor, which gives a total effect of R3.2 regardless of whether still air spaces are, or are not, achieved.

      Thank you for bringing this to our attention.

  • Linda Livesey February 26, 2020, 11:10 pm

    Three months ago I got a boiler installed, and they had to do under floor installation. I now have black mould creeping up my walls never happened before.L also have under bed storage and everything is covered in white powder a bit like salt. My husband has started with chest problems that he never had before.

    • ecoMaster April 14, 2020, 11:25 pm

      Hmmm, seems very curious. 1. You don’t mention what insulation was installed or in what manner it was installed. 2, It seems to me that this effect is more likely to be something related to the boiler than to underfloor insulation. 3. I have never ever heard of hydronic heating installers also installing underfloor insulation. Best to take a good look under the floor.

  • Kelvin Gebert March 18, 2020, 4:19 pm

    I have an overhanging (upper storey) underfloor application. My concern is that the polyester might absorb moisture (location not direct rain exposed though), which would deteriorate the insulation performance considerably. Can you comment?

    • ecoMaster April 14, 2020, 11:20 pm

      The polyester insulation does not in itself absorb moisture. If you pour water on it most of the water will go straight through. The small amount retained will simply dry out and the insulation will be good as new. It doesn’t matter how often you wet the polyester, it will always dry back to it’s original shape and size. You are quite right in thinking that wet insulation has degraded performance though. If your underfloor is not directly exposed then there will be nothing to be concerned about.

  • Belinda Cuzner April 14, 2020, 8:30 am


    Can you please advise if this material meets BAL 29 for bushfire rating. Thanks

    • ecoMaster April 14, 2020, 11:13 pm

      The BAL requirements for a timber subfloor are about the treatment of the perimeter of the floor to prevent ember attack under the floor. The perimeter must be treated with non-combustible material, typically fibre cement sheeting and air vents must be covered with metal mesh. There are specific requirements about the particular type of mesh and hole size of the mesh. So providing that the perimeter treatment is upt to BAL 29 standard then polyester underfloor insulation is perfectly fine and compliant.


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