Polystyrene Loft Insulation – Yes or No?

Let’s talk about loft insulation. I know – it’s truly thrilling stuff. Well, it actually is, because if you properly insulate your loft, you can have a cool little hideaway from the family. It really doesn’t get better than some nice, comfy peace and quiet, does it? And with the holidays coming up, it’s looking more and more enjoyable by the minute… Okay, jokes aside, let’s talk about insulation – specifically polystyrene loft insulation and its counterparts.

Polystyrene loft insulation is one of several potential materials with which to insulate your loft. It’s entirely safe, though a bit difficult to install properly without professional aid.

Now, without further ado, let’s talk about insulation!

Pros and Cons of Various Insulation

First things first, the belle of the ball – polystyrene. You’re here for a reason, after all, so let’s get right to it. There are several other potential materials that you could use, and each comes with its own pros and cons, as well as specialised uses. Let’s get going!


A lot of people have recently begun to go away from the older spray foam or fibreglass insulation for lofts. The primary reason for this is that it’s incredibly strong for its thickness, and it has excellent moisture control – an area that both spray foam and fibreglass struggle to compete.

Of particular note is polystyrene’s extremely high R-value. This refers to your insulation’s capacity to prevent the flow of heat in and out of your home. In other words, how well does your insulation do its job? The higher the R-value, the better insulation you’ll get from a given material. And boy, does polystyrene have a good one. Per inch, styrofoam extruded polystyrene has an R-value of 5 per 2.5 cm (1 inch).

The downside to polystyrene (like most insulations, unfortunately) is its ability to hold up to heat. Polystyrene isn’t technically a fire hazard, but if installed poorly, can be combustible. The caveat here is that if it’s installed professionally and properly, there is no undue or excessive risk of fire hazards in your loft when compared to other insulation mediums.


A warning before we get into this – I hate fibreglass insulation. Anyone who’s ever had to install it will agree with me here. It’s itchy, hurts your lungs when inhaled (please wear a respirator if installing any insulation, by the way) and can straight up give you splinters.

It has fallen out of use in recent years for a few reasons:

  1. It’s annoying and unpleasant to install, requiring a professional with a specialised machine to blow it in.
  2. It has a poor R-value in comparison to other materials, only rating at R-2.5 per 2.5 cm (1 inch). That’s half of polystyrene’s R-value per 2.5 cm.
  3. Once temperatures drop below -6°C, older forms of fibreglass lost a good amount of insulation value. This has changed with modern fibreglass, but still is a good point to reiterate.

Spray Foam

This is just about the best insulation available at the moment. Made of polyurethane, spray foam insulation can come in one of two forms: closed and open cell. Otherwise known as .9 kg (1 lb.) or 226 gram (.5 lb). Their insulation values are approximately R-6.5 (closed cell) and R-3.6 per 2.5 cm (1 inch). This is far better than polystyrene when comparing closed cell, and still better than the other options when comparing open cell.

Spray foam is also highly valuable in extremely cold environments. Minnesotan insulation is almost entirely closed-cell spray foam, considering their winters can reach -20°C and lower. Additionally, once your spray foam dries, air will be unable to make its way through. It has two major downsides that re directly tied to one another and the above information:

  1. It’s much more expensive than other forms of insulation. But then again, you get what you pay for.
  2. It cannot be installed by an amateur. It requires a specialised spray foam gun to install, and even professionals struggle to install it properly at times. When installed properly, it performs admirably, If installed poorly, though, it looses most of its insulation value.
    1. It will also need to be profesionally off-gassed to dissipate the toxic gasses that occur when spraying foam down. Again, don’t try to DIY this.


Also made of fibreglass, batts are the easiest way to insulate a loft or area above your home (such as the crawlspace in your roof). It’s also the worst type of insulation in general, so you (again) get what you pay for.

Its one upside is that all installation takes is purchasing a roll and spreading it out. It’s extremely easy to install, making it popular with DIY enthusiasts that live in temperate climates. If you live in an extremely cold or hot environment, I cannot stress this enough. Do not use batt insulation.


This is a step up from fibreglass batts but is still not a good option. Made from recycled and ground paper with boric acid added for fire resistance and pest control, it’s… insulation. You can install it yourself rather easily, though professional installation will serve a good purpose. Here’s why:

  1. Cellulose insulation kicks up a lot of dust when being installed. Professionals will wet spray it, essentially mixing it with water. This will both improve its R-value (slightly) and reduce the dust and following cleanup.
  2. Properly installed (professionally, that is) cellulose insulation carries a rough R-value of 3.5 per 2.5 cm (1 inch). This is assuming it’s been wet sprayed and properly installed.
  3. If installed correctly, it won’t form an air barrier, but its density will prevent a good amount of air flow. This will, in turn, “improve” insulation (though its R value won’t change for the better due to this). Better, though, its density will improve resistance to frost and moisture, especially compared to fibreglass – batt or not.

Final Thoughts

Polystyrene loft insulation is likely your best bet for a comfy loft. There are a lot of options out there, ranging from various types of fibreglass to polyurethane foam and cellulose. Each has its benefits and sacrifices associated, but in general – polystyrene is your best bet. It offers a higher R-value per 2.5 cm than most others and holds up well to moisture and heat alike.

Honestly, though, perhaps the best reason to opt for polystyrene insulation is installation – as long as you’re not laying fibreglass, you’ll be much happier. While it can’t be easily installed by an amateur, the benefits greatly outweigh the cost of hiring a pro to do the job. And please – if you opt to DIY the installation, wear a respirator. Regardless of the type of insulation you install, it can release either toxic gasses or actual material that you can inhale – neither is a good situation.

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About the Author Harry Thompson

Involved in home renovations throughout his life, Harry is an expert in everything to do with home and garden DIY. In his spare time, he enjoys cooking and tending to his garden.