Rigid insulation: the facts

It’s crucial to select the right insulation for your walls. Rigid foam is a great option for providing efficient thermal resistance (R-value) in a thin package. However, the different types have varied characteristics and performance – so choose wisely!

Expanded polystyrene (EPS)

If you’re insulating a concrete building, EPS insulation is the way to go. At approximately R-2.3 per cm, it actually has the lowest average R-value out of all the different forms of rigid foam insulation. This means that it’s not the best insulator of the bunch, but it’s also the cheapest, coming in at around $10 for a 2.4 m x 1.2 m sheet. You should also note that EPS absorbs water so while it can be laid on the ground – it’s best in the walls. When installed as sheathing, EPS should be used over house wrap and then treated to resist insects. Also, be warned that because most EPS is semipermeable, it is also fragile, so it should be handled with care.

If you’re looking for a middle-of-the-road insulator in terms of price and productivity, then XPS insulation could be the solution. Easily distinguishable by its colour (it comes in blue, green or pink!) XPS has an R-value of 5.0 and costs $2 per square metre. You can purchase it unfaced, or it also comes in a number of different facings, including plastic. Faced XPS is thicker and, therefore, stronger with a lower permeable rating. It still isn’t quite a vapour barrier, but it’s definitely a retarder.

Polyisocyanurate (ISO)

It’s a case of getting what you pay for when you opt for ISO panels. They are relatively expensive – especially considering the cost of the two alternatives – but with an R-value of between 5.6-8, it’s arguably worth the higher costs. ISO is unique in that it starts as liquid foam and has to be sprayed again to form a rigid panel. All ISO panels come in a range of different materials, which affect both durability and perm rating. Fibreglass is the standout for creating a vapour barrier, while foil-face ISO panels are less so. Applying ISO products as sheathing creates an exterior vapour barrier; they should never be used indoors.

Installation of rigid insulation

Insulation should ALWAYS be installed by a professional. It doesn’t matter how much time and effort you put into selecting the right insulation if you fail to engage a professional for the all-important installation. For guaranteed workmanship – and household safety – get quotes from qualified insulation installers now.