What Size Insulation Should I Use in My Ceiling?

ceiling insulation

When working on a ceiling insulation project, it’s important that you choose the right size and amount of insulation to fit your ceiling cavity. If ceiling batts are too thick for the cavity, you may end up compressing them and reducing their efficiency. If you order too much insulation, it could be a costly mistake, but if you order too little it will cause delays in your installation process.

Also Read – Improving Your Home with Ceiling, Floor and Wall Insulation

Choosing What Roof Insulation to Install 

To make sure you get it right, follow our tips below for selecting the ideal insulation size and amount for your project.

1. Choose the right R-Value

Whether you’re installing shed roof insulation or ceiling insulation in the family home, the ceiling/roof is responsible for the greatest heat loss and gain. That’s why it is especially important that you choose ceiling insulation with the right R-Value. The higher the R-Value you can afford the more efficient your insulation will be and the greater savings you can experience.

For ceiling insulation, we recommend minimum R-Values of:

  • R5.0 or R6.0 for those living in cooler climates such as Melbourne, Sydney or Adelaide.
  • R4.0 for those in warmer locations such as Perth, Brisbane or northern NSW.

Keep in mind that insulation batts with a higher R-Value are more dense and so you will get less square metres per bag than you would get with a lower R-Value product. Always check the details of your specific product to make sure you’re getting the right amount.

2. What type of insulation are you using?

Different products will vary in thickness for the same R-Value. For example, rockwool batts have higher R-Values than glasswool batts per unit thickness. Rigid foam board insulation also has high thermal properties for relatively little thickness.

If you decide to use insulation batts, these are cut to a width that fits between the ceiling joists of most Australian homes. However, the dimensions vary slightly between different brands.

Brands such as Knauf and Fletcher offer their ceiling batts in two widths: 430mm and 580mm, but the thickness of the product will depend on the R-Value you choose. For example, Knauf Earthwool R6.0 batts are 275mm thick while R6.0 Fletcher Pink Batts are 250mm thick. Always check the specificities of the product you intend to buy to ensure the right fit for your project.

3. Calculate square metres required

Now that you know which products you will use, it’s time to estimate the amount of insulation you need. Follow the simple calculations below to do so or check out our insulation calculator.

Step 1. Calculate square metres of your ceiling space by multiplying the length by width of the ceiling space.


Step 2. Take into account the joists and studs by multiplying the SQM from step 1 by 0.9.

SQM (ceiling) = L X W X 0.9

Step 3. Once you know the amount of SQM you need, divide your SQM value by the amount of SQM per bag of insulation.

Bags needed = SQM (ceiling) / SQM per bag

4. DIY? Do your research

Are you planning a DIY installation? Be sure to read up on the necessary safety precautions and best practices such as whether to install insulation or wiring first, how to use off-cuts to reduce waste and ensuring that batts are not compressed during installation.

Video: What to Check Before Buying Roof Insulation

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2 thoughts on “What Size Insulation Should I Use in My Ceiling?

  1. Caroline says:

    Hello i am retrofitting insulation into my ceiling. I need a bat that is about 11cm think. What is the highest R rating i can achieve ?
    thank you for your time,
    all the best,

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