Understanding How Much Insulation is Needed Per Square Metre

Insulation Calculation

Before installing insulation, you need to know how many square metres of coverage you need. Calculating this correctly is important because if you don’t order enough insulation it will cause delays, and if you order too much, it could be a costly waste of money. Check out our advice for correctly calculating how much insulation you need.

How do you calculate square meters?

Most basically, square metres is simply calculated by multiplying the height by the length of an area. For example, for insulating a particular wall in your home, you would calculate the distance from floor to ceiling then multiply it by the length of your wall.


However, things get more complicated when you consider that the joists and studs take up space. That means, simply multiplying height by length will overestimate how much insulation you need. At Pricewise Insulation, we suggest a good rule of thumb is to take off 10% for any timber framing.

For ceilings and underfloor applications, that means multiplying your SQM result from above by 0.9 to account for the timber framing.

SQM (ceilings & underfloor) = H X L X 0.9

For wall insulation, you will first need to minus the area of any doors and windows, and then multiply the result by 0.9 to account for the timber framing. The area of doors and windows can simply be calculated by using the calculation A=H X L.

SQM (walls) = [H X L – (area of doors & windows)] X 0.9

If you’re unsure, don’t worry! At Pricewise Insulation, if you know your SQM = H X L calculation, we can help you estimate the rest to get the right amount of ceiling, floor or wall insulation for your application.

Choosing your insulation

Once you know how much square metre coverage of insulation you need, you will need to figure out how many bags to purchase. Specifics differ from manufacturer to manufacturer, so be sure to keep these tips in mind:

  1. Higher R-Value mean less SQM per bag

    If you decide to upgrade to a higher R-Value, keep in mind you will get less SQM per bag than you would have with a lower R-Value. This is because insulation batts with higher R-Values are more dense. Check the details of your specific product to make sure you’re getting the amount you need.

  2. Differences between manufacturers

  3. Depending on the brand of insulation, there could be a difference in the amount of batts that fit into a bag. For example, Knauf wall insulation batts are packaged using advanced compression technology which increases the amount of batts per bag compared to other brands. Always check the fine details to know how much you’re getting per bag.

  4. Best practices to avoid waste

Besides calculating how much insulation you need, there are other practices you can use to minimise wastage. Ordering the right amount should lead to a better fit, but if you have any off-cuts, you can use these in smaller or hard to reach places. If you have excess insulation at the end, you may be able to use it as an extra layer in the roof or underfloor.

2 thoughts on “Understanding How Much Insulation is Needed Per Square Metre

  1. Kevin Sullivan says:

    Hi I’m having a music 🎶 sound room built ( Guitar 🎸)and am looking for the best sound reduction Batt that I can get for my money.

    What R rating do you recommend for this application ?
    What brand do you recommend ?
    What would be the approximate cost for a 3.2 x 3.2 M room.

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