Ceiling Insulation – Starting from the top

Since Covid-19 emerged everyone has been spending a lot more time inside their home during the day. During winter or summer it is a good time to check your insulation is up to scratch. There is plenty of information so first of we are starting with ceiling insulation.

We have done some research and put together a bit of information about home insulation with help from energywise.govt.nz.

Hot air rises, so making most homes easier and cheaper to heat properly starts with good ceiling insulation.

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New build wall and ceiling insulation

It’s worth checking your insulation occasionally to ensure it is still working effectively. It may never have been up to today’s standard, may have been damaged by people working in the ceiling space or slumped towards a heat source, creating a fire risk.

How much ceiling insulation?

How much insulation you need in your ceiling depends on how cold it gets in your area and on the thickness and condition of any existing ceiling insulation. The R-value is a measurement of the insulation’s effectiveness – the higher the R-value, the more effective it is at preventing heat loss.

DIY insulation check

If you can do it safely, peek through the hatch of your ceiling into your roof space to see if you have insulation.

You will need to add a top-up layer of insulation, if your existing insulation:

  • is less than 120mm thick (it should be thicker than the height of the ceiling joists)
  • doesn’t cover the whole ceiling
  • has gaps in it, or places where it is squashed or tucked in.

You need to remove the old insulation and start again with a new layer, if:

  • it is wet or damp in areas
  • has been damaged by rodents or birds.

Also check safety gaps between insulation and heat sources like recessed downlights, chimneys, flues or extractor fans for fire safety. Most recessed downlights installed before mid 2012 as well as heat/light/fan units require 100mm clearance from all insulation.

For more information check out New Zealand Standard NZS 4246:2016 Appendix A for full details of the requirements.

Choosing ceiling insulation

There are two common types of ceiling insulation: bulk, which fits between or rolls over ceiling joists, and loose-fill, which is blown in. Loose fill insulation should only be used where there is no other alternative. It has poor insulation properties unless installed perfectly and is very messy.

Bulk insulation


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Blanket insulation that covers ceiling joists prevents extra heat loss through the timber joists. When you’re topping up existing insulation, it can be easier to install blanket insulation than segments because you can just roll it over the top of what’s there already. With the blanket insulation covering the ceiling joists it is harder for a person to move around the ceiling space. Remember to be aware of light fitting and extraction fan clearances when installing.


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If you install segments between joists, you need to put in higher R-value insulation to make up for the heat that gets lost through the timber – check the table of minimum recommended R-values for existing homes below.

Minimum R-values for existing homes

The table below lists recommended minimum R-values for retrofitting ceiling insulation into existing homes. Always go higher if you can.

Minimum recommended R-Values for existing homesNorth Island (excluding Central Plateau)South Island and Central Plateau
Ceilings with no insulation, or up to 70mm of existing insulation R2.9 blanket or R3.4 segment insulationR3.3 blanket or R4.0 segment insulation
Ceilings with 70-120mm of existing insulation R1.8 blanket insulationR2.4 blanket insulation

Checklist for choosing ceiling insulation

To get a suitable, effective ceiling insulation product choose one that is:

  • intended for installation in roofs or ceilings
  • a high R-value – R-value is a measurement of the insulation’s effectiveness, the higher the R-value the better
  • the right width – you need the correct width for the spacing of ceiling joists, roof trusses or rafters, if you’re going to insulate in between
  • the right thickness (particularly for skillion roofs) – you need to keep at least a 25mm gap between the insulation and the roof underlay
  • compliant with the testing Standard AS/NZS 4859.1 – Look for the compliance statement on the insulation packaging.

Installing ceiling insulation

DIY insulation is relatively straight forward. See Installing insulation

DIY checklist

Things to consider before you begin

  • New Zealand Standard NZS 4246:2016 contains everything you need to know about installing insulation. It’s easy to follow and it’s free to download NZS4246-2016.pdf.
  • Safety – read and follow the Health and Safety advice in Appendix B
  • Ceiling space – check if your house has a ceiling space. If not, it could be tricky to install, but it may still be possible to put insulation in your skillion roof or cathedral ceiling.
  • Existing insulation – a quick look in your roof space will tell you if you have any insulation already. If there is, you’ll still want to check what state it’s in.
  • Leaks and electrical issues – check your roof and any plumbing in the roof space for leaks. Check that your electrical wiring and installations are in safe condition. You should fix any issues before insulating.
  • Insulating around heat sources – having the right gaps between ceiling insulation and heat sources like recessed downlights, chimneys, flues or extractor fans is important for fire safety. Refer to New Zealand Standard NZS 4246:2016 for detailed requirements.
  • Multi-storey homes – only the upper ceiling between your home and the outside needs to be insulated. You don’t need to insulate the ceiling between floors except above or below unheated rooms like laundries, workshops or garages.

Insulating skillion or flat roofs

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Insulating a home with no ceiling space is best done when re-roofing or renovating.

Ceilings with exposed rafters

If your skillion roof has exposed rafters, you can simply fit insulation between the rafters and line with plasterboard (or similar) on battens. Some rigid insulation even comes with plasterboard already attached that can be glued to your original ceiling. Try to use bulk insulation of the same thickness as the depth of the cavity, ideally with an R-value of R3.6 or higher. If the rafter depth is less than 190mm, consider making more room for insulation by fixing additional spacer battens to the underside of the rafters. For sloping ceilings, be aware that you’ll lose some ceiling height at its lowest point.

Ceilings with hidden rafters

If your skillion roof has hidden rafters and you’re planning on re-roofing or renovating, this is the time to fit insulation.

  • When re-roofing – fit insulation from above during the re-roofing work. This is best done by the roofer. Choose a roofer who is experienced and comfortable with installing insulation well.
  • When renovating – remove the ceiling lining to fit in insulation from underneath. If the rafter depth is less than 190mm, and you want to fit insulation with a recommended R-value of R3.6 or higher, you may need to fix additional spacer battens. Afterwards, line with plasterboard (or similar) on the battens.

Whichever way you do it, leave a 25mm gap between the insulation and the flexible roof underlay to avoid moisture problems.

New Zealand Standard NZS 4246:2016 contains everything you need to know about installing insulation. It’s easy to follow with lots of colour pictures, and it’s free to download here.

For more information check out underfloor insulation and wall insulation