Insulation Guide - Insulating your home


Prior to purchase of insulation for your home, sleepout or building there are a number of things you need to consider.

  1. What is R rating ?
  2. Which areas of your home to insulate first ? 
  3. How much insulation do I need in my ceiling?
  4. Assessing your existing ceiling insulation.
  5. How much insulation should I add to my existing insulation?
  6. What type of the ceiling insulations should I install?
  7. Biscuit vs blanket ceiling insulation
  8. What about underfloor insulation and vapour barriers?  
  9. Dampness in the home
  10. Installation
  11. EECA subsidies
  12. Draught stopping
  13. Other notes

What is an R rating?
The R rating of an insulation material refers to the thermal conductivity of an insulation product or in other words - the amount of heat the material will retain inside your home.  The higher the R rating, the greater the products ability to retain the heat in your home, thereby keeping you warm and saving you energy.

Selecting  areas of your home to insulate.
The best place to start insulating your home is the ceiling.  The ceiling is where your home looses up to 40% of it's heat because heat rises.  The next place to look it is under your floor.  Draught stopping your home  and hot water cylinder wraps should always be considered because the cost is low and  the savings will be high.  The cost of insulating walls can be high with the expensive procedures or relining and walls are likely to need redecorating after installation. If you are already redecorating then this is a great oppertunity to add insulation and the payback will make it a sensible investment.  When insulating your home it makes sense to have a higher R rating product in your ceiling than under your floor.  If insulating the walls the R rating should be less than the ceiling but more than the underfloor.

How much insulation do I need in my ceiling?
The amount of the ceiling insulation you choose will depend on your budget and the amount you wish to spend.
If you choose a basic level of insulation then your energy savings will be less, but you would expect to recover the money spent on insulation within in 1 to 2 years.  A good level of insulation has an approximate payback 3-5 years and the excellent level even longer than this.  It is important to realise that R rating is not linear and as the r rating increases, the heat loss and savings will increase at a lesser rate.  When considering the amount of insulation you require you may also wish to think of how long you will be living in the home.  It is almost certain that the price of electricity will increase in the future, therefore if you anticipate living a long time in your home, installing an excellent level of insulation could be a good investment.

Assessing your existing ceiling insulation
Before when you start, we recommend you measure the height of your existing ceiling insulation and use the table below to estimate the approximate R rating of your existing insulation.
Height of existing insulation Approx R rating of insulation once the new insulation has been installed over the top
Blown insulation                    R = zero
Less than 75 mm                  R = zero
75 mm -95 mm                     R = 1.5 – 1.8
100 mm-115 mm                  R = 2.0-2.2
120 mm-150 mm                  R = 2.5-3.2
155 mm-180 mm                 R = 3.6-4.0
Greater  than 180 mm         R >4.0
Most New Zealand homes that were insulated prior the year 2000 with fibreglass insulation will now have insulation that there is thinner than when it was first installed.  If the insulation is from the 1980s or earlier it will most certainly be less than 50 millimetres thick.  The reason that many fibreglass insulations sagged and many homes are now requiring an insulation top up or a complete new insulation is that the resins that held the fibreglass fibres together were not is good back then as they are today.  During the manufacture of fibreglass insulation, recycled glass and silica sand (in the case of lower quality insulations), are coated and with resin and it is this resin that seperates the fibres enabling them to trap air and maintain its loft (thickness).  It is the products ability to trap air that enables it to act as an insulator by slowing down the process of allowing warm air to escape through the ceiling of your home.  The reason an existing insulation of less than 75 mm is treated as having no insulation value is that this product is too old and the resin quality is too poor to hold the weight of the new insulation.  Once new insulation is installed over the existing this product will be further compressed to the point of being no use at all.  Unfortunately some overseas factories around the world are still manufacturing new fibreglass insulation using these old outdated resins.  New Zealand currently has no legislation to prevent the importation and sale of fibreglass insulation containing poor quality resins. It is a very sad fact that some people currently re-insulating their homes are unlikely to be doing it again within ten years.  Please refer to section about quality of insulation for more information.
If your existing insulation is a blown product like insulfluff, wool  or paper then it is unable to take the weight of any additional insulation material without seriously compromising its existing R rating.  Unfortunately even if you have a blown product that has been installed in recent years it will not be able to hold the weight of any additional materials and it is very likely to have experienced settling problems already.  The EECA program currently does not have any blown products on its approved product list. Most customers install the new full thickness insulation directly over the top of blown product as it can be very messy to remove.  A small minority of customers will go to great lengths to remove old insulfluff as they no longer wish to have this product in the homes.


How much insulation should I add to my existing insulation?
Once you have decided the level of insulation you would like to end up with and made an assessment on the R rating of your existing insulation you will be to calculate the remaining R rating that you need to add to the ceiling.  If using polyester ceiling insulation to top up then the additional  amount of additional R rating calculated,will be the actual amount to add.  If you’re using fibreglass ceiling insulation then you will need to go slightly higher.  Eg two layers of R2.3 polyester blanket will give a combined R rating of R = 4.6.  If the two layers were fibreglass then the R rating would be R = 4.4 - 4.5.  The reason for this is that polyester his much better lofting qualities than fibreglass.

What type of the ceiling insulations should I install?
Here are three main types of the ceiling insulation being used in New Zealand.
Fibreglass insulation
Fibreglass is by far the most popular choice and ceiling insulation in New Zealand.  The main reason for this is because the price is inexpensive.  Fibreglass is also known as glass wool and is made from recycled glass, silica sand or a combination of both.  Generally products that have a higher glass content will be less dusty and less itchy to work with.  This is because the fibre length is longer than that of product manufactured with a higher proportion of silica sand.  Products containing higher amounts of silica sand are also most likely to contain higher amounts of formaldehyde based resin resulting in a lower final product cost.  Today most fibreglass contains formaldehyde based resin although quality products are more likely to have stricter quality standards on its processing thereby ensuring a greater level of formaldehyde are flashed off during the processing.  New Zealand currently has no legislation to control the amount of formaldehyde present in insulating materials imported and sold in New Zealand. Fibreglass insulation manufactured without the use of any formaldehyde is also now available for purchase in New Zealand. An example of this is our Earthwool Glasswool range of fibreglass
Two lesser known disadvantages of fibreglass is that the product is usually not self supporting if stapled upside down (underfloor) and it will forever loose its loft if it become wet (roof leak).
Unfortunately there is a very broad range of insulation available in New Zealand. At the high end we have high quality, well controlled, manufactured products with virtually undetectable levels formaldehyde that will easily last the minimum durability period of 50 years. At the other end of the spectrum one can readily purchase (for often at a similar price) low quality, highly variable product that contains high levels of formaldehyde which is classified as carcinogenic in many countries. These products are often poor in quality and have sometimes had the resins partially destroyed by product being in its compressed packaging for longer periods of time. Insulation stock which is not well time managed and in its packaging too long will not loft correctly and hold its loft for the required amount of time

Polyester insulation
Polyester the insulation material of choice for many reasons.  Products are made from recycled Coke bottles and the material is the same product that is inside most synthetic duvet covers. This product is soft and non irritating to install contains absolutely no formaldehyde.  If polyester should ever become damp or wet, it will return to its full loft once the product dries out.  Polyester is a thicker material than fibreglass and more difficult to compress for transport.  Polyester is around 60 to 100 per cent more expensive than fibreglass putting it over budget for many installations.

Wool insulation
Although blown is not recommended by us there are some wool pieces available that contain a blend of polyester in order to maintain the lofting characteristics it does not naturally have itself.  Although some people like the idea of using a natural product that must be remembered that this product will rot if it remains damp over a period of time (eg. Roof leak)

Ceiling biscuits vs. blanket
Ten years ago the majority of the ceiling insulation sold in new Zealand was in the form of pre-cut biscuits, which were installed in between timber joists.  Nowadays many people use blanket material to install over the entire ceiling, including all the timber, to prevent the heat loss created when insulations is placed either side of the timber.  This heat loss is called thermal bridging.  When we use R3.6 blanket insulation and  install it as an envelope over the entire ceiling area, that ceiling area will have a system (or construction) r rating of around R=3.6.  If we use an R=3.6 ceiling biscuit installed either side of the timber then the system (construction) R rating that we create is around R=0.5 lower than its blanket equivalent -giving  around r=3.1.  Often council plans will specify a minimum R rating but will not take into account the whether the product used as blanket or biscuit. Consumers should be aware when purchasing their insulation that the blanket and biscuit if installed either side of the timber will give different ceiling system r ratings.  Blankets are often also recommended for cathedral ceilings we’re than timber spacing is not typical or in ceiling systems with metal battens where the insulation is installed prior to the gib being installed from underneath.

Underfloor insulation and vapour barriers
There are now 3 main products used underfloor in New Zealand

Polyester is a highly sound absorbing, breathable product which is self supporting for use underfloor. This is the product of choice for underfloors offering good value and being easy to install.  Products are easy to install with a traditional gun stapler and can be installed between the joists or underneath the joists. Being polyester this product is much nicer to work with when installing above your head. Polyester is also extremely popular for underfloor as it allows the timber under the house to breath and it is a very good sound absorbing material giving a noticeable difference to the cozyness and sounds in the home.

Fibreglass insulation
Fibreglass is not normally self supporting when installed upside down. Product specifically designed for underfloor requires a basic level of strapping and standard fibreglass blanket products require perforated foil or lots of strapping to support the poduct, but this method is not recommended. Product is normally only installed between the joists and care should be taken to ensure product does not get damp. Eg.Do not install vapour barriers that touch the fibreglass underfloor product to ensure it stays dry.

Polystyrene insulation
Polystyrene solid sheets are sometimes used underfloor. Prior to installation all electrical wiring should be covered with tape, as some polystyrene is known to react with the outside of some electrical cabling, heating up and creating a fire hazard. To install each distance between the joists should be measured individually and cut to friction fit. Product can be installed at the top of the joists to enable part of the joists to continue to breath or lower down for a slightly high r rating. Unfortunately polystyrene will hold moisture and reduces the timbers ability to breathe. When Polystyrene is installed near the bottom of the joists it is also know to have a drumming effect with the sound under certain conditions. All these factors coupled with the fact that it is much more difficult to install has seen a move away from Polystyrene for timber underfloors in recent years.

Vapour barriers are plastic sheets which are placed on the ground and taped together to provide a complete vapour barrier barrier to stop dampness in the ground from evapourating into the air under your home and absorbing up into the home, however not all home need this.

The soil under your home should be dusty and dry and in many New Zealand homes this is the case and no vapour barrier is needed. If you find that there is moisture in the soil under the house thenthe firstthing to look for is where the water may be coming from. There may be a leaking downpipe or an area of lawn or concrete sloping down toward the underfloor where water may be running from. In the first instance leaks should always be fixed and options looked at to route any rainwater away from the house. In some cases it may be impossible to remove the moisture from under your home beacause the land is low lying or the house is on a hillside - in this case is it recommended to install a vapour barrier.

200 micron thick polyesthylene is unrolled under the home and taped onto the piles and at the joins.

Always ensure any vents under the house are unblocked allowing the air to move under the house and dry things out.


Dampness in the home

Many New Zealand homes are too damp not only making the air harder and most expensive to heat, but enabling mould and dust mites to grow. The ideal relative humidty inside your home is 45-65%There is lots of information around about keeping moisture out of your home and some key points are:

  • Remove steam from cooking with an extractor fan
  • Dry clothes outside or vent a dryer to the outside of the home
  • Shower with the bathroom fan on or window open and shut the door after you have finished using your bathroom to enable to excess steam to keep venting out. Where there is inadequate ventilation in a bathroom it is common for the area of the house around the bathroom to be damp
  • Even people breathing produces moisture so open the windows for a few minutes a day to dry the air. Even if the air is cold outside unless it is raining outside the air will generally be dryer outside the home
  • Do not use unflued gas heaters

Whats not so well advertised is that some New zealand homes can be too dry.  If you are using a fireplace or heatpump this can dry the air easily in areas where the devices are heating and drying the air. Dry air is responsible for dry irritated skin, eczema and can created excessive couching when people have colds.

Many ventilation companies aggresively market expensive devices that claim to dry the air in your home often by reclaiming air from your ceiling cavity. We advise caution when dealing with such companies. Any device that moves air in your home will have an effect on condensation on the windows and remember that even a home of the correct temperature and humidity will have some condensation on windows where there are good curtains present. We have to remember than this lack of air movement overnight between the curtain and the window is what is keeping the rest of the home warm


We recommend that you purchase a basic maximum minimum thermometer with humidity meter for your home, most of which are available for $10-$20 from most hardware shops or the warehouse. This way you can find out whats really going on with the relative humidity in your home in all the rooms over a number of days before you make any large expenditure to control humudity in your home. 


Branz tested vs Branz appraised
Insulation products which are manufactured to New Zealand standards and sold in New Zealand are not required to be tested. As many New Zealand consumers have found out in recent years, some imported products do not actually meets the standard despite claims they do. The Building Research Association of New Zealand (BRANZ) perform 2 levels of voluntary testing of thermal conductivity (r rating) on building products.

Branz testing

Branz testing is carried out by some brands to demonstrate the actual R rating (thermal conductivity) of a test sample supplied to the lab by the manufacters or importer is correct. Branz does not asses the composition of the product and they have not tested the length of time the product will hold its loft although they do monitor the product overtime and data is being collected. A new test sample must be provided every 2 years. We suggest that you request a copy of a test report where product is advertised as branz tested as we have recently discovered many importers claim to have test reports but when asked produced out of date reports that were for products produced and sold over 5 years ago.

Branz Appraisals

Branz appraisal is a far more stringent process where test samples are randomly collected from the market and a companies must demonstrate an ongoing ability to adhere to the advertised r ratings. Where a product claims to be Branz appraised (approved, certified or other) we suggest you check the Branz apprasial register to confirm information is accurate. Branz also does some assesment over time to satisfy itself that the product will achieve the r rating for its minimum durability period of 50 years, which is really important for the customer as they will know they do not need to reinsulate again in the near future.

For all the other attributes not tested by Branz, we recommend you stick to the established brands that have a proven track record to ensure you are receiving a quality, healthy, freshly made product when buying fibreglass insulation in aprticular. Our customers are our main informants when poor quality products are being sold by others and we hope sometime soon legislation will be introduced to protect New Zealanders from poor quality, unhealthy insulation.

We are not aware of any health concerns with Woollen or Polyester insulation currently being sold in New Zealand but advise against all blown products as we know they will sag within a very short period of time. It is for the same reason EECA does not approve these.