Underfloor insulation

Good underfloor insulation can make a big difference to how comfortable your home is, and to the size of your energy bills. In homes with accessible underfloor spaces, it’s relatively cheap and easy to do.

Rough Costing

From www.expol.co.nz

An average-sized New Zealand home is around 100 square metres  and the cost of EXPOL R1.4 White Premium polystyrene underfloor insulation is approximately $11 per square metre (note – underfloor insulation cost will vary per retailer).

That makes the cost of insulating an average home around $1,100*. As an easy-to-fit insulation product, EXPOL also makes DIY a breeze – saving you money on installation.

DIY checklist

If you can access your underfloor area, have a look for insulation. There are 3 things you might find:

• Bare floorboards and no insulation – in which case you need to get some fitted.
• Foil-based products – don’t touch it if it’s held in with metal staples. There’s an electrocution risk if the staples have pierced electrical wires and the whole lot might be live. When checking or removing existing foil insulation, always turn off the power supply to the house and follow the Electrical Code of Practice ECP 55 which provides guidance for managing electrical safety risks of foil insulation. Get an electrician to help you if you’re not sure.
  If your foil appears to be well fitted and in good condition, then it’s probably working okay for now. Well-fitted modern bulk underfloor insulation can be much more effective. If the foil is in bad condition it needs to be replaced with bulk insulation. Retrofitting or repairing foil insulation in residential buildings is now banned under section 26 of the Building Act 2004.
• Bulk insulation – includes rigid polystyrene sheets, or softer products like polyester, wool and fibreglass. Check to see that any bulk insulation is tightly fitted against the underside of the floorboards with no gaps or pieces missing. If any has slipped or fallen out, you should replace it. You may need some clips or other fittings to hold it in place.
• While you are under the house it is a good time to install polythene sheeting to stop moisture rising up from the ground. For more info check out countryandcoast.nz/home-dampness/

Choosing underfloor insulation

There are two main types of underfloor insulation: bulk and foil insulation. We recommend using bulk insulation.

Bulk underfloor insulation

Polyester, wool, polystyrene and glass wool are some of the materials bulk underfloor insulation can be made from.

How bulk insulation works

Bulk underfloor insulation products are either friction-fitted between the floor joists, or held in place by fixings such as staples, strapping or clips.

Checklist

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

• intended for installation under suspended floors
• an R-value of at least R1.4 – R-value is a measurement of the insulation’s effectiveness, the higher the R-value the better
• the right width – for your floor joist spacing (this can vary under older houses, so you may need to measure between all joists)
• compliant with the Standard AS/NZS 4859.1 – so you know the insulation works as its says it does (look for the label on the insulation packaging)
• installed hard against the underside of the floor – there should be no air gap between the bottom of the floor and the top of the insulation
• able to perform in your location – if your subfloor space is not fully enclosed, you need a product that has been tested for performance and durability in windy conditions.

You may also want to check:

• performance guarantees offered by insulation manufacturers on their products
• manufacturers’ instructions for safely and correctly handling and installing (if you’re thinking of doing it yourself).

For rental homes, insulation must meet the Residential Tenancies (Smoke Alarms and Insulation) Regulations 2016.

Insulation regulations for rental homes – Tenancy Services website

Under Slab Insulation

When building a new home or extending your house it is good practice to insulate under the concrete slab. Talk to your architect or draughtsman about under slab insulation.

Foil insulation

Foil was the most common material used for underfloor insulation in New Zealand. Because of safety concerns, retrofitting foil insulation and repairing foil insulation in residential buildings is now banned under section 26 of the Building Act 2004.

Current warnings and bans issued under the Building Act – Building Performance website

Safety risks

If you install or repair foil insulation under an existing house you face serious risk of electrocution if the foil or staples used to fix it come into contact with live electric wires under the house. As foil is metal-based, it conducts electricity.
When checking or removing existing foil insulation, always turn off the power supply to the house and follow the Electrical Code of Practice ECP 55 which provides guidance for managing electrical safety risks of foil insulation.

Electrical Code of Practice ECP 55 – Energy Safety website

Performance concerns

The performance of underfloor foil is dependent on how well it is installed. The air gap between the floor and the foil needs to be well sealed which is very difficult to achieve. Any air movement between the floor and the foil will reduce the foil’s effectiveness, as will dust settling on the foil over time, which reduces its reflectivity. Foil insulation can also be damaged easily, e.g. by wind or cats. Damaged foil will need to be replaced with bulk insulation as you are not allowed to repair foil insulation in residential buildings

Information from https://www.energywise.govt.nz

For more information check out how to install various insulation.