It wasn’t until 1978 that wall and ceiling insulation became a required building elements in all new built New Zealand homes. Prior to 1978, ceiling and wall insulation was installed in less than 20% of houses. Oh, Christchurch City Council had started compulsory use in new houses in 1973.
Us Kiwis were a tough lot, considering there was usually inadequate heating and plenty of ventilation. You just wore more clothes inside.
40 to 50% of heat loss in a house is through the ceiling. If you want to reduce the cost of heating a house extra insulation in the ceiling is the place to start. The early insulation installations were minimal: R 1.9 was all that was needed and only in the ceiling. If the insulation thickness is less than 120 mm the ceiling insulation should be increased, and it is usual to keep the existing insulation in place.
Building standards are higher now. In Nelson, a standard pitched roof needs an R 3.3 minimum rating, 175 mm thick as individual batts, or 155 mm as a continuous blanket. There is a benefit from increasing the R rating.
Note that clearance must be maintained with any recessed lighting from the insulation to avoid the risk of heat buildup and fires. 12V ceiling lights are a particular problem due to the considerable heat produced and the need for a transformer for each light. Thermal efficiency is increased by replacing recessed lighting with surface mounted light fittings that do not need transformers.
Insulation is fairly easy to retrofit in the ceilings of houses built prior to 1978. You pass the batts up through the ceiling access panel and spread them out.
Sub-floor insulation should have a minimum rating of R 1.3. Maintenance-free materials are rigid polystyrene foam, polyester batts or rockwool. Reflective foils are unsafe due to the possibility of staples penetrating electrical cabling and the entire subfloor area being electrified. Yikes!!
Fibreglass batts under floors have two significant issues: they are not self-supporting and need to be held firmly in position, usually with plastic strapping, which deteriorates in time; and they are affected by moisture, they are considerably less effective when wet. Fibreglass batts can become displaced due to gravity or by air movement. Polyester batts are more stable.
Installing insulation in walls is clearly awkward. The interior wall lining to the exterior walls must be removed, might as well remove it all, install the insulation then re-line the walls, seal, and paint. Not so easy. Few houses built before 1978 have wall insulation unless this has been subsequently installed.
Also, there was a period when Insulfluff, insulation pellets, was “blown-in” between the ceiling joists. Over time this loose-fill insulation settles, or was blown around by draughts in the ceiling space, so is very much less effective. It is generally better to ignore any existing insulation and fit new insulation straight over the top.
Fibreglass is a cheap but unhealthy material: installation, or popping your head into the ceiling space, requires a good mask to avoid breathing in the fibres of glass blowing around. Fibreglass slumps over time, losing its insulating rating. It has a large energy requirement in its manufacture, various toxic chemicals can be used to bind the fibres more rigidly, and cannot be easily recycled.
Polyester, usually having a high recycled component, does not have these health issues and maintains its insulating properties. Naturally resistant to moisture, vermin, insects, mould, bacteria, and fire, but will burn if ignited with sufficient heat. It can be recycled.