Retrofitting roof insulation

By - , Build 99

Installing insulation retrospectively can be costly and difficult but it greatly reduces heating costs and improves home comfort. Retrofitting roof space insulation is usually the simplest, most effective option and is fully justified by the cost savings.

Feeding insulation into the roof space.

While many houses that were built before 1978 have had insulation fitted, there are still many older New Zealand homes that are not insulated. Installing insulation retrospectively can necessitate removing wall linings and in some situations underfloor insulation cannot be installed at all.

However it is generally relatively simple to insulate a roof space. With between 40 and 50% of heat loss through the roof, roof space insulation is also very effective. The cost can be recouped in around 4 years from savings on heating costs (see Build, February/March 2004, pages 8–9). But the most significant benefit is the improvement to home comfort, which may also result in improved health.

Range of options available

The principal types of insulation are:

  • segment or blanket material – glass wool, wool, polyester
  • loose fill – macerated paper (cellulose), wool
  • rigid and semi-rigid insulation (expanded polystyrene board).

Blanket or segment insulation is available in a range of thicknesses and different R-values. They are fitted between roof framing members. Blankets may also be draped over framing to increase the effectiveness of the insulation.

Loose fill macerated paper or wool is blown into the roof space to the thickness and density required to achieve a particular R-value.

Table 1: Recommendations for roof insulation R-values (adapted from SNZ/PA S 4244: 2003). 
Recommended roof insulation R-value    
    Code compliant (schedule method)BetterBest
Zone 1 (Northland and the Auckland region) Non-solid i.e. timber frame R1.9 R2.6 R3.3
  Solid R3.0 R3.5 R4.6
Zone 2 (rest of the North Island except the Central Plateau) Non-solid R1.9 R2.6 R3.3
  Solid R3.0 R3.5 R4.6
Zone 3 (Central Plateau region and all the South Island) Non-solid R2.5 R3.1 R3.5
  Solid R3.0 R3.1 R4.6
Table 2: Approximate R-values of different insulation materials (from the BRANZ House Insulation Guide, Appendix A). 
Insulation materialThicknessApproximate R-value*
Glasswool segment/blanket 100 mm R1.9
Wool/wool blend segment /blanket 100 mm R1.5
Polyester segment /blanket 100 mm R1.7
Macerated paper loose fill 100 mm R1.8
*The exact R-value for a product must be obtained from the supplier.

Expanded polystyrene boards must be tightly fitted between roof framing members and are only suitable for use where the access and roof space are generous.

R-value rating

Insulation is rated according to its thermal resistance or R-value. A higher R-value indicates a better thermal resistance and therefore more effective insulating capability.

NZBC Clause H1 Energy efficiency, which is currently under review, cites NZS 4218: 1996 Energy efficiency – Housing and small building envelope, sections 3.1 and 3.2 as a means of satisfying NZBC H1.3.1(a) for all buildings having a total floor area of no greater than 300 m2. The revised H1 Energy efficiency is likely to cite NZS 4218: 2004 Energy efficiency – Small building envelope which superseded NZS 4218: 1996.

Feeding insulation into the roof space.
Feeding insulation into the roof space.

Although retrofitting insulation does not require compliance with the Building Code, NZS 4218 Tables 1 and 2 provide a useful guide for R-value selection. (Note that insulation fitted as part of a renovation project will need to comply with current requirements.)

NZS 4218 gives three methods of demonstrating compliance based on three climatic zones – the schedule, calculation and modelling methods.

The schedule method provides tables of minimum R-values (see the code compliant values in Table 1).

The calculation method allows for a range of R-values in different parts of the construction, provided that a lower R-value in one part of the thermal envelope is compensated for by a higher R-value elsewhere.

Lastly, the modelling method uses computer modelling to calculate the energy use and insulation requirements of the building.

Both the schedule and calculation methods are useful for determining R-values for retrofitting insulation. Be aware that these R-values are minimum values and selection of higher values will increase thermal comfort.

Another useful document for determining R-values is SNZ/PAS 4244: 2003 Insulation of lightweight-framed and solid-timber houses. It provides prescriptive (but non-mandatory) specifications for ‘code compliant’, ‘better’ and ‘best’ insulation levels (see Table 1).

Approximate R-values achieved by different insulation materials are shown in Table 2.

Select higher R-value insulation

Selection of a particular insulation material and the R-value should be based on a number of considerations including:

  • cost
  • thermal performance
  • suitability of insulation e.g. glasswool is the most cost effective insulation for its thickness but should always be handled wearing gloves, mask, goggles and overalls
  • ease of installation e.g. loose fill should be considered where roof access is restricted or the roof pitch is low.

When selecting insulation, aim for a higher R-value than the minimum required by NZS 4218 (particularly in a retrofit situation when you are probably installing the only insulation that the house will have).

Insulation placement

All insulation must be kept clean, dry, and clear of water storage tank overflow trays and flues. When retrofitting insulation around downlights, refer to the New Zealand Electrical Code of Practice 54: 2001.

Blankets and segment insulation can be installed by the homeowner. They must be fitted snugly between framing without gaps, tucks or folds, and must not be compressed or packed tightly around electrical wiring.

Loose fill insulation must be installed by professional installers. It can be blown into inaccessible corners of the roof space and can also give total coverage across joists. It should be installed with extra thickness to allow for settlement. It must not be compressed by having any weight placed on it, so it cannot be used in roof spaces that are used for storage.

The benefits of retrofitting insulation into the roof space of an uninsulated house are immediately noticeable in terms of warmer indoor temperatures, lower relative humidity and a reduction in temperature fluctuation (see Build, September/October 1998, page 58). In addition, the longer-term cost savings fully justify retrofitting roof insulation.

For more

A new Standard, NZS 4246: 2006 ‘Energy efficiency – Installing insulation in residential buildings’, is now available.

Download the PDF

More articles about these topics

Articles are correct at the time of publication but may have since become outdated.

Feeding insulation into the roof space.
Feeding insulation into the roof space.