When winter arrives we do our best to keep warm and protect our health by heating our homes.

Whatever the heat source, all houses leak heat through the walls, floors and ceilings. The best way to prevent unnecessary heat loss is to insulate, preferably to a level above that required by the Building Code. The best time to install insulation is when a home is being built or renovated; existing homes are  considerably harder to insulate.

Typical heat loss percentages are quoted by the industry tor different parts of the building envelope (see Fig 2). Because a greater percentage of heat escapes the roof of an uninsulated house than any other part of the building envelope, it is thought wise to insulate the roof space first. The question is: if just one part of the building envelope is insulated e.g. the roof, is the stated heat loss rate for that component (42%) saved?




To answer this question Winstone Wallboards conducted heat retention tests using Sunrel, an acknowledged simulation design tool, on an existing five-bedroom home with no insulation. We then looked at bringing the house up to NZ Building Code Compliance by adding insulation into each part of the building envelope separately: walls, ceilings and floors. We also looked at the effect additional insulation might have on thermal comfort and possible energy savings (not dependent on occupant behaviour).

The test looked solely at the performance of the living area – the lounge, dining room and kitchen. The results show the number of days (% of year) that the temperature of that area can be expected to fall below 18°C. (The World Health Organisation recommends a minimum indoor temperature of 18°C.)

With no insulation, the living area remains colder than 18°C for 186 days of the year – slightly over half of the year. By adding code-compliant insulation to the floor only, this figure reduces by 7 days to 179 days to the ceiling space only, the figure reduces 22 days to 157 days to the walls only, the figure reduces by 22 days to 157 days.

Note that results for the ceiling space and the walls were the same, even though typical heat loss is reputed to be just 24% through the walls but 42% through the ceiling.

Obviously, a comprehensive approach to insulation obtains the best results. By adding insulation to both the ceiling and floor, the number of days colder than 18°C reduce to 157 per year. But by insulating walls, ceiling and floor the number reduces to 113 days per year – an overall improvement of two-and-a-bit months where the living area will remain above 18°C without additional heating.

So, even though it is wise to insulate your ceiling first, as this is an easy step to do, it doesn’t give a 42% improvement due to the associated heat loss. It appears that walls and ceiling have an equal effect on the thermal performance of the space and ideally you should insulate the entire envelope to make it truly effective.

Note: Tests were conducted using Sunrel, an energy simulation programme intended as research and design tool. Sunrel has been well tested through experimentation, practical use and is used as one of the bench marks for the International Energy Association testing procedure known as Best test. The results above are from a single Sunrel simulation. Variations can occur from house to house 

dependant on the orientation and design of the home. For further information on the Sunrel simulation results please see Winstone Wallboards Ltd.

WRITER JOANNE DUGGAN – Solutions Development & Partnership Manager