Ceiling Insulation and Under-roof insulation

by Tori
on 25 January 2017

Ceiling Insulation and Under-roof insulation: What should you choose?

Ceiling Insulation photo by Max

Installing above-ceiling insulation and under-roof insulation are two of the best ways you can reduce the heating and cooling loads in your home or commercial building, potentially saving up to 45% on heating and cooling energy, making it more energy efficient and cost-effective to run.

What are the factors you need to consider?

1. To sark or not to sark?

Sarking or sisalation is a reflective membrane that is placed under the roof to reflect the radiant heat from the sun back out of the roof space so that it doesn't penetrate down to the ceiling (and then into the house). Sarking is especially useful in warmer climates.

Reflective insulation only works where there is an air-space to reflect the radiant heat back into, so it needs to be installed with at least a 20mm space above it, or face downwards into the roof space.

Reflective insulation should not be placed above the ceiling as it is too much of a safety hazard in terms of it being a conductive surface in the same space that electricity cables are laid. Furthermore, the effectiveness of a reflective surface is reduced when dust gathers on the surface, so it is more effective in the long term to have it under the roof where dust is less able to gather on the reflective surface.

Reflective insulation of any type cannot touch a metal roof as it may conduct electricity to the whole roof.

Sarking can be installed by itself, or combined with bulk insulation for further advantages.

2.  Foil-faced blanket

A foil-faced blanket (also known as an anti-condensation blankets or builders blanket) is a combination of bulk insulation and reflective membrane.

Anti-con blankets are routinely placed under metal deck roofs in warmer climates to reflect the radiant heat in summer. They also have the advantage of providing an acoustic barrier as well as reducing condensations' harmful effects.

Condensation in well-sealed buildings is a problem in that the moisture from life (breathing humans, cooking, showers) can get trapped in the roof space and when the building cools down (at night for example), this moisture can condensate under the roof and run down the walls of the building. Damp within walls is not good practice. These anti-con blankets are 'anti-condensation' in that they stop the moisture condensing on the cool metal roof surface. A well-ventilated roof space (via gable vents or wind-driven ventilators) will allow condensation to escape.

Sarking or anti-con is not routinley placed under tiled roofs, as air is able to flow out of the natural spaces in a tiled roof, and sarking would cancel this advantage.

Safety factors to consider when installing any insulation are not covering the back of lights that will heat up and possibly cause fires, such as LED downlights, as well as ensuring that no foil sarking or foil-faced insulation is able to touch any electical wires or fasteners.

3.  Under-roof vs above ceiling – where to put the bulk?

question

Anti-condensation blankets come in a number of different thicknesses, equating to R-Values. This opens the question: where do you get the most 'bang for your buck' in terms of R-Values. Is a home-owner or commercial builder better off putting R1.3 anti-con under the roof and R2.5 above the ceiling, or placing R3.7 anti-con under the roof?

There are a number of factors to consider at this point:

a) Pre-build or post-build?

Under-roof sarking or anti-condensation blankets are definitely easiest at build stage, however, installing or increasing above-ceiling insulation can be done at build stage or as a post-build improvement.

b) Insulation Compression

One concern with high R-Value anti-con is that the bulk insulation my become compressed over the roof battens, thus decreasing the R-Value. Care needs to be taken that there is sufficient space for the anti-condensation blanket thickness, and if not, to opt for increasing above-ceiling insulation instead.

c) Cost

Buying one lot of thicker insulation is generally more cost-effective than the cost of two different types of insulation

d) House Design

The combination of roof slope and ceiling slope will have an impact on the effectiveness of the reflective properties of the sarking (more effective the flatter the roof).

e) Climate Zone

The climate Zone the home is in, especially the temperature differential between day temps and night timps will be relevant.

f) Roof Colour

Roof colour is relevant as a darker roof will absorb more of the sun's light energy.

With all the various factors that can impact on the effectiveness of roof and ceiling insulation, it is best to have a NatHERS Assessment performed by an Accredited Assessor to determine the best rating with the least amount of cost to home-owner or builder.

The NatHERS Software takes all of these factors (except for insulation compression and cost) into the Star Rating Calculation, and will generally rate homes with increased above-ceiling insulation higher.

Example Energy Rating Certificate Accredited Page 1

4.  Worked Example

The following is a table of insulation levels on a standard home in NatHERS Climate Zone 9 (around Brisbane)

Anti-condensation R-Value

Above-ceiling insulation R-Value

Total Bulk Insulation

NatHERS Star Rating

- (no additional sarking)

R2.5

R2.5

6.4

-

R2.5

R2.5

6.9

R1.3

-

R1.3

5.0

R1.3

R1.0

R2.3

6.8

R1.3

R1.5

R2.8

7.1

R2.5

-

R2.5

5.4

As the table above shows, it is better to have more of the insulation's bulk placed above the ceiling. Taking comfort and cost factors into consideration (both up-front and on-going energy costs), the best scenario for this home would be R1.3 anti-condensation under the roof, and R1.5 or higher above the ceiling.

For expert advice on your project, whether commercial or domestic, contact BERA for a quote.