Thinking

Foil insulation in flat ceilings - A reflective view

Matthew Parnell - Sunday, March 14, 2010

In all of this debacle that the installation of foil insulation to flat ceilings is inherently dodgy. Both as a result of the electrical dangers and also due to reduced insulation values. The insulation values of the top surface of the foil must be discounted due to dust build-up: in the old days we used to treat such surfaces as having zero reflectivity, thus zero insulation. Unfortunately, many of the houses with such insulation are  not only potentially dangerous, but are not delivering the insulation values claimed by the suppliers. The suppliers always count the full reflective value of their product, and do not account for the dust effect. This could reduce values by around R1 for summer conditions. Depending on what other insulation has been installed under roof cladding, foil insulation is clearly inadequate in any climate zone (see figures below). In all the media beat up about the insulation debacle, I have not seen any references to this point in any media discussion. Other criticisms are:

Traditionally, foil has never been installed in flat ceilings. I put the change down to aggressive marketing by the plastic bubble insulation manufacturers who spread the fiction that one layer of foil is a direct and equivalent substitution for bulk insulation. They claim this by the most optimistic calculations for insulation, which are often inapproriate for many specific locations.

Most of the images in the media of foil insulation installations are so poorly done that it reinforces the notion that foil is completely inappropriate in this situation. There are too many obstacles for a continuous foil membrane: cuts around truss chords; access for cabling and downlights. Further, the foil completely obscures the timber framing, so that any trades entering the roof space will find it harder to get a foothold. This means more tradies  and DIYers plunging through ceilings!

The insulation values obtained by a single layer of foil is also generally inadequate for all climates, even if the dust effect is not included.

The only foil product suitable for flat ceilings is the concertina-type that fits between ceiling joists or trusses, as it keeps the timber exposed, does not need to be fixed in place and thus avoids clips that may penetrate electrical cabling; and can be more readily cut around downlights and penetrations.

Note that foil in a flat ceiling  only produces insulation value for the bottom foil surface and associated air gaps: assuming a 90mm air gap if installed across tops of bottom truss chords, roughly R1.5 down (summer conditions) and R0.5 up (winter conditions). For plastic bubble foil, add another R0.14-R0.2 for the air bubbles.

For the concertina foil air gap 40mm nom, roughly R1.0 down (summer conditions) and R0.4 up (winter conditions).

For double layer foil batts with two reflective surfaces within cells, R1.2 down (summer conditions) and R0.45 up (winter conditions).

As a rule, do not take the claims of any foil insulation supplier on face value!

Post script: The other items not commented on in the media concerns the use of blown-in insulation, such as cellulose fibres. I have observed installers blowing this material into raked ceilings: this is also inappropriate because the material will settle “down the slope” over time, leaving the upper sections uninsulated. It is clear that such installers do not understand the material they are working with.

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About Dr. Matthew Parnell - With a built environment background, Dr. Parnell's specialty is buildings and their environmental impacts. Greensynergy Consulting is also active in bringing about change by developing the capacity of people, communities and organizations to adopt sustainable practices and develop strong sustainability cultures.
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