reading of 180�C, and an overall average no greater
than 140�C. The purpose of an insulation fire
barrier is to hold back both the fire and its heat
transmitted by all possible transfer mechanisms.
Insulation is therefore particularly important for pro-
tection under developed fire conditions where there
is especially a risk to life and a threat to the building
and contents.
A very effective and capable technology to
achieve both integrity and high levels of insulation is
based on a special intumescent glassy layer in a
sandwich structure between standard annealed
glass panes. In the event of fire, the interlayer reacts
and foams, expanding to produce a stable insulat-
ing layer that provides protection against the heat
of the fire. The layer turns opaque to block out the
fire. In all other respects, the fire-resistant glass
appears and functions as standard glazing.
Both full insulation and integrity can be achieved
with this intumescent technology. The insulation
capability is high over a wide range of classification
test times (from 15 minutes up to 180 minutes), and
test approvals cover a wide variety of different
framing systems and applications. The laminated
make up of the intumescent fire-resistant glass
coincidentally provides other desirable properties, in
particular good noise reduction � a tremendous
spin-off benefit for a product which is used
throughout a building's interior.
Providing sound insulation
The easiest way to increase sound reduction
is by increasing glass mass, i.e. thickness. For
example, a 10mm single pane has an Rw attenu-
ation index of 36dB compared with only 31dB for a
4mm pane and 32dB for a 6mm pane.
But sound insulation is not necessarily a question
of simply increasing thickness. Introducing a certain
degree of asymmetry into the make up of the
glazing by combining glass panes of different thick-
nesses is preferable to the use of two panes of the
same thickness. For example, a double glazed unit
composed of two 6mm glass layers with a 12mm
separating air gap (written as 6/12/6) has a Rw value
of 33 dB but a thinner 6/12/4 unit, using 6mm with
4mm glass, has a better insulation value of 34dB. In
the example of the two 6mm panes the resonant
frequencies of the two panes reinforce each other
causing a dip in the sound insulation at that
frequency. Improvements in sound performance can
be developed by changing the relative thicknesses
of the combined panes to minimise the coincidence
of resonant frequencies.
Sound insulation performance can also be
improved by combining glass in a laminated struc-
ture with a plastic interlayer to produce a damping
effect, such as polyvinyl butyral (pvb) or special
acoustic laminate foils. A standard 6.4mm glass
laminate with pvb, for example, provides a Rw value
of 32dB, a 6.8mm thick glass laminate with a
special acoustic interlayer (such as Pilkington
OptiphonTM) provides an improved index of 35dB.
Increasing the thickness of Pilkington OptiphonTM
readily produces further benefits (e.g. 12.8mm, Rw
39dB).
Comparing acoustic performance
Extreme care needs to be taken in comparing
declared acoustic indices for different products, and
especially in making assumptions regarding acoustic
performance. Estimates may be used in place of
measurements, and each measurement has an
expected variation from result to result, like all
measurements of physical parameters.
It is advisable to use indices measured and
recorded on the same basis; preferable to compare
acoustic performance as a spectrum over the sound
frequency range; and sensible to ask for an applic-
able acoustic test report in the same way that a fire
test report should be requested to confirm a fire
resistance classification . It is also useful to remem-
ber that a difference of 3dB in noise levels is
required to produce a sufficiently noticeable
difference for the human ear.
INTERNATIONAL FIRE PROTECTION 19
THE ADAPTABILITY OF FIRE-RESISTANT GLASS
FIRE RATED GLASS
of fire-resistant
Function Glass
Impact safety Pilkington OptilamTM
Impact safety & sound insulation Pilkington OptiphonTM
Impact safety, sound insulation & fire resistance Pilkington PyrodurTM & Pilkington PyrostopTM
Table 1: Summary

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