S E A L ED UNI TS & S E A L ANTS
Timber frame: accommodating
the differential
With sales of timber homes and buildings heading towards £bn in the next
months,Andy Swift, sales and operations manager, UK & ROI for ISOChemie,
considers sealant tapes for timber frame structures and accommodating
The latest figures from theStructural
Timber Association (STA) show that
timber frames currently account for
% of new housing starts in England,
% in Northern Ireland and % in Wales.
Indeed, as some industry experts predict
offsite construction overtaking traditional onsite
methods due to sustainability factors – with
panellised and modular homes being made from
recyclable materials and using up to 6% less
energy in the building process than equivalent
traditional onsite builds – the outlook for timber
frame does indeed look rosy.
Timber frame has many attractions: it
allows for greater flexibility in design and, as
a sustainable material, is cost-effective which
combined, aid the effort to reduce construction’s
carbon footprint. Built offsite in a factory, with
fittings included, structures such as modular
homes can be installed on site faster than other
methods of house building.
However, when it comes to effective weather
sealing of fenestration and external facades,
which might have to last upwards of to
years, new build timber frame structures
can experience a huge amount of settlement
occurring during the first 8 to ’ months of
their life.Therefore, consideration must be given
to this condition and factored in as part of the
design at the critical product development stage.
Timber frame dwellings have unique
requirements regarding differential movement
between the internal timber wall and the external
masonry wall. Basically, the internal timber wall
will shrink in height due to a combination of
loading and moisture loss, while the external
wall grows in height as the bricks swell due to
moisture gain. As the windows are generally fixed
to the internal timber wall this means that the
windows also have a large amount of downward
movement within the aperture of the masonry
wall.The UK Timber Frame Association’s
‘Differential Movement Calculator’ document,
which lays down compliance requirements for
house builders to follow, suggests that even on a
two storey dwelling, the first floor windows will
move by upwards of 6 mm while second floor
windows on a three storey house can move by
almost as much mm.
IMPLICATIONS
What does this mean for the fenestration
industry? As a window manufacturer, the quality
of the window installation is paramount because
any problems are usually laid at your door;
while installers might see a poorly sealed unit
tarnish their reputation with the house builder or
indeed, the occupants.It also means that using
a silicone solution to provide a seal, which gives
a MAF (Movement Accommodation Factor) of
between œ% - œ% to accommodate the timber
framemovement, could be largely ineffective –
unless extremely large gaps around the window
are required to accommodate movement.
Alternatively, the use of advanced foam
sealing tapes can be beneficial, adding value
to the quality of the final finish. The current
generation of tapes, such as ISO-Chemie’s Iso
Bloco T-Max, can accommodate up to 6 mm
of movement (MAF = œ%) while remaining
weather proof to Storm Force .
When sealing the windows in timber
frame houses, the gap at the head increases
as settlement occurs while the gap at the
cill decreases. Sealing the vertical sides also
becomes problematical due to sheer movement.
This can be overcome using sealant tapes.The
window should initially be located higher in
the aperture than the expected final position,
which means that the head tape will be in a more
compressed state while the cill tape will initially
be in a more expanded state. This will equalise
or reverse following the settlement movement,
but will still remain intact and weather tight.
Horizontal gaps should also be installed first to
prevent any problems forming post settlement.
Tapes can be used to reduce vertical side
gaps, too. They will adhere to the window frame
only, allowing it to creep down the brickwork
during the settlement period, accommodating
the substantial sheer movement forces that can
rip apart a silicone seal.
ADDED BENEFITS
Using a sealant tape means that the days when
an installer would have to return to a house to
remove a damaged post settlement silicone seal
are gone. Neither does he have to go over the
seal with a tool to get an acceptable smooth
finish because tapes already have a flat finish
when supplied.
This is due to an open cell structure
which ensures the surface remains perfectly
smooth and flat, regardless of the expansion/
compression cycle – the tape always expands
and compresses with the same footprint /width
as it was originally installed with.
Impregnated foams can normally be installed
quickly and simply with sufficient compression to
provide a weather proof seal commensurate with
the type of tape chosen, the average being 6
Pa - at this level a joint can resist wind driven
rain up to violent Storm Force (68 mph wind).
Furthermore, because the tapes do not rely on
adhesion to provide an effective seal they can be
installed in the wet, saving time and money – a
‘breathable’ quality ensures that any trapped
moisture in the joint will evaporate when the dry
weather returns.
It’s clear that perimeter seals around
fenestration and movement joints in timber
frame as well as brickwork structures perform a
vital function. However, the correct specification
of jointing materials is essential in ensuring that
the structure remains capable of accommodating
changes and variations in gap size and long-term
performance.
differential movement.
July www.ggpmag.com
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