Insulate Magazine Issue 11

www.insulatenetwork.com
Space Consideration
Compromises can also be
caused by on-site constraints,
a good example being that of
hybrid roofs. Imagine a flat roof
constructed on timber deck
and joists: it’s not surprising
that people sometimes look at
the space between joists and
wonder if it can be filled with
additional insulation. If there is
a restriction on the thickness
of insulation that can be
accommodated above the
structure, then the temptation
to utilise the extra space is
even greater.
However, problems can
occur if a standard warm roof
is designed and constructed,
but with additional insulation
incorporated below the deck.
Not only is a vapour barrier or
vapour control layer (VCL), a
vital component of the roof, it
must be correctly positioned -
on the warm (internal) side of
all the insulation. There are
numerous reasons why
incorrect placement of
insulation relative to the VCL
should be avoided, perhaps
best summarised in the
foreword of BS 5250:2011 +
A1:2016 Code of practice for
the control of condensation in
buildings, which states:
‘Bearing in mind that
occupants often fail to use
buildings
There are numerous reasons
why incorrect placement of
insulation relative to the
Vapour Control Layer (VCL)
should be avoided
in the manner intended, be it by choice, lack of
understanding or force of circumstance,
designers are advised to err on the side of
caution and adopt robust, fail-safe solutions.’
Label with care
To that end, section H.2 of the standard lists
three acceptable scenarios for the placement of
insulation in a flat roof: cold roof, warm roof and
inverted roof. Nowhere does it support the
design and construction of hybrid flat roofs.
Some insulation manufacturers, however, are
happy to advocate hybrid constructions, usually
when the balance of the two insulation
thicknesses is considered to be “correct”, i.e. the
layer of insulation between the joists does not
have higher thermal resistance than the layer of
insulation installed over the deck.
A condensation risk carried out in accordance
with the method detailed in BS EN ISO 13788
insulatenetwork
27