October 2022

Ask The A Proctor Group
MEETING AIRTIGHTNESS
REQUIREMENTS
In this series, the A. Proctor Group examines some commonly held myths in the
construction industry concerning thermal and acoustic insulation, condensation control,
and fire protection. In this article, the team tackle the question: Is it true that the best
way to meet airtightness requirements is by using an internal VCL?
AVCL, or vapour control layer, is a layer
within a building construction that
prevents water vapour passage. The VCL
is positioned on the side of a construction
element that would have the highest vapour
pressure and therefore protects the insulation
and structure from the ingress of water vapour.
In the UK, vapour drive flows from inside to
outside, so a vapour control layer should be
applied to the warm side of the insulation. VCLs
are typically made of polyethylene, or foil, which
are impermeable to both water vapour and to air.
Therefore, they are often called Air and Vapour
Control Layers, or AVCLs. As these AVCLs are air
impermeable, many constructions incorporate
them as the primary airtightness line for the
building, though this does present some
challenges that must be overcome.
Being positioned on the internal face of a
construction means that the AVCL interfaces with
many building services which could damage it.
For example, services such as plumbing or
electrical fixtures in the external walls will need
to puncture the airtight line. Typically, these
require specialist airtight backing boxes or other
seals which need to be taped to the AVCL to
ensure the continuity of the airtight line. Any hole
in the airtight line that has been improperly
sealed is detrimental to the airtightness of the
construction. Therefore, the VCL and
plasterboard fixings need to be carefully
considered during the design phase.
Additionally, the internal lining of the building is
subject to punctures created by the building user.
Things like picture hooks and television brackets
cause unexpected penetrations in the airtight
membrane, which can lead to air leakage that
has not been accounted for in the original design
of the building.
Internal airtight membranes also require careful
and accurate detailing around floors, ceilings,
and internal walls. The level of airtightness that
an internal membrane can provide to a building
depends on how well it is detailed, with small
holes potentially creating significant issues.
Externally wrapped
An alternative and more effective option is to
make the building airtight from the outside. Using
an airtight vapour permeable membrane means
the entire structure can be wrapped externally.
This external membrane means that a
homogenous airtight line is formed around the
entire construction, which results in interaction
with far fewer building services.
Using an external airtight membrane provides the
benefit that it only needs to interact with the
fixing brackets and possibly external insulation.
This results in a more robust construction, which
maintains its effective airtightness even after
completion.
A commonly held belief is that all airtight
membranes are vapour control layers, which
would cause condensation if used outside of the
insulation. As part of its extensive highperformance
range of construction membranes,
the A Proctor Group offers Wraptite, a unique,
fully self-adhered airtight and vapour permeable
membrane. The Wraptite membrane is typically
adhered to the sheathing board. The membrane
can span across concrete frames and columns,
allowing for complete encapsulation of the
building with detailing that is far easier. Because
Wraptite is self-adhered, it doesn’t require
fixings, and in exceptional circumstances that do
require fixings (e.g. for a rainscreen façade), the
adhesive will provide a measure of airtightness.
Depending on the type and placement of the
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44 TC OCTOBER 2022