February 2024

Ask A. Proctor
BREATHER MEMBRANES & ROOFING
UNDERLAYS: WHAT’S THE DIFFERENCE?
The team A. Proctor Group explain all...
Many people use these terms
interchangeably. BS 5250:2021,
Management of moisture in
buildings – Code of Practice, defines a
breather or vapour permeable membrane as
one with a vapour resistance lower than
Sd 0.12m (0.6MNS/g). These are generally
considered walling
membranes.
Underlays are then split into
two main categories, High
Resistance, or HR underlays
and Low Resistance, or LR
underlays. HR underlays are equivalents to the
traditional 1F felts and will require full ventilation
below them. LR Underlays must have a vapour
resistance below Sd 0.05m (0.25MNS/g).
There are other key performance criteria that are
required, i.e. water resistance. Walling underlays
require a minimum Water Hold Out Classification
of W2, whereas roof underlays are required to be
W1 which is more onerous to achieve. A further
distinction is the air permeability or airtightness
of the underlay, both have benefits depending on
the design.
What are the benefits of air permeability
in a roof?
Air permeable low resistance (APLR) underlays
allow air to circulate through the layers of the
membrane, in combination with having low
vapour resistance. Airtight underlays, the most
common type in the UK, allow vapour to disperse
via diffusion through the layers, having been
made with a microporous film layer in the middle
of the underlay. Air permeable underlays allow
vapour dispersal by two mechanisms, diffusion,
as well as in the air passing through the underlay.
This is down to the unique meltblown core.
Images: Proctor Air Valley detail (above),
eaves detail (left).
Assessments carried out by
BRE and Glasgow
Caledonian University have
shown that air movement
through the underlay increases the vapour
removal from the roof and can be similar to that
achieved by both eaves and ridge ventilation. The
ability to remove ventilation from the roof can be
a huge benefit with the increased need for
insulation, and the difficulty in maintaining
ventilation pathways, stopping cold bridging, as
well as making complex and historic roofs easier
to detail and more aesthetically pleasing.
“Underlays are NOT
designed to be primary
weatherproofing
layers – they should
not be relied on as a
temporary protection
during adverse
weather”
If you are building a project that is NHBC
registered, the use of an air permeable underlay
can also mitigate the requirement for high level
ventilation in cold roof constructions – this
ventilation is required where an airtight underlay
is used.
Proctor Air falls into the category of APLR and
therefore can be used without vents. For
project specific advice our Technical Team
are on hand to review your designs.
How does Proctor Air perform in
extremes of weather?
Developing Proctor Air allowed us to look at
what the membrane needed to do, which
therefore meant we could enhance attributes that
are important for the UK climate, i.e. wind, rain
and UV exposure.
Underlays are NOT designed to be primary
weatherproofing layers, and as per BS
5534:2014, Code of practice for slating and tiling,
they should not be relied on as a temporary
protection during adverse weather, such as
storms and freeze/thaw cycles. In these
conditions, especially re-roofs, the use of a
protection layer, i.e. tarpaulin, should be
considered. UV exposure guidance relates to the
effects of sunlight on the membrane and does not
mean that the membrane will remain water
resistant for this length of time. Proctor Air
contains hydrophobic additives in all three layers
to help improve water resistance.
Wind uplift resistance is also excellent with
Proctor Air. At 345mm batten centres it can be
used in Zones 1-4 in the UK and with centres at
250mm, or laps sealed with Wraptite Tape, it can
be used in Zones 1-5.
Does Proctor Air have a warranty?
Yes, it does. We are delighted to be able to offer a
15-year warranty on Proctor Air. Details of this can
be found in our literature and installation guide.
Contact A. Proctor Group
A.Proctor Group Ltd.
proctorgroup.com
20 TC FEBRUARY 2024