February 2024

Ventilation
IN-ROOF SOLAR ROOFING: SHINING A
LIGHT ON NHBC’S NEW GUIDANCE
Matt Neary, from leading roofing products manufacturer Glidevale Protect, takes a closer
look at the new NHBC guidance and the choice of roofing underlay and ventilation solutions
available to help lower condensation risk when using integrated, in-roof solar PV panels.
At the end of 2023 the National House
Building Council (NHBC) published
new standards for 2024 which
included guidance on the use of integrated
in-roof solar PV panels, how they are
classified and what associated ventilation
strategies are therefore required, in
alignment with requirements set
down in BS 5250. The standards
apply with immediate effect to
every new home registered with
the NHBC where the foundations are
started on or after 1st January 2024.
So, what do contractors need to know?
Air permeable or air impermeable? That is
the question!
It’s important to have clarity on whether roofs
with solar panels are classed as being air
permeable or air impermeable, and the new
NHBC guidance aims to address this head on.
The new NHBC Standards contain revisions to
clause 7.2.15 ‘Ventilation, vapour control and
insulation’, stipulating that where arrays of
integrated in-roof solar PV panels are used, the
whole roof covering should be treated as air
impermeable unless the solar panel
manufacturer is able to demonstrate their system
is air permeable.
As a result, roofs with integrated PV panels will
now be classed as air impermeable as standard,
even if the roof tiles themselves are air permeable.
So a small number of PV panels would render the
whole roof impermeable and should be treated as
such, unless it can be proven otherwise. And as an
air impermeable roof covering, there is a higher
risk of interstitial condensation on the underside of
Left: Matt Neary. Above: Protect A1 Solar underlay.
the underlay, the external covering and
behind the solar panel arrays if the correct
ventilation strategy is not used.
What does this mean in terms of roofing
underlays?
When a low vapour resistance (LR) membrane or
vapour permeable ‘breather’ membrane underlay
is used on a warm or cold roof in conjunction with
integrated, in-roof solar PV panels, Glidevale
Protect’s recommended best practice is to install
counter battens. Using 25mm deep counter
battens with a minimum free ventilation area of
not less than 25,000mm²/m at eaves or low level,
and 5,000mm²/m continuous air gap each side of
the ridge or at high level is advised. On a cold
roof, ventilation is also required under the
membrane at 7,000mm²/m at eaves level with a
normal ceiling, or 3,000mm²/m at eaves with a
well sealed ceiling. Alternatively, it is also
accepted to ventilate with 5,000 mm²/m at ridge
instead of 3,000 mm²/m at eaves if there is a
well sealed ceiling. Without the use of counter
battens with an LR underlay, the risk of interstitial
condensation forming on the underside of the
solar panels increases, as the water vapour will
pass through the membrane but cannot escape
into the atmosphere.
One alternative option when using an LR
underlay on a cold roof however, as detailed
in BS 5250, is not to counter batten but
ventilate underneath the underlay as if it was
an HR underlay, thereby requiring eaves level
ventilation and additional ridge ventilation,
the latter only required subject to the roof’s
pitch, span and if classified as a mono pitch.
An alternative solution to using an LR underlay
where in-roof solar is being installed would be to
use a high vapour resistance (HR) or impermeable
non-breather underlay which is designed to prevent
moisture from transferring through the underlay
and reaching the exterior layers above the
membrane, thereby not requiring the use of counter
battens. Ventilation would still be required
underneath the underlay on both cold and warm
roofs, with cold roofs requiring 10,000mm²/m at
eaves level and 5,000mm²/m at ridge, the ridge
ventilation only being required when the roof pitch
is over 35 degrees, a span over 10m or a mono
pitch. Warm roofs require additional airflow with
25,000mm²/m required at eaves or low level and
5,000mm²/m at ridge or high level.
The key takeaways
When it comes to installing integrated PV panels
as part of the roofing structure, the appropriate
ventilation strategy needs to be factored in from
the outset. To discuss your individual project
requirements and how the updates to the NHBC
Standards will affect new roofing specifications
using in-roof solar PV, please contact Glidevale
Protect at technical@glidevaleprotect.com or
0161 905 5700.
Contact Glidevale Protect
www.glidevaleprotect.com
22 TC FEBRUARY 2024