We have produced
Technical Guidance on
IEV and DCMEV. This is
available to members, and the
calculators are included in the
For Path C, where the retrofit design includes any EEMs for the improvement
of the building fabric (e.g. insulation, air-tightness, replacement windows) and/
or a ventilation upgrade (see Annex C), it shall also include an appropriate air
tightness standard for the dwelling after the work has been completed. There
is also a requirement for the Retrofit Installer to demonstrate compliance with
the air tightness standard by means of an approved test (e.g. fan pressurization
testing) in accordance with the standard published by the Air Tightness Testing and Measurement
Path B is covered by PAS 9.2.5 and air tightness testing may not be required. Only assessment of
the adequacy of the existing ventilation system and upgrade in accordance with Annex C is.
Path A is covered by PAS 9.1.12.
For all paths, Annex C 3.2 covers where the ventilation is to be upgraded. Testing may be
needed when the ventilation is upgraded. Where intermittent extract ventilation or passive stack
ventilation have been installed as part of the design, a test may be used to show that the air
permeability is greater than 5 m3/m2h at 50pa.
At present, only SWIGA meet
The Insulation Assurance
Authority (IAA) is currently
working on something too.
The more properties a project
contains, there is a higher risk
of error and defects occurring.
This is because replicating
mistakes at scale increases
its impact on residents and
The PAS2035 Risk Assessment states that
• Fewer than 10 dwellings = Risk Path A
• 11 – 30 dwelllings = Risk Path B
• More than 30 dwellings = Risk Path C
Retrofit Coordinators are reminded that it’s the highest assessed grade across the five criteria that
determines the overall risk path.
So whether or not your project is spread across a wide area or multiple streets, it’s the overall number
of properties that matters
Verge trims and cappings are
not compliant with PAS 2035,
and may not be used, even on
park homes. IAA guidance on
this point is incorrect. SWIGA
is preparing new guidance.
Yes, fire doors should not have
undercuts or grilles. However,
internal doors are rarely fire
doors, except in HMOs.
If the intention of the project is to reduce the air permeability of the house below 5
m3/m2hr @ 50 Pa, or it might be reduced below 5 m3/m2hr @ 50 Pa by virtue of the
insulation and/or airtightness measures, then the ventilation has to be upgraded to
a continuous system. It doesn’t have to be dMEV, it could be cMEV, MVHR or PIV,
but PIV is not recommended.
On the cooker hood point, if the cooker hood is an extracting model (not a recirculating model) then
it can be counted as part of an intermittent extract ventilation (IEV) system. It cannot be counted
as part of a continuous ventilation system unless it runs continuously, i.e. 24/7
Good question! There is
less risk in other spaces,
unless they are unheated, but
you should apply the same
principles as are set out in the
Yes, The Retrofit Academy Centre of Excellence already has software that covers
improvement option evaluation and medium-term improvement plans, and we are
working to improve the formats. The latest version of the software accepts inputs
from Full SAP or from PHPP.
Of course you need the
customer’s consent – it’s
their home! If you don’t
install the required heating
and ventilation the partial
insulation will lead to
condensation and mould
growth in those rooms, eventually damaging
the building fabric and the occupants’
health. So unless you install the heating and
ventilation you may not install the partial
You should use the insulation industry best practice details published by NIA/
SWIGA/INCA and/or the details in the Retrofit Pattern Book published online;
both are referred to in PAS 2035. Your specifications should comply with the
manufacturers’ installation requirements and recommendations, and with the
requirements of the products’ or systems’ BBA certificates.
It has to be possible to heat
the partially insulated or
uninsulated rooms to at least
18oC during the heating
Yes, if you test after you
have installed any proposed
insulation and airtightness
The new flow chart referred to is the one for use with partial IWI only. The PAS
is correct in all other circumstances, but the new flow chart imposes continuous
ventilation if IWI and/or EWI are not applied to 100% of the wall area of the dwelling.
In all other circumstances, you can use an airtightness test to demonstrate that you
don’t need to upgrade from intermittent to continuous ventilation (i.e. Q50 > 5 m3/
m2hr), but the least risk way of proceeding would be always to install continuous
ventilation. Decentralised mechanical extract ventilation (dMEV) fans are a straight
swap for intermittent fans in wet rooms, and they cost very little more.