Q: What is the minimum insulation that should be in a roof to meet the minimum U-value of 0.35WmK threshold value?
A: Roughly 140-150mm for “quilt” type insulation and 75-80mm for rigid board type insulation depending on material/manufacturer.
Q: What is the thickness of insulation that will give a U-value of 0.18W/m2K?
A: Roughly 270mm for quilt type insulation and 120mm for rigid board insulation depending on material/manufacturer.
Q: What is a ‘cold roof’?
Q: What is a ‘warm roof’?
For a pitched roof, it means insulation is placed between the ceiling
joists. For a flat roof, it means insulation is placed between the roof
joists, with cross-ventilation between the insulation and the deck.
For flat roofs in particular, it can be a costly method of retrofitting
insulation due to the work required. If not done correctly and with
adequate ventilation, it can lead to problems with condensation.
For a pitched roof, it means insulation is placed at rafter level. For
a flat roof, it means that insulation is placed between the supporting
deck and the waterproof outer layer. This is the recommended method for
When stripping an old roof and the existing roof is close boarded, is
there a requirement to upgrade the insulation underneath?
A: If the total U-value including the existing insulation does
not meet the threshold requirements of Part L of the Building
Regulations (i.e. U=0.35), then yes the insulation will need to be
The roof refurbishment job I'm looking at involves a loft which has
been converted into a room, and the rafters have been covered over with
plasterboard. Do I need to remove some boarding to look at the
insulation thickness and assess the U-values?
Yes - you must know if the existing insulation meets the threshold
value of 0.35 U-value, i.e. about 150mm of quilt. If it does then well
and good – if not then the roof should be insulated to 0.16 – which will
mean stripping the boards filling between the rafters with extra over
the top and then counter–battens and battens to ensure airspace under
Of course it
may not be technically possible to do this because you will raise the
roof finished level; in that case you will have to leave the insulation
and boarding as it is. If they do not have an air and vapour control
layer (AVCL) between the plasterboard and the insulation then there may
be a possibility of condensation occurring if more insulation is added.
If a house has a fully boarded loft with insulation between the ceiling
joists, can extra insulation be added over the boards if required to
A: Yes - extra insulation can be applied over the boards – but ensure of course that ventilation is not blocked at the eaves.
Q: For flat roofs when the timber deck is not being replaced, is there a requirement to upgrade the insulation underneath?
A: If the total U-value including the existing insulation does
not meet the requirements of Part L of the Building Regulations, then
yes the insulation will need to be upgraded if technically and
Q: A flat roof needs to be refurbished; if the waterproofing is replaced what is the minimum thickness of insulation required?
A: Approximately120mm of rigid board.
Q: I am NOT stripping the waterproofing off a flat roof – do I need to upgrade the insulation?
A: No - if you are simply overlaying with new waterproofing
leaving the existing waterproofing in pace then this is a repair not
refurbishment and upgrading the insulation is not required.
Additionally, this job doesn’t need to go through CompetentRoofer as it
is not reportable work.
How do I install or upgrade insulation if the material is not a good
fit, bearing in mind that insulation material becomes more expensive
depending on how thin it is?
Insulation should only be installed or upgraded where it is technically
practical and economically viable (payback on energy costs saved over
15 years). Whatever insulation is used, the roof must also be
ventilated enough to avoid condensation. If it is not technically
possible to install full insulation, for instance it would affect
threshold heights then it would be expected that any insulation would be
installed which does not adversely affect the existing building.
A flat roof that is a thermal element (separation of hot and cold
space) needs to be refurbished and I don’t know how much insulation
there is in the void between the ceiling and the roof. What should I
A: The extent of the insulation must be determined to
see if it meets the requirements of the Building Regulations. Any
insulation is unlikely to meet the minimum standard if it is between the
roof joists i.e. a ‘cold roof’ and so the contractor will need to
address this. The contractor will usually create a ‘warm roof’ by
removing insulation, sealing any ventilation, applying a vapour control
layer (VCL) to the deck, adding insulation to the top of the roof and
then applying new waterproofing.
Churches and graded/listed buildings
Q: When refurbishing a church roof – does the insulation need upgrading?
Q: I am stripping and replacing the tiles on a Victorian church hall. There is about 100mm of insulation in the roof space, must the insulation be upgraded; the church has very little funds?
A: Churches i.e. the place of worship are outside the requirements of Part L of the Building Regulations so would not; however church offices, halls, meeting rooms etc are included and would need insulation brought up to scratch if below the threshold.
A: Unfortunately yes, unless the roof was the actual church i.e. a place of worship it must be upgraded to current standards.
Q: I am tendering to strip and re-slate a Grade II listed building. I intend to use Spanish slate rather than Welsh slate like the original. Am I allowed to do this and do I need to upgrade the insulation?
A: The first part of the question is a planning not building regulations issue. Change to the appearance of a listed building should be checked with the planning department of the local council and English Heritage.
As regards insulation most historic or listed buildings will still need to comply with building regulations and upgrading the insulation is likely, however the work must not prejudice the character of the building or cause degradation of the fabric by using inappropriate materials or design.
The fabric of historic buildings function differently to modern construction and this must be taken into consideration. It may be possible to get an exemption from applying the full requirement of Part L if the technical demands are proven.