Insulation queries

CompetentRoofer has around 450 members who completed around 900 jobs in February and registered another 1200 jobs in March 2015, double for the same period in 2014. The volume of work going through CompetentRoofer has increased dramatically in the past year and this has led to an increase in the number of enquiries of a technical nature, particularly with regard to insulation. Our Technical Manager, Ian Henning, explains.


Those asking queries recently have had two main challenges – insulating boarded up pitched roofs, and fitting insulation into a flat roof which wasn’t designed to take insulation in the first place.

This may be because while thermal efficiency of building has been a requirement in the Building Regulations for many years, it was only in the revised 2006 edition of Part L1 and 2B that consequential improvements came in. This meant that any roofs being renovated had to be checked for thermal efficiency.

As Technical Manager I’ve answered many queries regarding insulation and some of my answers have been published on our FAQ page.  Here are a few of the queries I’ve answered recently:

Pitched roofs

Q: 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 upgraded.

Q: The roofing 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?

A: 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 filling between the rafters with insulation and possibly extra over the top and then counter–battens and battens to ensure airspace under the underlay.

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 top up?

A: Yes - extra insulation can be applied over the boards – but ensure of course that ventilation is not blocked at the eaves.

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: Unfortunately yes, unless the roof was the actual church i.e. a place of worship it must be upgraded to current standards.

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.

Flat roofs

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 work reportable to the LABC.

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 when creating a warm roof, i.e. insulation over the deck and VCL.

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 economically possible. See question below.

Q: A flat roof that is a thermal element (separation of warmed 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 do?

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  existing insulation, sealing any ventilation, applying a vapour control layer (VCL) over the deck, adding insulation  and then applying new waterproofing.

Q: How do I install or upgrade insulation if there is little space or other restrictions?  

A: Insulation need only be installed or upgraded where it is technically practical and economically viable (payback on energy costs saved over 15 years).  If it is not technically possible to install the full thickness of insulation to meet the regulations – for instance it would affect threshold height of a doorway leading out onto the roof – then it would be reasonable to install the maximum amount without adversely affect the existing building. Ultimately the work must NOT make the current situation worse.

See our FAQ page for more solutions and links to useful pages.

Further information about CompetentRoofer, and about how join the scheme, is available on this website.  You can also email us at or call on 0207 448 3189.

Mar 30, 2015 - All news

CompetentRoofer can provide:

  • Quotes from the CEO and key people from within the company
  • Images of members' work, events and exhibitions
  • Media information from our members.

Press Releases:
You can find an archive of all our press releases by clicking here. Please contact the PR & Marketing Coordinator if you wish to be on the press distribution list.

Image & Logo Requests:
We have a number of images that are available upon request. To request the use of the the logo for articles, press releases, please use the contact details listed below.

Press Contact

Laura Mason - PR & Marketing Coordinator