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On conversions and refurbishments to existing buildings, if insulation is missed around window reveals, condensation and unsightly mould can occur on the internal room side of the wall finish.
This article is based on questions and answers in order to determine a satisfactory standard in buildings being converted or refurbished. Please also refer to BRE 262 Thermal Insulation: Avoiding the risks.
Applicable sections of the Technical Manual:
Section 6 – External Walls
Section 8 – External Windows and Doors
Appendix D
Insulation is often missing as the thickness of insulation required on a reveal is often greater than the depth of the window frame itself, resulting in builders reducing or omitting the insulation.
Part L (conservation of fuel and power) of the Building Regulations recommends that the minimum performance of any insulated element should have a U value no worse than 0.7 w/m2K. This U value should be used as the minimum standard for an internal reveal.
This article does not attempt to answer that question as it depends on the thermal performance of the existing structure and the quality of the insulation product but as an example with a 225mm solid brick wall and using a high performance insulation board, a typical insulation thickness could be 20-25mm or more.
Vapour control: A vapour control provision should be continuous throughout the insulation layer. An EPDM bonded to the masonry reveal face and lapped to a DPC stapled around timber windows might address the risk of water ingress around window units but could create a condensation risk within the construction. Condensation should be negated by the introduction of a VCL and a thermal analysis of the proposed reveal make-up is strongly recommended.
There are a number of products on the market including some multi foils that are thinner which may be used. Any product to be proposed, must have third party certification.**
It should be noted, some, multi-foil insulation products require two air gaps of about 25mm per gap to function correctly.
**Where multi-foil insulation is proposed, the installation requirements and scope of approval on the third party accreditation certificate should be checked prior to installing (the certificates scope of approval might not cover the type of installation being proposed).
Other solutions may employ Aerogels which are very thermally efficient, naturally hydrophobic and can achieve the same thermal performance as PIR insulation using a thinner amount of material.
Where the jambs are too narrow and it is not practical to provide insulation, a condensation risk analysis could be provided by the developer to support the proposed solution, however the waterproofing detail around the frame still needs to be robust enough to prevent lateral damp penetration.
Our Technical Services Department will pick this up during the course of the ‘Refurbishment Assessment’ and discuss our concerns with the Developer during the course of this process.
Yes. Unless the reveal incorporates an insulated cavity closer.
No, but the external wall insulation must be continuous and finish at the junction with the window or door to prevent the cold bridge.
For refurbished buildings, interstitial condensation around the reveals are a common issue we come across. The method of insulating the reveal should be discussed with all interested parties early on in the design process.
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Every care was taken to ensure the information in this article was correct at the time of publication. Guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. For the most up to date Premier Guarantee technical guidance please refer to your Risk Management Surveyor and the latest version of the Premier Guarantee Technical Manual.