If you are converting or refurbishing a building, knowing when to insulate reveals and what’s acceptable is key to ensuring defects don’t occur. Because let’s face it the last thing you want is a build-up of condensation leading to potential mould growth.

We have put together some guidance to help you determine the necessary standards when insulating window reveals in refurbished buildings.

What is an acceptable level of insulation to a reveal?

Building Regulations Approved Document Part L1B 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 a window reveal.

So what does that equate to in insulation thickness?

This depends on the thermal performance of the existing structure and and the quality of the insulation product.

As an example if insulating a 225mm solid brick wall using phenolic or polyisocyanurate (PIR) insulation, then a typical insulation thickness would be 20-25mm.

What other considerations should be taken into account?

  • Provision of vapour control - should be continuous throughout the insulation layer.
  • Width of window frame jamb - needs to be wide enough to support the reveal.
  • Plaster dabs should be avoided - particularly where the damp proof course (DPC) around the window is not evident.
  • Applying insulation directly to the wall - some insulation products can be applied this way. If not the independent lining should be continued around the reveal.

What if the existing windows are to be retained and the jambs are too narrow?

There are a number of products on the market which are thinner for use with narrow reveals, including some multi foils. However before considering any of these products make sure they have third party certification.

If this is not possible, you should carry out a condensation risk analysis to support the proposed solution.

What if the walls are to be thermally upgraded but not the windows?

This poses a greater risk as the high performance of the upgraded walls could further exacerbate the problem so it is important to obtain a specialist report.

In the case where the walls are to be upgraded externally, the internal reveal doesn’t need to be insulated, but the external wall insulation must be continuous and finish at the junction with the window or door to prevent a cold bridge.

What about cavity walls that have been insulated using blown insulation?

You will still need to insulate the reveal unless the reveal incorporates an insulated cavity closer.

Can a robust detail be acceptable for this construction?

No, the robust details scheme only applies to new construction work and is not suitable for internal solid wall insulation.

If you have any questions about insulating reveals we’d love to hear from you.


BRE 262 Thermal Insulation: Avoiding the risks
Building Regulations Approved Document L1B

By Frzana Ferguson

Please Note: Every care was taken to ensure the information in this article was correct at the time of publication. Any written 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. However, for the most up to date LABC Warranty technical guidance please refer to your Risk Management Surveyor and the latest version of the LABC Warranty technical manual.

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