Cover your exits – How to prevent condensation and water penetration in refurbishment and conversion projects

The two principle causes of problems with reveals, sills and heads of existing windows are condensation and water / damp penetration.

If these areas of the refurbishment are not sufficiently protected it can be costly to rectify so we have explained what causes these issues and how to prevent them.

What causes condensation in openings?

It is the difference in temperature between the internal and external environments that causes condensation to form on reveals, window heads and glazing.

Why are refurbishments more prone to condensation?

Many refurbishments involve converting previously non domestic properties into habitable spaces.  This is likely to change the whole ventilation / condensation dynamics of the building with regard to the dramatically increased production of water vapour from typical human occupation and activity. The previously adequate ventilation may no longer suffice.

What causes water penetration?

Water penetration is generally not as prevalent as problems caused by condensation but can arise where there are severe exposure zones locally and / or nationally because the condition of the masonry is not adequate enough to prevent infiltration from windblown rain.

This is especially the case with new replacement windows where they are usually supplied considerably undersized to facilitate easy fitting. This leaves unacceptable gaps around the windows that actually reduces the performance of the new window over the existing tight fitting window.

When defective masonry is covered up using render it can actually exacerbate the risk of water penetration particularly around openings. If the detailing is not correct, the previously absorbent masonry has been covered with a semi impervious layer that will cause windblown rain to run down its surface, concentrating in some locations somewhat like a river.  This concentration of water will easily find defects in the render details surrounding windows and doors, and rapidly penetrate.

What can you do to minimise water penetration?

Replacement windows and doors should not have any gaps greater than 10mm around the sides, head and reveal and must be recessed from the face of the external envelope by a minimum of 38mm. It is permissible to increase the perimeter gap / tolerance to a maximum of 15mm for larger doors such as patio doors.

It is imperative that rain water running down the face of the window is rapidly directed away from the envelope of the building by an effective window sill that overhangs the face of the building by at least 25mm and contains a drip at least 10mm from the face of the building.

To prevent issues with water penetration where render has been used it is imperative that the render overlaps the side of the window jambs and that the water running down the face of the building is directed away from the soffit of the head of the window by using a bellcast drip feature or placing the soffit angle in a positon to enable an upward slope to the head of the window.

What happens if a cold gap is bridged incorrectly?

One solution for minimising cold bridging is to construct an insulated independent internal lining. However, this may not be the best method for refurbishment and conversions because it can actually increase the temperature between the internal and external environments. This results in an increase in condensation forming where cold bridging is likely to be an issue.

Once the air within a dwelling reaches the limit of the amount of water vapour it can hold, it becomes saturated, this is known as the dewpoint. When saturated air comes into contact with a surface which is at a lower temperature than itself, the air is chilled at the point of contact and sheds its surplus water vapour on that surface – initially in the form of a mist and, if excessive, eventually in the form of droplets of moisture.

If this process continues for any length of time it will eventually lead to the deterioration of the finishes or even the substrate in the area of the cold bridge. This problem normally occurs with solid masonry or stone structures but can also happen with cavity construction if there is inadequate separation in the form of insulated DPCs or cavity closers between the external leaf and internal leafs on the reveals, heads and sills.

What can you do to prevent saturation and subsequent deterioration of finishes due to a cold bridge?

Firstly, create some form of thermal and if possible, physical separation between the external and internal environments.

Secondly, ensure that there is adequate ventilation within the dwelling to prevent the internal air becoming saturated with water vapour and to promote the drying out of the masonry. The constant changing of the air will enhance this occurring.

It is imperative that the internal insulation to the main external walls is returned around the reveals and heads of the window and that there is some form of separation between the external and internal window sills.

If there is limited space around the window jamb to extend the internal insulation, it is usually possible to install some specialised reduced thickness insulation to the reveal and head that will have an adequate impact on the window jambs and head. This would need a Condensation Risk Analysis or professional opinion of a specialist historic building consultancy to support the proposed solution.

The required thickness of the insulation product depends on the thermal performance of the existing structure and the quality of the insulation product.

As an example, with a 225mm solid brick wall and using a high performance insulation board such as Celotex or Kingspan, a typical insulation thickness for the reveal should be 25mm. However, there are a number of other products on the market including multifoils that are considerably thinner.  Any alternative product used should have some form of acceptable third party accreditation to confirm its suitability and performance for use in this specific situation.

For more information on refurbishments and conversions please refer to Chapter 12 of the LABC Warranty Technical Manual.


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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