When fitting partial fill cavity wall insulation, such as Kooltherm K8 Plus Cavity Board, on a new build project, it is important to check what width of clear cavity (also known as residual cavity) is needed between the outer face of the insulation and the external leaf. The minimum width of this cavity may vary from 20 mm to 75 mm depending on several factors including the insulation material being installed.
In this article, we’ll look at:
- why a clear cavity is needed
- how exposure risk is assessed
- when Kooltherm K8 Plus Cavity Board can be fitted with a 20 mm cavity
- what residual width is typically recommended.
Why is a clear cavity needed when fitting partial fill cavity wall insulation?
Cavity wall constructions were originally developed to prevent damp issues. By forming two separate leaves of masonry with a cavity between, moisture which enters through the external construction can be channelled down and out through weep holes at the base of the wall – keeping the internal leaf dry.
Since the cavity wall constructions rose to popularity a century ago, energy performance targets have been set and repeatedly tightened for buildings. To meet these, insulation is now typically also fitted within the cavity to limit heat losses. When fitting partial fill insulation boards, it is necessary to maintain a minimum clear cavity width. This is to prevent moisture being drawn through the insulation and into the inner leaf or forming interstitial condensation within the construction.
The minimum width for the cavity depends varies depending on project specific factors including the:
- building height
- permeability of the outer construction
- location (specifically the Building Regulations that apply as these are different in Northern Ireland and the Republic of Ireland)
- cavity insulation product being installed
- exposure risk of the property.
How is exposure risk assessed?
In the Republic of Ireland, a Driving Rain Index map is provided within Figure D.1 of S.R. 54: 2014 / A2 2022. This assigns an exposure factor based on the local topography and rainfall. In Northern Ireland, a map of exposure zones for driving rain can also be viewed in the relevant NHBC guidelines. Areas are ranked in four exposure zone categories:
- Sheltered
- Moderate
- Severe
- Very Severe
Properties located in areas with a higher exposure factor in Republic of Ireland, or more severe exposure zone in Northern Ireland, are considered to be at higher risk of driving rain. This is one of the main routes for moisture to enter a building cavity and may therefore lead to stricter requirements around the cavity width and external leaf design and construction.
Whilst these maps provide a reference point for assessing exposure risk, it is important to be aware that local factors can significantly affect the risk for a specific property. For example, a home in an elevated position may be at greater risk of driving rain than the exposure risk maps suggest. On the other hand, properties located in the centre of a development may be more sheltered than expected. This risk should be assessed by a qualified individual.
When can Kooltherm K8 Plus be installed with a 20 mm residual cavity?
Kooltherm K8 Plus has undergone testing to BS 4315-2: 1970. This can allow it to be fitted with a clear cavity of just 20 mm – a reduction over the standard recommendations shown below. Keep in mind, a greater clear cavity may be needed in some applications depending on the exposure risk, building height and the type of external construction and finish.
For further advice please contact the Kingspan Insulation Technical Service Department.
What clear cavity width is typically recommended in the Republic of Ireland?
Under the Building Regulations, a minimum cavity width of 40 mm is recommended for buildings up to 12 metres tall. A reduced cavity width may be deemed acceptable by Building Control depending on the exposure risk, the external leaf design and construction (for example, if an external render is applied or roof overhands extended) and if a product such as Kooltherm K8 Plus (which has undergone additional testing to BS 4315-2: 1970) is used. Refer to the product’s BDA certification for further details.
For buildings over 12 metres and up to 25 metres in height the exposure factor must not exceed 120, calculated in accordance with IS EN 1996- 3:2006 (with masonry or concrete inner and outer leaves) and using the Driving Rain Index. In all other cases a minimum 50 mm residual cavity must be retained.
What clear cavity width is typically recommended in Northern Ireland?
The Building Regulations in Northern Ireland recommend a minimum 25 mm residual cavity width between the insulation and the outer leaf for wall heights up to 12 metres. Again, it may be possible to agree a reduced cavity width with Building Control depending on the exposure zone, design, wall construction and insulation material.
A minimum 50 mm residual cavity width is recommended for wall heights greater than 12 metres and up to and including 25 metres.
In addition to the Building Regulations, the NHBC and Zurich Building Insurance Technical Standards set further requirements which developers need to be aware of. These normally require 50 mm residual cavity width in areas of Severe exposure to wind driven rain and a minimum 75 mm residual cavity width in areas of Very Severe exposure to wind driven rain, where nominal standards of tolerance and workmanship are accepted.
The NHBC and / or Zurich Building Insurance Technical Standards are able to accept a 50 mm residual cavity width for Kingspan Kooltherm K8 and K8 Plus in all exposure zones including areas of Very Severe exposure where the outer leaf is fair faced masonry with tool flush joints.
What is BS 4315-2: 1970?
BS 4315-2: 1970 describes a test to assesses water penetration through permeable wall surfaces.
For the test, a wall construction is fitted on a test rig and fitted with pressure-box test equipment. The equipment maintains a positive air pressure (simulating driving wind at a rate of around 45 mph).
The construction is sprayed with water for 1 minute, every 30 minutes over a 48-hour period. The results are typically assessed using three methods:
- Recording, by time-lapse photography, the increase in area of dampness
- Recording the change in weight of the specimen
- Collecting and recording the amount of leakage through the specimen.
