Designing for UK Exposure Zones

Wind Driven Rain

Exposure and Wall Design

As a general rule, the more exposure to wind driven rain the more robust your wall construction will need to be. In the following section we will explore the recommended wall build ups along with technical section examples for masonry cavity with both full-fill and partial fill insulation. Key features of different features in robust wall constructions are listed below and ordered from most to least robust:

MOST ROBUST

Rendered finish to the external face of the wall – this helps to increase the robustness of the facade, stopping the rain from forcing its way into the wall systems whilst allowing water vapour to move from the inside to the outside of the structure.

Mixed finishes – Where you have both render and facing brickwork, you should assume the requirements of the least robust and build the cavity to those specifications.

Partial fill cavities – Partial fill cavities tend to be a more robust option against wind driven rain as any water that filters through the facade can drain through that cavity. However, there are some options for full fill cavities for high exposure zones. 

Full fill cavities – Full fill cavities are becoming more popular due to higher u-value requirements. Although they tend to be a less robust option compared to partially filled cavities for wind driven rain, they can be used if specified with the right cavity width. 

Facing brickwork with flush joints – This is the most robust type of facing brickwork and is the only type suitable for high exposure zones. 

Recessed mortar joints – Different types of recessed mortar joints such as concave, vee, struck, raked and weathered should only be specified in areas of low exposure. 

Flush sills and copings – This is when copings and sills do not extend 45-50mm over the external face. In areas of low exposure this is permitted as it is assumed that these areas will not receive much wind driven rain.

LEAST ROBUST

The following wall details have been drawn using Table 4 – Maximum Recommended Exposure Zones for Insulated Masonry Walls shown above, found within Building Regulations Part C. These recommended cavity or residual cavity widths are minimums based on exposure and are not based on Building Regulations (BR) Part L. U-value requirements to meet BR Part L will most likely require thicker insulation widths in most exposure areas.

The table provides 140 variations of wall build-ups in total and 56 for built-in full fill and partial fill cavities. We are showing 4 possible build-ups below. More options are available at the bottom of this post. The key features of a cavity masonry wall for each exposure zone are detailed below:

Exposure Zone 4 – Very Severe

100 or more litres/m2 per spell

Within this exposure zone, cavities should be made as wide as possible, this is helped by the updated u-values required for new construction. The most robust external wall finish is a rendered-finish although facing brickwork is accepted with wider cavities. Traditionally only partial fill cavities were allowed although full-fill cavities are allowed if at least 150mm. In the example above, to achieve the required u-value, insulation thickness may need to be increased, but the clear cavity must remain a minimum thickness of 50mm.

Exposure Zone 3 – Severe

56.5 to less than 100 litres/m2 per spell

Within this exposure zone, cavities can be less wide than for very severe locations, however, this is more dependent on the u-value requirements than exposure. Most full-fill cavity widths within a flush facing brickwork wall are accepted at this exposure zone. All residual cavity widths with partial fill insulation are suitable to this zone. A wall can always be designed for a higher exposure zone but only for a lower zone if the wall does not face into the prevailing wind. For buildability however, you may want to design all walls to the maximum exposure on the site, rather than having different facade thicknesses and finishes. In the example above, insulation thickness may need to be increased to achieve the required u-value within the building regulations.

Exposure Zone 2 – Moderate

33 to less than 56.5 litres/m2 per spell

Within this exposure zone, in general, cavities can be thinner when using flush mortar joints. The use of recessed mortar is allowed in some cases with wider cavities or residual cavities, as are flush sills and copings. In the example above a minimum of 100mm residual cavity is necessary due to the recessed mortar joints. If using flush mortar joints, only a 50mm residual cavity is required. To achieve the required u-value, insulation thickness may need to be increased, but the clear cavity must remain the minimum thickness within BR Part C.

Exposure Zone 1 – Sheltered

Less than 33 litres/m2 per spell

Within this exposure zone, in general cavities can be even thinner. The use of recessed mortar is allowed in most cases as are flush sills and copings. While the minimum requirement of 50mm is specified in Part C, it is likely this will not achieve the u-value requirements of Part L, therefore the thickness of the full fill insulation may need to be increased in order to comply. Thicknesses are also dependent insulation specification and should be installed to supplier / manufacturer’s recommendations.

1. Where local conditions accentuate wind effects, such as open hillsides or valleys. Where the wind is funnelled onto the wall, add one to this exposure zone value.
2. Where walls do not face into the prevailing wind, subtract one from this exposure zone value. 

The exposure zone outlines the minimum cavity or residual cavity thickness based on exposure and finishes. A number of other factors may also affect the thickness of a masonry cavity wall including:

• U-value requirements
• Any structure protruding into the cavity
• Lintels / steel support dimensions
• Doors and window frame thicknesses

Exposure, Level thresholds and drainage

High Exposure

For high exposure areas, drains can be designed to abut the door system or external wall to maximise water draining within a level threshold as well as any water that hits the window or door system and drains downward.

Many window and door systems have an internal drainage channel within the base frame or sill which can be connected to a drainage system. Some sliding door systems even come with their own drainage channel which will clip onto the base frame or sill. The flow rate of the drainage channel should match the requirements of the location and exposure required. Visible drains with a removable grill will allow for easy maintenance, cleaning and removal of any debris such as leaves.

It may also be necessary to install a trench drain to the external walls as described in the low exposure section below.

Medium Exposure

For areas of medium exposure a proprietary external drain may be fitted between or under the door sill and external finishes. This includes slot drains or drainage channels which are hidden under the external finishes. These will not be able to be accessed from above for maintenance but can still have inspection areas to the sides.

Low Exposure

For this case, what is sometimes referred to as a french, trench, land, filter or perimeter drain can be installed. This consists in a PVC drain pipe with perforated holes built into a trench filled with larger stones. Rainwater can then filter through the stones, reaching the pipe and being redirected to the main waste water drain or a soakaway. If this type of drain is being covered by earth, a non-woven geotextile fabric should cover the stones before earth is placed on top, preventing the soil from filtering into and blocking the drain.

This type of drain can also be used in higher exposure areas on the perimeter to further divert water away from the building. Where there are basements or parts of a building below ground level, this type of drainage may also be used alongside tanking membranes.

Very Low Exposure

For areas of low exposure or for small sections of the facade, it may not be necessary to have any drainage and simply design paving away from the external wall to alternative drainage zones.

Summary

We have compiled a selection of cavity masonry wall options showing different widths and finishes based on exposure zones. These can be downloaded here [Include link to file.]

There are also a number of different drainage options to choose from when designing level thresholds. While the building regulations only advise over high exposure zones, there are a number of options depending on the aesthetics, door system choice and sill choice. We have included a few within this post but more details can be found on the Detail Library’s website.

Links and Image Credits

Source URL: https://studiomr.co.uk/peasburge-barn/

Image URL: https://i.pinimg.com/564x/f3/57/d2/f357d2efe65def162025aed4eb46d5a8.jpg

Architect credit: Studio MR [https://studiomr.co.uk/]

Photographer credit: Studio MR [https://studiomr.co.uk/]

Source URL: https://www.iqglassuk.com/products/flush-threshold-drain-for-doors/s62497/

Image URL: https://i.pinimg.com/564x/f6/39/7c/f6397caf9abaaac019c9a6f2df699586.jpg

Architect credit: IQ Glass [https://www.iqglassuk.com/]

Photographer credit:  IQ Glass [https://www.iqglassuk.com/]

Source URL: https://www.oliverleech.com/parsons-green

Image URL: https://dvqlxo2m2q99q.cloudfront.net/000_clients/67972/page/w1000-oliver-leech-architects-st-maur-road-9-f27248.jpg

Architect credit: Oliver Leech Architects [https://www.oliverleech.com/]

Photographer credit: Tim Crocker [https://www.timcrocker.co.uk/]

Source URL: https://lofthome.nl/stalen-huis-voorbeelden/

Image URL: https://lofthome.nl/wp-content/uploads/2021/12/lofthome_nlst41_07web-1325×1458-1.jpg

Architect credit: bkvv architecten [https://bkvvarchitecten.nl/]

Photographer credit: bkvv architecten [https://bkvvarchitecten.nl/]