Products > Roof

RIBA Core Curriculum
- Design, construction and technology
- Legal, regulatory and statutory compliance

Knowledge level
- General Awareness

Condensation, caused by water vapour coming in to contact with cold surfaces which then forms water droplets, can be harmful to buildings and cause issues such as mould and mildew growth. This course examines how to control and avoid excessive condensation from building up within a roof structure by providing passive airflow from the outside of a building and by limiting airflow from within the living spaces.

In this section
- Cold roofs with impermeable underlay
- Cold roofs with vapour-permeable underlay
- Warm roofs with impermeable underlay
- Warm roofs with vapour-permeable underlay
- Examples of well-sealed ceilings

Definftions
High Resistance (HR)
Low resistance (LR)

            
                    
Navigate through each step of the CPD using the left and right arrows to review the content. At the end of each section are some questions, these are required if you wish to obtain a certificate upon completing the course.                     
                                     
            
    

Cold roof with impermeable (HR) underlay
If a building is less than 10 metres wide and the roof pitch is less than 35 degrees, then 10mm eaves ventilation is required.

Cold roof with impermeable (HR) underlay
If a building is 10 metres wide or more or the roof pitch 35 degrees or above, then 10mm eaves ventilation is required together with 5mm ridge ventilation.

In practice, adding high level ventilation creates a far more efficient mechanism for air movement than eaves ventilation on its own.

Air passing over the ridge encourages air circulation through the eaves vents.

The problem with eaves to eaves only ventilation is that it requires more external air movement and the path between opposite eaves is frequently blocked by objects that the homeowner puts in the loft.

It is also known that air passing from eaves to eaves tends to pass close to the ceiling only and does not circulate within the higher roof space.

Cold roof with impermeable (HR) underlay
If the roof pitch is 15 degrees or less then 25mm eaves ventilation is required.

Cold roof with impermeable (HR) underlay
For a lean-to or mono pitch 10mm eaves ventilation is required together with 5mm ridge or top edge ventilation.

Cold roof with impermeable (HR) underlay 5
If the roof pitch of a lean-to or mono pitch is 15 degrees or less then 25mm eaves ventilation is required together with 5mm ridge or top edge ventilation.

Cold roofs with vapour permeable underlay

Cold roof with vapour permeable (LR) underlay
Using vapour permeable underlay reduces the need for ventilation. Although, in practice, no-one manufactures a 3mm continuous vent, it does mean that less vents would be needed if, say, tile vents were being used to provide ventilation at eaves

For cold roofs with vapour permeable underlay, if a building has a well sealed ceiling then 3mm eaves ventilation is required.

If a building has a normal ceiling then 7mm eaves ventilation is required.

In practice, a commercially available 10mm eaves ventilation system would normally be used.

Cold roof with vapour permeable (LR) underlay 1.jpg
5mm ridge ventilation can be used if a building has a well sealed ceiling – making the use of a ventilated dry ridge system an ideal solution.

National House Building Council, usually known as the NHBC, standards require 5mm ridge ventilation where a vapour permeable underlay is installed without eaves ventilation.

This does not apply to underlays that are third party assessed as being vapour and air permeable.

Cold roof with vapour permeable (LR) underlay 2
For buildings larger than dwellings, 5mm eaves ventilation is required with a well sealed ceiling or 10mm eaves ventilation with a normal sealing in addition to 5mm ridge ventilation.

For buildings larger than dwellings, the NHBC Standard does not provide specific examples; it simply states that in larger or more complex roofs it might be necessary to provide additional vents at high level.  If in doubt, it is wise to include high level ventilation.

Good practice for any building over 10 metres wide or over 35 degrees in pitch, is that high level ventilation is used.

Warm roofs with impermeable underlay

Warm roof with impermeable (HR) underlay 1
There should be a 50mm gap between the insulation and tops of rafters or counterbattens. The actual gap will reduce slightly with the drape of the underlay.

25mm eaves ventilation and 5mm ridge ventilation is required.

Warm roof with impermeable (HR) underlay 2.jpg
Where there are obstructions to the air flow, such as firewalls or valleys etc, additional ventilation gaps of 25mm above and 5mm below the obstruction are required.

Warm roof with impermeable (HR) underlay 3
Where the insulation only partially follows the roof slope, such as dormer windows etc, 25mm eaves ventilation and 5mm ridge ventilation is required.

Warm roofs with vapour permeable underlay

Warm roofs with vapour permeable (LR) underlay 1
If the building has a well sealed ceiling and efficient Air and Vapour Control Layer (AVCL) then ventilation is not required.

Warm roofs with vapour permeable (LR) underlay 2
If the building has a normal ceiling then 25mm eaves ventilation and 5mm ridge ventilation is required.

Example well-sealed ceiling details

It is generally accepted that installing a totally convection-tight ceiling is unattainable.  BS 9250 was written to give designers and builders practical ways of installing a well-sealed ceiling – in other words, to provide something that is possible to do, given current construction methods.

BS 9250 provides example details of how to create seals at junctions, such as the junction between the wall and ceiling.  In the following example, it shows the use of adhesive or sealant to seal the gap behind the plasterboard.  If a continuous seal is not in place here, then there is potential for a lot of air flow behind the plasterboards, which will reduce the energy efficiency of the building.

BS 9250 - Code of practice for design of the airtightness of ceilings in pitched roofs
Creating an efficient air and vapour control layer and sealing wall/ceiling junction

BS 9250 - Code of practice for design of the airtightness of ceilings in pitched roofs 2
Example detail of a cable penetration with support and grommet

BS 9250 - Code of practice for design of the airtightness of ceilings in pitched roofs 3
BS9250 explains how to create an Air and Vapour Control Layer (AVCL) using film, and gives examples of how to deal with junctions. This is an example of an airtight AVCL junction detail at ceiling level

Summary

Key points for the control of roofspace condensation

  • Minimise the transfer of moisture within a building to roof space
  • Assume occupants may not always use building as intended and provide robust solutions
  • The use of vapour permeable underlays reduces the amount of roof space ventilation
  • Most roof tiles and slates do not require batten space ventilation*
  • The airtightness of the ceiling determines how much heat, air and water vapour passes into the roof space by convection
  • The vapour resistance of the ceiling determines how much water vapour passes into the roof space
  • Warm pitched roofs do not require ventilation if an efficient AVCL at ceiling is provided

* Examples of roof coverings requiring addition ventilation include some fibre cement slate products (but check with the manufacturer) and metal tile and sheeting products.

Further reading

  • Building Regulation Approved Document C: 2004
  • BS 5250: 2011: Code of practice for control of condensation in buildings
  • BS 5534+A1: 2010: Code of practice for slating and tiling
  • BS 9250: 2007: Code of practice for design of the airtightness of ceilings in pitched roofs
  • BBA or other third party certificates for construction products
  • NHBC Part 7 Pitched Roof Standards: 2012
  • BRE Digest IP/5-06: modelling airflow and condensation in pitched roofs: April 2006
  • Surevent: www.surevent.org.uk

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For cold roofs with vapour permeable underlay, if a building has a well sealed ceiling then how much eaves ventilation is required?

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For warm roofs with impermeable underlay, how much gap should there be between the insulation and tops of rafters or counterbattens?

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