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7.2.15 Ventilation, vapour control and insulation
Roofs shall have adequate precautions against condensation and cold. Issues to be taken into account include:
- ventilation
- vapour control
- insulation
- pipework.
7.2.15.1 Ventilation
This guidance on ventilation should be read in conjunction with the definitions table at the beginning of this chapter.
Pitched roofs should be adequately ventilated to limit the risk of interstitial condensation. Roof ventilation should be in accordance with BS 5250:2021 Management of moisture in Buildings – Code of practice.
The roof ventilation guidance in this clause is taken from BS 5250:2021 ‘Management of moisture in Buildings – Code of practice’ for simple roof designs, where vents on opposite sides of the roof are likely to provide adequate ventilation. Where it is not possible to implement a ventilation strategy in accordance with BS 5250 due to the complexity of the roof design, specialist advice should be sought.
New-build homes can be at risk of a significant additional moisture load from the construction process and therefore roof-space condensation may occur during the first heating season. The builder may consider additional precautions than those stated below, to help reduce the potential for condensation during this period eg by providing 5000mm2/m high-level ventilation in cold roof construction, where high-level ventilation is not required in the British Standard.
The roof ventilation strategy should be selected dependent upon:
- the roof covering ie, air permeable or air impermeable
- the underlay type ie, Type LR or Type HR
- the roof type ie, cold or warm roof
- the pitch of the roof
- the ceiling type ie, normal or well-sealed (where applicable).
Roofing underlay
Roofing underlays, Type HR or Type LR, should be selected with consideration for the roof type, the outer weatherproof covering and the design of the property, in order to ensure that the required ventilation provision can be met.
Where a Type LR underlay is fully supported on sarking sheets or boards which offer a high resistance to the passage of air or water vapour, such as plywood, OSB, chipboard, or tongue and grooved sarking boards, the Type LR underlay and sarking should be treated as a Type HR underlay, for roof ventilation purposes.
Where Type LR underlays are laid on open-jointed square edged sarking boards, typically 150mm wide with a minimum 2mm gap between each board, these may be treated as Type LR underlays, for roof ventilation purposes.
Air permeability of roof coverings
The manufacturer’s information should be checked to confirm the air permeability of the roof covering. BS 5534 Slating and tiling for pitched roofs and vertical cladding. Code of practice (Annex L) provides information on testing the air permeability of tiles or slates.
Where the permeability of the roof covering is unknown, the roof covering should be treated as air impermeable.
Where arrays of integrated solar roof panels are installed forming the roof covering, the whole roof covering should be treated as air impermeable, unless the panel manufacturer is able to demonstrate that their system is air permeable. Integrated solar roof panel manufacturers may also require a larger air space beneath the panel than stated in this clause, to increase ventilation and cooling of the panel.
Roof ventilation strategies
Roof ventilation should:
- prevent the entry of birds, etc (fabrications with 3mm-10mm openings are acceptable)
- ensure that ventilation pathways remain clear ie, not blocked by insulation or the structure
- have a spacer in the eaves to allow insulation to be installed over and beyond the wall plate to minimise the thermal bridge without blocking the ventilation path (the spacer should be of sufficient length to maintain ventilation throughout the thickness of the insulation)
- incorporate correctly sized, proprietary eaves ventilators, which are fixed in accordance with the manufacturer’s instructions.
For the purposes of health and safety, it may not be necessary to provide ventilation to small roof areas (3m2 or less) over porches or bay windows. Where no roof ventilation is proposed, a condensation risk analysis should be undertaken to ensure the risk of interstitial condensation is mitigated. Alternative designs for small roofs maybe more appropriate, for example:
- warm roof designs, or
- the use of air and vapour permeable underlays.
To avoid condensation in larger roofs, pitched roofs which incorporate insulation should be designed to limit the risk of interstitial condensation. Guidance on roof ventilation strategies can be found using the flow chart below:
Air permeable outer weatherproof coverings
This section provides guidance on the roof ventilation strategy where an air permeable outer weatherproof covering is used.
Outer weatherproof coverings of concrete and clay tiles are typically classed as air permeable; manufacturers' information should be consulted.
Table 11: Cold roof ventilation (Type HR underlay(1) and air permeable(2) outer roof covering)
Note(s)
1. Where referred from Table 15, the ventilation strategy in Table 11 may also be applicable where no batten space ventilation is provided in a cold roof with an air impermeable outer roof covering. In this scenario a Type HR or Type LR underlay may be used.
2. The ventilation strategy in Table 11 is also applicable where an air impermeable outer roof covering is used.
3. Based on the longest horizontal dimension of the roof.
Table 12: Warm or hybrid roof ventilation (Type HR underlay(1) and air permeable(2) outer roof covering)
| Roof pitch | Minimum eaves/low-level ventilation (underneath underlay) | Minimum ridge/high-level ventilation (underneath underlay) | Additional requirements |
|---|---|---|---|
| 10° to <75° | 25,000mm2/m | 5,000mm2/m | AVCL required
Minimum 25mm clear ventilation pathway required(3) |
Note(s)
1. Where referred from Table 16, the ventilation strategy in Table 12 may also be applicable where no batten space ventilation is provided in a warm roof with an air impermeable outer roof covering. In this scenario Type HR or Type LR underlay may be used.
2. The ventilation strategy in Table 12 is also applicable where an air impermeable outer roof covering is used.
3. Minimum 25mm clear ventilation pathway is measured from the lowest point of the underlay drape or underside of sarking.
Figure 46: Hybrid roof — Room-in-roof
Figure 47: Hybrid roof — Room in roof (flat roof dormer)
Figure 48: Warm roof
Table 13: Cold roof ventilation (Type LR underlay and air permeable outer roof covering)
| Roof pitch | Ceiling type | Minimum eaves/low-level ventilation (underneath underlay) |
|---|---|---|
| 10° to <75° | Normal(1) | 7,000mm2/m(2) |
| 10° to <75° | Well-sealed(1) | 3,000mm2/m(2)(3) |
Note(s)
1. A normal ceiling typically has an air permeability of 300mm2/m2 (0.3%).
A well-sealed ceiling conforms to BS 9250 and typically has an air permeability of not more than 30mm2/m2/(≤0.03%).
2. Based on the longest horizontal dimension of the roof.
3. Alternatively, a high-level vent 5,000mm2/m based on the longest horizontal dimension of roof can be provided.
Where no ventilation is proposed to the cold roof void with air permeable outer roof coverings, the roofing underlay (Type LR) must be a low water vapour resistance and air permeable underlay and hold current certification for use in a non-ventilated application, from an appropriate independent technical approvals body acceptable to NHBC. Such membranes should have a water vapour resistance, (Sd), not exceeding 0.05m (0.25 MN∙s/g) and a minimum air permeability of 34m3/m2.h at 50 Pa, or more.
Table 14: Warm or hybrid roof ventilation (Type LR underlay and air permeable outer roof covering)
| Roof pitch | Roof type | Minimum eaves/low-level ventilation (underneath underlay) | Minimum ridge/high-level ventilation (underneath underlay) | Additional requirements |
|---|---|---|---|---|
| 10° to <75° | Warm roof | None | None | AVCL required (1) Underlay drape should be maintained |
| 10 to <75° | Hybrid roof | 7,000mm2/m | 5,000mm2/m | AVCL required A minimum 25mm clear ventilation pathway is required (2) |
Note(s)
1. Where a continuous AVCL is impractical to install, an additional 25,000mm<sup2/m eaves or low-level ventilation and 5,000mm2/m ridge or high-level ventilation should be provided below the underlay. A minimum 25mm clear ventilation pathway is required below the underlay.
2. Minimum 25mm clear ventilation pathway is measured from the lowest point of the underlay drape or underside of sarking.
Figure 50: Warm roof — LR underlay with air permeable covering
Figure 51: Hybrid roof — LR underlay with air permeable covering
Cold roof with flat roof apex
Where a cold pitched roof meets a cold flat roof, there is a risk of condensation occurring on the underside of the flat roof deck. Ventilation beneath the underlay should therefore be provided in accordance with Figure 52.
Air impermeable outer weatherproof covering
This section provides guidance on the roof ventilation strategy where an air impermeable outer weatherproof covering is used.
Outer weatherproof coverings of fibre cement slates, sheet metal, continuous bitumen or plastic membranes are typically considered to be air impermeable; manufacturers' information should be consulted.
Roofs with a Type HR underlay and air impermeable outer roof covering should be ventilated in accordance with the following guidance:
- cold roof — Table 11
- warm and hybrid roof — Table 12.
Roofs with a Type LR underlay and air impermeable outer roof covering should be ventilated in accordance with:
- cold roof — Table 15
- warm or hybrid roof — Table 16.
Table 15: Cold roof ventilation (Type LR underlay and air impermeable outer roof covering)
| Roof pitch | Ceiling type | Minimum eaves/low-level ventilation (underneath underlay) | Minimum batten space ventilation using minimum 25mm deep counterbattens (above underlay)(1) |
|---|---|---|---|
| 10° to <75° | Normal(2) | 7,000mm2/m(3) | 25,000mm2/m
at eaves/low level and 5,000mm2/m at ridge/high level |
| 10° to <75° | Well-sealed(2) | 3,000mm2/m (or 5,000mm2/m ridge or high-level ventilation)(3) | 25,000mm2/m at eaves/low level and 5,000mm2/m at ridge/high level |
Note(s)
1. If no batten space ventilation is provided, the Type LR underlay should be treated as a Type HR underlay and ventilation provided in accordance with Table 11 above.
2. A normal ceiling typically has an air permeability of 300mm2/m2 (0.3%). A well-sealed ceiling conforms to BS 9250 and typically has an air permeability of not more than 30mm2/m2 (≤0.03%).
3. Based on the longest horizontal dimension of the roof.
Table 16: Warm or hybrid roof ventilation (Type LR underlay and air impermeable outer roof covering)
| Roof pitch and type | Minimum eaves/low-level ventilation (underneath underlay) | Minimum ridge/high-level ventilation (underneath underlay) | Minimum batten space ventilation using minimum 25mm deep counterbattens (above underlay) | Minimum ridge/high-level ventilation (above underlay) | Additional requirements |
|---|---|---|---|---|---|
| 10° to <75° Warm roof | None | None | 25,000mm2/m at eaves or low level(1) | 5000mm2/m | AVCL required Underlay drape should be maintained |
| 10° to <75° Hybrid roof | 7000mm2/m | 5000mm2/m | 25,000mm2/m at eaves or low level | 5000mm2/m | AVCL required A minimum 25mm clear ventilation pathway(2) |
Note(s)
1. If no batten space ventilation is provided then the Type LR underlay should be treated as a Type HR underlay and ventilation provided in accordance with Table 12 above.
2. Minimum 25mm clear ventilation pathway is measured from the lowest point of the underlay drape or underside of sarking.
Figure 54: Warm roof – Type LR underlay with air impermeable covering
Figure 55: Hybrid roof – Type LR underlay with air impermeable covering
Pitched roofs with no ventilation provision and air impermeable outer weatherproof covering
Pitched roofs with no ventilation provision and air impermeable outer weatherproof covering will not be acceptable to NHBC.
7.2.15.2 Vapour control
AVCLs should be provided in accordance with the design and be:
- placed on the warm side of insulation
- used in warm and hybrid roof construction.
AVCLs should be:
- installed once framing timbers have a moisture content of less than 20%
- installed once the building is weathertight
- selected in accordance with the design, eg a minimum 500 gauge (125 micron) polyethylene sheet, vapour control plasterboard or a product assessed in accordance with Technical Requirement R3
- fixed at 250mm centres to framing members, including laps and around openings; boards should be fitted in accordance with Chapter 9.2 Wall and ceiling finishes
- lapped into openings, ie, roof windows, dormers, etc
- sealed around service penetrations, where used, downlighters should be specified and sealed to limit air leakage
- made good where damage has occurred.
Joints in the AVCLs:
- should have 100mm minimum laps
- should be located on rafters
- may be sealed with adhesive tape for enhanced airtightness (but joints should still occur over rafters).
Where vapour control plasterboard is used, joints should be:
- positioned on rafters
- cut with care to avoid displacing the vapour control material
- filled, taped and finished, in accordance with the design and manufacturers' recommendations.
Where the ceiling below a cold pitched roof includes an AVCL, the design should ensure adequate ventilation is provided to the habitable areas to prevent condensation problems in the home.
Access hatches to cold roof voids should have:
- an air leakage rate not more than 1m3/h at a pressure of 2 Pa when tested to BS EN 13141-1, or
- a push-up cover with a minimum weight of 5.5kg and compress a closed cell seal or ‘O-ring’ between the cover and frame (clamps may also be required to ensure that the cover compresses the seal).
Proprietary hatches should be fitted and sealed to the surrounding construction in accordance with the manufacturer’s instructions.
7.2.15.3 Insulation
Insulation should be of sufficient thickness to meet the requirements of Building Regulations and laid over the whole loft and wall plate.
The thermal performance of any access hatch should contribute to the overall thermal performance of the ceiling or wall in which the hatch is located and avoid cold bridging.
Table 17: Suitable materials for roof insulation
| Material | Standard |
|---|---|
| Mineral wool | BS EN 13162 |
| Blown mineral fibre | BS 5803-2 |
| Blown cellulose fibre | BS 5803-3 |
| Rigid polyurethane foam | BS EN 13165 |
| Proprietary products | Technical Requirement R3 (1) |
Note(s)
1. Propriety products should hold a satisfactory assessment by an appropriate independent technical approvals authority acceptable to NHBC
7.2.15.4 Pipework
To reduce the risk of freezing or condensation forming on pipework in roof voids, the following precautions should be taken:
- where possible, water pipes should be below the main roof insulation
- water pipes should be insulated in accordance with Clause 8.6.3
- roof insulation should be placed above and around water tanks, but not below them
- ‘cold rising’ pipework above ceiling level should be insulated, even where it is below the main roof insulation.
Last updated: 18th February 2025