Pitched Roofs: Ceiling Level - Technical Detail


Pitched roofs – ceiling level

Insulation at ceiling level design


It is possible to achieve very high levels of insulation in pitched roofs where the insulation is positioned at ceiling level because the insulation thickness is largely unrestricted by construction considerations.

A major factor influencing the design of pitched roofs with insulation at ceiling level, is the type of roof tile underlay chosen. Traditionally, roofs with cold loft spaces were ventilated to allow for the removal of moisture-laden air from the roof space.

The substitution of traditional roof tile underlays such as 1F sarking felt (which have high water vapour resistance and require ventilation) with modern roof tile underlays that have low water vapour resistance (and require no or low levels of ventilation), require the designer to incorporate various design features into the ceiling and pitched roof which are dictated by the type of roof tile underlay specified.

Ventilation and control of condensation

Where insulation is placed in roofs with insulation at ceiling joist level there is a risk that condensation will form on surfaces on the cold side of the insulation in the loft space.

Condensation is most likely to occur where warm moisture laden air (laden with water vapour) is able to pass to the cold loft space but is prevented from dissipating to the atmosphere by the roof structure.

The key steps that need to be taken to prevent the formation of condensation are to restrict the passage of warm air and water vapour through the structure to the cold loft space and allow for its removal if it enters the cold loft space.

The former is achieved by ensuring that the ceiling is well sealed and has a high resistance to the diffusion of water vapour the latter by either ventilating the cold side of the insulation or ensuring that the roof construction will allow water vapour to disperse through its structure and dissipate to the atmosphere.

Reference should be made to BS5250:2021 - BS 5250: 2021 provides guidance for pitched roofs with two types of tiling underlay:

  • Type HR (high water vapour resistance) such as traditional sarking felt
  • Type LR (low water vapour resistance) less than 0.25 MN s/g

Where an LR underlay which has third party certification by the British Board of Agrément, (for use as a tiling underlay for a pitched roof with a cold loft space and insulation at ceiling joist level) is used, any water vapour that does pass through the insulation layer can disperse through the tiling underlay to the outside air. This is known as a ‘breathing’ roof and is recommended by Knauf Insulation for new dwellings.

Where a HR underlay, such as traditional bitumen based sarking felt, is used as the tiling underlay, it is necessary to provide cross ventilation to the cold side of the insulation to enable water vapour to dissipate to the atmosphere and thus prevent condensation forming in the roof construction. BS 5250:2021 places great emphasis on the air tightness of the ceiling to prevent water vapour entering the loft space.

A BS 5250 ‘well sealed’ ceiling

In BS 5250, a ‘well sealed’ ceiling requires the following:

  • The design should avoid holes in the ceiling and constructional gaps, especially at the junction with dry lined external walls

  • No access door or hatch or downlighters should be located in rooms where large amounts of moisture vapour are produced, such as kitchens or washrooms

  • The access hatch should include draught seals that are compressed when the hatch is closed and have an air leakage rate no worse than specified

  • Penetrations, such as those for services and roof lights, should be permanently sealed with suitable proprietary products

  • The ceiling should be sealed to the external walls to limit any leakage through cracks

  • Recessed light fittings should either comply with BS EN 60529 and be rated IP60 to IP65 (depending on room use), or incorporate an appropriate sealed hood or box

  • The head of any cavity in any wall or partition should be sealed to prevent transfer of warm moist air into the roof space

  • A well-sealed ceiling is likely to be a prerequisite of achieving a satisfactory air leakage rate when a new building is tested

  • The air leakage rate through an access hatch, including its frame, when tested in accordance with BS EN 13141-1:2019 should be less than 1m³/h at a pressure of 2 Pa.

BS 9250: 2007 Code of practice for the design of the airtightness of ceilings in pitched roofs gives detailed advice

BS 5250:2021, defines LR underlays as having a water vapour resistance of less than or equal to 0.25 MN.s/g and recommends that only LR underlays with technical approvals given by UKAS accredited technical approval bodies (e.g. BBA) for this type of application are used without ventilation.

If it is proposed to use a LR underlay without this type of technical approval, then ventilation is required as set out in Table 1.

Note that if there is likely to be high initial moisture load in the building due to water introduced into the building during the construction phase, or the designer has any doubts about the viability of constructing a well-sealed ceiling then consideration should be given to installing 5mm high level ventilation irrespective of whether the LR underlay has a technical approval or not.

Where no eaves ventilation is provided, the ceiling level insulation is usually pushed up tight against the LR underlay to prevent air leakage into the loft at eaves level.

Table 1

    Size of ventilation openings at:
  Type of ceiling Eaves High level
No technical approval Normal 7mm -
No technical approval Well sealed 3mm 5mm
With technical approval Normal See relevant technical approval See relevant technical approval
With technical approval Well sealed As above As above



OmniFit® Rolls  are made from glass mineral wool and formed into rolls which are lightweight, flexible, resilient and non-combustible.

Rocksilk® Flexible Slab is a multi-use, flexible, rock mineral wool slab designed for friction- fitting in a range of acoustic, thermal and fire resistant applications.

Typical construction

A suspended and ventilated timber ground floor. The insulation is placed between the joists and supported on polypropylene netting.

The netting should be positioned to support the insulation so that there is no gap between the insulation and the underside of the floor deck.

The floor joists running parallel with masonry walls should be spaced at least 35mm away from the wall to allow insulation to be placed next to the wall.

The wall insulation should start a minimum of 200mm below the top of the floor insulation to minimise thermal bridging.


If the insulation is the full depth of the floor joists, staple the support netting to the underside of the first joist and unroll the netting, stapling to the underside (or side) of each joist as the netting is unrolled.

Where the joist is deeper than the floor insulation, mark the depth of the insulation on the side of the joists. Staple the support netting along this line and pull taut to the adjacent joist and staple again. Pull the netting over the top of the joist and staple to the depth of the floor insulation. Repeat the process until there is netting support to the whole floor.

Install OmniFit® Rolls ensuring there are no air gaps between the insulation and the underside of the floor deck.

Fix the chipboard floor deck in the usual way, using waterproof PVA glue at the joints, and allow a minimum 10mm gap at the room perimeter. If in doubt refer to the chipboard manufacturer's instructions.

When fixing the skirting board, apply a self- adhesive foam strip to the underside of the skirting and two beads of sealant to the back surface. Apply pressure to ensure the foam strip is compressed immediately before fixing the skirting in place.


Thermal performance

Omnifit® Roll 40 has a thermal conductivity of 0.040 W/mK.

OmniFit® Slab 35 has a thermal conductivity of 0.035 W/mK.

Rocksilk® Flexible Slab has a thermal conductivity of 0.037 W/mK, except 140mm thickness which has a thermal conductivity of 0.035 W/mK.

Fire performance

OmniFit® Roll 40, OmniFit® Slab 35 and Rocksilk® Flexible Slab are classified as Euroclass A1 to BS EN 13501-1, which means that they are non-combustible and will not contribute to the development of a fire or flame spread.

Typical specification

Polypropylene netting to be (draped over and between the joists and stapled to the sides of each joist*/stapled to the underside of the joists*). (* delete as appropriate)

OmniFit® Roll 40*/ Rocksilk®Flexible Slab* of ......mm thickness and of width to suit joist spacings, supported on the netting and to be in contact with the underside of the floor deck. OmniFit® Roll 40*/ Rocksilk®Flexible Slab* to be cut and placed to fully fill in the gap between the last joist and the perimeter wall. Flooring grade t and g chipboard to be fixed to the floor joists, all as specified by the designer.

Alternatively, consult the National Building Specifications, Standard version clause/ clauses…P10/250……………

Knauf Insulation specification clauses can be downloaded here.

U-values of suspended timber ground floors insulated between joists

    U-values (W/m2K)
Thickness (mm) Product Ratio of permineter (m) to area (m2)
    0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
300 OmniFit® Roll 40 0.10 0.12 0.12 0.13 0.13 0.13 0.14 0.14
250 OmniFit® Roll 40 0.11 0.13 0.14 0.14 0.15 0.15 0.15 0.15
200 OmniFit® Roll 40 0.12 0.15 0.16 0.17 0.17 0.18 0.18 0.18
150 OmniFit® Roll 40 0.14 0.17 0.19 0.20 0.21 0.22 0.22 0.23
100 OmniFit® Roll 40 0.16 0.21 0.24 0.26 0.27 0.28 0.29 0.30
250 Rocksilk® Flexible Slab 0.10 0.12 0.13 0.14 0.14 0.14 0.14 0.15
200 Rocksilk® Flexible Slab 0.11 0.14 0.15 0.16 0.16 0.16 0.17 0.18
140 Rocksilk® Flexible Slab 0.14 0.17


0.20 0.21 0.21 0.22 0.22
100 Rocksilk® Flexible Slab 0.16 0.21 0.23 0.25 0.26 0.27 0.28 0.29


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