Pitched Roof - Ceiling Level - Technical Detail

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Pitched roofs – ceiling level

Insulation at ceiling level design

Introduction

Insulation in cold lofts is installed in a number of layers with the first layer being laid between ceiling joists, and subsequent layers being laid at right angles to the ceiling joists, with all edges butt jointed to prevent thermal bridging and unwanted heat loss. The principle consideration is thermal performance.

Our loft insulation solutions provide excellent levels of thermal resistance in relation to cost of installation as thickness is largely unrestricted.

Knauf Insulation Loft insulation Glass Mineral Wool

Products

Knauf Insulation Loft Roll 44 and Knauf Insulation Loft Roll 40 are made from glass mineral wool and formed into rolls which are lightweight, flexible, resilient and non-combustible, their manufacture has a very low impact on the environment.

  • Insulating at ceiling level provides most cost effective insulation solution in terms of ratio of cost to energy saved 

  • Very low U-values can be easily and economically achieved

  • Products are compression packed to reduce transport related CO2 emissions

  • Non-combustible with Euroclass A1 reaction to fire rating

  • A+ Generic BRE Green Guide Rating

Typical construction

A pitched roof of timber trussed rafters with tiles or slates on battens and roof tile underlay optionally on sarking board. Plasterboard ceiling below.

Knauf Insulation Loft Roll 44 or 40 is installed in two layers, the first layer between the joists and the second layer across the joists. This minimises thermal bridging through the ceiling joists.

Ventilation requirements

If using an accredited type LR roof tile underlay with a well-sealed ceiling – no or less ventilation is required.

In all other circumstances ventilation at eaves and ridge is required.

Installation

The first layer of Knauf Insulation Loft Roll 44 or 40, the same depth as the ceiling joists, is laid between the joists. The insulation should be taken over the wall plate to link up with the wall insulation. If the roof requires ventilation this is most easily done just before the eaves ventilator is fixed, during installation of the roof tile underlay.

The second layer of Knauf Insulation Loft Roll 44 or 40 is laid at right angles to the ceiling joists, with all edges butt jointed. The insulation should be pushed up tight against the roof tile underlay, or tight against the eaves ventilator, if no ventilation is required, take care not to block the ventilation air path, if ventilation is required.

Electric cables should not be covered with insulation in case they overheat. Cables should be lifted up and ideally fixed to the structure above the insulation or laid on top of the loft insulation. If in any doubt consult with a suitably qualified person, such as an electrician.

Performance

Thermal performance

Knauf Insulation Loft Roll 44 has a thermal conductivity of 0.044 W/mK. Knauf Insulation Loft Roll 40 has a thermal conductivity of 0.040 W/mK.

Fire performance

Knauf Insulation Loft Roll 44 and Knauf Insulation Loft Roll 40 are classified as Euroclass A1 to BS EN 13501-1, which means that they will not contribute to the development of a fire or flame spread

Cold Roof – Other Precautions

All tanks and pipes in the loft should also be insulated to prevent freezing. Do not insulate directly under the cold water tank.

The loft hatch should also be insulated and draught stripped. The thermal performance of the access hatch should contribute to the overall thermal performance of the ceiling in which the hatch is located, and avoid cold bridging.

Air Tightness

Where enhanced airtightness performance is required the designer should consider the specification of a separate and fully sealed air leakage barrier to the underside of the ceiling joists, i.e. a polythene sheet. This reduces the transfer of both heat and moisture, improving the energy efficiency of the building and minimises the risk of interstitial condensation.

    Typical U-values

    Knauf Insulation Loft Roll 44

    U-value (W/m2K) Thickness (mm)    
      Between joists Over joists Total thickness
    0.09 100 400 (2x200) 500
    0.10 100 350 (150 +200) 450
    0.11 100 300 (2 x 150) 400
    0.12 100 250 (100 + 150) 350
    0.13 - - -
    0.14 - - -
    0.15 100 200 300
    0.16 100 170 270

    Note: joist sizes assumed to be 100 x 47mm at 400mm centres, default timber bridge fraction, 12.8% as per BR443. Assumed 12.5mm standard plasterboard and cold ventilated roof with felt or sarking boards. All dimensions are nominal.

    Typical U-values

    Knauf Insulation Loft Roll 40

    U-value (W/m2K) Thickness (mm)    
      Between joists Over joists Total thickness
    0.08 100 400 (2x200) 500
    0.09 100 350 (150+200) 450
    0.10 100 350 (2x150) 400
    0.11 - - -
    0.12 100 250 (100 + 150) 350
    0.13 100 200 300
    0.14 - - -
    0.15 - - -
    0.16 100 150 250

    Note: joist sizes assumed to be 100 x 47mm at 400mm centres, default timber bridge fraction, 12.8% as per BR443. Assumed 12.5mm standard plasterboard and cold ventilated roof with felt or sarking boards. All dimensions are nominal.

    Typical Section

    TYpical roof section

    Typical specification

    The whole area of the ceiling to be insulated with two layers of Knauf Insulation Loft Roll 40*/44* of total thickness ......mm.

    (*Delete as appropriate)

    Width of first layer of insulation to be appropriate to the joist space and of a thickness equal to the joist depth. The insulation to be laid between the joists and over wall plate on external walls.

    The second layer to be laid at right angles to the first layer. All joints between the rolls of insulation to be close butted.

    If an HR roof tile underlay is installed maintain, a 25mm ventilated airspace between the insulation and the sarking felt/ sarking board at the eaves.

    Do not insulate under cold water tanks unless they are elevated. Cold water tanks and pipes to be separately insulated.

    Loft hatch to be insulated with a minimum 100mm depth of insulation.

    Alternatively, consult the National Building Specifications, Standard version clause/clauses… P10/125 and 135……………

    Knauf Insulation specification clauses can be downloaded from knaufinsulation.co.uk/nbs

    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.

    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: 2021places great emphasis on the air tightness of the ceiling to prevent water vapour entering the loft space.

    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

    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 rooflights, 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

    Ventilation requirements for underlays

     

    Type of ceiling

    Size of ventilation openings at:

     

     

    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

    BS 5250: 2021, defines LR underlays as having a water vapour resistance of less than or equal toMN.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.

    Ceiling level insulation with LR underlay - Using LR underlays can obviate the need for ventilation in a pitched roof.

     

     

     

     

     

     

     

     

     

     

     

     

    Ceiling level insulation with HR underlay

     

     

     

     

     

     

     

     

     

    Ventilated roof design

    With a ventilated design, the insulation is placed at ceiling level and cross ventilation of the loft space is used to disperse moisture vapour. It is important to allow a clear, unobstructed ventilation path at least 25mm wide between the insulation and the tiling underlay to allow outside air to enter the loft space and remove the water vapour.

    The ventilation requirements for roofs with a duo pitch are shown above. Lean-to and mono-pitch roofs should have a continuous gap equivalent to 10mm at the eaves and high level or ridge ventilation equivalent to a continuous gap of 5mm.

    Ventilation openings should prevent the entry of insects. A 3mm or 4mm mesh across the ventilation openings should be incorporated. Gaps and holes in the ceiling should be sealed to restrict the amount of water vapour that enters the loft space. Draft seal the loft hatch and provide catches or bolts to compress the draft seal and prevent air leakage from wind uplift.

    Other precautions for ceiling level insulation

    All tanks and pipes in the loft should be insulated to prevent freezing. Refer to BS:5422: 20099 or the TIMSA guidance for achieving compliance with Part L of the Building Regulations for the type and thicknesses of insulation required.

    Do not insulate directly under cold water tanks unless they are elevated. The loft hatch should also be insulated to a minimum depth of 100mm and draught stripped.

    Construction Details

    To avoid thermal bridging the roof insulation should butt up to or lap the wall insulation. The designer should consider at what stage this ‘linking’ insulation is installed, as this will affect the detailing of insulation at the eaves.

    In cold roofs, one way of achieving this in practice is to place a short length of insulation quilt over the wall plate (and cavity closer, where applicable) immediately before the tiling underlay is fixed. This avoids having to push the insulation into place from inside the roof once the roof covering has been completed.

    The gap between gable/separating walls and the first joist should be insulated to avoid thermal bridging. In cold roofs, the second layer of insulation should be butted up against the gable and separating walls to avoid thermal bridging. Knauf Insulation products are supplied in widths to suit standard joist spacings.

    Recessed light fittings

    Where recessed light fittings are to be used, specify LED compact fluorescent or low voltage tungsten lamps to minimise heat build up. Locate the fittings in enclosures that provide at least 75mm clearance around the fitting for air to circulate. Seal the enclosure to prevent air leakage into the loft and, if necessary, ventilate to the room.

    Loft with storage deck

    Where a boarded out storage deck is provided above the loft insulation, a check should be made to ensure that the ceiling joists are adequately sized to support the anticipated loadings. A minimum 50mm unobstructed void should be maintained between the underside of the storage deck and the loft insulation to allow for ventilation which will prevent the buildup of interstitial condensation.

    Solutions optimiser and path finder

    Knauf Insulation Solution Product Construction Code U-value Range (High) U-value Range (Low)
    Between and above joists two layers Loft Roll 44/40 Pc01 0.17 0.08

    Contact our Technical Support Team