'Ventilated' roof construction
The vast majority of existing pitched roof buildings that include habitable roof spaces are of the ‘ventilated’ construction type. Equally, the type is often preferred when designing for loft conversions and where replacing the roof covering is either impossible or undesirable.
Ventilated roofs are characterised by a ventilated air space between the between-rafter insulation and the underside of the sarking felt. Ventilated roofs differ from unventilated roofs in their different types of sarking. Traditional sarking, or high resistance (HR), sarking felt is usually based upon a bitumen compound and is impervious to transmission of air and moisture. In a ventilated roof, any moisture is evaporated into the ventilated air space and passed to the outside. A ‘Breathable’, or low resistance (LR), sarking membrane on the other hand, transmits any moisture-laden air through its fabric, whilst preventing water penetrating in the reverse direction. In an ‘un-ventilated’ roof, moist air is passed from the roof construction, through the breather membrane, out into a ventilated air space formed by counter battens.
U-value target
U-values normally achievable for the rafter-level insulation of a pitched roof are between 0.20 - 0.16 W/m2K depending on existing rafter depth and available space.
NB
• The roof insulation methods illustrated below are for general guidance only. The performance of products and methods of fixing vary. When specifying, confirm with the product manufacturer their installation methods and expected performance.
• Beware when combining different insulation materials. Check with respective manufacturers regarding possible condensation risks.
Insulation and ventilation
The existing roof condition
• In addition to the possibility of defective / decaying materials, existing roofs are unlikely to meet with current regulations – even less with best practice standards of thermal performance and air permeability.
• Ensure to check the condition of timbers, sarking felt, and roof tiles / slates before proceeding with the works.
• If sarking is not present or roof battens are defective, a new 'ventilated' or 'un-ventilated' roof covering should be considered.
The converted / refurbished roof
It is very unlikely that an existing roof can be brought up to regulation or best practice standards by the installation of insulation between the rafters alone. Naturally, rafter dimensions vary considerably with some Victorian housing featuring rafters of as little as 75mm (labour = cheap, materials = expensive) and with the requirement for a continuous 50mm air gap beneath the sarking, there is often very little space to exploit.
The only realistic way to upgrade the thermal performance of a roof and maintain the existing covering, is to add insulation to the underside of the rafters. Sometimes this can be achieved by effectively extending the rafters, but the commonest way is to fix rigid insulation with an integral vapour control layer to the underside of the rafters.
Typical U-values: between and below rafters
| Between (mm) | Below (mm) | U-value W/m2K |
| Expanded polystyrene (EPS) between and below* | ||
| 100 | 50 | 0.25 |
| 195 | 50 | 0.16 |
Sources: *Vencel Resil;
Ventilation
• There must be a continuous 50mm gap above the between-rafter insulation which is vented to the eaves and the ridge. When locating the insulation between the rafters, it is common to include 50mm battens attached to the rafters in the ventilation zone up to which the insulation can butt.
• Ventilation is required at the eaves equivalent to a 25mm continuous strip and at the ridge equivalent to a 5mm continuous strip.
Air tightness
• In most instances, a combined air tightness / vapour control layer (VCL) should be located on the warm side of the insulation.
• It is essential to maintain the air tightness of the construction. If services are run behind the air tightness layer, punctures are to be expected through pipes and wires entering the habitable space. These punctures should be located through rigorous site inspections and sealed accordingly.
• An alternative is to form a ‘service zone’ (shown in the above example) by adding battens to the underside of the VCL before covering with plasterboard. (This strategy is usually only available if space is not at a premium.)
General design and installation notes
• Before commencing work, examine the roof timbers for rot, damp and infestation
• Confirm with the insulation manufacturer the expected u-values and suitability of planned construction particularly where condensation control is required.
• Ensure that fire protection is provided to the underside of the rafters by using12.5mm plasterboard or similar.
• When using rigid insulation boards, preference should be made for interlocking boards. All joints should be taped and the joint and the junction between the rafter and insulation is sealed with foam (leave a 5mm gap).
• Air cavities where occurring between the vapour control layer and the u/s of the insulation should be sealed top and bottom.
• Ensure that the space between the wall and roof insulations is continuous at eaves.
• Avoid recessed lighting
• Cables less than 50mm away from plasterboard should be enclosed in metal conduits
Publications
Building Research Establishment (BRE)
• Thermal Insulation: Avoiding Risks, C.Stirling, BRE Press, 2001
• Installing Thermal Insulation, BRE Press, 2006
Further information
• Insulation materials compared
• TRADA
• National Insulation Association
• British Urethane Foam Contractors Association
Pitched roof insulation products on GreenSpec
Disclaimer
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