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Pitched Roof - Rafter Level

Pitched roofs are an extremely popular method for roofing many types of commercial and public buildings.

Overview

Many designers of commercial and public buildings (especially offices and schools) use the height offered in the pitch of the roof to create light, attractive interiors with high level sloping ceilings which include light wells and roof windows. The thermal efficiency of the roof along with the desire to create interesting and attractive interiors, can make the specification of insulation for this application a complex issue.

Knauf Insulation Products

Unifit 035 & Unifit 039 are high performance rolls of glasswool. It is easy to cut and friction fit between irregularly spaced rafters.
Classic 040 & 044 isa roll of unfaced glasswool, available in range thicknesses to suit various building applications. It is lightweight, flexible, resilient and non-combustible.
TP116 Slab is a flexible, semi-rigid non-combustible slab of glasswool for use between rafters.

Summary

Knauf Insulation supply products for a range of pitched roof insulation systems. A performance vapour permeable underlay and insulated lining boards are also available for use alongside our standard insulation products

Detailed Design Considerations

Utilising the Roof Void

If part of the roof void is to be utilised it is most likely that insulation will be placed at rafter level. Traditionally insulation is placed between the rafters from below, leaving an air gap between the insulation and the sarking felt for ventilation. Finally, the ceiling is lined with a vapour control layer and plasterboard.

The minimum structural rafter depth, the roof pitch and the thickness of insulation needed to meet the desired thermal performance all need to be considered in designing the roof insulation solution.

Building Regulations

Elemental U-value requirements for pitched roofs.

Controlling the Condensation Risk

Where insulation is placed in roofs there is a risk that condensation will form on surfaces on the cold side of the insulation.

The main way of preventing this is to install a vapour control layer on the warm side of the insulation. This limits the amount of water vapour emitted from the building that can enter the insulation layer.

The vapour control layer must be continuous, well sealed at joints and should preferably be placed behind services, such as electrical cables, to avoid puncturing.

Where a vapour permeable membrane is used as the tiling underlay, 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 buildings.

Where a traditional bitumen based sarking felt, or other impermeable underlay, 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.

Position of Insulation

There is a choice of insulation position in pitched roofs:

At ceiling level - see (add link)

Between Rafter Insulation

Insulation between the rafters can be designed in two ways:

‘Breathing’ roof with vapour permeable underlay as tiling underlay
Ventilated design

Breathing Roof Design

Insulation fully fills the rafter space without an airspace between the insulation and tiling underlay, which must be vapour permeable. If a thin layer of insulation is installed, it is recommended that an insulation/plasterboard laminate is used as the internal lining to prevent thermal bridging. This is not necessary where rafters are at least 140mm deep and fully filled with insulation.

A combined airtight/vapour control layer should be placed on the warm side of the insulation. This not only makes the ceiling convection tight, but also restricts the amount of water vapour passing through the ceiling.

Where cables and piped services are to be installed, the plasterboard lining may be battened out to provide a suitable duct. The services should be routed on the inside of the vapour control layer to avoid any puncturing.

‘

Breathing’ Roof

With a breathing roof, moisture vapour from the building can easily diffuse through the vapour permeable underlay, eliminating the need for cross ventilation. This has several advantages:

The ‘vapour open’ construction avoids the risk of condensation within the roof timbers
The vapour permeable underlay also acts as a windproof layer on the outside of the rafters
The avoidance of draughts, improves the energy efficiency of the roof construction.

Knauf Insulation recommend the use of breathing roof construction for all pitched roofs.

Ventilated Design

With this design, a 50mm ventilated airspace must be provided between the insulation and the tiling underlay. Should the rafter depth be insufficient to accommodate both the required thickness of insulation and the 50mm ventilated airspace, an insulated dry lining is recommended. This has the added benefit of minimising thermal bridging.

Ventilation openings should be provided at each and every roof void at both low and high level. At the eaves, ventilation openings should be equivalent to a 25mm continuous gap. At the ridge the ventilation opening should be the equivalent of a 5mm continuous gap each side of the ridge.

A vapour control layer must be applied to the warm side of the insulation.

Insulation Below Rafters

In each case the insulation performance provided by fully filling the rafters can be improved by the use of insulated plasterboard to the ceiling.

If the insulation board over the rafters has a high vapour resistance, the use of mineral wool between the rafters is not recommended unless the designer is confident that an effective vapour control layer can be provided to the underside of the rafters.

Where only rigid insulation boards with a high vapour resistance are used, such as extruded polystyrene, a vapour control membrane is not normally required unless the building is liable to high humidity. An airspace is not required between the insulation and tiling underlay if the latter is a vapour permeable underlay.

The rafters should be faced with 12.5mm plasterboard, or similar, to provide fire protection.

Upgrading an Existing Pitched Roof

An overall insulation thickness of 250mm to 300mm of glasswool is recommended when upgrading the thermal insulation of an existing roof.

In most pitched roofs, any existing insulation is likely to be between the ceiling joists. Where the existing insulation reaches the top of the joists, the new insulation should simply be laid across the joists at right angles. Where the existing insulation is below the joist height, add extra insulation between the joists so that it reaches the top of the joists. Then lay a second layer at right angles to the joists.

Existing roofs are most likely to have an impervious sarking underlay, such as a bitumen felt, so it is very important to maintain the cross ventilation of the roof void from eaves to eaves.

Thermal Bridges

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/rafter 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/rafter spacings.

Recessed Light Fittings

Where recessed light fittings are to be used, specify ones with 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 roof void and, if necessary, ventilate to the room.

Installation

Contractors should follow the guidance given in HSE 33 when using and installing fragile materials.

Typical Specification Clauses

Glasswool Between Rafters

The whole area of the pitched roof to be insulated with Unifit 035 or Unifit 039*/ Classic 040* ......mm thick, friction fitted between the rafters. The insulation to be pushed over the wall plate at the eaves to link up with the wall insulation. (*Delete as appropriate)

A minimum 50mm airpath must be maintained between the insulation and the sarking felt.