Pitched roofs can be insulated to create either warm or cold
roof spaces.
Insulation placed at flat ceiling level creates a cold roof space.
This is the lowest cost option for insulating a roof and uses low
cost readily available glass wool quilt insulation. Very high
levels of thermal insulation can be achieved because there is
virtually no restriction on the thickness of insulation that can be
installed.
Insulation placed in the plane of the roof pitch create a warm roof
space. This allows the attic to be utilised as extra living space
enhancing the value and desirability of the dwelling without
significantly altering the building envelope.
Knauf Insulation supply products
for a range of roof insulation systems giving options that will
comply with Building Regulations and are suitable for use in the
preferred method of construction and required level of thermal
performance. Further to insulation products, a high specification
vapour permeable underlay and insulated lining boards for use
alongside insulation products are available.
The Table below illustrates the wide number of options for creating
warm roof spaces. Also included in this section are details of
options for insulating cold attic roofs.
If part of the loft area is to be utilised as living area, it
is most likely that sloping ceilings will be created. These cannot
be insulated in the same way as the normal loft. Traditionally this
has been resolved by placing insulation between the rafters from
below, leaving an air gap between the insulation and the sarking
felt for ventilation, before lining the ceiling with
plasterboard.
With the high levels of thermal performance demanded by the
latest Building Regulations it is necessary to examine the
solutions to be adopted to ensure that the required performance is
met at reasonable cost without compromising the design of the
dwelling. 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
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 from the dwelling 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, such as Knauf Breatheline, 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 dwellings.
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.
Building Regulations set out adequate levels of ventilation to
avoid the risk of condensation within the roof space. Approved
Document F Ventilation gives specific guidance, as does Part G of
the Building Standards (Scotland) Regulations. Both allow BS 5250:
2002 to be used as an alternative.
Position of
insulation
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.
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.
Ventilated design
With this design, there is a 50mm ventilated airspace between
the top of 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.
Over Rafter
Insulation
Insulation over the rafters provides a completely insulated
external envelope to the roof construction, and allows the maximum
usable space within the roof enclosure.
The insulation boards are laid across the rafters. It is important
that there should be no gaps in the insulation layer and no
ventilation between the outside and the attic space. A vapour
permeable underlay must be provided to direct wind driven rain and
snow to the gutter.
Counter battens are fixed through the insulation boards and vapour
permeable underlay into the rafters. Where the over rafter
insulation is over 50mm thick, consideration should be given to the
fixing of the counter battens.
Further insulation will normally be required between rafters to
meet Building Regulation requirements. 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.
2. Insulation at Ceiling Level
Ceiling Level
Insulation
This is the most common and economic way to insulate a pitched roof
and creates a ‘cold roof’. It has the advantage of minimising both
the heated volume and the quantity of insulation material
needed.
Cold ‘breathing’ roof.
The features of a cold breathing roof are:
- a vapour control layer at ceiling level to limit the amount of
water vapour entering the loft
- a vapour permeable underlay as the roof underlay, to allow
dissipation of water vapour from the loft space and, just as
importantly, act as a windproof layer
- 25mm deep counter battens to provide an airspace above the
vapour permeable underlay for dissipation of moisture vapour
- ceiling level insulation that is pushed up tight against the
vapour permeable underlay to prevent air leakage into the loft at
eaves level
Insulation is laid at ceiling level in two layers, the first
layer between the joists and the second layer across the
joists.
Cold ventilated
roof
With the ventilated design, the insulation is also placed at
ceiling level, but cross ventilation of the loft space is used to
disperse moisture vapour.
It is important to allow a clear, unobstructed ventilation path
between the insulation and the tiling underlay to allow outside air
to enter the loft space and prevent the build-up of moisture.
The ventilation requirements for roofs with a double 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 holes should be incorporated. A
separate vapour control layer is not necessary, but all 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.
Cold Roof – Other
Precautions
All tanks and pipes in the loft should also be insulated to prevent
freezing. Tank insulation jackets should meet the standards for
HEES. Do
not insulate directly under the cold water tank. The loft hatch
should also be insulated to a minimum depth of 100mm and draught
stripped.
Upgrading an Existing Pitched
Roof
An overall insulation thickness of 250mm to 300mm, 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 loft space 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.
Typical Details
Typical Specification Clauses
1) Mineral wool between
rafters
The whole area of the pitched roof to
be insulated with Crown FrameTherm 35*/Crown Rafter Roll 32* .....
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)
Alternatively, refer to NBS clauses: P10/140 and 320
2) Insulation between rafters with
Polyfoam Linerboard
The whole area of the
pitched roof to be insulated with Crown FrameTherm*/Crown Rafter
Roll 32*/ Polyfoam Raftersqueeze* ..... mm thick, friction fitted
between the rafters. The width of the insulation should be
appropriate to the spacing of the rafters.
Polyfoam LinerBoard of thickness ......mm with integral 9.5mm
vapour check plasterboard nailed to the rafters at maximum 300mm
centres and finished using standard drylining techniques. (*Delete
as appropriate)
Alternatively, refer to NBS clauses: P10/140 and 320, and
K10/245
3) Rocksilk Pitched Roof Slab and
Crown FrameTherm
Insulation laid over rafters
to be Rocksilk Pitched Roof Slab, ....mm thick, located over the
rafter. All boards tightly butted and with staggered joints. The
insulation to be held in place by 50 x 32mm counter battens nailed
through the boards, into the rafters using ..... fixings.
Knauf Breatheline should then be laid, slightly draped, over the
battens with the edges lapped. The tiling battens to be nailed into
the counter battens.
Crown FrameTherm 35*/Crown Rafter Roll 32*..... mm thick, to be
friction fitted between the rafters. The width of the insulation
should be appropriate to the spacing of the rafters.
Alternatively, refer to NBS clauses: K11/670 and P10/140 and
320
4) Polyfoam Sarking
Board
Insulation laid over rafters to be Polyfoam Sarking Board,
....mm thick, located over the rafter via the rebated edges. Knauf
Breatheline is then pulled taut and laid directly over the
insulation and held in place by 50 x 32mm counter battens nailed
through the boards, into the rafters. The tiling battens to be
nailed into the counter battens.
Alternatively, refer to NBS clauses: K11/670 and P10/140 and
320
5) Polyfoam Pitched Roofboard and
Polyfoam Raftersqueeze
Insulation laid over
rafters to be Polyfoam Pitched Roofboard, ....mm thick, located
over the rafter. Knauf Breatheline is then pulled taut and laid
directly over the insulation and held in place by 50 x 32mm counter
battens nailed through the boards, into the rafters. The tiling
battens to be nailed into the counter battens.
Insulation between the rafters to be Polyfoam Raftersqueeze,
supported on battens nailed to the side of the rafters.
Alternatively, refer to NBS clause: K11/670 and P10/140 and
320
6) Ventilated cold
roof
The whole area of the ceiling to be
insulated with two layers of Crown Loft Roll of total thickness
......mm. Width of first layer of insulation to be appropriate to
the joist spacings and of a thickness equal to joist depth. The
insulation to be laid between the joists and to finish 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.
Maintain an airspace between insulation and sarking felt/sarking
board at the eaves.
Do not insulate under cold water tanks. Cold water tanks and pipes
to be separately insulated.
Loft hatch to be insulated with a minimum 100mm depth of
insulation.
Alternatively, refer to NBS clauses: P10/120 and 130
7) Sealed Cold
Roof
The whole area of the ceiling to be
insulated with two layers of Crown Loft Roll of total thickness
......mm. Width of first layer of insulation to be appropriate to
the joist spacings and of a thickness equal to joist depth. The
insulation to be laid between the joists and to finish over wall
plate on external walls and pushed up tight to Knauf Breatheline.
The second layer of Crown Loft Roll to be laid at right angles to
the first layer.
All joints between the rolls of insulation to be close
butted.
Do not insulate under cold water tanks. Cold water tanks and pipes
to be separately insulated.
Loft hatch to be insulated with a minimum 100mm depth of
insulation.
Alternatively, refer to NBS clauses: P10/120, 130 and 320
8) Upgrading Existing
Insulation
The two alternative
specifications:
a) Between joists (where applicable)
The whole area of the existing ceiling to be insulated with Crown
Loft Roll, thickness .....mm. Insulation of width appropriate to
the joist spacings to be laid over the existing loft insulation and
dressed over wall plate.
b) Across the joists
The whole area of the existing ceiling to be insulated with Crown
Loft Roll, thickness ......mm. To be laid over the joists, with all
lateral joints close butted.
For both methods all end joints to be close butted. Maintain
airspace between insulation and sarking felt/sarking board at
eaves.
Do not insulate under cold water tanks. Cold water tanks and pipes
to be separately insulated.
Loft hatch to be insulated with a minimum 100mm depth of
insulation.
Alternatively, refer to NBS clause: P10/120 and 130