Converting the loft is a popular and relatively low cost method
of increasing the floor area of an existing property and increasing
its value.
Overview
The loft space of most
homes is almost always unused space.
Converting the loft is a popular and relatively low cost method of
increasing the floor area of an existing property. The extra space
can be used to form extra rooms, or to enable the enlarged property
to be converted into flats.
Loft conversions can create high quality living and working space
with good levels of light and comfort that will enhance the value
of a property.
Advantages
- Loft conversions allow for the increase of living space in a
building without altering the shape of the building envelope. It
utilises space that normally remains unused.
- A loft conversion is usually the most cost effective method of
increasing the useable floor area of a property. It may be possible
to complete the work without removing the existing roof covering or
making any major structural alterations.
- A good quality loft conversion will enhance both the usefulness
and the value of a property, providing the project has been
properly designed. A building design professional should always be
consulted before commencing any project as should the Local
Authority Planning and Building Control who will give guidance on
planning and Building Regulations.
Knauf Insulation Products
- Crown FrameTherm is a lightweight unfaced glass
mineral wool available in three thermal conductivities and in slab
and roll form.
- Crown Loft Roll is a lightweight unfaced glass mineral
wool available in roll form.
- Polyfoam Raftersqueeze is a high performance, 100%
ozone friendly, extruded polystyrene, rigid board insulation. It is
lightweight and consists of a flexible infill piece of insulation
to fit between rafters, even allowing for some timber
variation.
- Polyfoam Linerboard is a laminate of Polyfoam high
performance, 100% ozone friendly, extruded polystyrene, rigid board
insulation and 9.5mm tapered edge plasterboard.
- Crown Rafter Roll 32 is a semi-rigid unfaced glass
mineral wool roll, with the characteristics of a semi-rigid slab
when unrolled.
Note: All the solutions in this section are tailored to
projects where the existing roof covering is not being
replaced.
If the roof structure is to be substantially renewed then it will
be necessary to comply with insulation standards for new build
construction that are shown in the "Pitched
Roofs: New Build and Upgrade – Residential” section. |
Summary
Knauf Insulation offers a wide range of loft conversion products to
meet all design and Building Regulation requirements.
Detailed Design Considerations
Converting an existing roof
When converting an existing pitched roof it will be necessary
to comply with Building Regulation requirements for thermal
insulation and ventilation, as well as structural stability, fire
resistance and design of the new staircase.
Thermal insulation
The maximum allowable Elemental U-values for pitched roofs where
there is a ‘material alteration’ are shown below:
| |
U-value
(W/m2K) |
| |
Rafter Level |
Ceiling Level |
Dwarf Wall |
| England, Wales & NI |
0.20 |
0.16 |
0.35 |
| Scotland |
0.20 |
0.16 |
0.30 |
| Ireland |
0.20 |
0.16 |
0.27 |
Ventilation and condensation
Building Regulations require that adequate provision be made
to avoid the risk of condensation in a roof space or in a roof void
above an insulated ceiling. 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.
Ventilation of the roof space or voids is designed to enable water
vapour to dissipate to the outside and help prevent
condensation.
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 50mm ventilated airspace must be provided to each and every space
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.
A vapour control layer must be applied to the warm side of the
insulation.
Insulation position
The insulation for a room-in-the-roof conversion is a
combination of ceiling level insulation and insulation between the
rafters and in dwarf walls. The horizontal areas of ceiling are
insulated with rolls of insulation quilt. The rafters in the
sloping ceiling and the studs of the dwarf wall can be insulated
between the timber members with either a plasterboard or an
insulation plasterboard laminate.
When installing insulation between rafters, the rafter depth is
crucial, as the depth must be sufficient to accommodate the
thickness of insulation required and a 50mm ventilated airspace. If
necessary, the effective rafter depth can be increased by adding
timber battens to the inner face of the rafters over the
appropriate area.
The dwarf walls are treated as conventional timber framed walls and
must meet Building Regulation requirements for exposed walls. The
insulation used in the sloping part of the roof can also be used in
the dwarf walls.
The insulation at ceiling, sloping ceiling and dwarf walls should
be closely butted to form a continuous layer of insulation to the
habitable rooms.
Vapour control layer
In order to restrict the amount of water vapour passing
through the ceiling and dwarf wall, a continuous vapour control
layer should be provided on the warm side of the insulation.
Typical Details
Typical Specification Clauses
1) Between rafters: Crown FrameTherm 35/Crown
Rafter Roll 32
The whole area of the sloping ceiling and dwarf walls to be
insulated with Crown FrameTherm 35/Crown Rafter Roll 32* ..... mm
thick, friction fitted between the rafters/timber studs, leaving a
minimum 50mm wide ventilation airpath behind the insulation. The
insulation to be butted up against insulation at ceiling level.
*Delete as appropriate)
2) Between and below rafters: Crown FrameTherm
35/40 and Polyfoam Linerboard
The whole area of the sloping ceiling and dwarf stud walls to be
insulated with Crown FrameTherm 35/40* ......mm thick, friction
fitted between the rafters/timber studs, leaving a minimum 50mm
wide ventilation airpath behind the insulation. The width of the
insulation should be appropriate to the spacing of the rafters and
studs. (*Delete as appropriate)
Polyfoam Linerboard of thickness ......mm with 9.5mm plasterboard
to be nailed to the rafters and timber studs at maximum 300mm
centres and finished using standard drylining techniques.
3) Between and below rafters: Polyfoam
Raftersqueeze and Polyfoam Linerboard
The whole area of the sloping ceiling and dwarf stud walls to be
insulated with Polyfoam Raftersqueeze ......mm total thickness,
friction fitted between the rafters/timber studs, leaving a minimum
50mm wide ventilation airpath behind the insulation.
Polyfoam Linerboard of thickness ......mm with 9.5mm plasterboard
to be nailed to the rafters and timber studs at maximum 300mm
centres and finished using standard drylining techniques.
4) Ceiling level insulation
All flat ceiling areas 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.
NBS clauses
- Insulating between rafters is covered by NBS clause
P10/140
- Ceiling level insulation is covered by NBS clause P10/120,
130
- An insulated lining below rafters is covered by NBS clause
K10/245