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How-to Guide
Loft insulation
An essential part of your self-build project will be insulation. This is not only
for the sake of the environment, but also for your wallet. Good loft insulation
can save around £200 per year in energy bills, as well as over a tonne
of CO2. It’s cheap and easy to do, and is a quick way to stop your house
from losing over a quarter of its heat.
25% HEAT LOSS Standard Insulation
Loft
as living
space insulation
If you’re planning to use your loft as extra living space then the insulation must be at rafter level.
It will also require a vapour membrane and plasterboard over it before decorating. If you’re not
planning on using your loft as an extra room, then the simple step-by-step guide below will show
you the best ways of saving heat and energy.
TOOLS you will need
MATERIALS you will need
• Sharp knife or scissors
• String
• Strong adhesive tape, ideally waterproof
• Expanding foam filler aerosol
• Tape measure or stapler
• Solvent-free adhesive
(if using board insulation)
• Protective clothing, face mask and gloves
• A good light source. If the loft does not
have electric lights, then at least an
extension cord and safety light
• Heat diffusers if using downlights
in rooms below
• Insulation material: insulation blanket
rolls, loose lay or board insulation –
the current recommended thickness is
a 270mm layer
• Split foam sleeving or pipe lagging
material
• Jacket for water tank (if applicable)
• Strong boarding to create a load-bearing
floor for storage areas of your loft
• Cable clips to fix electrical
wires out of the way
• Foam strip draught
excluder for the hatch

PREPARATION
• First, decide what you want to use the loft for
as this will determine what kind of insulation
you will need.
• The simplest way of insulating is by rolling out a
double layer of insulation blanket between the
joists then cross-wise over them.
• If you plan to use the loft for storage, then you
will need strong boarding, making sure each
piece is big enough to cover at least three joists
for stability
• Next, measure your loft. Check the space
between joists and buy insulation at the
appropriate width and length.
• Make sure you’re fully protected with overalls,
mask and gloves. Insulation can be irritating to
the skin and its fibres must not be inhaled.
• Check that your work area is properly lit
• Ensure all other work to the loft is complete and
safe (such as plumbing and wiring).
• If your loft isn’t fully boarded, make sure you
tread carefully, as the spaces between the joists
won’t hold your weight. This will avoid damage
to the ceiling below and injury to you.
DOING THE JOB
ALTERNATIVE MATERIALS
• First, check that you won’t be covering any
electrical wires, as this can cause overheating.
• Fasten any wires to the sides of joists with the
cable clips.
• If there are any downlights inset into the ceiling
of the room below, make sure you fit downlight
heat diffusers to these before laying the insulation
– again to avoid overheating.
• Next, unroll the insulation blanket from the eaves
to the centre making sure it completely fills the
space between the joists. Leave a gap at the very
edge of the eaves to ensure natural ventilation
and eliminate condensation.
• Roll out to the centre, then cut and repeat from
the other end, making sure the ends butt snugly
against each other.
• Once this layer is complete, repeat at 90°
from the previous layer, going perpendicular
to the direction of the joists. Do not compress
either layer, as this will compromise its thermal
performance
If the spaces between your joists are not uniform,
then a roll of insulation blanket may not be
appropriate. In these cases, it may preferable to
empty sacks of loose-lay material into the spaces
between joists.
Before doing this, make sure none of the
material can disappear down the cavity wall
by blocking the eaves with lengths of timber
between the joists. As above, be careful not
to block this area completely, so as to avoid
condensation. Also, seal any holes in the ceiling,
such as those through which pipes or wiring are
fed, with expanding foam filler.
Next, empty sacks of loose-lay into the spaces.
Use the edge of a piece of wood to smooth and
level off the layer. Once this is laid, be aware
that every time you open or close the loft the
draught can blow the material about, so a layer
of blanket insulation or board insulation over the
top is advisable.
LAYER 1
JOISTS
LAYER 1
JOISTS
LAYER 2
LAYER 1
JOISTS
LAYER 1
JOISTS
LAYER 1
• Insulate over the cold-water tank, but not under it.
If it’s too well insulated from heat below it
may freeze (see other essentials below for
more information)

BOARDING
• If you intend to use some or the entire loft for
storage, you will need to lay boarding across
the joists.
• In this case, you will only be able to lay
insulation up to the level of the tops of the joists.
For further insulation performance you can lay
one or two layers of board insulation between
the joists and the load-bearing boarding.
• Cut it to shape with a sharp blade and fix it in
place with a solvent-free adhesive.
• Then either nail or glue the load-bearing
boarding (usually chipboard) on top.
OTHER ESSENTIALS
• Don’t forget to insulate the cold-water tank and all
pipes. If these are a uniform shape and size then
ready-made jackets are available to fit straight
onto the tank, and split-foam pipe sleeves will just
slip straight onto the pipes.
• If the tank and pipe runs are more complicated
shapes the tank can be wrapped in insulation
blanket (take care to ensure no fibres drop into
the water).
• The pipes must be wrapped in pipe lagging.
Wrap it round in a spiral motion, including all
taps & joints, and fasten with string, wire or tape.
• Cut two sections of insulation blanket – one for
the lid of the water tank and one for the loft
hatch. Wrap both in plastic and secure with
staples or tape. Finally, fix a length of foam
draught excluder all round the entrance to the
loft hatch.
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