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How you insulate your attic depends on whether you want a "cold" or "warm" roof space. A cold roof requires insulation at joist level to stop heat escaping through the unused roof space. A warm roof is insulated between and under the rafters of the roof itself. The recommended depth for insulation has been increased recently, so you may have to increase the depths of the joists or rafters if you want to create usable platforms of storage space.
Recycled insulation is a green and non-itch alternative to conventional blanket insulation. The techniques for laying this are the same, but non-itch insulation makes it a more comfortable process. Check the instructions on the product to ensure that you install it to the right depth requirement.
One of the most widely used forms of attic-insulation, blanket insulation is simple to work with — although you should always wear protective clothing, as it can be uncomfortable to handle. Before starting, ensure that you have measured the surface area of your attic accurately, and that you bear in mind the recommended depth requirements and order accordingly (manufacturers supply blanket insulation in many different depths, so you must remember to order to the correct depth as well as to the necessary surface area). Always consider the recycled and natural alternatives to conventional blanket insulation.
Sweep away any debris from between the joists. Determine whether a vapor barrier is needed. If the drywall surface is silver-backed, you won't need an extra membrane — if not, it is advisable to install one.
Roll out the vapor barrier, cutting and laying lengths in between each pair of joists. Staple the barrier to the sides of the joists using a staple gun (Image 1). Cut holes in the barrier to accommodate any electrical hardware.
Do not unpack the insulation blanket until you are in the attic (Image 2). This will restrict the presence of insulation fibers to the work area.
Roll out the insulation blanket between the joists, taking care not to compress it. Tuck it in against the sides of the joists (Image 3).
Butt the lengths of insulation up against one another, making sure that there are no gaps between each of the lengths (Image 4).
Cut a hole in the insulation blanket to allow for electrical hardware. This is an important step to prevent the electrical components overheating.
Lift any cables or wires above the insulation to stop them overheating. Any heat they do give off will rise harmlessly into the cold roof.
If required by regulations, run a second layer of insulation at right angles to the first to increase depth.
Excerpted from Do It Yourself Home Improvement
© Dorling Kindersley Limited 2009