The two most common forms of loft insulation are blanket insulation and loose-fill insulation. Blanket insulation is made from glass fibre, mineral fibre or rock fibre and is sold in large rolls. All three materials are non-flammable and are treated against vermin, damp and rot. The rolls are usually 6 - 8m long and up to 400mm wide. The thickness of the material can also vary from 100mm to 200mm, but the thicker insulation the less heat will be lost.
The most common forms of loose-fill insulation are exfoliated vermiculite and mineral fibre. Loose-fill insulation is supplied in bags, which will cover an area of approximately 1sqm. Modern loose-fill insulation material is very light so it's not recommended for loft spaces that are extremely draughty as it will get blown about, reducing its effectiveness.
Although extremely effective for insulation, remember that blanket insulation materials and loose-fill materials can cause discomfort and irritation when they come into contact with the skin; therefore it is advisable to wear a long-sleeve shirt, long trousers, thick work gloves and a disposable dust mask.
Before laying the insulating ensure there is adequate lighting in the loft to carry out the job. Also that areas between the ceiling joists are swept to remove debris; and that all gaps around vents, pipes and wiring entering the loft are sealed using flexible mastic.
To calculate the amount of insulation you require, multiply the length of your house by its width. This area may need to be increased depending on the thickness of the blanket material you are using and the current Building Regulations recommendation for the thickness of loft insulation.
The average ceiling joist in modern houses is around 100mm deep, so to meet current Building Regulations it may be necessary to increase the depth of the material by adding another blanket layer on top of the first but at right angles to it.
Starting at one end of the loft begin to rollout the blanket between the joists ensuring there is a gap of about 50mm from the eaves to allow air to circulate. This will help prevent condensation. Press the blanket down lightly as you move across the loft but avoid compressing it. On reaching the opposite end of the loft cut the material, remembering to leave a 50mm gap.
When one roll finishes, begin another by butting the two ends together, making sure there are no gaps. Avoid covering loose electric cables as they could overheat. Instead, push the material underneath them or where possible fix them to the joists. To prevent an electrical fitting from overheating, cut a hole in the insulation to allow for the fitting.
When encountering pipes the material must be cut tightly around them. Never lay the insulation material under the cold water tank, as the warm air from below prevents the water freezing.
Repeat the process between all the other joists
It is easier to remove the wrapping from the rolls in the loft, as the material is compressed for transportation and expands when unwrapped.