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Insulation of roofs and Dormers

Why Important

Heat is lost through a typical roof and in this age of spiralling fuel prices and demands to reduce carbon footprints, it is sensible to insulate the roof space.

What to look for

In an attic, insulation can be laid between and over rafters. However it is important to maintain ventilation within the roof space.

In roofs that have no underslating felt this occurs naturally through gaps between the slates and sarking boards. It is therefore important to check if a slating felt has been laid.

When to look

How to fix

There is a bewilderingly wide range of insulation materials available.

It is beyond the scope of this advice to look at each in detail, but generally natural materials (such as sheep’s wool, hemp or recycled newspaper) work better in older properties, as they are able to absorb and disperse moisture and prevent the condensation problems that can occur when retrofitting insulation. Always seek advice from manufacturers.

Current best practice is to use ‘breathable’ felt underlay as this will still allow air movement through the gaps as described above.

In the recent past, plastic non-breathing under-slating felts were regularly laid, and before that, hairy felt bituminous underlays.

If either of these are present when insulating, additional ventilation will need to be introduced into the roof space above the insulation to avoid a build-up of condensation.

Avoiding creating problems

Historical background

insulation_of_roofs_and_dormers.txt · Last modified: 2022/01/04 10:18 by