Are You Insulated?

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It is important to check how your roof space is ventilated and to ensure that any air passages are kept clear. This may mean in some cases fitting boards or proprietary plastic inserts at the eaves to hold the insulating material back. Any water pipes protruding above the insulation need to be lagged with closed cell plastic tubes and any storage tank wrapped or boxed with blanket type or rigid sheet insulation. For fire safety reasons all insulation should be kept 3" away from heat and light fixtures.

Lived-in Attic Space or Lined Lofts

If your attic space is habitable or is going to be some day, then it is the rafter zone that needs insulating. There are three choices: Insulation can be installed either below, between, or above the rafters (See illustrations above.).

Below the Rafters

If the rafters are open and unplastered, it is a simple matter to place rigid sheet insulation on the inside and fix a board finish underneath. This method of insulation reduces the room height and width. This in itself may inhibit the thickness of insulation and the minimum thickness to be considered should be about 2". Sheets of plastic foam insulation can either be laid between cross-battened timber, machined to the same thickness as the insulation, or simply pinned in position before fixing boarding. In the former the plasterboard or matchboarding can be fixed to the cross battening and in the latter, fixings need to be made through the insulation to the joists beneath. I have personally used 3" polystyrene insulation, which has the advantage of being highly water resistant and as a result, no vapor barrier is required. Plastic insulation cuts easily with a sharp, long-bladed knife.

Additional insulation can be fitted between the rafters as well. However, it is important not to fill the whole depth of the rafters to allow proper air circulation to them. This may well determine the thickness of the insulation possible. There is an advantage if at least one of the insulation boards is covered with a layer of aluminum foil to add to the effectiveness of the insulation.

If your attic space is already lined, the insulation can be placed directly on the underside of the lining. Alternatively, your roofing tiles or shingles may need replacing, and in that case insulation could be added between the joists while the roof is stripped. You could do this by either laying rock-wool or similar material between the rafters, remembering to leave the 2" gap between the insulation and the roofing felt, or, if the rafters are shallow, to use rigid board plastic foam insulation with a high R-value.

Between the Rafters

This is the most usual place to install the insulation. However, your rafters may not be deep enough to accommodate a reasonable thickness of insulation with the required 2" air gap above. To increase the thickness a timber lath can always be fixed to the underside of the rafters as shown in the illustration.

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