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Between and Beneath rafters
The method of insulating a pitched roof is particularly suited to loft conversions and refurbishment where the rafter depth is insufficient to accommodate the thickness of insulation required whilst providing a 50mm ventilation void (a requirement of the Building Regulations/ Standards for 'cold roofs').
This option is preferable to insulating only between the rafters (provided there is sufficient head room to accommodate the insulation) as it limits the cold bridge effect through the rafters. For this application we would recommend the use of Seconds between the rafters and Seconds beneath the rafters.
Fixing details
- To maintain a 50mm ventilated void above the insulation, side nail battens to the rafters to provide a 'stop'. This will prevent the insulation boards moving into the void.
- Boards cut individually to fit the rafter spacings may be temporarily fixed to the battens using large headed clout nails.
- Ensure accurate trimming to achieve close butting joints and continuity of insulation. Measure the space between the rafters before cutting the boards as spacings can vary.
- Fix the insulated plasterboard at right angles to the underside of rafters. Boards should be fixed with galvanised clout nails, long enough to allow 25mm penetration of the timber. These should be placed at 150mm centres and not less than 10mm from the edges of the board along all supporting timbers.
- All edges of boards must be supported. This will necessitate the use of noggins placed between the rafters to correspond with the long edges of the boards.
- Ensure the continuity of the insulation at the ridge.
- Ventilation should be provided in accordance with Approved Documents F, F2 (Condensation in Roofs) of the Building Regulations or Technical Standard K (Ventilation of Buildings, Regulation 23) of the Building Standards (Scotland).
- In certain instances it may be appropriate to attach all of the insulation to the underside of the rafters fixing - see fig. 3.
- Boards should be cut using a sharp knife or a fine toothed saw.
- To prevent a cold bridge between the cavity closer and the rafters, a vertical section of insulation should be installed between the rafters above the cavity closer.
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What an odd size eh ,Now let me tell you a story they cut them this size so they would fit in shipping containers,can't tell you where they were all going because it's a secret and I'd have to give them you. These are the better insulation value boards with a foil on one side and a tissue face on the other and if you can use the odd size they are a real bargin.
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