Installing Insulation in Roofs
Where the ceiling is either flat or has a slope of less than 25 from horizontal, any of the bulk insulations can be used. (Figures 1 & 2) If the ceiling has a slope of more than 25 from horizontal, loose fill insulation might move unless supporting joists are added. Alternatively batts, rolls or sheets of insulation can be used.
Depending on the climate, a suitable level of insulation should be installed, as recommended in Australian Standard AS 2627.1 - 1993.
Pitched Roof with Horizontal Ceiling
The roof cladding may be tiles, metal sheeting or even fibrous cement. The pitch or slope of the roof usually means there is adequate space in the roof for a person to move about and install insulation. Insulation is usually placed between ceiling joists, in contact with the top of the ceiling.
The bulk insulation should finish over the top of the wall plate around the perimeter of the building, but should not restrict ventilation of the roof space from the eaves (Figure 1)
Where better summer performance is desired, foil backed insulation batts can be used. In this case, the foil batts are placed on top of the rafters, with the foil side facing down. It is important to fill in the gap between the ceiling and the insulation around the perimeter of the building, and also around any penetrations through the insulation. To prevent air exchange between the spaces above and below the insulation, the insulation must be installed so that there are no gaps between the insulation batts or sheets.
A reflective foil sarking is often used under tiles to provide weather proofing, and also to reduce radiant heat transfer from the tiles in summer.
In this case, provision should be made for ventilation near the highest points of the roof. This can be achieved by removing the sarking for a width of up to 300 mm on both sides of the roof ridge, or by installing thermostatically controlled roof ventilators, together with ventilation openings under the eaves. Alternatively, ventilators may be installed in gable ends.
A foil backed blanket is often used under a metal clad roof, mainly to reduce rain and other noise from the metal cladding. As with tiled roofs, the reflective surface on the underside of the blanket helps to reduce radiant heat transfer in summer, and acts as a vapour barrier.
It is recommended that placing of the insulation be done early in the day before the roof space becomes uncomfortably hot. Where necessary, a sharp knife and cutting board should be used for cutting insulation to fit neatly. A short plank placed over the ceiling joists makes it easier and safer to move about the ceiling. When installing fibrous insulation materials the use of a face mask, goggles, gloves, long sleeves and trousers is recommended.
Roof with Ceiling Parallel to the Roof Surface
With this type of roof there is usually not enough space for a person to gain access to the roof to install insulation.
The recommended procedure is to remove enough of the roof so that the insulation can be put into place. Insulation batts should be placed between ceiling joists, and in contact with the top of the ceiling.
Loose fill insulation may be blown into the roof / ceiling space where the slope of the ceiling is less than 25% from horizontal. This avoids the necessity to remove much of the roof cladding, as mentioned above. However, it is difficult to be sure that the ceiling is properly covered. In addition, overfilling with loose fill insulation may restrict ventilation of the roof/ceiling space. As with pitched roofs, a reflective foil sarking can be used under tiles, and a reflective backed insulation blanket can be used under a metal clad roof.
Ventilation is important because it can be the only means of controlling or
avoiding condensation of water vapour on the underside of the roof. Excessive condensation may cause staining, mildew or mould growth on the underside of the ceiling. It is important not to exhaust moist air from bathrooms, laundries and kitchens into the roof space, as this will magnify any condensation problems.
Condensation of water vapour can take place whether there is insulation in
the roof space or not. It is a potential problem, particularly in sarked tiled roofs, or metal covered roofs. If fresh outside air is able to flow through the roof space, the circulating air will allow condensation to evaporate and be carried away, This can be achieved by providing enough openings under the eaves and at the highest point of the roof, ie at the ridge. A gap should be left in the sarking of up to 300 mm on both sides of the ridge.
Vapour barriers need to be used with metal clad roofs, in any climate, to prevent condensation from forming on the underside of the metal cladding. For metal clad roofs in temperate climates, a reflective foil sarking or a foil backed insulation blanket is used immediately under the metal roof cladding.
In most installations, this will not provide a sufficient insulation level for the locality, and therefore bulk insulation is still required next to the ceiling lining. In cold climates, a reflective foil laminate is commonly used as a vapour barrier with both tiled and metal clad roofs, and this is installed between the ceiling and the underside of the bulk insulation, that is, on the warm side of the insulation. Foil backed plaster board, used as a ceiling with the foil side facing up can also be used as a vapour barrier.
In hot climates, where air conditioning is used, a vapour barrier should be installed on the top of the bulk insulation, that is, on the warm side of the insulation, for both tiled and metal clad roofs. This prevents condensation from forming inside the insulation.