The options available to you for insulating your attic depend on whether you would like to insulate at ceiling level or at the rafters (at the slope of the roof). The attic should be insulated at the rafters only if you intend to use the area as a living space as it the heat will rise through the uninsulated ceiling into the attic and it is wasteful to keep the attic heated. Typically, Irish attics are not used as living spaces so detailed below are the measures available for insulating at ceiling level.
At ceiling level, you can lay fibre quilt between and over the joists. This is the cheapest and simplest installation option. The U – value requirement under the 2002 building regulations on insulation for a pitched roof with insulation at ceiling level is 0.16Watts per m2K. That requires 250 – 300 mm of insulation, if you are using rock wool, glass wool, or mineral wool insulation. Obviously there are higher performance insulations, but in any event, you should plan for at least 150 – 200 mm.
In Canada, some people put 600 mm of insulation in the attic. The building code does not require this amount but if you would like to lose even less heat and increase indoor comfort, then increase the amount of insulation you install. You don’t need quite as much in Ireland’s climate but 400 mm of mineral wool is probably optimal. Make sure the load isn’t too heavy for the ceiling (place planks or boards onto the beams and insulation on top of them) and that you still have access to any tanks, pipework etc.
The quilt should be placed in two layers to minimise thermal bridging or heat loss through the joists themselves. Any element that is a poorer insulator, and allows more heat to pass through it than the material in surrounding area, is called a thermal bridge. The first layer is placed between the joists. The second is laid at right angles across the joists.
This insulating fibre loses some of its insulating properties if it is compressed. Therefore, you can’t keep anything on top of this insulation. If storage area is required in the attic space, and a second roll across the joists is not possible, make certain that the storage is over a part of the house with less of a heating requirement, e.g. the bedrooms rather than the living room). The full thickness of insulation should be installed over the living room – the warmest room in the house.
Installing the insulation properly is very important. Ill-fitting or compressed insulation will not perform to its potential. When fitting insulation between rafters it is very important that it is cut tight, with no gaps. Gaps between the insulation lead to thermal looping, which causes the insulation to become ineffective. Good workmanship is key; otherwise you will not get the full benefits of the insulation. Also, when laying insulation in the attic, make sure to leave the openings at the eaves unblocked since it is important to allow for ventilation to prevent mould or damp occurring.
Another option is to have glass fibre, mineral fibre or cellulose fibre (i.e. recycled paper) blown into your attic by a professional between and above the ceiling joists. While this type is a better insulator than the other DIY quilting, it is also more expensive to install, as a professional must be hired. Storage in your attic is also reduced and it can look messy.
At the rafters, you could install a polyurethane foam insulation system or spray-on cellulose fibre between the rafters but if you choose to insulate between the rafters in this way, please ensure that you insulate the underside of the rafters also, to prevent the heat escaping along this path. Another option available here is to use semi-rigid boards with the insulation on the back. – See more at: http://www.seai.ie/Power_of_One/FAQ/Insulation/What_is_Roof_Insulation.html#sthash.ftjHQxya.dpuf