Insulation of the roof and walls in an old attic
In old buildings, the first insulation layer is usually installed between the joists and mounted using elastic polyurethane foam. An unbroken insulation layer is then installed under the joists so that the second layer of Finnfoam boards covers all seams of the first layer, leaving no cold bridges.
It’s usually best to use two layers to insulate old attics as the distance between the support beams may vary considerably. A sensible compromise has to be established regarding the thickness of the insulation layers to avoid sacrificing too much room height and space. The room type – is it a heated or semi-heated space, for example – must also be taken into account.
The old regulations that came into force in 2007 required the roof to have a U-value of 0.15 and the walls to have a U-value of 0.24. According to the present regulations, however, the roof must have a U-value of 0.09 and the walls 0.17. As Finnfoam's cell structure is completely homogenous and closed, no undesirable air leaks due to warm air rising occur inside the boards. As long as the joints are sealed properly using elastic polyurethane foam, the structure is unbroken and leakproof. This prevents heat loss due to air leaks: heat is transferred only by means of conduction and radiation. At ordinary room temperatures, the role of heat radiation is negligible. Most of the heat is transferred through conduction, which is not dependent on gravity but occurs equally through the walls and the roof. For this reason, the same insulation thickness should be used in the walls and the roof.
The old 2007 regulations have been considered the minimum level that should always be achieved when insulating structures. The example below is also designed so that these regulations are met. Space permitting, however, using a thicker installation layer is recommended today as energy costs are expected to increase significantly in the future.
The U-value of the lower insulation layer (170 mm) will be approximately 0.19 to 0.20, depending on the spacing of the support beams. In the walls, 130 mm should be sufficient to meet the 2007 requirement of 0.24. The roof, however, would need a 210 mm layer to meet the 2007 U-value requirement of 0.15. 'Under-insulation' (-40 mm) of the roof can, however, be compensated by 'over-insulating' the walls (+40 mm). The payback period for 'over-insulating' the walls by 40 mm is much shorter than the cost of insulating the roof (170 mm + 40 mm = 210 mm).
The U-value does not tell the whole story of a structure's insulation capability, however. The structure's air leak value (n50) is almost as important. The current average air leak value of wood-frame houses is 4. Using a leakproof structure, however, the air leak value can be reduced to 1. Leakproof structures may be used to reduce the building's energy consumption by an amount equal to the amount achieved by adding approximately 25% more insulation. When aiming to improve thermal insulation and tightness, ventilation must also be taken into account: does the new structure allow sufficient ventilation?
Phase 1: Finnfoam is installed between the joists.
The insulation board is cut to a shape approximately 1–2 cm narrower than the installation space and wedged between the joists using pieces cut out of Finnfoam so that the lower surface of the board is level with the joists. A ventilation gap of at least 50 mm must to be left between the Finnfoam boards and the sub-roofing. Space permitting, more than one insulation layer can be installed between the joists.
Phase 2: the joints are insulated using elastic polyurethane foam.
A generous amount of elastic polyurethane foam is applied between the joists and the Finnfoam boards. We also recommend applying some polyurethane foam to the horizontal joints between the two boards in order to seal them. When the boards are rubbed briefly against each other, the polyurethane foam's tendency to expand is eliminated and the foam only functions as glue. Once the polyurethane foam has hardened, dried excess foam and the Finnfoam wedges can be cut away.
Phase 3: insulation is installed under the joists.
The objective here should be to build an unbroken layer of insulation under the joists (FL-200 boards with half-interfitting edge profiles should be used to make the joints leakproof) to minimise cold bridges. The Finnfoam boards can be attached lightly to the Finnfoam layer above using screws. A small amount of PU foam is applied to all joints and the next board is installed immediately before the foam has hardened. The positions of the joists should also be marked on the Finnfoam boards.
Phase 4: Insulation is mounted.
The unbroken layer of Finnfoam boards is mounted securely to the joists using boards and long screws. Temporary attachment screws may be removed as the final mounting progresses and can be reused as applicable. A small amount of PU foam is applied to the temporary mounting holes to seal them.
Phase 5: interior wall materials and electrical wiring are installed.
An interior wall material, such as plasterboard or panelling, is mounted onto the wooden boards. If the spacing of the underlying boards is approximately 900 mm and plasterboard is used, shorter spacing (450 mm, for example) should be used – otherwise the plasterboards may sag from the centre in the ceiling. If additional wooden boards are needed for adjusting the spacing, the boards may be attached to the Finnfoam layer using glue and/or coarse-threaded (large-pitch) screws. A large-pitch screw holds well in the Finnfoam layer and has a tensile strength of over 20 kg. The principal support points of the interior wall material, however, should be the boards that are mounted to the joists. A small ventilation gap is left between the interior material and the Finnfoam layer. The gap also contributes to the insulation and is, even more importantly, very useful for concealing electrical wiring and other cables.