Thermal insulation of pitched roofs can be applied only to the interior side or in combination, between+below the rafters. Jointless placement of XPS below the rafters guarantees avoidance of thermal bridges. It can be installed in existing structures, as well, for posterior loft development or for energetic modernization of roof structures.
If the roof was built without thermal insulation inserted above the rafters, the most economic way for reducing the thermal bridges and improving the thermal protection is provided by a combination of thermal insulation between the rafters and fixed to the bottom plane of the roof structure; it can be used expansively in refurbishments.
What concerns the supporting structure, the XPS is self-supporting until the completion of the interior cladding; subsequently, it is squeezed between the cladding and the rafters. The tongue and groove or staggered edge finishing of the STYROFOAM™ XPS 300 STG panels ensures composite action of the structures, however, it is no substitute for structures designed to ensure lateral rigidity of the roof structure, e.g. wind-bracing placed between the rafters.
The thermal insulation panels are arranged bottom-to-top. From building physics aspects, highly efficient thermal insulation capable also for damp prevention can be achieved during the installation of the XPS panels.
The damp insulation layer improves also the air tightness and can be fitted between the STYROFOAM™ XPS plates or directly behind the interior cladding, depending on the ratio of thickness of the thermal insulation layers laid in front of and behind the foil. In the presence of vapor permeable underlay plating, the vapor barrier layer could, in general, be done without, owing to the relatively high vapor diffusion resistance and the tongue and groove edge finishing of the XPS plates, however, the vapor barrier layer is required for ensuring air tightness of the layered composition. The interior cladding can be built by using wooden panels fixed to branders or gypsum plasterboard panels fixed to lathing or a system of profiles, etc.
INSTALLATION ADVICES FOR ROOF RENEWALS
If no secondary watertight foil (i.e. underlay plating) had been built into the roof structure, such omission must be rectified. In case of posterior loft development, one should deliberate replacement or repair of the shell-plating or the lathing. If the lathing must be replaced, the new damp barrier system can be laid on top of the rafters.
The XPS plates must be installed according to a bottom-to-top arrangement with offset binding, by ensuring tight fitting of the tongue and groove edges. The elements can be fixed to the rafters by screws & washers or thermal bridge free fittings fixed with stainless steel clips.
Following the completion of the thermal insulation, the interior cladding can be placed, by continuous laying of the air and damp tight foil.
Prior to making detailed designs, the standard checking procedures, incl. verification of the structure's flammability parameters, must be completed, like in case of any other layered structure used for space delimitation.