Platta på mark för Passivhus - Fuktsäkerhet och Värmeisolerande förmåga

Sammanfattning: The legislation for energy consumption in homes has been strengthened in recent years. In pursuit of energy-efficient homes various forms of low-energy houses have been developed, including Passive Houses. Progress has been made regarding walls, windows and roofs to create a low U-value, while the foundation of the house has been put aside from energy saving point of view. For that reason the aim with this report is to illustrate alternative solutions to slab on grade with good thermal insulation and a minimized thermal bridge. The foundation is an important part of the building that should have good bearing capacity, thermal insulating ability and serve as the building's moisture barrier against the ground. Today we seek to achieve these functions primarily through the following layers of materials, described from the soil and up to floor level. 150 mm of macadam as draining layer and to prevent capillary action EPS foam, normally 300 mm, as thermal insulation and as a complementing preventive layer of capillary action Concrete slab with reinforcement as bearing material Slab edges are often constructed with an L-shaped concrete mould of EPS foam (L-elements), which is practical and good from a strength and moisture-resistant point of view, but they create a substantial thermal bridge along the edges. The thermal bridge increases the total U-value significantly and should be minimized in order to reduce energy consumption during the usage stage of the house. According to the new legislation from 2009 by BBR thermal bridges must now be included in energy calculations. Two foundation constructions, created to provide good thermal insulation and moisture safety, are selected for closer examination; “U-min grund” (passive foundation) from the company Supergrund AB and “Koljerntekniken” from MRD Sälj och Bygg AB. The study of the constructions will be focused on their thermal insulation ability and moisture safety but also with consideration to economy and strength. “U-min grund” is constructed as a traditional Swedish slab on grade with concrete slab and EPS foam insulation under the slab. To achieve a low U-value the layer of EPS foam is 400 mm thick and the slab edges are constructed with U-shaped concrete mould of EPS (U-elements) to break the thermal bridge. The thick thermal insulation creates a large temperature difference between the ground and the surface of the concrete slab. This gives a low relative humidity RH in the upper part of the concrete slab, 53% according to our calculations, which makes the foundation very good from moisture point of view. “Koljerntekniken” is constructed by cellular glass held together with metal profiles to form a 400 mm thick board. Cellular glass should serve both as a bearing and thermal insulating material. Furthermore cellular glass is fully air-and moisture-tight according to the construction company Pittsburgh Corning, which makes the foundation water proof. Both foundation constructions have good results in energy calculations. According to our calculations, with the software HEAT2, the thermal bridge is minimal in “Koljerntekniken” which results in a U-value, including thermal bridges, of 0.089 W/(m2K). “U-min grund” creates a more significant thermal bridge along the edges but has essentially the same U-value of 0.094 W/(m2K). However, through a discussion with Lars Sentler, Professor of Building Construction at Lund University, there are indications that the strength in both “Koljerntekniken” and “U-min grund” is unstable. The weakness is located along the slab edges where the load from the building is led down. A rough disposition of the production cost shows that “U-min grund” is only slightly more expensive than a classic slab on grade with L-elements and 300 mm EPS foam. “Koljerntekniken” however is more than twice as expensive to manufacture, largely because of the production cost of cellular glass. One of the conclusions of this report is that the slab on grade technique used today is not suitable for future Passive Houses. The problem of minimizing the thermal bridge, while maintaining other functions along the edges, turns out to be difficult to solve. The report reveals that more focus must be put on the design of the foundation, to create a well functioning building with low energy consumption and long lifetime.