Building performance, energy
STRUCTURES
Concepts
The problem of heat flow and vapour diffusion
Protective layer
Insulating layer
Loadbearing layer
Temperature gradient Dew point
Inside +20°C Vapour barrier/check
Outside -10°C
Fig. 12: Heat flow through a wall (facade) Thickness (m)
The phenomenon of vapour diffusion Cold air contains little water vapour (outside – dry air), hot air contains considerable water vapour (inside – high humidity). When hot air meets cold air or is quickly cooled, moisture in the air condensates as water (dew point). This can happen as a result of the temperature gradient within a layer of insulation (6t = 21.1°C) within the construction. Moisture in the construction leads to damage to the building fabric: - rotting (wood) - mould growth - breakdown of the microstructure (materials) - disruption to the loadbearing structure - damp thermal insulation is useless Condensation within the construction (interstitial condensation) must therefore be prevented, or all moisture must be allowed to dry out or escape.
The following symbol is used on drawings to indicate the position of the vapour barrier/check:
Measures Specific technical measures to prevent interstitial condensation, in the thermal insulation especially, are as follows: Measure 1 Internal loadbearing layer made from a vapour-tight material, e.g. in situ concrete, glued panels (sandwich panels in timber construction), internal lining of sheet steel; or Measure 2 Vapour barrier membrane attached on the warm side directly in front of the thermal insulation; or
Basic principles A “vapour barrier/check” must be integrated in order to prevent condensation. Two rules must be observed in conjunction with this: - The vapour barrier/check must be attached to the warm side (inside) prior to fixing the thermal insulation. - The imperviousness (to vapour) of the materials must decrease from inside to outside. “Sealed loadbearing layer on the inside, vapour-permeable protective layer on the outside.”
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Measure 3 Thermal insulation made from a vapour-tight insulating material, e.g. cellular glass; or Measure 4 Ventilated cavity between insulating layer and protective layer; condition: good air circulation (thermal currents) in the cavity, width of cavity: 3–4 cm
Building performance, energy
STRUCTURES Concepts
Protective layer Insulating layer Loadbearing layer
Loadbearing layer
Protective layer Insulating layer Loadbearing layer
Protective layer Insulating layer Loadbearing layer
Insulation concepts Diagram of layers
Protective layer Insulating layer Loadbearing layer
Protective layer Insulating layer Loadbearing layer
Protective layer Insulating layer Loadbearing layer
Protective layer Insulating layer Loadbearing layer
Protective layer Insulating layer Loadbearing layer
In finalising a draft design the question of a suitable insulation concept arises in conjunction with the intended architectural appearance of the building. Insulation is not automatically “thermal insulation” but can also include sound insulation, for example. Thermal insulation between the interior and the exterior climates is used above all in the facades, in the roof and in the foundations, or rather the “floor over the basement”. Sound insulation is employed primarily between the storeys (in the floors) or in the walls between sound compartments, e.g. between apartments, offices, etc. At the start the architect is faced with the choice of a thermal insulation system. In synthetic systems or compact systems individual elements provide several functions, e.g. insulating and load-carrying. Examples of this are single-leaf masonry walls and timber panel elements. By contrast, there are complementary systems split into a hierarchy of layers with the functions of loadbearing, insulating, and protecting. Starting with the position of the structural elements in relation to the insulation, complementary systems therefore require a further refinement of the insulation concept according to “loadbearing layer inside” or “loadbearing layer outside”. When choosing a complementary system the diagram of layers serves as a reference for the constructional analysis of a building. It is suitable for checking the continuity and coherence of the insulation concept and for localising problems. Loadbearing layer, insulating layer (thermal and sound insulation) and protective layer are shown schematically on plan and in section, with the rule being that the individual layers should not be interrupted. Openings (doors, windows), changes of direction (projections, rooftop terraces, etc.) and nodes (junctions) in the layers demand special attention. The insulation concept is elaborated when these key points are designed in detail, or – if particularly serious disadvantages are discovered – the concept is discarded.
Protective layer Insulating layer Loadbearing layer
Protective layer Loadbearing layer Insulating layer
Example: unheated basement (= no thermal insulation) Loadbearing layer
Fig. 13: Diagram of layers (template) External walls, floors and roofs are first drawn schematically with three layers. The dimensions of the individual layers are not defined here, they are determined by building performance, structural and architectural criteria.
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Building performance, energy
STRUCTURES
Concepts
Insulation concepts Complementary systems – loadbearing layer inside
Edge of roof
Parapet Rooftop terrace, roof garden
Edge of terrace
Terrace–facade junction
Projections and returns Loggias, balconies, open walkways
Loggia rainwater drip
Soffit of loggia, junction with lintel
Openings Doors, windows
Window junctions, lintel, spandrel panel, reveals
Floor–wall External wall, internal partition
Floor junction
Protective layer Insulating layer Loadbearing layer
Protective layer Insulating layer Loadbearing layer
Protective layer Insulating layer Loadbearing layer
Roof Flat, pitched
In this concept the loadbearing layer is exclusively on the “warm side”, completely enclosed by the layer of insulation. The outermost layer serves, in the first place, to protect the insulation against mechanical damage and climatic effects and has no loadbearing function. Various materials may be used, from a thin layer of render to suspended stone slabs to facing brickwork or fair-face concrete. Accordingly, the thickness of the protective layer can vary considerably. Penetrations through the thermal insulation are confined to the fasteners for the insulating material and the external cladding or the ties attaching a self-supporting external leaf to the loadbearing layer. The ensuing thermal bridges are minimal. Owing to the uninterrupted development of the insulation layer and the minimal thermal bridges, the “loadbearing layer inside” concept does not present any problems in terms of the building performance and is one of the most common facade arrangements. It is also frequently used in the refurbishment of uninsulated or poorly insulated buildings.
Diagram of principle
Construction detail
Fig. 15: Case study: rendered external insulation, wall–floor junction The protective layer consists of render applied to the insulation. This form of construction results in a thin wall but the protective layer provides little defence against mechanical damage, which can lead to problems around the plinth in particular (damage to the insulation caused by feet, vehicles, etc.).
Junction with surrounding Plinth “underground house” ground “platform house” “house raised on stilts”
Diagram of principle
Fig. 14: Diagram of layers, loadbearing layer inside The insulating layer continues uninterrupted as a “second leaf “. The circles designate the transitions where the different layers are joined together; these key details must be resolved in detailed drawings.
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Protective layer Insulating layer Loadbearing layer
Ground slab, junction with foundation
Protective layer Insulating layer Loadbearing layer
Foundation Pad, strip, raft
Protective layer Insulating layer Loadbearing layer
Construction detail
Fig. 16: Case study: double-leaf masonry, wall–floor junction The protective layer is realised as a self-supporting masonry leaf, e.g. using clay or calcium silicate bricks, and partial tying back to the loadbearing layer is necessary owing to the instability of the non-loadbearing external leaf in the case of multistorey buildings. The use of double-leaf masonry results in the thickest wall construction.
Building performance, energy
STRUCTURES Concepts
Insulation concepts Complementary systems – loadbearing layer outside The “loadbearing layer outside” concept is used primarily on buildings with a fair-face concrete or facing masonry external facade, or those with a single interior space.
Edge of roof
Parapet Rooftop terrace, roof garden
Edge of terrace
Terrace–facade junction
Projections and returns Loggias, balconies, open walkways
Loggia rainwater drip
Soffit of loggia, junction with lintel
Openings Doors, windows
Window junctions, lintel, spandrel panel, reveals
Loadbearing layer Insulating layer Protective layer
Floor—wall External wall, internal partition
Protective layer Insulating layer Loadbearing layer
Loadbearing layer Insulating layer Protective layer
Roof Flat, pitched
The insulation in this case is on the inside. The transfer of loads from floors to the external loadbearing structure in multistorey buildings means that the insulation layer is interrupted at every floor. To reduce the ensuing thermal bridges the soffits of the intermediate floors have to be insulated for a distance of at least one metre around the perimeter. Combined thermal and impact sound insulation can be incorporated on the top of the floor. Fair-face concrete structures can also make use of corrosionresistant chromium steel anchors which enable a structural connection between wall and edge of slab but also leave a cavity which can be filled with a compressionresistant insulating material. The continuity of the insulating layer is guaranteed here, but the (closely spaced) anchors do represent discrete thermal bridges. Owing to their “false vapour-tightness sequence” (most permeable layer on the inside, densest layer on the outside), constructions with internal insulation must include a vapour barrier on the inside of the thermal insulation in order to prevent condensation.
Floor junction
Diagram of principle
Construction detail
Fig. 18: Case study: floor support not separated, discontinuous insulating layer To compensate for the interruption in the insulation layer a strip of insulation at least 100 cm wide must be attached to the soffit around the perimeter (either laid in the formwork or fixed to the underside of the floor). Disadvantage: the soffit must be plastered or lined (“facing quality”). Combined impact sound/thermal insulation must be incorporated on top of the floor. The vertical loadbearing layer can be in concrete or masonry.
Protective layer Insulating layer Loadbearing layer
Ground slab, junction with foundation
Diagram of principle
Fig. 17: Diagram of layers, loadbearing layer outside The system chosen for the floor connections (with chromium steel anchors) makes possible an uninterrupted insulating layer. The circles designate the transitions where the different layers are joined; these key details must be resolved in detailed drawings.
Loadbearing layer Insulating layer Protective layer
Foundation Pad, strip, raft
Junction with surrounding ground
Loadbearing layer Insulating layer Protective layer
Plinth “underground house” “platform house” “house raised on stilts”
Construction detail
Fig. 19: Case study: floor support separated, continuous insulating layer This type of construction is only possible in reinforced concrete because the chromium steel anchors must be integrated into the wall and floor reinforcement. Compression-resistant insulation must be incorporated between the face of a wall and the edge of the floor. Such insulation is often included with the respective anchor system (e.g. Schöck-Isokorb).
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STRUCTURES
Building performance, energy Concepts
Seven rules for the design of a low-energy house
What are the key factors when planning a low-energy house? The following seven rules are intended to provide an overview and a guide. 1. Work according to a concept The form, location, and interior layout of a building have a major influence on the energy consumption. Strive for clear, simple solutions. If you are not inventive by nature, assemble your house (intelligently) from inexpensive, readily available parts. 2. Plan a high degree of insulation… The thermal insulation of a low-energy house is at least 20 cm thick. Depending on the type of construction, the complete external component can be between 25 and 60 cm thick in total. … and avoid thermal bridges The problem of thermal bridges occurs wherever the insulated building envelope is penetrated by components which allow the passage of heat from inside the building. Many buildings lose more heat via avoidable thermal bridges than over the entire uninterrupted wall. Transitions and junctions require special care: – between window and wall, roof and other windows, – between door and wall, – between wall and roof, – between roller shutter and wall, – via shafts and flues at wall and roof, – via thresholds, window sills, lintels at floor and wall, – via fasteners, e.g. for balconies.
5. Cover the residual heating requirements with renewable energy media Solar energy, wood, and ambient heat are ideal for lowenergy houses because small installations (heat pumps, collectors) are adequate for low energy requirements, or only a small amount of fuel (wood) is necessary. 6. Store and distribute the heat with a low temperature level... The lower the temperatures of the heating media, the smaller the losses; this applies to both the generation and the distribution of heat. ... install the heat storage media in the heated part of the house... Every storage medium loses heat; this heat must be used in a low-energy house. ... and insist on short lines In some low-energy houses the supply and return pipes (due to their large surface area) heat up more than the radiators being supplied. This can lead to problems in the regulation of the heating system and to unnecessary energy losses. 7. Use energy-saving household appliances The use of energy-saving household appliances reduces emissions and environmental loads at the power station locations.
3. Exploit solar heat gains Include large windows on the side facing the sun, provided their energy audit is positive. Adequate storage capacity is necessary in order to absorb the radiation. This means that a heavyweight form of construction is preferable for internal partitions and floors. Position permanently habitable rooms, e.g. living room, children’s rooms, on the sunny side whenever possible. 4. Build airtight... No house without convection safeguards! The occupants breathe, not the walls, nor the roof. Ensure airtightness and check the workmanship, particularly at troublesome details. ... and install mechanical ventilation This will increase the quality of life in the house and reduce energy consumption because the heat losses can be recovered (heat exchanger). The ventilation plant must be carefully sized, and disturbing noise can be reduced with sound attenuation.
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Excerpt from: Othmar Humm: NiedrigEnergie- und PassivHäuser, Staufen bei Freiburg, 1998
Building performance, energy
STRUCTURES Example
Low-tech – high tectonics
Andrea Deplazes
Camouflaged energy concept One example for energy-saving construction within the costs framework of conventional building methods: What was originally intended as a conventional school design at the tender stage changed during the planning phase to a concept complying with the Swiss “Minergie” Standard. In doing so it was possible to avoid delegating the energy problem to the building services and instead to achieve a synthesis with the tectonics of the structure. A visitor to the school in Vella would be unable to discover anything that could be deemed unusual in a school. The buildings employ a solid form of construction, with fair-face concrete walls internally and solid timber wall panelling for the classrooms and the sports and assembly halls. The buildings are enclosed in a layer of thermal insulation 12 cm thick, which in turn is protected by a layer of render about 3 cm thick – exactly as used in the traditional timber houses not far from the school, which are clad with a thin render “membrane”. The internal layout corresponds exactly with typical school requirements. But upon closer inspection our attentive visitor would make a few discoveries: no radiators in the rooms, no centralised heating plant in the basement, no solar collectors anywhere in the building or on the roof! Instead, a mechanical ventilation system ensures a supply of fresh air with a low air change rate (0.5) and is intended to prevent uncontrolled ventilation losses (e.g. windows left open unintentionally). A heat exchanger has been installed downstream from this system to introduce waste heat from the exhaust air into the incoming fresh air. That is it, the only technical component in the school; this belongs to the – in architectural terms – less interesting part of the concept. More conspicuous are the ribbed concrete floors, the solid floor finishes of Vals quartzite stone slabs (also in the classrooms) and the large-format windows with their hopper-shaped reveals whose timber frames are screened externally by the thermal insulation. This is where the inconspicuous energy concept begins – with the use of passive solar energy. A technical problem? Soon after beginning the planning it was discovered that the location of the new school would be really ideal for
exploiting solar energy. Although nothing of this kind had been allowed for in the budget, the local authorities approached us, the architects, with the wish to integrate solar collectors into the roof surfaces. (“However, it mustn’t cost more.”) We were not impressed by the idea of the “badge of enlightened energy consciousness”, which all too often is placed conspicuously in the foreground. After all, the addition of technical equipment to the building would have disturbed not only the architectural surroundings of this mountain village with its splendid, archaic houses. To greater extent it disturbed our understanding of our role as architects – trying to combine diverse, often conflicting parameters in the design process – in that we would have to come to terms with an aesthetically successful integration of collectors into roof and other surfaces. A tectonics solution We therefore developed the concept of storing the solar energy in solid components. The appealing notion here is that we can use the same wall thicknesses and floor depths as in a conventional design – provided that the components are of solid construction so that they can absorb the incoming solar radiation (through the windows) as quickly as possible and thus prevent overheating in the interior. However, as the walls in the classrooms would be needed for all sorts of blackboards, magnetic notice boards, cupboards and showcases, and hence would not be available as a storage medium, we opted for ribs on the absorption surfaces and the optimisation of the floor mass distribution in line with the recognition that the dynamic penetration of heat radiation into solid components is about 10 cm (primary storage). During periods of good weather lasting a few days in the winter the storage media can be continually charged (secondary storage). Multiple use strategy This is coupled with additional, satisfying multiple uses. Provided with ribs, the floors easily span the 7.5 metres across the classrooms with little material consumption. At the same time, the profiled soffits create an extremely effective acoustic diffusion so that other acoustic measures (absorption) are unnecessary. Inexpensive energy-saving
Figs 20 and 21: School building and multipurpose hall (left), south facade with large area of glazing (right) Bearth & Deplazes: school complex, Vella (CH), 1997
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STRUCTURES
Building performance, energy Example
Fig. 22: Section through classroom wing
Project:
School complex with multipurpose hall, Vella (CH) Local authority of Vella, Lugnez (CH) Architects: Valentin Bearth, Andrea Deplazes, Chur Energy concept: Andrea Rüedi, Chur Building services: Nold + Padrun, Chur
lights are easily installed between the ribs without creating any glare. And finally, the ribbed floors create a rich architectural motif which can certainly be regarded as a transformation of the Baroque ceilings in the aforementioned houses of this district. Just one last component was missing in order to redirect the maximum amount of solar radiation up to the soffit – light-redirecting louvres on the inside of the window panes. But as specially designed light-redirecting systems would have been too expensive, we made use of conventional aluminium louvres which we threaded onto the operating cords and rotated 180°. These louvres are let down in winter just enough so that the pupils nearest the windows are not disturbed by the shallow, intense incoming sunlight, which is heightened by snow on the ground. However, the foremost one-third of the floor surface directly adjacent to the windows can still absorb heat and correspondingly “charge up” like a sheet of blotting paper across the depth of the room. The louvres can be rotated into position to reflect the sunlight over the heads of the pupils and up to the underside of the ribbed floor slab. This allows not only the heat absorption of the floor slab to be exploited to best effect, but also improves the natural lighting across the depth of the room, which in turn reduces the amount of electrical energy required for lighting. And the fact that in this position the louvres are still “open” and thus permit a view of the surrounding countryside should not be underestimated.
Fig. 23: Classroom with ribbed concrete soffit
Client:
Key parameters Recommendations of SIA 380/1 “Energie im Hochbau”, 1988 edition; target value : 260 MJ/m2a SIA brochure D 090 “Energiegerechte Schulbauten”; standard target value: 150 MJ/m2a optimised target value: 76 MJ/m2a Value calculated for Vella according to “Handbuch der passiven Sonnenenergienutzung”, SIA/BEW document D 010: 24 MJ /m2a Measured results for Vella (IBT diploma thesis 98/99): 34 MJ/m2a The deviation of the measured energy consumption values from the calculated ones for the school complex in Vella lies within the tolerances of the method of calculation. Storage capacity (reserve for poor weather): During a period of poor weather lasting 4 days, an outside temperature of -5°C and decreasing solar gains the storage media discharges from an average 21°C to 19°C. At this point the descending temperature gradient intersects with the preheated (with the heat exchanger) air temperature curve of the mechanical ventilation system such that the value can be maintained. (Measured values from 12–15 Jan 1999, measurements taken in winter 1998/99)
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Versatile concept As a concept for the use of solar energy through storage in solid components such as floors and walls, which have to be constructed anyway, this method is not confined to schools. The multiple use strategy of components is the condition that must be fulfilled in order to remain competitive – in terms of price – with conventional methods of building. It could be the right time to switch from the modernistic understanding of complementary architectural systems comprising monofunctional individual parts to synthetic, complex, polyfunctional components. That is what we call holistic thinking. Only in this way can we achieve added value in economic, energy, and cultural terms “in one fell swoop”, which is nothing other than “sustainability”. The entire energy concept with solid storage media would have been architecturally meaningless for Vella if the necessary massiveness could not have been combined with the theme of plasticity and the “monolithic mass” of the building, in the play of the surfaces, interior depth, and thin-wall facade skin, both in the corporeal expression of the building and in the motifs of the detailing, and with the urbanistic structure of this mountain village and its powerful, cubic, stocky houses.
Excerpt from: Bulletin, Magazin der Eidgenössischen Technischen Hochschule Zurich, issue No. 276, “Energie – im Umbruch”, January 2000, pp. 32–33.