Framing - inside or outside the thermal envelope?
Framing
A strong frame is a necessity in Portuguese buildings in order to comply with seismic regulations. Typically this can be: - a reinforced concrete frame structure, with steel covered by at least 5cm - Light Steel Frame, integrated to the wall structure - timber frame - reinforced concrete embedded in insulating formwork (ICF) - reinforced concrete surrounding insulation (Baupanel)
Timber framing is not used often, although it does seem to gain some market share based on factory modules made in Eastern Europe.
By far the most common form is the reinforced concrete frame. In many older buildings, this frame defines the wall thickness and causes significant thermal bridging. A naive view would be that Portugal has a very temperate climate and this should not be a problem. However, the reality is that much of Portugal does have an average winter temperature that is lower than a comfortable internal termperature, and in moutainous regions there can indeed be snow. In addition, the summer temperature is often uncomfortably warm and buildings are subject to significant direct solar gains. This is in effect a bigger problem than heating in many cases.
Lost Formwork for Frames
The reinforced concrete frames can be built with formword shuttering or using 'lost formwork' where the columns and lintels use spaces cast into blocks which are typically 20cm deep and 40cm long.
Thermal Mass
It may seem obvious that we would ideally place such a frame inside the thermal envelope. Having done so, we would gain from the heat capacity of the material, which should help buffering against temperature swings inside the building.
However, the effect on a daily cycle is quite limited unless the frame elementsare exposed to the living space. Even modest coverings by materials with poor aerial heat capacity will be enough to diminish the effects significantly and we typically do need to create some sort of cavity or brick covering into which we can embed services.
The thermal capacity of such 'covered frames' will still result in a buffering effect and protect against temperature swings - but the time period over which it is effective will be longer than the daily cycle.
Internal Frames
External Architectural Features
However, it is quite common to desire architectural features that are quite hard to implement without introducing significant thermal bridges, because we need to attach to the frame. Typical examples include projecting balconies that provide shading to the ground floor and a wide balcony to rooms on the first floor, and projecting shades extending a flat roof over such a balcony on the first floor. A further feature seen quite often is that the upper floor is amaller than the lower floor and some of the space is given over to an extended balcony as a roof terrace.
It we allow additional pillars (or piloti) outside the structure then such architectural features could in principle be a second independant structure that is merely adjacent to the insulation installed around the frame. However in many cases we wish the elegance of an externally unsupported cantilever structure.
In many partially constructed Portuguese houses we see the complete reinforced concrete structure is created first,including: - pillars and ground beams - flooring for the fist floor extending outwards to create terraces and balconies - additional pillars to support a flat roof - flat roof structure extended to provide shading
Clearly such a structure is likely to suffer thermal bridging effects unless all the exposed concrete surfaces are insulated. In the case of a projecting flat concrete surface, we would need to insulate the top and bottom and the insulation on the upper surface would need to be robust enough to support the floor surface, which is typically tiled.
The labour involved in ETICS is signifianct, and properly insulating such a feature is likely to be expensive, and result in a very thick structure which may be inelegant.
Condensation
If the thermal bridging were solely related to heat loss or gain, then it may be reasonable to be quite relaxedabout it unless we have a particular desire to achice passive operation - whether or not PHI certified.
However, a problem with condensation is likely to occur in Spring and Winter.
The issue in this case is that the interior of the house is likely to be warmer than the outside - perhaps significantly so just before dawn. The seasons are typically very wet and thee overall humidity levels are high. This means that: - air drawn into the building has high relative humidity - there are additional humidity sources within the building from cooking, washing, and people breathing - consequently the relative humidity of air inside the building is also high - the parts of the concrete structure that are within the thermal envelope but are subject to significant thermal bridging may be cold enough to cause water from the air inside the thermal envelope to condense - such condensation will be in the floor and ceiling structures near the outside walls or underneath a terrace - internal walls in contact with such surfaces may also suffer condensation
Eliminating such condensation risks is difficult although an artificially increased rate of ventilation - and such vintilation carries significant costs in termsof heating load unless a very efficient MHRV unit is used, which will itself be expensive to acquire and fit into a structure of this sort.
External Frames
An alternative aproach is to regard the framing as part of the external skin of the building, so that the insulating layer is within the frame and external architectural features are attached directly to it without creating thermal bridges.
The problems are not eliminated- we now need to support internal walls, floors and ceilings and either support them independantly of the building's seismic frame or treat the joints from them to the external frame as potential thermal bridges.
- There are some potential features on our side in this case:
- the 'touch points' are somewhat reduced in scale,so we can use more expensive point solutions if necessary
- the internal structures like intermediate floors can be lighter and supported by internal walls
- if necessary, we can create a somewhat independant internal frame structure: it does not have the same loadings from mass or wind, and is not exposed to the elements
- we can use wooden structures so long as they are not directly exposed to cold surfaces with condensation
Condensation with an External Frame
We must assume that the internal surfaces of the frame and the external skin are all cold and subject to condensation forming. It can form when there is: - a body of trapped warm air with high humidity from a wet Spring day, that will shed water as the temperature falls overnight - warm moist air from the inside of the house that leaks out slowly - water vapour that diffuses through permeable elements of the building structure
The amount of insulation that we need to place between the outer leaf structure and internal to prevent condensation structure is modest: - we need to use a high strength insulation with closed cell structure - we need to make sure that the temperature gradient is such that the internal structure is not cold enough to form condensation - a relatively small amount of insulation will suffice if the concern is solely condensation: this might be the case if we use a wooden flooring system, since it will provide some degree of thermal resistance itself
- Candidate insulation products include:
- foam glass
- PUR/PIR
- very high density EPS, such as Compacfoam
- purpose-build thermal break systems, typically used for balconies in the UK
The key feature that we need is a bolt system that can attach the internal structure to the external frame through the insulation layer without creating a thermal bridge that causes the internal structure to have a sufficiently low temperature at a perimeter to cause condensation.
This enables us to attach a thermally isolated inner beam to the inside of the external frame, and we can the use this beam to support the interior floor structure.
Wooden Joist Systems
If we use a wooden joist system then the simplest solution for the internal perimeter support beam is to attach a piece of timber, and use a joist system that can rest on it and be attached to it with brackets.
- Candidate high-performance engineered wooden joist systems include:
- I-Joist, such as Finnjoist (availablefrom Jular)
- metal-web joist systems (in the UK, known as Posi-Joist, Ecojoist, Easi-Joist, Spacejoist)
In either case we extend the upper flange over the support beam and then attack both flanges with brackets. The extended flange enables us to position and support the weight of the joist while the brackets are attached: under load it should be the brackets that provide support, since they fix bothupper and lower flanges.
Both of these systems offer straightforward services routing since they contain space in the web or can be drilled to create space.
Concrete Beam Systems
An alternative to the timber systems is to use pre-stressed concrete beams with an infill structure: the structure is then screeded above and plastered below. They provide a strong base with significant internal thermal mass.
Service provision is not straightforward without a further false ceiling below. Some of the infill systems in concrete or Leca do provide channels but they will only run parallel to the support beams and routing services is tricky.
These systems are somewhat cheaper in Portugal than the wooden systems.
Conclusions
It may be more practical to forgo the thermal capacity of the structural frame in favour of reduced thermal bridging if the architecture would make preventing such bridges difficult.
- In effect we need to consider the relative practicality of thermal bridges:
- with external architectural features
- with internal ceiling and flooring
The decrement effect of placing the mass outside the insulation is reduced (see Design criteria for improving insulation effectiveness of multilayer walls) but the absolute benefit of decrement is somewhat limited if the static insulation levels is very high.
It is relatively straightforward to add aerial heat capacity in targetted locations in a building; materially eliminating thermal bridging can be difficult. Neither is inexpensive.
An interesting solution is the Baupanel system, which places a thin reinforced concrete layer on each side of an EPS core. The 40mm or so on the inside has a high aerial capacity and the overall static U value is good for the wall thickness. The decrement behaviour is rather poor - but for a good static insulation level, that might not be important. The addition of even a modest amount of external insulation using Aislone or ISODur is sufficient to completely change the decrement behaviour.