REDUCED HEATING REQUIREMENTS

In essence SIPs takes the frame out of timber frame building and thereby minimises cold bridging. There are still some cold bridges around openings where windows and doors are fitted to timbers and in some cases timbers are used in the panels to 'beef up' the structure and form door and window openings. Typically cold bridging is reduced to around 5% compared to 15-30% with a timber frame. The roof has around 1% of bridging timber compared to 6% in a trussed roof with insulation between the rafters. These comparisons are approximate because it is of course possible to reduce cold bridging on a trussed roof by using external insulation as a warm roof.

Having a highly insulated and relatively air tight building may cut down on heating bills but it can be an uncomfortable and unhealthy living environment so ventilation must be an essential part of the design. If the ventilation has built in heat recovery you can almost do away with the need for a heating system. Heat given off by, people, lighting, cooking and an increasing array of electrical equipment could, in theory, be enough for all but the coldest days.

Before committing to a heat recovery system it is worth looking carefully at the performance efficiency. Manufacturer’s figures should not simply be taken at face value without some independent verification. It is common for systems to claim 80% efficiency but 80% of what and at what temperature differentials? Without a graph of inside and outside temperatures plotted against fan speed an efficiency figure can be misleading. Heat losses for a SIPs house can be worked out from U values in the usual way but if there are complaints from SIPs owners they are usually that the structure is too warm. With such a well insulated structure you also need to look at heat gains. If there are enough south facing windows then the solar heat gain and retention through modern low e sealed unit will produce a build up of heat on most days of the year. The problem is that the heat will be too much in the middle of the day and not enough in the evenings because SIPs, and timber frame for that matter, lack thermal mass and there is nowhere for this heat to be stored.

The lack of thermal mass has been overcome to some extent by SIPCRETE, a company that manufactures mesh covered PUR cores which are erected and then sprayed or rendered in situ with concrete. The concrete does the same job as the OSB face does in timber based SIPS. The result is a very strong and heavy structure with thermal mass and a reassuring feeling of solidity. More importantly it offers greater fire resistance.

Another ingenious solution to the lack of thermal mass is Smart board from KNAUF. This is a gypsum board, used instead of ordinary plasterboard. The gypsum is impregnated with a phase change material called Micronal, a paraffin wax like material manufactured by BASF. You can best imagine this ingredient as a lava lamp heating up and distributing heat throughout the board in the day time and releasing it as the temperature drops. The board can be skimmed with finish plaster and is then indistinguishable from the standard plasterboard. but at over £100.00 for a standard sheet you would not want your dry liners treating is as if it were ordinary plasterboard.

Phase change materials (PCMs) such as Micronal are destined to become a major part of building in the very near future. Ten m2 of Smart Board, containing 3kg of Micronal, stores 1kW/h of heat energy and the release is self regulating. In a well insulated house a flow of 1kw/h is enough for most living areas but you might be looking for five hours in the winter.

ON SITE CONSIDERATIONS

Using cladding to form a structure rather than having a skeletal frame is not in itself a new principle. Aircraft and racing cars have been built using this principle and most insects are structured in this way. The fact that it works in other areas does not of course mean that it is a proven structure for building houses. Tests to give data on Racking, Vertical UDL, Uplift, Horizontal UDL, Eccentric compressive loads, Compressive Strength, Creep, Concentrated load, thermal conductivity, tensile, sheer, peel strength have been carried out by SIPTEC.

The way that SIPs work is not always clearly understood. Using two co-operating faces produces a panel that is typically 5 times stronger in racking than a Timber Frame Structure, and significantly stiffer. The operative word here is 'co-operating'. When subjected to loading, a good Structural Insulated Panel acts like a steel I-beam. The tensile strength of the skins resist the stretching and crushing forces of tension and compression. The insulating core does not carry any load but its adhesion over the entire surface area is an important factor in preventing deflection of the panels. If the insulating core is not well stuck and the panels buckle the strength is lost immediately. Hemsec Sips MPF cores claims a super high-strength adhesion spread forces across the surface area of the panel to resist shearing and slippage between the skins. The company says that it becomes possible to create arches, vaulted ceilings and shapes that would be too costly to produce in traditional methods.