Warm as Toast and Cool as a Cucumber

18.08.2014


George Pickard, FCIOB, chair of the sales and marketing committee of the Precast Flooring Federation advises on how beam-and-block manufacturers have risen to the challenge and provided solutions that can exceed the requirements of the new Approved Document Part L.

Hardly a day goes past when one is not reminded of the green ecological merits of this or that product. With the introduction of the new Part L Document, we take one further step towards this goal in the construction industry. The three key drivers of the changes are:

 

  • Facing up to concerns about depending on imported gas and oil.
  • Reduced CO2.
  • Realisation that fuel is expensive.

The link between political will, legislation, specification, design and buildability has never been more effective.

In April this year, far-reaching changes to energy regulations came into force, specifically improving insulation and airtightness. At the heart of the changes is a fabric-first approach. For new domestic buildings, the revisions to Part L introduce a new fabric energy efficiency rate (FEES) to close the loophole that allowed renewable and low-carbon technologies to offset poor fabric performance. As a result, the changes to the regulations now focus firmly on achieving a good thermal performance for the building’s fabric. As Cliff Fudge, technical director of H&H Celcon, puts it, “It’s far more cost-effective to focus on a building’s fabric at the outset rather than trying to improve thermal efficiency later”. Which explains the popularity of masonry construction, whose thermal mass and high insulation provide a pleasant year-round living environment.

Today, it appears that after a 40-year gap, the benefit of the Salford low-energy house has not gone wasted. The brainchild of Dr J E Randall at the University of Salford and J M Hoyle an architect with Salford City Council, the revolutionary houses consumed about 25% the energy of the general housing stock at the time and exceeded current building regulations. Central to the design is a high thermal capacity internal structure protected by insulation in a well-sealed envelope.

Members of the PFF have been working with industry to provide buildable solutions for floors for over 20 years, with insulation values improving up to 0.08W/m2K, regarded as the absolute value for floors.

Insulating between beams and on the underside (to avoid cold bridging) has led to several systems on the market today: these are generally tested in accordance with NHBC criteria, achieve third-party accreditation and certification under a range of BBA certificates.

Improving U-values in the floor, wall, roof and window elements puts greater pressure on the thermal efficiency at construction joints where thermal bridging can occur. The average house has 300m of construction junctions – the equivalent area of a garage door – where a third of all heat saved by insulation could be lost.

The heat loss associated with thermal bridging is referred to as its psi-value (where psi is the Greek letter not pounds per square inch). This is calculated by computer modelling using THERM, BISCO and TRISCO, all of which estimate the heat flow through the various materials and construction joints in both 2D and 3D. The construction joint between the floor and the wall can be catered for with the enhanced detailing and, if adopted, can provide psi-values of 0.06W/mK.

Using Trisco modelling technology reveals the thermal resistance achieved with this enhanced detailing.

Empowered with this knowledge, a best-fit build model can be investigated using the resources of the quantity surveyor and energy assessor. A likely notional solution to the fabric would be a U-value of 0.13W/m2K for floors and 0.18W/m2K for walls, with a robust psi-value. John Thorn berry, MD of Moulded Foams, observes “Many clients future-proof their homes with uprated thermal insulation, knowing that investment in additional insulation pays for itself within the first 13 years and choosing to go for the absolute value of -0.08W/m2K for the floor, which is often the smallest fabric area and cheapest element to improve”.

The day Mr and Mrs Average in their three- bedroomed semi with their 2.4 children can expect to be as warm as toast in the winter and cool as a cucumber in summer for just £250 a year draws ever closer.

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