Don’t lag behind the times

When choosing construction products today the focus is on achieving best value overall. It is vital to look at whole life costing (WLC), a method of assessing the long-term performance and attributes of materials by comparing lifespan, installation, running and maintenance costs.

There are considerable savings to be made by using the most appropriate HVAC ductwork. The traditional method of providing insulated ductwork is to use galvanised sheet steel, which is then lagged. There are a number of issues arising from this in terms of weight, space and time, all of which have been researched in the past. However, the WLC aspect has not previously been addressed.

Kingspan Insulation commissioned independent consultant Cyril Sweett to look at the whole life costs of a traditional HVAC ductwork system compared with a modern pre-insulated system.

Cyril Sweett based the comparison on a system comprising 308m of ductwork of a wide variety of dimensions. The initial cost of insulated galvanised sheet metal was £47,154, while rigid phenolic pre-insulated ductwork was £37,000.

The figures were based on the use of pre-insulated ductwork constructed of 22mm rigid phenolic insulation panels and an aluminium grip flange jointing system versus 40mm mineral fibre lagged galvanised sheet steel ducting. There was an initial capital saving of 21.5% on the part of the pre-insulated ductwork (see Table 1).

Cyril Sweett then calculated the operating cost over a 30-year period, taking into account the energy saving, maintenance, cleaning costs and general wear and tear. The estimated costs are £85,058 for insulated galvanised sheet metal and £67,672 for rigid phenolic pre-insulated ductwork.

Again, the pre-insulated ductwork represents the most cost-effective solution, with a 20.4% saving over the 30 years.

One major factor is that maintenance or repair was easier with pre-insulated ductwork, as it was simply a matter of cutting into the panel and replacing the damaged area, instead of having to replace an entire section of sheet metal ducting and then insulating it.

Adding together the initial cost and the whole life costs produces a WLC saving of £27,540, or 20.8%, if pre-insulated ductwork is used rather than the sheet metal system (see Figure 1).

Earlier independent research commissioned by Kingspan Insulation had already shown that significant savings could be made because of the exceptional air-tightness achievable with pre-insulated ducting, which can reduce the size of the fan and energy output needed to power the HVAC system, with potential annual savings of over 30% on running costs. This formed part of the data upon which Cyril Sweett based the whole life costing.

The initial review, by independent consultant BSRIA, was of the implication of ductwork specification and different duct air-leakage rates on energy consumption. The methodology in this report was used to compare a 100m long run of galvanised sheet steel ductwork and a 100m long run of rigid phenolic pre-insulated ductwork. Both had cross-sectional internal dimensions of 600 x 600mm. The results are indicated in Table 2.

Big energy savings

BSRIA’s methodology showed that the rigid phenolic pre-insulated ductwork can save 31% of the energy required to run a fan in an HVAC system with a typical design flow rate of 7.5m/s. Savings can be greater for lower rates.

A further review by independent consultant Bucknall Austin was then commissioned on the implications of reduced fan power on actual energy usage. This concluded that for a 100m long run of ductwork, with internal dimensions of 600 x 600mm and an air velocity of 7.5m/s, rigid phenolic pre-insulated ductwork could save 3,100kWhr a year.

There are other advantages to using pre-insulated ductwork in terms of durability. The excellent thermal performance of closed cell rigid phenolic insulation is not prone to long-term degradation and is unaffected by air movement or water ingress. It is not easily compressed, and maintains its performance over the whole 30-year “life” period. This is a particularly important attribute in light of the Energy Performance of Buildings Directive, which also takes a long-term view, albeit relating to carbon output rather than cost.

Pre-insulated ductwork is also lightweight and takes up less space, making it ideal for refurbishment projects. It requires no specialist equipment and it can be installed up to three times faster than sheet metal ducting. This also has an impact on the whole life costing by speeding up programme and moving the installation of services off the critical path, enabling earlier completion of projects. Last-minute changes to the design and unforeseen problems can be more easily dealt with as the sections of ductwork can be cut and assembled on site.

Kingspan Insulation has produced a series of papers examining the differences between sheet metal and pre-insulated ductwork. Copies of these and the independent reports are available from the marketing department, tel 0870 733 8333 or email literature.uk@koolduct.kingspan.com