Debate heats up on insulation usage under roof sheeting

“The cheapest, most reliable source of new energy is saved energy,” says Peter Turnbull of PG & E. The promulgation of SANS 10400 XA requires that insulation should be installed to achieve prescribed R-values in the roof space.  

The obvious methods of installing insulation are to lay it under the roof covering or above the ceiling. If laid under the roof covering, the logical response is to lay it over purlin.

However, the use of insulation of any bulk presents issues of stability for the roof sheeting, and the performance of insulation, bulk or rigid insulation in particular is questionable if it is severely compressed.

If used under pierced fixed sheeting such as IBR, the fastener is longer, and is put under high stress. If used under concealed fix sheeting, the concern is that a thick blanket insulation will bulk up under the sheet and could force the sheet off the clip securing it.

Various methods have been developed by the roofing industry to address these challenges:

  • U-shaped stirrups or spacers have been used. They press through the insulation and rest on the purlin, providing some structural support to the fasteners and the roof sheet. However, stirrups are laborious to install and are prone to rock. They also do little to reduce the stress on the longer fasteners and do not adequately support the span of the roof sheeting.
  • Another commonly used method is to install a row of timber purlins or packers, which are fixed to the lower row of purlins over the insulation. Not only is this practice a fire hazard, but it results in excessive compression of the insulation. And seldom is a timber spacer in perfect alignment or of perfect dimension with all other packers, providing a sub-standard base for the roof covering, compromising the performance of the insulation.

Various other methods include metal straps, secondary metal purlins and more.

Every method above faces the same unresolved issue

For insulation to perform to its optimum level, a continuous layer must be installed with minimal compression or thermal bridging. None of the above methods achieve this, and the performance of excellent insulation material is compromised by the method of installation. Architects and home owners do not achieve what is claimed or being paid for, and the entire thermal rating of the building is downgraded.

However, a properly engineered roof spacer system such as Ashgrid will, by the creation of a defined cavity or roof space, accommodate blanket insulation at its full depth. Such systems are pre-engineered for rapid and easy installation, and have varying leg heights for various depths of insulation. The resulting R-values of the insulating material may therefore be taken at full design value as claimed by the manufacturer.

Tests on the performance of blanket insulation over Purlin

Safintra South Africa recently undertook tests to determine the performance of an insulation blanket installed with their respected Saflok concealed fix system.

A 135mm fibreglass blanket with a density of 12kg/mᶟ was installed under the sheet and over purlin. Straining wires were installed with sufficient slack to allow the insulation to maintain as much loft as possible between purlins, within practical considerations. Saflok 700 was installed as the roof covering.

Two methods were compared in the test. The first test was conducted without a roof spacer system. The second test was conducted using the Ashgrid roof spacer system.

The tests clearly showed that

  1. Even without a roof spacer system, the integrity of the Saflok system was unimpaired, with the clips securely locking the sheeting over the blanket which had compressed to 15mm
  2. However, compression of the insulation blanket at the purlin compromised the thermal performance of the insulation by almost 40%, whereas the roof spacer system allowed the insulation to perform to its full design value.

There is no doubt that the cost of roof construction has been increased by the requirements of SANS 10400:XA. This cost increases by the cost of insulation materials and labour at time of construction, but is more than amortised by energy savings over the first seven years of the occupied life of the building.

The cost of a roof spacer system with properly performing insulation is in most instances actually lower than the cost of additional layers of insulation to achieve the required R-value in the roof space.

Speak to Safintra about roofing systems that deliver as they should – for owners, for occupants and for the environment.

Safintra South Africa
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