SA Roofing September 2017 // Issue: 93 - Page 30

REGULARS: QUALITY MATTERS Assessing African construction methods Insulated roof construction using straining wire and compressing the flexible bulk density faced insulation between the roof sheet and the purlin, is as standard as it gets in some parts of Africa I’ve worked in. But do they really achieve the required R-Value? By Richard Polling of MRC Group It is the air trapped between the fibres that provides the ability for the glass fibre to behave as an insulator and not the actual fibres themselves. If the insulation is compressed then the air is squeezed out from within the insulation, and the ability of the glass wool insulation to insulate is dramatically reduced. Proper insulation is necessary for the upkeep of a building. What is glass wool insulation? Glass wool is made from sand and recycled glass and is pulled through a fine mesh through centrifugal force. The strands dry and cool when they become in contact with air. These fibres are layered and arranged to produce a wool and is held together with a range of differing binders. The differing thickness and densities of products provide differing thermal (R-values) and acoustic performance. How does glass wool insulation work? Is it the amount of glass wool fibres within the insulation roll? As some people would declare, or is it the density of the material? Or something else? The fibres, that are bound together, create air voids where air is trapped. The voids ‘trap' air and slow down convection reducing heat loss and gain. Heat passing through the fibres and the isolated air pockets therefore can't penetrate very far as the material itself does not conduct heat very well. 28 SEPTEMBER 2017 RESIDENTIAL // COMMERCIAL // INDUSTRIAL What is the insulation performance of the standard insulated roof systems using straining wire and compressed flexible bulk density? As we have discovered by compressing the insulation between the roof sheet system and the steel or timber purlin, the thermal performance of the flexible bulk density insulation is dramatically reduced. Tests have proved that compared to the stated uncompressed R-values when compressed between roof sheets and purlins at 1.8m centres, the R-value can be decreased by up to 72%. If you take the amount of insulation in your roof space and reduce the thermal performance by 72% you may be alarmed to see what thermal performance (R-value) your roof really provides. The question that then must be asked is whether the roof build up is achieving the regulated SANS 10400 part XA and SANS 204? Who pays for this thermal reduction? The impact of this revelation will impact many parts of the building including the HVAC calculations. Ultimately the building occupier pays, as the amount of energy to warm or cool the internal spaces within the property, is higher than anticipated, which costs the building owner more in rising electricity costs. Will putting more insulation in the roof solve this problem? Yes, adding more insulation does provide better thermal performance however, the thicker the insulation, the less you can compress it between the purlin and the roof sheet. But now your roof system is structurally unsound, as with the case of pierced fix roof system, you cannot fix the roof system tight back down to the purlin, because there is now a gap and in the case of a secret fix roof system reliant of a clip, there is now real danger of the compressed insulation forcing the roof sheet off the clip. For good practice the maximum thickness of compressed flexible bulk density insulation used between 'secret fix' roof sheeting clip and the purlins should be no more than 75mm, without a bar and bracket supp