Insulated roof construction using straining wire and compressing the flexible bulk density faced insulation between the roof sheet and the purlin is pretty much as standard as it gets in Africa, as far as constructing insulated roofs is concerned …… and yet do they REALLY achieve the required R-Value?
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), acoustic and fire performances.
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 thereby 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.
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.
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 1.8 metre centres, the thermal performance (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 actually what thermal performance (R-Value) your roof is really providing!
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.
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, which sounds good!
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 a really danger of the compressed insulation forcing the roof sheet off the clip.
Maximum thickness of compressed flexible bulk density insulation used between ‘Secret Fix’ roof sheeting clip and the purlins should be no more than 75mm.
MRC Group installs over-roofing systems for refurbishment projects, built up roof and uncompressed insulated roof systems for new build projects, using a structural bar & bracket support system. This system allows you to create a structural zone that insulation can be retained in its uncompressed state, providing the highest thermal and structural performance.
This method of construction provides the building owner the thermal performance (R-Value) that is stated therefore reducing the monthly energy costs of keeping the building cool or warm.
Contact us for metal roofing refurbishment and new build projects in Africa.
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