Roof construction methods and thermal performance

Are African construction methods of roof construction failing the building owners? What is the real thermal performance of your insulated roof? Richard Polling of the MRC Group provides insight into these two burning questions.

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?

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 come in contact with air. These fibres are layered and arranged to produce a wool that is held together with a range of differing binders. The differing thickness and densities of this product provide for differing thermal (R-Values), acoustic and fire performance.

How does glass wool insulation actually 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 is it 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.

Performance of the standard insulated roof systems using straining wire and compressed flexible bulk density faced insulation

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 40%.

If you take the amount of insulation in your roof space and reduce the thermal performance by 40% you may be alarmed to see what thermal performance (R-Value) your roof is really providing.

Who pays for this thermal reduction?

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, 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 real danger of the compressed insulation forcing the roof sheet off the clip.

Maximum thickness of compressed flexible bulk density insulation used between a ‘secret fix’ roof sheeting clip and the purlins should be no more than 75mm.

The Solution is not to compress the insulation

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 and bracket support system. This system allows you to create a structural zone in which 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.

For more information about the MRC Group, visit This article was published with permission from the MRC Group. Read the original article here.



  1. Lyndsay

    Thanks for the insights Richard. It is such an important issue in our energy constrained environment.

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