During this winter’s cold spell, many householders encountered problems of condensation forming on the inside surface of roofing felts, which collected and dripped down onto the ceiling, causing dampness to rooms below. Many complained of ice forming from this condensation on the underside of the cold roofing felt. When this started to melt, it dripped down and caused puddles of dampness to form.

So where does this condensation come from?  Well it is not from the outside environment, or from the cold- water storage tank, even many may be uncovered. Some of this vapour could have been generated by small expansion tank, venting the central heating system, often located in the attic space. But by far, most of this condensation is formed from warm vapour-laden air that drives under pressure from heated habitable rooms into the cold attic to condense on the coldest and least ventilated surfaces, such as roofing felt.

You can easily notice where this vapour starts to condense in the insulation quilt by dark stains discolouring the insulation, caused by moulds forming from sustained damp conditions.  In the winter, after a very cold night, if you lift this insulation in the attic you may notice that it is damp on the underside. You can imagine the damage and heat loss this condition can cause, when high-humid air and vapour from the warm interior air migrates under pressure into the structure and forming interstitial condensation and build up of dampness.

Next week’s blog topic: Improving Air Tightness

-Duncan Stewart