Interstitial Condensation

What is this new "buzzword" that keeps cropping up? Why only now? How come this was never a problem in older buildings? Well, the answer is simple. Additional insulation, if installed incorrectly, is one thing that can lead to interstitial condensation.

Mould behind dry liningInterstitial condensation is very simply, condensation within the structure of a building, ie. not on the surface and not clearly visible until often, it’s too late and the damage is done. Warm air can hold a lot of moisture, so air within a dwelling for example, is perfect for moisture-laden air, especially as we create so much of it – through washing, cooking, breathing, etc. Condensation occurs where this warm, moisture-laden air is suddenly cooled and can no longer hold the high levels of moisture. In a building, this is usually when the warm air meets a cold surface such as a mirror, window or external wall. If the wall is uninsulated, then it can be extremely cold in winter time and condensation will be visible on the surface...


But what happens if the wall is fitted with internal wall insulation (dry lining), for example? Now the surface is warm and there's no longer condensation...or is there...? The moisture contained in air is water vapour and is usually completely invisible to the human eye, as it is in such tiny particles. This water vapour can easily pass through most building materials such as plaster, slabs, insulation, etc. And so, they do just that...

So, your wall is insulated on the inside. This makes the external wall even colder, as now the heat cannot get out to heat it up. But the moisture can....Now, the moisture-laden air is cooled even further than before and dumps even more condensation, down along the wall surface, exactly as before...only now, the old wall surface is within the overall structure of the wall (including the dry-lining).

This is called "interstitial" condensation, as it happens within the wall’s structure. Any moisture within a structure is not good. It leads, first of all, to nasty moulds and fungi to grow within the structure which leads to very poor air quality within the building. This can have severe health implications. Dampness also leads to structural damage, whether it's rotting timber, degraded insulation, cables or pipes/fitting corroding, etc. It's a huge problem because it can't be seen for maybe several years but by then, it's far too late to do anything about it. This can equally be a problem in new and existing buildings, so great care should be taken, especially when altering the make-up of any building element eg. wall

The good news is that correct construction methods such as: using an appropriate "vapour check", a barrier against water vapour; placing the insulation in a different position, such as outside the wall, or introducing the appropriate ventilation where condensation may occur; can eliminate the possibility of interstitial condensation. It is critically important to consider what construction methods are employed before work is carried out. Unfortunately, unscrupulous or ignorant contractors may try to lead you down the wrong path as the incorrect and easy methods are generally much cheaper, quicker and easier to install.

Our advice is to do your homework first, consider all options, be sure you find a knowledgeable contractor (they are out there!) and build it right, first time! Contact us for further information.

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Barbara Bradby, Ballycroy

External Insulation, Loft insulation, DPC

Our little house by the sea in Co Mayo was a dream, but the damp was a nightmare. I looked into external insulation first as a solution for the damp which was breaking out on all the internal walls. The walls were concrete, there was condensation... Read more

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Brendan Coyne, Snugboro, Castlebar, Co. Mayo

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