I’ve just returned from a meeting with a large basement waterproofing membranes provider. They have always provided the plastic dimpled membrane for cavity drainage, as have most industry providers, and condensation has always been a possible danger with such membranes. Since the widespread use of this general kind of waterproofing, the industry has struggled with this issue for decades.Do you want to learn more? look here
It was industry standard practise in the 1970s and 1980s to suggest that the air gap between the membrane and the plasterboard lining be vented top and bottom to avoid condensation. This recommendation was usually modified to ‘don’t ventilate the cavity’ throughout the 1990s and early part of the twenty-first century, since it may actually promote condensation on the membrane by bringing a continuous stream of damp air into touch with the cold surface of the membrane itself. As a result, the suggestion altered, but the issue remained.
While the introduction of high-quality, low-cost dehumidifiers, which are now widely accessible in most electrical shops, has helped, condensation on a cold plastic surface remains a serious danger. Insulating in front of the membrane increases this danger. ‘Why? ‘Surely, if I insulate something, it would stay warmer?’ you may wonder. ‘It was hearing that same statement today that prompted me to write this piece, particularly because it came from a big plastic membrane provider.’
I’m not a scientist, and I’m not sure whether it’s the first or second rule of thermodynamics (it doesn’t matter), but I do know that energy cannot be produced or destroyed – that much is common knowledge. So… If you add an insulation barrier to make anything WARMER, you must also add an insulation barrier to make something COLDER by the same amount. Insulation is not a source of heat. It has no effect on the temperature. It simply halts or delays the transmission of heat from one location to another. So, if the room is warm and the ground outside is cold, and the membrane is on the outside wall, and insulation is placed between the warm room and the cold wall, the wall and everything else on it (the membrane) will become COLDER while the room will remain WARMER. And if you make a vapour barrier cooler, you increase the likelihood of condensation.
The difference between insulating a membrane as described above and a ‘insulated’ membrane is that in an insulated membrane, the insulation is an integral part of the membrane, not a separate element in front of it; in fact, the insulation is BEHIND the vapour barrier, i.e. between the cold wall and the vapour barrier itself, so that the vapour barrier is kept warmer rather than cooler. That’s all there is to it. Insulating in front of a membrane in the mistaken belief that you are keeping it warmer is a simple error to make, but one that can be avoided with a little consideration.