Ventilation

My question is not aimed at questioning the facts, but is merely to gain a better insight so as not to make a mistake in my build that I am planning as per a different thread. Where and when is ventilation (to prevent mould etc) necessary and when is it NOT necessary? Let's start with a roof. Is the space between a roof and the first of 2 ceilings a necessity because of temperature exchanges between outside and inside worlds and also due to the greater risk of external moisture getting there first - or is there another reason? What then happens between the 2 x ceiling masses then - or between wall spaces ... I would assume the fact that they are supposed to be completely airtight negates any worries? What happens on a concrete floor at ground level in the scenario where floated floors are not the way to go and flooring is directly on this concrete as such. What may seem dry before build might develop moisture problems afterwards - what happens to this trapped moisture from a mould-danger perspective? So basically, what needs to breathe, what doesn't - and at what point do you discern between the variables and choose to consider ventilation or not? Thank Steve

turnitup wrote:

So basically, what needs to breathe, what doesn't - and at what point do you discern between the variables and choose to consider ventilation or not? Thank Steve

This is not ever up for consideration as ventilation is a requirement of the building. It is not part of acoustics and is a science on it's own. Building has been turned into a science and it has nothing to do with acoustics, on the face of it although all these elements will weave together in small part. One of the biggest issues I see that we each struggle with is this mass/spring/mass as it applies to the ceiling. There is so much talk of a complete seal in order to get the best isolation. So this leads people to ask when and where and wtf can I do here to keep my building healthy and my isolation stable? If there were an easy answer it would not keep coming up. But the structure dictates what you will do in this respect and the structure is always a moving target because it is always made of different materials, in different countries, short and tall height, cold regions and hot regions. So it is confusing..it is still confusing to me but I can say a few things that cannot be refuted. The air of a building is different then the air for a mass/spring/mass assembly but they can sometimes be the same air. But there are modifications (fireblocking) that may have to be installed in a typical residential type building. This does not become a barrier as such but impedes the flow of air in the event of fire. This type of situation would only apply to a real room in a room type double leaf, mass/spring/mass build. Even at that, the air that is part of the roof becomes the same air that is part of the room just described, only that it is fire blocked with a dense rigid insulation. If you have a typical residential building and you have installed mass to the walls and are trying to get what you think is a mass/spring/mass in your ceiling by installing more mass on the top side of the joists this can work. It can work due to the same reason that people install plywood over the top of the joists to acquire more attic space. It works I can only assume because it is >not< completely sealed. In effect you have a coupled mass typical of a standard wall the only difference being that air flow still exists. The problem, and this is the problem we are addressing, is that when you close something (seal up) entirely that should be opened in some respect to air you remove the benefit of convection. And once this is done, then you are no longer building first, but building from an acoustical point of reference. And that will not work, ever.

Understood - thanks X-Space. I have added one final question to this issue, which is then specifically about applying this to my project on my original thread, (to which you have also kindly answered). Steve

Quick update. Been scouring, phoning and questioning as much as possible on this issue and have found a feasible way to resolve this by ensuring a "warm roof" that, if done correctly, negates the need for any ventilation in the ceiling space beneath the roof deck. Will be adding this to my "Corrugated Plastic Roof thread". Steve