CADcaster

In the last paper I gave, I hope I outlined the types of insulation that are on the market, in this second paper, I will look at the effect the insulant  has on the structure and  one of the factors  should you be looking at, ie Condensation.

It has always amazed me that contractors and indeed many designers just stuff as much insulation into any crevice, just because there’s space for it, with no thought to whats actually happening to the structure, when you place such a powerful material in the construction. Let me explain, insulation is usually installed because the client needs to keep warn or cool, and wants to stop the transfer of heat but this is not the only thing going to happen, a hole host of other items also come into play, such as the condensation risk analysis, and for this we need a quick lesson in Doltons laws of partial pressure..

Air, the stuff we breath is made up of may gasses, one of which is water vapour, Just how much water vapour depends upon the temperature of the air, and that will also influence the pressure of that air water vapour,, So outside there might be a temp of 10 C at 80% RH in other words the air is holding 80% of its capacity of water vapour at 10 C, giving a vapour pressure of    20Pa Wiki has a nice easy to understand example of this. Also go to this chart to find the RH value of air. But the internal temp could be as high as 22 C with a humidity of say 30% looking at a cycrometric chart will show that there is a pressure difference between inside to allow the internal water vapour pressure to force its way to the outside, once it comes into contact with a colder section of the structure, it cools  and the RH rises to the point the air cannot hold that amount of water vapour and it starts to drop out as beads of water on the nearest cold surface.

To prevent this happening, the construction needs to be altered to prevent the water vapour and ait migrating across the structure, hence the invention of the vapour barrier, its sole job being to stop the passage of moisture laden air, and in most cases its very good at its job, but be aware of one major flaw, it has to be fixed, and it comes in sheets that have to be joined and sealed, and its here that its integrity falls down and air laden with moisture leaks into the structure and condensation arrises. The more heavily insulated the structure is the worse the problem.

But there are ways to help the vapour barrier to work, first is to keep the RH down, but adequate ventilation to areas were condensation is at its highest, Kitchens, bathrooms, and the like, also the use of small condensation machines to mechanically reduce the rh.

Perhaps the alternative in certain cases is to allow a small amount of vapour to escape and deal with its in the design, this if often the case in pitched roof structure, were a vapour permeable sarking layer is installed that allows vapour to pass through it, but not water, the vapour condensing in a place were it can do no harm.
So just placing insulation into a structure is going to lead to many problems unless you take a carefull look at what is going to happen given air and water vapour will want to migrate across the structure. Fortunately many of the more responsible insulant manufacturers will come to your help and complete a condensation risk analysis for you looking at one moment in time and show just what might happen if you do not install vapour barriers or ventilate correctly. its usualy a free service, or you can search online to find a free service, or maybe pay for it, its not a huge expense. Or you might want to do a full real time condensation risk analysis based upon a your 3D CAD drawing, this will look at a whole year of the building given its life, use and just were it will be located.

This is not intended to be a full article on condensation rick analysis, but I hope I have given you sufficient knowledge to understand that its an important subject and needs further research and understanding, print this of and start making notes about the different types of insulant, look at the way vapour can transmit across its structure, and think what effect is this going to have, can I deal with it, with a vapour barrier,  should I change the insulant, or indeed the position.

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Source: Konstrukshon CPD Weblog – Architectural Technologist – Insulation and its effect on condensation
Go to Source: Konstrukshon CPD Weblog