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Housing - New Build: Fully filled cavity wall with AAC block - interstitial condensation
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- CommentAuthorondrejg
- CommentTimeMay 10th 2012
Hi, I tried to model a simple cavity wall with facing brick, 100 mm mineral wool and AAC block (0.18 W/mK) inner leaf in BuildDesk U and it shows an interstitial condensation risk according to BS EN ISO 13788. As far as I know this type of wall construction is widely used so I don't understand how it is possible that it does not pass the condensation risk check. Am I doing something wrong? Can anyone help me to understand this? -
- CommentAuthorDarylP
- CommentTimeMay 10th 2012
... where is the dew point?
If it is in the outer leaf, then any 'solid' water will drain down the inside of the outer leaf and away, or evaporate from the outer face of the brickwork...
But remember there is also p/b or plaster on the inner leaf of the aac block. Have you included this layer as well?
Cheers... -
- CommentAuthorondrejg
- CommentTimeMay 11th 2012
DarylP, thanks for your answer. The condensation occurs on the inside of the outer leaf. I went through Rockwool brochure and indeed it says that the insulation has water repellent fibres and "Any water penetrating the outer leaf will drain down the surface of the batts". This confirms what you wrote. Cheers -
- CommentAuthorCWatters
- CommentTimeMay 11th 2012
It seems to me that the better the insulation in the cavity the more likely you are to get condensation on the inner surface of the outer leaf as it's colder. -
- CommentAuthortony
- CommentTimeMay 11th 2012
I have 300mm in my cavity. no problems but I have lime in my masonry cladding, the condensation usually forms on or near the outside face of the outer leaf, in rare and extreme circumstances it can form on the inside of the outer skin but certainly not often enough to b a problem.
I do get it on the outer leaf metal lintel when it is below -2C outside.
The thicker the insulation in the cavity the warmer you will be and the less energy you will need
Risk analysis has to look at worse cases...... -
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CommentAuthorSteamyTea
- CommentTimeMay 11th 2012
Posted By: tonyRisk analysis has to look at worse cases......
Not really, it just looks at the probability of something happening below a fixed level and then the associated consequences.
So if you are looking at condensation risk, first you look at the one tailed temperature profile to 2 standard deviations and then you look at the RH levels to the same resolution, from that you can calculate the probability of hitting the dew point.
If you want to look at worst case, then you just look at the extremes and how often they happen.
Very different methodologies even though the dataset is the same.
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