Relative Humidity Stability

[Darren ODea]

Hi there,

I have been undertaking some research in some Australian climates (I don't think this is an Australian context discussion). I occasionally get drastic, seemingly overnight transitions from high RH in the outer 10 mm of the insulation layer to acceptable levels.

The example below is a lightweight south-facing external ventilated facade (35 ACH) with a vapour-permeable membrane and 90mm insulation layer followed by a 10 mm plasterboard layer. Canberra is the climate and driving rain is included. ASHRAE 160 internal loads (between 65 - 70%RH)

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Can the sudden drop in RH within the 10 mm insulation layer be explained numerically as it appears incorrect?

[Christian Bludau]

Hi Darren,
that may be a numerical problem or also a jump in the temperature for example. In the first step I would calculate for a longer time, maybe at least three years. If this behavior repeats every year, its probably no numerical issue.
Christian

[Darren ODea]

Hi Christian,

Thanks for the response.The data has been taken from a 5 year study and is from the second year. It repeats identically year on year.

I cannot identify an increase in the temperature of the 10 mm of insulation that directly correlates with the extreme RH drop.

canberra.png (59.67 KiB) Viewed 5736 times

Not with standing that RH has been maintained at high levels and this wall system is failing (ASHRAE 160), it still 'feels' incorrect/misleading. It only happens on the southern (low drying, non-equatorial facing) walls. North, west and east are fine!

Have you any further thoughts?

canberra2.png (65.87 KiB) Viewed 5736 times

[Christian Bludau]

Hi Darren,

it looks to me as if dew water is forming during this period. You are evaluating a whole cm of insulation here, aren't you? If you only look at the outermost grid element, you will probably find a relative humidity of 100%, which indicates that condensation is forming in the pore volume.

Christian