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# Dew point for wall structure

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## Dew point for wall structure

(OP)
Regards to all of you.

I have a small problem that I can't really solve fully at the moment.

I am trying to determine whether there will be a condensation in/on an insulated concrete wall.

I've managed to determine the heat transfer rate thru the wall and temperature between each layer.

Now, this is where I got stuck. I've managed to find water vapour saturation pressure for each of the given temperature between the layers, but I do not know how to determine realistic 'flow' of water vapour pressure thru the wall in order to see if condensation will occur.

The bit of text written below are basics for my calculations, that describe my problem accurately:

Wall consist of a 300 mm heavy rebared concrete (Lambda=1.55 W/mK) and 80 mm of rockwool (Lambda=0.035 W/mK). Temperature and RH are 18 degrees Celcius and 50% on the side of the concrete and 0 degrees Celcius and RH of 90% on the other side (side with rockwool).

I've assumed that convective heat transfer coefficient is 20 W/(m2K).

All of this gives me an U value of 0.388 W/(m2K).

### RE: Dew point for wall structure

In my experience to determine whether or not condensation will form on an exterior wall you have to determine what your minimum surface temperature will be on the inside face of the wall (which it seems like you have) and compare it to the dewpoint of the air inside the space.

It seems like you are more doing an analysis on your water permeability through the wall? I haven’t done that before but if it’s also a valid concern hopefully someone else can answer that.

Somehow if your permeability calculation came up with a water flow rate, you would then calculate if you have enough air movement in the room with enough water absorption capacity to absorb that flow rate - and if so no condensation should form on that wall, assuming it is not in a stagnated corner of the room.

### RE: Dew point for wall structure

Water vapor vapor need not permeate the rockwool and concrete layers in order for condensation to occur on the bare side of the concrete wall. Looking at water vapor pressure curves, if the 50%RH air at 18degC on the bare concrete surface side could reach approx 8degC, water vapor can condense out from the air onto the concrete surface.

So run a heat transfer calc to see what the bare concrete side surface (or film) temperature will be. By inspection alone, it should be obvious that condensation cannot occur in the rockwool layer, since only water vapor from the cold side can diffuse into rockwool. Concrete is nominally impermeable to water vapor, so diffusion in the opposite direction is relatively negligible.

In the calc for overall U, also take into account the natural convection htc values at the cold rockwool side and similar for the bare concrete side. This will then lead to Q, and from this you can extract a value for the bare concrete side surface temp.

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