DewPoint Calculation within the wall Assembly

I had a question for @coachadamcoers after today’s workshop on Wall Assemblies regarding Dewpoint Calculation within the wall assembly. I came across a few questions previously where the given information was a section of a wall, the different components of the wall assembly, their R or U values, interior temp, exterior temp asked to calculate if the Vapor where the vapor barrier would go and to calculate where the dew point with the wall would be, like a hotspot type question. Can you guide me beyond the general answer of warm side (interior) in cold climate and outside in warm climates please?

3 Likes

You will mostly need a vapor barrier in very cold weathers. You must place it closer to the interior side of the wall assembly because the warm air inside a room in (for ex. Canada) may cool down and become moist (dew point) as soon as it goes through the assembly if it is not stopped. Similar things happens in humid places such as Florida where the exterior is very hot and so people put their AC to cool down the interior space. The vapor barrier is placed closer to the exterior of the wall assembly so that warm air does not reach the interior of the assembly which is getting cooled down by the AC, thus preventing the condensation.
Hope this helps.

1 Like

Hi @najia.hashim ,

There was also a similar question posted awhile back that might be helpful to you! Check it out at the link below.

-Darion

1 Like

In MEEB p. 296 there’s a good example that walks you through the calculation. Also Building Construction Principles chapter 5. Basically its the cumulative R/Total R value of the assembly x temperature difference (outdoor-indoor). This number is subtracted from the indoor temperature to provide the temperature at the cumulative R reference point within the assembly. If you know what the dew point temperature is you can determine where in your assembly condensation will occur. The dew point would be determined by locating the dry bulb and wet bulb temperature on a psychometric chart and drawing a horizontal line to the left.

1 Like

thanks. this is the answer i was looking for. I also found a very good and step by step demo here:
https://www.constructionspecifier.com/illustrating-the-dew-point-an-effective-way-to-determine-its-value/3/

1 Like

While this is interesting, I do not think they will request this sort of calculation. Anyone thinks different?

@najia.hashim - Sorry for the delayed response.

Effectively the formula to calculate this as someone previously mentioned is:

T(cavity) = Interior Temp (Ti) - The difference between interior and exterior Temp (Delta T) multiplied by the percentage that of Insulation in relationship to the overall R-Value (RBatt/R-Wall)

I have attached a link to a very beneficial article that walks through the process, but one thing to keep in mind, is that if the wall assembly introduces a continuous layer of Insulation (as is required by the International Energy Conservation Code) the batt insulation within the stud cavity begins to not have as much value in the overall system as the continuous layer of insulation on the exterior will hold a greater impact in the energy loss off the assembly.

I hope this helps. Best of Luck on Sunday!

https://www.buildingscience.com/documents/digests/bsd-controlling-cold-weather-condensation-using-insulation#Fig01

1 Like

Hi @najia.hashim ,

I’m afraid I had to delete your comment as it does violate the NCARB candidate agreement. You can talk about topics you encountered, but try to avoid talking about literal questions.

Thanks,
Darion

6300 N. Northwest Highway P.O. Box 31846, Chicago, IL 60631-9998 / All rights reserved © 2020 Black Spectacles.