How do I calculate the temperature of the wall from exterior to interior
Hello all, If I am given a wall with the exterior and interior temperature and I'm asked to determine the temperature let's say at the midpoint of the insulation....how do I calculate this?
Thanks!
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There are two methods, ratio method and a formula.
Ratio Method: At the point in question, if 80% of the R value is on the inside of the wall, the temp at that point is 20% less than the outside temp.
Formula: Temp @ point = Temp Outside + [ (R outside / R total) x delta T ]
I struggled with this one too and delved a lot into it, but the ratio method is what I think is the best because questions on the test will likely be in a simplistic format.
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You will never be asked for the temperature at a given point within the wall assembly without also being given the temperatures on either side of the wall. It’s not possible to solve otherwise.
As an aside, it might help to think about why an architect would go through this exercise in the first place. Usually the reason is that we’re concerned about condensation within the wall assembly. (This probably won’t be part of the question, but like I said, knowing the reason for this type of calculation in real life helps).
Condensation in walls occurs when moisture (usually moisture-laden interior air) reaches a cold surface. Precisely how cold the surface needs to be for condensation to occur (the dew point) will vary depending on how much moisture is in the air (its relative humidity) and its temperature. The surface within the wall that we usually care the most about is the interior face of the exterior sheathing. This is because that surface is exposed to warm, moisture-laden interior air, AND that surface is potentially also cold because it’s (usually) outboard of all or some of the insulation in the wall. (And moisture + cold surface = condensation risk).
But let’s get back to the calculation. We can use it to find out the temperature of various surfaces in our proposed wall… but we have to know the temperature on each side of the wall first, and temperatures change over the year! So what do we do?
In NCARB-world, the temperature will probably just be given. But in professional practice, the building code gives us some guidance: It tells us to take the average temperature of the three coldest months for wherever we’re building (usually December, January, and February) and use that for the exterior temperature. I think it also tells us to assume 72 degrees on the interior. The code then tells us that if we run through the calculation and the surfaces within the interior parts of the wall are 45 degrees or warmer, our wall is considered low-risk. If we have a surface that’s colder than 45 degrees (and also exposed to interior air), we may have a problem and other code requirements to control condensation may kick in (like an interior vapor retarder, or a requirement for more exterior insulation).
To be clear: even walls that “pass” the code’s 45 degree test can get condensation. The calculation is based on average temps, not coldest temps. The code just uses both the calculation and the temperature threshold as a rule of thumb. Mostly it’s better at identifying low-risk assemblies than telling us how to design higher risk assemblies.
You don’t have to know how to explain all that to answer these types of questions on the exams and you don’t need to know the code reference! But hopefully knowing some of the logic behind the calcs and that the code addresses this (it doesn’t make you guess) will help you in the exams and in professional practice.
I’m almost positive there is a 40 Minutes of Competence (free study session through Amber Book) on this topic.
Good luck!
Christine Williamson
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