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Condensation at a basement window (C) Daniel Friedman Dew Point Data for Building Wall Cavities

Dew point psychometric chart & dew point calculation equations

Dew point: Definition of dew point & relative humidity; how to calculate the dew point.

This article explains how the wall cavity dew point, the point at which moisture condenses out of air onto a surface, is calculated for a building cavity such as inside of an insulated wall.

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How to Calculate or Predict the Dew Point in a Building Wall or Ceiling Cavity

Psychometric chart Question about how we calculate the dew point in buildings

The question-and-answer article about calculating the dew point in walls and a discussion of mathematical models of moisture condensation, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss. Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Our page top photo shows severe moisture condensation on a basement window and window frame.

All of the literature I have read on condensation within building wall cavities warns of the problem of cavity moisture (a potential source of mold contamination, insect attack, or structural rot). But the literature does not provide any clues as to how to predict wall cavity condensation.

Is there a formula which will determine the dew point inside a building wall when both inside and outside relative humilities and temperatures are known? - J.L.B., G4reenfield Center, NY.

[Click to enlarge any image including this psychometric chart]

How to Predict or Calculate a Wall Cavity Dew Point or Condensation Point in Buildings

Yes, JLB. Editor Daniel F. includes dew point calculations, formulas, and mathematic models later in this article.

First let's understand what the dew point is. The dew point and relative humidity are the two most-widely used ways that people describe the amount of moisture that is in air.

Definition of Dew Point: Why moisture condenses out of air.

The dew point, or properly, dew point temperature, is the temperature to which air will have to cool to reach its saturation point. The air saturation point is the point at which the air can hold no more water - it is fully saturated.

Warmer air can hold more water than cooler air

While it is not technically-correct, you can think of warm air as having more space between the gas molecules that comprise the air, giving room for water molecules.

What's actually the case is that there's plenty of space for water molecules in both warm and cold air. It is rather because in cooler air the water molecules (in air as a gas form) as well as other gas molecules making up air are absorbing less energy than in warmer air.

When the water molecules absorb less energy they are less excited, they zoom about less, they bang into one another with less force, so they begin to condense, first into micro-droplets (fog) and finally into visible droplets that form on first on cooler surfaces with which air is in contact (because air there is most-cooled).

When the air temperature and the dew point temperature are the same, the air is fully saturated.

If we hold the barometric pressure and water vapor content of the air constant, then when we cool the air below the dew point, the water that the air can no longer hold will be condensed out of the air and onto cooler nearby surfaces.

In fact a cool wall or ceiling surface will cool warmer moist air close to the wall surface, thus causing that air to cool and thus causing moisture to condense out of the air onto the surface.

Definition of Relative Humidity (RH) & Relationship of RH to the Dew Point

Relative humidity (RH) is defined as the actual amount of water vapor present in air expressed as a precent of the maximum quantity of water that that same air could hold at a given temperature.

The maximum amount of water that air could hold for a given temperature (and actually barometric pressure but we'll ignore that) is also defined as the saturation point of the air.

RH is the ratio of the actual water vapor pressure in air to the saturation vapor pressure of that air a fixed temperature and barometric pressure.

Relationship of Dew Point to Temperature

RH is expressed either as the ratio of actual vapor pressure to the saturation water pressure. Saturation water pressure appears in some texts as the "equilibrium water pressure" since that's the point at which the effects of temperature and vapor pressure are balanced: the air can't accept any more water. Raising the temperature will allow the air to accept more water, thus raising the dew point.

Lowering the temperature will squeeze water out of the air, lowering the dew point. However the exact relationship between temperature and dew point is not linear (look again at the psychotic chart earlier on this page). Lawrence (2005) suggests a rule of thumb that might work for ranges of temperature and relative humidity when the air is high in moisture:

td = t - {100 - RH / 5} for air at RH > 50%

Lawrence is saying that for moist air, the dewpoint temperature td will decrease about 1°C for every 5% decrease in RH (starting at td = t, where t = the dry bulb temperature when RH = 100%)

So what the heck, J.L.B., you can see before we even start that these relationships are not linear and not trivial. So there won't be a trivial formula either. Lawrence goes on to explore the mathematical basis of the linear approximation of the psychometric chart (Steve calls it the psychotic chart). You can find his article (Lawrence 2005) atReferences or Citations .

As Steve B. originally replied:

Mathematical models exist for computing the place and accumulation of moisture condensation inside building walls. Their usefulness, however, is limited for a number of reasons.

Dew point calculations or tables vs. real world conditions: role of air leaks vs moisture diffusion

First, the [dew point or psychometric chart] models are based exclusively on moisture diffusion theory (moisture molecules moving through building materials).

In reality, air leaks into and out of wall cavities, rather than moisture diffusion, accounts for the largest portion of moisture transmission in buildings. Because of variations in workmanship, construction details, uses of sealants and caulks, and similar variables, the relative contributions of diffusion and air leakage in building walls and ceilings is unpredictable.

Second, the moisture condensation mathematical models assume that the building wall is continuous (no holes or penetrations) and that the environmental conditions (temperature, moisture, wind, air pressure) are unchanging.

Actually, conditions constantly change inside and outside of buildings, and cold spots occur at leaks to the outdoors, lapses or omissions of insulation at building corners, air leaks occur around openings for doors and windows, and at thermal short circuits are caused by highly conductive materials such as metal, glass and concrete.

These are the places where the problematic wall or ceiling cavity condensation is likely to occur. So you can also see that the occurrence of wall or ceiling cavity is certainly non uniform in space (building walls or ceilings) and time.

Also the prediction of building wall condensation does not necessarily indicate an actual condensation problem.

The length and severity of winter and the ability of building materials to safely store and later expel moisture are important factors in determining whether a building cavity moisture problem will actually occur.

With this in mind, the best defense against building wall or ceiling moisture damage is a good offense: proper air and vapor barriers, caulking, and thermally-broken door and window components.

To do the wall condensation or dew point calculations, you need to know the temperature and vapor pressure gradients through the wall(or ceiling). These are directly proportional to the resistance's of the wall's components to heat flow and moisture vapor flow (and air leaks). At any point where the calculated vapor pressure exceeds the saturation vapor pressure (derived from the temperature at each point), condensation may occur.

Below we provide links to further information on dew point calculation from ASHRAE and the National Bureau of Standards. For greater accuracy in predicting wall cavity condensation, the vapor pressure curve is recalculated for each plane of condensation in an iterative procedure.

A Quick Look at the Psychometric Chart Relating Dew Point, Temperature, Humidity

Psychometric chart

[Click to enlarge any image]

What is the Dew Point?: the dew point (Tdp) is the temperature at which water vapor just starts to condense out of air that is cooling - for example when warm moisture-laden air contacts a cool surface inside of a wall cavity.

Above the dew point the moisture stays in the air. At or below the dew point moisture leaves the air and in buildings, condenses on the cooler surface that the air is contacting.

This also means that if you are measuring the relative humidity in a room, the RH number only has meaning if you measure the room temperature at the same time and location.

In the table above, the left-most curve, the 100% relative humidity line offers a simple case - that's air that is 100% saturated. So on the chart below, notice that on the left-most curve, the wet bulb temperature equals the dry bulb temperature - that is, when the air is fully saturated at 100% RH, no more air water can be evaporated out of the air.

Original Article

...

Approximate & More Accurate Formulas to Calculate the Dew Point in a Building

Question: how do I actually calculate the dew point in a building?

(May 31, 2015) Mike said:
I love how this article avoids the answer to the actual question being asked by saying yes, there is a mathematical formula but avoids giving the formula.

2015/12/28 Robin said:
What Mike said.

Reply: how to calculate the dew point if we know the relative humidity (RH) and teperature

Thanks Robin and Mike you are quite correct - I'll have added dew point calculation advice into this article.

Relative humidity is the ratio of how much moisture air is actually in the air compared with how much moisture the air could hold for a given temperture.

As you'll read in the psychometric chart above, this relationship is a logarithmic curve rather than a simple linear one.

Barometric pressure is also a factor to be considered.

That is, the dew point - or maximum amount of moisture that air can hold before water starts condensing out on cooler surfaces - is exponentially greater at higher temperatures (dry bulb temperature) and at higher vapor pressures.

If you take a look at the dew point chart given above (the psychometric chart) you'll see that the chart presents dew point data as a function of temperature and indoor relative humidity along a logarithmic scale - that is the dew point in a building is not a simple linear function. The actual calculations or formulas are themselves approximations of a more complex environment and don't consider possiblyi overwhelming effects of building air leaks and other variables.

On a review of moisture models and calculations you'll see why for practical purposes many people prefer to read the dew point off of a handy psychometric chart rather than work in log scales and calculations.

I have added your painful but fair critique along with calculation advice into the article and provide details and expert dew point resource citations below. You'll see that because the calculation is troublesome, using the psychometric chart looks ever so much more attractive.

See DEW POINT TABLE - CONDENSATION POINT GUIDE for the chart approach. That said, let's take a look at two dew point calculation approaches:

How to Calculate the Approximate Dew Point - simplified equation

Dew Point Temperature = Td = T - ((100 - RH)/5.)

where

This equation is attributed to a 2005 proposal from Mark Lawrence cited below and is considered a reasonably accurate estimate provided the RH is above 50%. I also recommend Devres (1994) for an excellent article on calculating the dew point or the psychometric properties of air.

How to Calculate the Dew Point more Precisely

The following procedure is derived from information provided by Columbia University and cited below:

Relative Humidity - RH = 100% x (E / Es)

This is an approximation of a more complex and more precise Clausius-Clapeyron equation where we set E and Es as follows:

E = E0 x exp [(L/Rv) x {(1/T0) - (1/Td)}]

Es = E0 x exp[(L/Rv) x {(1/T0) - (1/T)}]

and

Given a temperature in Kelvin, solve for Es, substitute that equation into E and solve for Td to obtain the dew point.

Since these calculations work in Kelvin I include below formulas to convert from Kelvin to Celsius and from Celsius to Farenheit degrees.

To convert Kelvin to Celsius use

To convert Celsius (tc) to Farenheit use

References on Dew Point Calculations


...

Continue reading at DEW POINT TABLE - CONDENSATION POINT GUIDE or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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Comments

Ryszard Jankowski · Apr 22, 2023

Would the calculation for determining Td hold for determining the temperature at which a 66%RH occurs (as opposed to 100%?). ie 66% point Temperature = T66 = T - ((66 - RH)/5.) I'm no mathematician, and I don't know if the mathematical relationship that the equation is based on would be proportional in this way. I work in damp and mould consultancy and knowing the '66% point' of a surface would be useful.

InspectApedia Editor (mod) · Apr 22, 2023

@Ryszard Jankowski,

Thank you for the helpful question.

I'll review our research on this as it's been a while.

Meanwhile, keep in mind that NO contemporary on-site measurement can tell us prior conditions, so your wall could be nowhere near it's dew point right now, yet there could be a moldy wall cavity present.

So IMO absent visible mold or wet conditions, building leak history, materials, ventilation, etc. are critical.

Anonymous · Apr 22, 2023

@InspectApedia Editor , thanks. Here in the U.K. we have a lot of buildings with solid brick walls and I’m writing a report on one such property now. We often discuss dewpoint in the context of lifestyle condensation that left unventilated leads to surface mould developing. I would LOVE to work out a calculator that, given RH & temp would spit out dewpoint AND ‘mould risk point’ (66%+). It would be a really useful tool to help people understand what’s happening on surfaces in their home. Latest thermal imaging cameras can actually display this 66% threshold visually, so the maths is out there… somewhere

InspectApedia Editor (mod) · Apr 23, 2023

@Anonymous, Ryszard

Thank you for the helpful discussion.

Nice idea but take care not to allow your readers to misconstrue your data, as I warned in my earlier comment.

I've found thermography a very useful tool but terribly abused by make-a-buck (make-a-pound) folks who promote imaging as a "mold scan" - which is about as reliable as predicting the geographic coordinates of future asteroid strikes on the dark side of the moon.

In the UK construction you cite I'd be most-concerned about false walls of drywall installed over un-insulated solid brick or other solid masonry walls. You can't see the wall cavity interior and drywall over brick is a mould-friendly construction.

The simple dew point formula that we gave, also stated as

Td = T - ((100 - RH)/5.)

where Td is the dew point in Celsius, and of course RH is Relative Humidity, holds rather well for any humidity value above 50%, so that's a formula you can use.

In the U.S. the ASHRAE Fundamentals text gives a thorough exposition of this problem.

But here's a common misleading statement we read across various articles:

"... we can prevent condensation problems in walls by finding the wall's temperature profile over its surface area and then calculating the dew point, perhaps using a psychometric chart."

Nonsense! Finding the dew point does not prevent condensation in the wall cavity.

Rather it is the maintenance of indoor building conditions such that the the dew point is never reached inside the wall cavity that will prevent condensation therein.

So I would love to see a copy of your paper, and I hope you'll encourage your readers to take the steps to walk through the gap between dew point theory and dew point practical application. That means we need to know a heck of a lot more than just those numbers. For example:

1. what is the air leakage rate for the wall and where are air leaks occurring

2. what are other water or moisture intrusion sources in the wall - such as wind-blown rain penetration of the masonry exterior, affected by the presence or absence of a drainage plane and weep openings

3. what are the ranges of outdoor and indoor conditions that affect the building.

Let's translate interesting dew point theory into what actions people can take to

- determine if there is a mould problem in the building
- determine its cause
- remove problem mould and fix its cause

InspectApedia Editor (mod) · Apr 23, 2023

@Anonymous, Ryszard

Here is a nice recap of dew point calculation, courtesy of Michael Bell at Columbia University, original source: iridl.ldeo.columbia.edu/dochelp/QA/Basic/dewpoint.html

Michael recommends

Lawrence, Mark G. "The relationship between relative humidity and the dewpoint temperature in moist air: A simple conversion and applications." Bulletin of the American Meteorological Society 86, no. 2 (2005): 225-234.
...The relative humidity (RH) and the dewpoint temperature (td) are two widely used indicators of the amount of moisture in air. The exact conversion from RH to td, as well as highly accurate approximations, are too complex to be done easily without the help of a calculator or computer. However, there is a very simple rule of thumb that can be very useful for approximating the conversion for moist air (RH > 50%), which does not appear to be widely known by the meteorological community: td decreases by about 1°C for every 5% decrease in RH (starting at td= t, the dry bulb temperature, when RH = 100%). This article examines the mathematical basis and accuracy of this and other relationships between the dewpoint and relative humidity. Several useful applications of the simple conversion are presented, in particular the computation of the cumulus cloud-base level (or lifting condensation level) as zLCL >> (20 + t/5)(100 – RH), where zLCL is in meters when t is in degrees Celcius and RH in percent. Finally, a historical perspective is given with anecdotes about some of the early work in this field.

Junior Research Group, Department of Atmospheric Chemistry, Max Planck Institute for Chemistry, Mainz, Germany

Publisher’s Note: This article was amended to correct an error in the value for coefficient A1 at the bottom of page 226. The correct value for coefficient A1 is 17.625.

CORRESPONDING AUTHOR: Mark G. Lawrence, Max Planck Institute for Chemistry, Junior Research Group, Department of Atmospheric Chemistry, Postfach 3060, 55020 Mainz, Germany, E-mail: lawrence@mpch-mainz.mpg.de

Here is a copy:

inspectapedia.com/Environment/Dew-Point-Calculation-Lawrence.pdf

InspectApedia Editor (mod) · Apr 23, 2023

The exact con-
version from RH to td, as well as highly accurate ap-
proximations, are too complex to be done easily with-
out the help of a calculator or computer. However,
there is a very simple rule of thumb that I have found
to be quite useful for approximating the conversion
for moist air (RH > 50%), which does not appear to
be widely known by the meteorological community:
td decreases by about 1°C for every 5% decrease in RH
(starting at td = t, the dry-bulb temperature, when RH
= 100%)

RE-Posting for Ryszard (mod) · Apr 24, 2023

@InspectApedia Editor ,

I’d be happy to send you a redacted copy of the client report, but would need an email to do so.

I understand the issues you mention fully. Our consultancy evolved out of fire and flood restoration work and training. We teach and work to ansi/IICRC standards/courses and are often called in by clients already convinced, or at least suspecting, mould is causing them health problems, often after consulting with functional medicine practitioners, and our reports address the state of the property as a whole… including air & surface sampling if suitable (big subject). We survey building pathologies and vulnerabilities, as well as internal ‘lifestyle’ vapour generation factors to help them understand and solve their problem.

With regards the proposed T66 or Tmr (mould risk) I adapted 2 approximation formulas yesterday and found that the T66 point each arrived at was similar AND mapped out also gave a good approximation of the Td I already had from plotting HR and temp (from on site instrumentation) on a psychrometric chart.

The tables may be a little opaque… but it does seem to work and serves as a good illustration to the client about WHY mould is cropping up where it is and why penetrating/rising damp risks are so important to understand (like climbing plants all over a north facing solid brick wall, bridged DPC etc).

This may be a little amateurish, but here’s my two calculations that suggest that it is possible to roughly estimate the T66 or Tmr at time of my visit. Both rooms have a t66 of between 16-17C… so walls under 16c would be safe to say are at increase of risk of mould if conditions remain stable. Thermography revealed many such cold spots and areas that should lead to actions (proposed in report) to reduce risk of mould reoccurring.

Happy to share the redacted report if it can be done in a secure way

We have a consumer information course ‘Mould SOS’ on YouTube and would welcome feedback. youtube [dot] com/channel/UCmLUnB6Lhq3hb37maS8I4vA

InspectApedia Publisher (mod) · Apr 24, 2023

@ Ryszard,

Our email is at the page top or bottom CONTACT US link

inspectapedia.com/Admin/Contact-InspectApedia.php

I took a look at two of the videos in your online information about mould - from what I saw you were outlining at a very general level what you consider the scope of a proper investigation - frankly it seemed more of an advertisement for your services than an actual online course in mould and IAQ investigations.

Still I was glad to see the breadth of scope. In my experience far too many "mould investigators" simply stop by to make a couple of measurements or "do a mould test" - which, as I've said ad-nauseum, are close to worthless without a thorough onsite investigation of the whole building and its leak and moisture history, an understanding of its construction and thus where it's worth invasive inspection methods to check for hidden trouble at the most-suspect locations, combined with an interview and understanding of the building occupants, health risks, complaints, etc.


Anonymous · Apr 25, 2023

@InspectApedia Publisher, understood - and I agree it does serve that dual purpose. The information we repeat to every potential client takes HOURS to explain over the phone so we recorded it all so that we speak to ‘informed consumers’… general knowledge on these issues is v poor. We found ourselves educating every potential client one to one over the phone… hence the series of explainer videos. It’s a win/win. Especially when the caller has brain fog, is experiencing anxiety and poor memory. It’s all there as a reference for them.

It really is a ‘basics and fundamentals’ of understanding mould in the built environment and the most common questions and misconceptions we hear on calls from, often desperate, people.

People call us having scared themselves silly on the internet and very confused. We ask them to watch the videos then call back. Some get far more technical than others such as ‘mould triggers’ which goes into all the Td and RH interactions etc.

I’ll send you a redacted report later today or tomorrow but as you say it addresses the building as a whole including history and we do indeed interview the client based on a U.K. ISO guidelines document for IAQ

InspectApedia Publisher (mod) · Apr 25, 2023

@

Thank you for the discussion. It's gratifying to realize there are at least some people in the field who understand what's necessary to do the job correctly as safely.

We often find ourselves warning people that the fear and anxiety around these topics may be itself a greater health hazard as well as a money hazard.

danjoefriedman (mod) · Jan 25, 2019

Frank:

Bottom line: for a basement that has no discernable sources of water entry you might still want to use a dehumidifier, add some heat, and when insulating, use enough R-value that the wall and wall cavity won't reach the dew point.

I prefer solid closed-cell foam insulation that's more resistant to both air passage and to mold growth within the insulating material. You could use foil faced foam and foil tape the joints to keep air from moving into the wall cavity.

Frank C · Jan 25, 2019

I read this wealth of knowledge understanding how critical the calculations are. My solid concrete foundation walls are not insulated presently. My goal is insulate with rigid foam R10 to eliminate my high humidity in my fully dry basement other wise . No leaks no water
Except high humidity in the summer time which creates mold.. Current calculations
Won’t be same after insulation is added .
My thought was laminate R10 4x8 sheets right against inside foundation walls to change the entire present dew point and humidity conditions. I live in NY .. I’m rusting on my math I’m finding this science a challenge. I’m just trying to eliminate the high humidity from the non insulated walls hoping I don’t make a mistake using this method I feel maybe the best way possible using perfect contact points for no air transfer.
Lastly my goal is to prevent mold by lowering humidity using only one method my research says This method is the only way to absolutely have the least to almost no amount of air between foundation and insulation.
I’m releluctant to move forward I don’t like failure my goal is a non mold environment.
Low humidity in summer without HVAC system needed..

danjoefriedman (mod) · Dec 29, 2015

Thanks Robin and Mike you are quite correct - I'll add some calculation advice into the article above.

If you take a look at the dew point chart given you'll see that the chart presents dew points along a logarithmic scale - that is the dew point is not a simple linear function. that's why most users prefer to read the dew point off of a handy psychometric chart rather than work in logs.
I'll add your painful but fair critique along with calculation advice into the article above later today,.

 

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