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Are Dew-Point Calculations Really Necessary?

How to perform dew-point calculations — and why it's possible to design a high-performance wall without performing calculations or consulting a psychrometric chart

Posted on Sep 17 2010 by Martin Holladay, GBA Advisor

Most builders understand that condensation can form when warm, moist air encounters a cold surface. Condensation is bad, and builders want to avoid it. There’s a solution, though: According to building scientists, we can prevent condensation problems in walls by determining a wall’s temperature profile and performing a dew-point calculation. This calculation may require the use of a psychrometric chart.

A few brave souls, striving to educate themselves, may consult a copy of ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. Fundamentals to learn more about dew-point equations (see Image 1). That’s what I did — briefly, before I decided to close the book and put it back on the shelf.

To wade through this thicket, I’ll attempt to answer a few questions:

  • What’s a dew-point calculation and how do I perform one?
  • Does such a calculation yield useful information?
  • Are there simpler ways to design walls that perform well?

Understanding temperature profiles
Building scientists sometimes talk about a wall’s “temperature profile” or “temperature gradient.” The idea is to estimate the temperature of different wall components, assuming certain indoor and outdoor conditions.

For example, consider the wall of a house on a cold winter day. If it is 72°F indoors and 0°F outdoors, the siding temperature will be close to 0°F, while the drywall temperature will be close to 72°F. The other wall components will be at temperatures ranging between these two extremes.

If we draw a cross-section of a wall, we can calculate the theoretical temperature of any point within the wall. However, since these temperature profiles usually fail to account for air leakage, they are usually inaccurate. Moreover, they represent a theoretical one-dimensional model; since the real world has three dimensions, this model has limited value.

What’s a dew-point calculation?
Builders or designers perform dew-point calculations to determine whether a certain component of a wall, ceiling, or roof — in most cases, the sheathing — will stay warm enough during the winter to avoid condensation problems.

To answer this question, we need to know the indoor relative humidity and temperature of the sheathing during the winter. Of course, OSB installed in a house without any exterior foam sheathing will be colder that the interior face of foam sheathing (or OSB covered with exterior foam sheathing).

Nobody likes a psychrometric chart, but sometimes you have to use one
When builders hear about the need to consult a psychrometric chart, there are two possible reactions:

  • What’s that?
  • I saw that chart once in an introductory course on building science, and I never want to see it again.

It's true — a pyschrometric chart is notoriously confusing to the uninitiated. But a psychrometric chart is a useful graph; it can be consulted to determine many values, including dew points. Here are some basic definitions and pointers to help you navigate a psychrometric chart.

HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. engineers refer to ordinary air temperature as “dry bulb temperatureAir temperature as measured by an ordinary thermometer..” On most psychrometric charts, dry bulb temperatures are indicated by vertical lines; the dry bulb temperature scale is at the base of the chart.

Wet bulb temperature” is determined by swinging a sling psychrometer — that is, a special thermometer equipped with a bulb wrapped in moist cloth — through the air. The evaporation of the moisture in the cloth cools the thermometer below the dry bulb temperature. On a psychrometric chart, the wet bulb temperature scale is located on the curved line on the left side of the chart. Straight lines that slope down from the upper left to the lower right indicate wet bulb temperatures.

The dew point temperature is the temperature at which moisture in the air begins to condense on hard surfaces. The dew point temperature scale is located along the curved line at the left of the psychrometric chart. The dew point temperature scale can also be found along the right hand side of the chart (with lower dew point temperatures at the bottom of the chart, and higher ones at the top of the chart).

On the psychrometic chart, curved lines representing conditions of equal relative humidity extend from the lower left to the upper right. The 100% relative humidity line is the last (upper) curved line on the chart, corresponding to the wet-bulb and dew-point temperature scale line.

Several Web sites — for example, this one, this one, this one, and this one — provide directions for determining dew points using the psychometric chart.

If I have 28°F sheathing, is it below the dew point?
If you know the temperature of your sheathing — let’s say it’s 28°F — you might want to know if it is below the dew point. Assuming that interior air can reach the sheathing, the answer to your question depends on your interior conditions.

If the interior of the building is kept at 70°F and 35% relative humidity, we can use the psychrometric chart to determine the dew point. On the bottom of the chart, find 70°F. Follow the vertical line up from 70°F at the base until you intersect a curved line corresponding to 35% relative humidity. From that intersection point, follow a horizontal line to the left side of the chart, until the horizontal line intersects the curved line indicating 100% relative humidity. You can read the dew point temperature along that curved line; it’s about 40°F. You just determined the dew point for 70°F air at 35% relative humidity.

Your 28°F sheathing is below the dew point, which means that as long as these conditions continue, the sheathing is likely to accumulate moisture.

If, on the other hand, the interior of the building is kept at 65°F and 20% relative humidity, the psychrometric chart tells us that the dew point is 24°F. So our 28°F sheathing is above the dew point — as long as the interior conditions don’t change.

What are the outdoor conditions in winter?
While some builders perform dew-point calculations for the coldest day of the year — that is, the design heating temperature — ASHRAE has developed a simplified calculation method based on a temperature that isn’t quite so low. (Details can be found in ASHRAE Fundamentals, chapter 27.) According to building scientist Joe Lstiburek, calculating the dew point for the coldest day of the year isn’t particularly useful, since a small amount of condensation for a few hours during the winter won’t lead to any problems. (The condensation just dries out when the weather warms up.)

In fact, multiple simulations using WUFI software (a sophisticated modeling program developed in Germany) and years of U.S. and Canadian field research have shown that no harm will occur even if sheathing is below the dew point for 2% of the winter.

A simplified way to tell whether your sheathing is above the dew point
The simplified method for performing dew-point calculations for wall assemblies is explained in “Robust Walls,” a useful article by Ted Cushman. Cushman interviewed Joe Lstiburek, who explained, “Take the average temperature for December, the average temperature for January, and the average temperature for February — and you average those, and use that average as your design temperatureReasonably expected minimum (or maximum) temperature for a particular area; used to size heating and cooling equipment. Often, design temperatures are further defined as the X% temperature, meaning that it is the temperature that is exceeded X% of the time (for example, the 1% design temperature is that temperature that is exceeded, on average, 1% of the time, or 87.6 hours of the year). for outside. You set your interior design condition as 70°F and 35% relative humidity. Then you do a simple calculation to make sure that the condensing surface doesn’t drop below the dew point.”

Although the calculation method has been criticized as a little rough-and-ready, Lstiburek defends it. “When someone says, ‘Yeah, but that’s not really what’s going on’ — well, that’s true. But it’s a very good approximation. It gets us 98% accuracy with one easy calculation.”

So if you know where to look up the monthly mean temperatures for December, January, and February for the location where you are building — data are available for many locations at the Web site of the National Oceanic and Atmospheric Association (NOAA) — you can calculate the winter temperature to use for your dew-point calculation.

An example, step by step
Cushman’s article noted that in Boston, the mean temperatures for December, January, and February are 33°F, 28°F, and 30°F respectively; the average of these three numbers is 30.3°F. So in Boston, 30.3°F is outdoor temperature you should use for your dew-point calculation.

To take an example, let's look at the following wall assembly for a house in Boston: interior drywall, 2x6 studs filled with cellulose, OSB sheathing, 1 in. of XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. foam, a rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. gap, and wood lap siding. We need to know whether the interior side of the OSB will get cold enough during the winter for moisture to accumulate.

Here’s what you do:

  • Determine the delta-TDifference in temperature across a divider; often used to refer to the difference between indoor and outdoor temperatures. (ΔT) — that is, the difference between the outdoor temperature (30.3°F in our example) and the indoor temperature (70°F).
  • Calculate the percentage of the insulation that is on the interior side of the sheathing by dividing the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the cavity insulation (the insulation between the studs) by the total R-value of the wall.
  • Now calculate how cold the sheathing gets, using this formula: Temperature of the sheathing = Indoor temperature - (Delta-T * Percentage of the insulation that is on the interior side of the sheathing).

In our example, the delta-T is 39.7 F°. The percentage of the insulation that is on the interior side of the sheathing is R-19/R-24 = 0.79. The temperature of the sheathing is 70°F - (39.7F° * 0.79) = 70°F - 31.4°F = 38.6°F .

If you follow Lstiburek’s advice and use his indoor conditions — 70°F and 35% relative humidity — you don’t need to look up the dew point in a psychrometric chart, because Lstiburek tells you that the dew point for these conditions is 40°F. So the sheathing in the Boston house is below the dew point — making the wall assembly risky. This wall needs thicker exterior foam to keep the OSB above the dew point.

Are dew-point calculations useful?
It’s certainly useful to know whether your sheathing will be above the dew point or below the dew point in winter. When sheathing is below the dew point, it’s likely to accumulate moisture. Warm sheathing is better than cold sheathing.

Unfortunately, though, temperature profiles and dew-point calculations have been misunderstood and misused for years. In his excellent book, Water in Buildings, William Rose wrote, “The language ‘reaching dew point’ seems to indicate that one could plot a temperature profile through a wall, find the point where that profile intersects a horizontal line indicating indoor dew point temperature, and expect burgeoning water at that location. This impression is decidedly incorrect. If water accumulates, it does so on the surfaces of materials, not within the thickness of materials.”

Rose goes on to explain that misunderstandings arising from dew-point calculations are caused by a failure to consider the saturation vapor pressure. I’m happy to report that, for the purposes of this discussion, understanding saturation vapor pressure is unnecessary. (For those who care, Rose explains, “A description and example of the profile method is maintained in ASHRAE Handbook — Fundamentals, Chapter 23. … If at any point the vapor pressure value exceeds the saturation vapor pressure, reset the vapor pressure at that point to the value of the saturation vapor pressure. After all, having vapor pressure exceeding saturation is quite rare.”)

Dew-point calculations are often misused
Anton TenWolde, a supervisory research physicist at the U.S. Forest Products Laboratory, made the same point at a workshop at a 2002 EEBA conference. TenWolde’s discussion of the issue is worth quoting at length:

“The perceived importance of condensation has been bolstered by the wide misuse of the dew-point calculation. … Many of you are familiar with a chart like this: you project the temperature profile through the wall to calculate saturation vapor pressures. Then you calculate vapor pressures based on the permeance of the materials, and you come up with a profile like this.

“I have seen hundreds of these profiles, and many seem to show condensation occurring in the insulation. This has encouraged a lot of research into the performance of wet insulation. But the picture is wrong, because the vapor pressure has to be below the saturation pressure. You need to make a correction, and if you do that, if you redraw it, the condensation does not occur in the insulation. We thought there would be a problem with condensation in the insulation, but all the action happens on the sheathing and the interior vapor barrier. We’ve confirmed this by opening up walls. The action is never in the insulation.

“I have a problem with the way we perform dew-point calculations. The method cannot handle hourly calculations. It doesn’t take anything into account except vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. . It doesn’t take into account moisture storage, air movement, liquid water movement, or rain. It doesn’t take into account more than one dimension — it thinks the wall is flat. It doesn’t take into account the variability of material properties, or the effect of the sun. In other words, it doesn’t take into account the real world.

“A moisture problem occurs when wetting exceeds drying over a long period of time. But it is important to know several things: How wet does it get? How long does it stay wet? And what is the temperature while it is wet? Because if it is cold enough the mold won’t grow well, and decay organisms won’t do well. How does this information translate into the design of a building?

“You need to assume that the building will get wet, somehow, at some point in time. Stuff happens. So you need a moisture-tolerant design. The question is, how much water should a building component be able to handle? Which leads us to the question, what good is building science?

“For one thing, it is not very good at predicting how wet buildings get. The dew-point calculation was an attempt at doing that. But the dew-point calculation is terrible at predicting if something gets wet, much less how wet it gets. Wetting is an unpredictable, singular event. I don’t think we should let building science anywhere near this question.”

Wall sheathing actually gets wet from both directions
To elaborate on TenWolde's point: even if a builder performs a complicated dew-point calculation to be sure that OSB sheathing doesn't get wet due to diffusion of water vapor originating from the interior of the home, the calculations won't prevent the OSB from getting soaked by wind-driven rain leaking through defective flashing.

Sophisticated hygrothermalA term used to characterize the temperature (thermal) and moisture (hygro) conditions particularly with respect to climate, both indoors and out. modeling programs like WUFI take into account a tremendous number of variables, including the orientation of the wall, the width and height of the roof overhang, the amount of rain striking the wall, the amount of sun hitting the wall, the amount of air leakage through the wall, and differing indoor conditions. Compared to a WUFI simulation, a simplified one-dimensional model based on a temperature profile through a wall and a dew-point calculation is of limited value.

It's nevertheless worth performing the calculation, because we really don't want our sheathing to be cold enough to accumulate moisture. Field studies have shown that Lstiburek's simplified dew-point calculation method is adequate to avoid diffusion-related moisture accumulation in OSB or plywood sheathing. That said, we shouldn't pretend that this calculation can predict the actual moisture content of the sheathing; at best, we can say that the method works well enough to avoid problems from moisture originating from the interior of the house.

Standards and training, not calculations
In his 2002 EEBA presentation, TenWolde continued, “We are a little better at predicting how buildings dry, because we can do diffusion calculations. It involves physics. Drying is much more predictable than wetting events. We can talk about evaporation. However, air movement is still a problem. There are a zillion ways that air can move through a wall, so it is very difficult to predict. But we can do diffusion calculations up the wazoo.

“I don’t think we should do much building science on wetting. We should address wetting — the stupid stuff — with standards and training. You don’t need differential equations for this.”

So why would I want to perform a dew-point calculation?
The main reason for a builder to perform a dew-point calculation is to determine whether exterior rigid foam is thick enough to prevent moisture accumulation in your wall sheathing.

If you’re installing exterior rigid foam, you don’t want an interior vapor barrier, because foam-sheathed walls need to dry to the interior. To get around existing building code requirements for vapor retarders, you will probably follow the provisions of Table N1102.5.1 in the 2007 Supplement to the International Residential Code (IRC). This table allows the use of Class III vapor retarders (ordinary latex paint) on the interior side of foam-sheathed walls (rather than more restrictive vapor retarders) — as long as the foam sheathing is thick enough.

If you use Table N1102.5.1, you can bid dew-point calculations goodby
The good thing about Table N1102.5.1 is it provides an easy check to be sure your foam sheathing is thick enough. If you follow Table N1102.5.1 — something your building inspector should be requiring anyway — you don’t have to do any dew-point calculations. That’s good. (Anton TenWolde is probably pleased that builders can consult this table without performing any calculations whatsoever.)

Table N1102.5.1 is reproduced on this page as Image 4 below. (Click the image to enlarge it; click the "plus" sign if you want to make it even larger.) To use the table, find your climate zone. The table lists the minimum R-value of foam sheathing for 2x4 walls and 2x6 walls. For example, if you’re building in climate zone 7, Table N1102.5.1 tells you that a foam-sheathed 2x4 wall needs foam with a minimum R-value of R-10 (for example, 2 in. of XPS), while a foam-sheathed 2x6 wall needs foam with a minimum R-value of R-15 (3 in. of XPS).

That's simple, isn't it?

For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

Last week’s blog: “New Green Building Products.”

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Image Credits:

  1. ASHRAE Fundamentals
  2. Energy Design Update
  3. International Residential Code

Sep 17, 2010 8:05 AM ET

More foam on 2x6 walls.
by ryan evanczyk

1 Helpful?

So if I am reading this right....why does the N1102.5.1 table require thicker insulation for 2x6 walls vs 2x4 assuming you filled the stud bays up with the same interior insulation (whatever) that may be?

Sep 17, 2010 8:15 AM ET

Psychrometrics & Hygrothermals
by Armando Cobo

1 Helpful?

As a residential designer, I perform dew calculations on every house to determine wall and roof assemblies, just like I use a MEs for HVAC calcs., SEs for foundations on bad soils and sometimes when framing gets a little wacko. Why, risk management for my clients and I. Maybe is not all perfect, but in my mind I know I am doing and using every tool possible to provide my clients with the best solutions. It’s a matter of pride to know that my designs, may not be 120% perfect, but have the best information my clients hire me for; and that makes me sleep better at night.

Sep 17, 2010 8:23 AM ET

More foam on 2x6 walls...
by Armando Cobo

1 Helpful?

It takes more heat and/or less humidity to warm-up the sheathing on a 2x6 thick wall than a 2x4 wall; therefore more outsulation keeps the sheathing bellow the dew point.

Sep 17, 2010 8:31 AM ET

Response to Ryan Evanczyk
by Martin Holladay, GBA Advisor

0 Helpful?

Ryan Evanczyk,
The thicker the insulation in the stud bays, the colder the sheathing -- since the insulation in the stud bays is preventing the indoor heat from reaching the sheathing. Cold sheathing is risky, because it tends to accumulate moisture.

The safest wall puts all of the insulation on the outside of the sheathing, and leaves the stud bays empty. This construction method is called PERSIST. For more information on PERSIST, see
Getting Insulation Out of Your Walls and Ceilings

Sep 17, 2010 9:13 AM ET

air leakage
by J Chesnut

0 Helpful?

Thanks Martin for this detailed explanation.

If air leakage is an x-factor in the wetting process do US building scientists recommend a maximum ACH50 target. There is the recurring debate in the Q&A about how tight is appropriate. I think it was Jesse Thompson that recently replied to Robert Riversong that the reasoning behind the Passivhaus .6 ACH50 included limiting the wetting of very thermal resistant wall assemblies. Should we expect different recommendations for different climate zones?

Sep 17, 2010 9:29 AM ET

Response to J Chesnut
by Martin Holladay, GBA Advisor

0 Helpful?

J Chesnut,
Although various energy-efficiency and green building programs have published envelope air leakage targets, I think it is impossible to generalize about the effect of air leakage on wall condensation problems.

For example, air leakage and condensation won't damage some wall assemblies (CMU walls, ICF walls). Some wall components are very vulnerable to moisture -- OSB being the classic example -- while others are more robust.

Moisture accumulation in sheathing might be a problem in a wall without a rainscreen, but the same level of moisture accumulation might not be a problem if there is a ventilated rainscreen between the sheathing and the siding.

As with almost everything in building science, the answer is once again, "it depends."

That doesn't mean that air-tightness targets aren't a good idea. They are. There's just not one number for every type of wall -- at least not one number that can be justified by condensation concerns.

Sep 17, 2010 10:19 AM ET

The ******* method
by Bill Rose

0 Helpful?

Hello Martin,

The repository of the “dewpoint method” is ASHRAE Handbook Fundamentals Chapter 27. As Handbook Chair of ASHRAE TC4.4, I’m the person sort of in charge of what that chapter says, for better or worse. The material is under revision at present, so this post and the comments are helpful.

First of all, to the aggravation of my colleagues, I try to avoid loose and non-specific uses of the term “condensation”. It is defined in the soon-to-appear chapter on Building Envelopes in the Applications Handbook as follows:

(Moisture) condensation is the change in phase from vapor to liquid water. Condensation occurs typically on materials such as glass or metal that are not porous or hygroscopic and on capillary porous materials that are capillary saturated. Use of the term “condensation” to refer to change in phase between vapor and bound water in capillary or open porous materials is discouraged.

Most people, if asked why they consider doing a “dewpoint method” on a wall section, will answer that they are trying to ensure they won’t have condensation. But if the wall is made of sorptive materials like wood or brick, condensation (strictly speaking and with some qualification) doesn’t happen. What does happen involves short-term effects and long-term effects. In the short term, the moisture content of materials like wood will dictate what the humidity in a cavity will be and not vice versa. The ASHRAE method acknowledges this: it requires the user of the method, whenever apparent vapor pressure exceeds saturation vapor pressure, to change the apparent vapor pressure downward and calculate the resulting rate of accumulation. That, after all, is what nature does. Most users of the method are misusers: they calculate the temperature profile and convert it to saturation vapor pressure, then they calculate the vapor pressure profile from permeance values, and if at some point the vp exceeds the svp they go “Aha! Condensation! No damn good! Need a vapor barrier!” This is a serious misuse of the method. To determine long-term effects, it is important to use long-term average conditions, such as those suggested by Lstiburek described in your post. In summary, the shortcuts will likely be as good a predictor as this method.

I am not a defender of the method for design purposes. The following disclaimer was added to 2009 Handbook Fundamentals where the “dewpoint method” is discussed:

ASHRAE does not recommend the dew-point method as the sole basis for hygrothermal design of building envelope assemblies. ASHRAE Standard 160[P] is [being] developed to assist in hygrothermal analysis for design purposes. The dew-point method is presented here for reasons of historical continuity, and because it serves as an illustration of the fundamental principles of conduction in heat transport and diffusion in moisture transport.

In the current revision period, I may argue to drop the term “dewpoint method” and substitute “ASHRAE profile method”. The method uses temperature gradients, vapor pressure and saturation vapor pressure to calculate profiles, but at no point does it use dewpoint calculations.

Sep 17, 2010 11:17 AM ET

by Jesse Thompson

0 Helpful?

Another note on attempting to calculate moisture behavior and just how difficult this actually is, and this pertains to WUFI: I'm seeing lots of people starting to try and use WUFI for this purpose all the sudden, but it's shockingly easy to model a wall that WUFI says will turn to oatmeal (thank Martin...), but these walls just aren't failing out in the field like the software says they would be.

WUFI raises all sorts of red flags to me, it makes interesting looking scary charts really easily, but I'm not sure at all how valid these results are in the hands of amateur users (and I put myself and others who might have had a half day walkthrough of the software in this category). It's just way too complicated to set up correctly, and far too easy to make minor mistakes that could lead to false results.

Interesting to play around with to learn more about moisture movement as a general phenomenon, but we have to keep our eyes on what is actually happening in real buildings as built and keep learning from the built examples.

I guess this is also a pitch for more building monitoring and open publishing of the results. It's such a shame there is so little actual research going on in the US in the construction field, we need so much more considering how much money gets spent by this industry.

Sep 17, 2010 12:51 PM ET

Response to Jesse
by Martin Holladay, GBA Advisor

-1 Helpful?

You're right. Not only does every software program need to be validated by various field measurements, its results need to be checked against common-sense observations made by experienced remodelers.

As Anton TenWolde said, "What good is building science? ... It is not very good at predicting how wet buildings get. ... Wetting is an unpredictable, singular event. I don’t think we should let building science anywhere near this question.”

Sep 17, 2010 1:29 PM ET

Armando- your calculations
by Hunter Dendy

0 Helpful?

Armando- what methods are you using for your dew point calculations? Those in the "ASHRAE Fundamentals" or do you have another resource? Or are you outsourcing that too? I'm also a residential designer and would like to perform this kind of check on all of my wall assemblies too. For risk management as you said and for a better first hand knowledge, rather than just relying on hearsay from past experiences. Of course case studies are a great tool too (I'm not completely ignoring them).

Sep 17, 2010 4:46 PM ET

My calculations...
by Armando Cobo

1 Helpful?

The Dew-Point method or Assembly Profile Method is based on ASHRE Fundamentals. I learned that from BSC long a go and I teach in seminars and at the Advanced Green Building classes for NAHB.
First, I look a psychrometric chart for the interior dew point at temp 70°F and RH30% in Dry climate, 40% in Mix-Humid climate and 50% in few occasions (its not a rule, it’s like a “feel”). Then use Climate Consultant 5 (freeware) to download the temps from where the design is from. I take the average of the 3 coldest months of the year and use that as my exterior temp. Then I figure the Rvalue of each wall component and finally I follow the formula Ts=Ti-DT(Rc/Rt) where: Ts= Sheathing temp., Ti= Interior temp., DT= (Delta T) difference of temps inside and out, Rc= Thermal resistance of cavity and Rt= Thermal resistance of assembly.
In Albuquerque, NM the 3 month ave. is 37°F and if my wall assembly is 1”R5 insul, ½” OSB, 2x6 Cellulose and Sheerock then: Ts= 70°F-43°F(R21/R26), Ts=70-43(.80), Ts=70-34.4, Ts=35.6° which is bellow the 37°F interior dew point at the OSB from the psychromatic chart.
If the clients tell me they like to keep their winter temp at 75°F all winter, then I use that figure; but 70°F is reasonable for most cases. I hope I didn’t get you more confused now.

Sep 17, 2010 4:51 PM ET

Adding to...
by Armando Cobo

0 Helpful?

As others have said, this is not a perfect sytem or tool but it raises yellow, orange and red flags if the numbers are not there.

Sep 17, 2010 5:09 PM ET

by Armando Cobo

0 Helpful?

I knew something went rong... DT=33 in ABQ, therefore TS=43°F which is higher, which makes my sheathing warmer than the dew point. It happens when in a hurry to leave the office. Sorry!!!

Sep 19, 2010 4:24 AM ET

by Martin Holladay, GBA Advisor

0 Helpful?

[Bill Rose sent me an e-mail with this comment, which he asked me to post:]

I should point out that ASHRAE has developed Standard 160 "Criteria for moisture design analysis in buildings" as the go-to method for making design decisions using hygrothermal analysis. The standard creates a framework for using transient hygrothermal analysis programs such as WUFI for design purposes, by specifying default inputs where design inputs are not available, and by specifying criteria by which an assembly can be deemed acceptable or not. AntonTenWolde, whom you quote in your post, spearheaded the formation of this standard. It's a nice piece of work, and is being adopted more widely, especially in government work.

The steady-state profile (dewpoint) method is fine for teaching purposes and simple illustration. It is really not useful for design purposes.

The profile method does not account for storage, air movement or bulk water. Transient modeling and 160 account accurately for storage. Wetting and air movement are somewhat accounted for, though they will always be wild cards in any analysis.
Bill Rose

Sep 20, 2010 9:18 AM ET

1968 Twin Cities 2 Story
by Doug McEvers

0 Helpful?

I am just finishing a window replacement for a 1968 vintage home in a Minneapolis suberb (7876 hdd). The wall is 2x4 with full fibeglass batt insulation, the exterior sheathing is 3/4 fiberboard and the siding is 12" masonite lap with no felt between the sheathing and siding. The one oddity for this house is the drywall is foil backed which creates a warm side, low perm vapor barrier. This wall must dry to the outside (cold side) through the sheathing and siding.

The hardboard siding is original and in good shape, the inside of the walls are like new, even under the windows, I can see a bit of filtering through the insulation around the outlets but these walls are dry inside, even during an intense bout of warm humid days with AC running full time.

Sep 20, 2010 11:55 AM ET

What about high performance walls?
by Larry

0 Helpful?

How does all this play out with 12" thick cellulose walls that are meticulously sealed, zip sheathing on exterior and gaskets with dry wall and latex paint on inside? It seams most of the thrust of the dewpoint analysis is related to conventional wall systems. Has anyone done research on high performance walls?

Sep 20, 2010 12:09 PM ET

Response to Larry
by Martin Holladay, GBA Advisor

1 Helpful?

Professor William Rose and I agree on this point: you don't need to perform a dew-point analysis to design a wall. That was the point of my blog.

Rose wrote, "The profile method does not account for storage, air movement or bulk water. ... The steady-state profile (dewpoint) method is fine for teaching purposes and simple illustration. It is really not useful for design purposes."

Concerning your hypothetical 12-inch thick cellulose wall: if you don't have exterior foam sheathing, your sheathing will be able to dry to the exterior. Neverthless, OSB is a much riskier sheathing than plywood in this application. Because of the 12 inches of cellulose, the OSB stays cold over the winter; that means that the OSB is at risk of moisture accumulation. Building scientist John Straube often raises warnings about the risk of OSB-sheathed 12-inch walls insulated with cellulose.

To protect your sheathing, specify plywood, not OSB. And be sure to include a ventilated rainscreen gap between your sheathing and your siding, to speed drying of the sheathing if it ever gets wet.

Sep 20, 2010 1:23 PM ET

Armado, another question about your calc.
by Hunter Dendy

1 Helpful?

Thanks for the description. One question though- it looks like the Rc (thermal resistance of cavity), is this R-value of the cavity insulation (6" cellulose in your example) and Rt (thermal resistance of assembly) is the entire assemblies R value across the cavity section. Is that correct?

Sep 20, 2010 3:51 PM ET

Sheathing for 12" wall
by Doug

-1 Helpful?

Katrin Klingenberg's affordable passivehouses in Illinois used 12" thick walls filled with cellulose. They used asphalt-impregnated fiberboard as the exterior sheathing--it's highly permeable. The structural OSB layer was applied to the inside face of the I-joist walls.
After building the first house, they realized that a stud wall erected inside the OSB made electrical, plumbing, and even holding up the second floor system much easier, compared to trying to run all the systems in the exterior walls. So, they added a stud wall on the first level which supports the second floor system.
So, it ends up being a sort of PERSIST-esqe wall, with (from the inside) an uninsulated 2x4 wall with wiring and pipes in it, then the structural sheathing on the warm side of the insulation (also the lowest-perm layer), then the main insulation, with increasingly permeable materials from the structural OSB sheathing out. Only, the insulation here is cellulose packed in stud bays, instead of foam as used in PERSIST/REMOTE houses.
It hadn't occurred to me that asphalt-impregnated fiberboard would actually be useful for something before learning about these houses! But if you're looking for a sheathing that's highly permeable and inexpensive, there you have it.

Sep 20, 2010 3:55 PM ET

Response to Hunter
by Armando Cobo

0 Helpful?

Rc is the cavity insulation; Cellulose R21. Rt is the wall assembly insullation; 1" Rigid R5 + Cellulose R21. I don't give any insulation value to the OSB or Tyvek.

Sep 20, 2010 3:58 PM ET

by Armando Cobo

-1 Helpful?

I know you and Mr. Rose do not like the profile method, I agree is not perfect, however, the new ASHRAE160 also gives you 3 methods to calculate the dew point and the first one, if I'm not mistaken, is very empirical; and if you look at the ASHRAE 160P, they had 3 different design temperatures and humidity that finally was agreed to compromise to a single one. Goes to the point that there is not one certain way to see things.
IMHO, many of these methods are not perfect and unless you are willing to spend some serious money on an expert, they do help raise some concern or Safeway. For that matter, look at the whole building science disagreements that are very frequent on many camps.

Sep 20, 2010 4:03 PM ET

by Doug

1 Helpful?

In my experience, which certainly is that of an amateur, WUFI not only says certain assemblies should fail (yet they don't), WUFI also sometimes doesn't show failures in assemblies that are actually failing.
In particular I have a wall where sun-driven vapor builds up in wallpaper-covered drywall, and no matter whether I use realistic or grossly exaggerated parameters I can't get WUFI to show the wall failing. (Nor could an engineer hired by the homeowners for that matter.)
It does make me feel better that "building not very good at predicting how wet buildings get.". That's been my experience here in the mid-atlantic mixed climate, where substantial numbers of buildings "break the rules" (easy to do with vapor drive reversing twice a year), but most don't fail.

Sep 20, 2010 8:39 PM ET

Response to Martin re: 12" wall
by Larry

0 Helpful?

Martin, do you have a link to John Straube's OSB warnings? We're using Zip wall panels and tape, they are suppose to breath but they are OSB at the core. We're also using a vented rain screen design. Many thanks.

Sep 20, 2010 9:54 PM ET

by Armando Cobo

0 Helpful?

So here is a new wrinkle: if the Vapor Permeability of OSB and Plywood is .75 on Dry Cup but 2 & 3.5 respectively on Web Cup, would that tell me that IF the sheathing gets wet, in the winter time I would have a higher risk of moving the dew point in the wall and therefore create different conditions for condensation? Would higher vapor permeability means higher air infiltration? If that is true-true, why should I install plywood before I install OSB?

Sep 21, 2010 3:49 AM ET

Response to Larry and Armando
by Martin Holladay, GBA Advisor

0 Helpful?

Larry and Armando,
1. I have had two phone conversations with John Straube on the issue, and I haven't seen anything in writing yet. I have it in mind as a future topic for one of my blogs.

2. The issue is not the permeability of OSB; the issue is whether the winter wetting of the OSB exceeds the ability of the OSB to dry during the summer. If wetting exceeds drying, you can have rot. OSB sheathing installed without a rainscreen gap will dry much more slowly than OSB sheathing installed with a rainscreen gap.

3. If OSB and plywood are exposed to identical wetting conditions, the OSB will rot faster than the plywood. This has been confirmed by several studies; I can track some down if you want.

4. If you like ZIP sheathing, it's probably OK to stick with it -- it's hard to determine or evaluate the level of risk. But for goodness' sake, don't forget the ventilated rainscreen gap!

Sep 21, 2010 9:08 AM ET

plywood, OSB, perms, infiltration
by Hunter Dendy

0 Helpful?

The delta T from one side of the sheathing to the other is going to be negligible, so any vapor that makes its way to the sheathing (plywood or OSB) would condense at the first surface it touches, so I don't think the difference in permeability would move the dew point. However, it seems to me that the higher perm of the plywood would be beneficial because it means the plywood has more ability to move the condensation through to dryer air, or store it until drying conditions occur. The OSB will saturate quicker and moisture would just sit at it's surface.
Also, the vapor permeability differences of the materials doesn't affect their air sealing ability- that will be more of the installation method/details, unless your air barrier is the drywall, then it wouldn't matter.
Martin, I've read several times here and at Bldg. Science, the idea of winter wetting and summer drying. That concept makes me a little uneasy, it seems like a long time for any wood product to hold moisture.

Sep 21, 2010 10:02 AM ET

Climate & OSB
by Doug

1 Helpful?

Any areas/climates where OSB is usually OK? Any where it often fails?
I suppose I mean, "except where leaks keep it wet"--I'm assuming pretty good workmanship is required in any climate.
How cold does it have to be for the "cold OSB" problem to happen?

Sep 21, 2010 11:14 AM ET

Response to Hunter Dendy
by Martin Holladay, GBA Advisor

0 Helpful?

Hunter Dendy,
Summer drying can balance winter wetting in many cases. The reason it works: mold can't grow, and wood doesn't rot, when it is cold. Mold and rot both like warm temperatures. So winter wetting isn't necessary dangerous.

Sep 21, 2010 11:17 AM ET

Response to Doug
by Martin Holladay, GBA Advisor

-1 Helpful?

There are many situations where OSB works well.
1. If OSB is covered with exterior foam of adequate thickness, it stays warm and dry.

2. In a poorly insulated building, OSB may be warm enough to avoid getting damp. (Leaking building heat helps keep it dry.)

3. In a home with an impeccable interior air barrier, good exterior flashing, and a rainscreen gap, the OSB will rarely get wet, and will dry relatively quickly after it does get wet.

Sep 22, 2010 1:27 PM ET

Tool for newbies
by Daniel

0 Helpful?

I have a flash-based psychrometric slider that I built for getting a visual and tactile understanding of what the psych chart is showing. Just slide the temperature and dewpoint, or temperature and humidity, and see how the changes affect one another. Happy to share the SWF file if anyone wants it.

Sep 22, 2010 4:41 PM ET

Green Building
by Pam

0 Helpful?

Hi Martin, This comment is random. I write it to you because you are my favorite blogger here -- and oh so reasonable. I've been thinking: You all at GreenBuildingAdvisor have a problem, I think. There is no such thing as green building. Unless it involved mud huts. Ok, and I guess on your pyramid there are things to be done that would have a net improved carbon footprint. So: "Greenifying" would be okay. But "green building" if it involves building a house from scratch (and there is plenty of talk about that here including 3,100 s.f. houses etc. blah blah blah) -- nope. What do you think about my latest hypothesis?

Sep 22, 2010 5:29 PM ET

Response to Pam
by Martin Holladay, GBA Advisor

1 Helpful?

I agree with the thrust of your comments.

You note, "There is no such thing as green building. Unless it involved mud huts." I wrote something similar: "Plenty of green buildings aren’t durable -- hippie houses made from salvaged materials, yurts or gers, tipis, old-time Alaskan log cabins, and third-world favela shacks. One might perceive a trend in these examples: when it comes to greenness, size matters more than durability." That was in my blog on durability.

You also note, "Greenifying would be okay. But green building, if it involves building a house from scratch (and there is plenty of talk about that here including 3,100 s.f. houses etc. blah blah blah) -- nope." I wrote something similar: "The best approach is not to build. Since the number of people per household in the U.S. has been dropping for years, a strong argument can be made to support the proposition that the U.S. already has too many houses. It’s better to renovate an existing building than to build new." That was from my blog titled "Energy Use Is the Most Important Aspect of Green Building."

Sep 23, 2010 8:24 AM ET

"setting" the interior relative humidity
by Peter Yost

1 Helpful?

Great blog Martin.

I just want to add that it can be deceptively easy to "pick" the interior relative humidity for those three winter months, when in fact the occupants and their behaviors drive that determination and the average interior relative humidity is a driver in the calculations.

Since most if not all people don't have a very strong or accurate sense of relative humidity and relatively small changes (say, 5%) can have big impacts on both diffusion and air-transported moisture, we should add to the building codes a requirement for HVAC sensors that have a hygrometer reading right next to the thermostat readout.

If you run your wintertime interior relative humidity at 50% (and I have been in plenty of homes which do, some consciously and others blissfully unaware), it changes a lot in the calculations and in the performance of building assemblies.

Sep 23, 2010 8:40 AM ET

Response to Peter Yost
by Martin Holladay, GBA Advisor

0 Helpful?

I agree completely that builders have no idea what the interior RH will be once a home is occupied. If anything, this fact only reinforces my conclusion that we all need to be skeptical of vapor-profile or dewpoint-calculation conclusions.

I've heard designers say, "This was performs very well, based on an interior RH of 30%." In some cases, however, an interior RH of 50% throws the wall into failure -- at least according to the calculations. Well, that wouldn't let me sleep well at night. If the resident decides not to run the HRV, that means the walls are going to rot? How do you explain that to a jury when you end up in court?

If you're designing a finely balanced wall that depends on a low indoor RH, you're too close to the cliff. It's time to choose a more robust wall assembly.

Sep 23, 2010 10:40 PM ET

The good thing about Table N1102.5.1 is it provides an easy...
by John

0 Helpful?

Is "Table N1102.5.1" available on-line somewhere?



Sep 24, 2010 4:04 AM ET

Response to John
by Martin Holladay, GBA Advisor

1 Helpful?

Q. "Is Table N1102.5.1 available on-line somewhere?"

A. It's Figure 4 on this page. Click the last illustration at the bottom of the blog; click the "plus" sign if you want to expand the image.

Sep 27, 2010 10:30 AM ET

finding climate zone
by Hallie Bowie

-1 Helpful?

Can you please provide a link for where to verify the climate zone in your area?

Sep 27, 2010 10:39 AM ET

Response to Hallie Bowie
by Martin Holladay, GBA Advisor

-1 Helpful?

Hallie Bowie,
On the GBA Web site, the official DOE climate zone map can be found on this encyclopedia page:
Insulation Overview

Many Web sites have reproduced the DOE climate zone map, including this one:

Sep 28, 2010 9:17 AM ET

by Gene

1 Helpful?

One thing to remember, vapor pressure and gas laws state the moisture wants to equalize ie it perms to the outside (even through brick) keeping the moisture inside the house when a lot of people add humidity to their houses is required the reverse is true in the south keeping moisture outside is a requirment for the design of a/c systems

Sep 28, 2010 9:22 AM ET

Response to Gene
by Martin Holladay, GBA Advisor

0 Helpful?

I'm not sure what you are trying to say. Maybe some punctuation would help?

Sep 28, 2010 12:00 PM ET

I'm converting a 105 year old
by David Jenny

1 Helpful?

I'm converting a 105 year old summer cottage in Midcoast Maine to year-round use. Some walls are 2x4, some are 2x6. All stud bays are open to the interior. Exterior is either tongue and groove or 7/8 boarding board - all is now covered with Typar. New windows and old are flashed and the exterior is ready for a layer of cedar shingles. Thermal bridging is a major issue for the following reasons: The original structure was framed atop 6x6 perimeter beams propped up on posts (I've installed an insulated foundation to replace the posts); All corners were framed with 4x6 lumber; Additions and numerous changes to window and door placements have added a lot of extraneous lumber to exterior walls. To further complicate things architectural details make the structure look like a castle. I don't see how I can add 2-4 inches of ridgid foam insulation to the exterior of this structure in its current state. What do you recommend to most effectively insulate these walls (oh yes, the spacing between the studs varies greatly as well)? I can easily get to the space where the perimeter beam supporting the floor joists meets the foundation wall to spray closed cell foam. Might this be a case for a layer of interior foam board with perhaps blown-in cellulose between the bays? Do you see a problem with this structure being planned to dry to the outside? (no need for air conditioning here)
Thanks for your help.

Sep 28, 2010 12:19 PM ET

Response to David Jenny
by Martin Holladay, GBA Advisor

-1 Helpful?

David Jenny,
The building you describe sounds like an excellent candidate for thick exterior foam sheathing. But if you have already installed your windows and Tyvek, it sounds like it's too late for that.

It's a shame that you installed your Tyvek and windows before you had a plan for insulating your walls.

You can certainly install interior rigid foam over your studs. It sounds like your wall will be able to dry to the exterior; board sheathing is certainly less risky than OSB.

You could also use closed-cell spray polyurethane foam, although spray foam won't solve your thermal bridging problems.

Sep 28, 2010 1:31 PM ET

Little bit of knowledge
by David Jenny

0 Helpful?

Yep - a litle bit of knowledge is a dangerous thing. I had been thinking that loose cellulous or closed cell spray foam would be adequate. Until reading this thread and others here I had also thought of adding fan-fold foam to the exterior to smooth out the surface and to act as a thermal break. ( I have often wondered just how thick a thermal break would be needed to foil the heat transfer through wood studs.) But, if I am understanding this, it seems that in my situation (the corner I have painted myself into) whatever type of foam board I apply needs to go on the interior so the wall can dry to the exterior.

I'm leaning toward one inch of polyiso rigid foam and blown in cellulous. I also have a stock pile of polyiso seconds of various thicknesses (with what appears to be kraft paper facing) that I could cut to fit into the bays and seal in place with a flash coat of spray closed cell. A third option might be to cut strips of foam to nail to the studs and then fill the bay with blown-in cellulous. I must say that I have some affinity for the idea of having the wall unit be able to absorb the moisture ladden air as needed and to dry out in both directions

Sep 28, 2010 2:36 PM ET

Unforgiving Wall Assemblies
by Andrew Homoly

1 Helpful?


I would like to expand on your idea of wall assemblies being "too close to the cliff" in terms of not allowing for the ignorance of builders / owners in terms of indoor relative humidity. I am a builder in Kansas City and 99% of homes in our area have no system to monitor indoor relative humidity. Most people do not know about HRV's and have not changed the setting on their humidfiers for years. As our homes get tighter and tighter, this needs to be addressed as a few well publisized moisture failures could undermine a lot of hard work for better energy efficiency.

As an example, there is a 6 year old SIPs home in our area that appears to be a total loss. It is in litigation right now, so none of this information is official, but it appears as though humid air got between the SIPS panels and condensed on the back side of the stucco finish. The home did not have an HRV and potentially had a humidifier. The outside layer of the OSB disintegrated over time (starting at the joint areas) and now the structural integrity of panels is so bad that the homeowner had to move out. Exterior flashing appeared to be good and the stucco was applied over a wire mesh layer of bulding paper plus Tyvek with drainage crinkles (in other words, it looks like all the damaging water came from the inside).

I was a big SIPs supporter until I heard about this case. Now I even question the used of closed cell foams for fear that moisture cannot dry out to the inside. Granted this case was probably a perfect storm of problems, but I think there is something to be said for building a "forgiving" wall section that does not count on a homeowner properly running their HRV, humidifiers, etc.

My market is high end custom Green homes. My clients want the best possible design and they are willing to pay more money up front for the best long term design. I am leaning towards a 2x6 wall with Icynene (to allow it to dry to the inside) with rigid foam insulation on the outside (to minimize the condensing effect on the layer of OSB and to reduce the thermal bridging effect of the studs). A semi-permeable rigid foam would be optimum to allow drying to the outside. What wall would you recommend? I would also be interested in other people's viewpoints of their "perfect wall".

Sep 28, 2010 3:43 PM ET

Response to Andrew Homoly
by Martin Holladay, GBA Advisor

0 Helpful?

Andrew Homoly,
The failure mechanism you describe occurred on a large scale in Juneau, Alaska, where many SIP homes with poorly sealed panel seams allowed exfiltrating air to condense against roof underlayment and wall cladding. The result was extensive OSB rot near the panel seams.

Stucco is a particularly unforgiving cladding, and thousands of stucco-clad U.S. homes have wall rot. Stucco over OSB is particularly problematic. I am now writing a blog on these failures -- and on stucco details that will help builders avoid problems. Look for the blog in a few weeks.

Sep 28, 2010 8:54 PM ET

Response to Martin Holladay
by Andrew Homoly

0 Helpful?


I look forward to your article on stucco. However, can you answer my question on the optimum wall system? Would you recommend open cell insulation in the stud cavity plus rigid exterior foam insulation to provide a great balance of energy efficiency and moisture forgiveness? Would your answer change if the exterior cladding was stucco or say Hardie siding?

Sep 29, 2010 4:14 AM ET

Second response to Andrew Homoly
by Martin Holladay, GBA Advisor

1 Helpful?

Andrew Homoly,
Q. "I am leaning towards a 2x6 wall with Icynene (to allow it to dry to the inside) with rigid foam insulation on the outside (to minimize the condensing effect on the layer of OSB and to reduce the thermal bridging effect of the studs). A semi-permeable rigid foam would be optimum to allow drying to the outside. What wall would you recommend? ... Would your answer change if the exterior cladding was stucco or say Hardie siding?"

A. Your suggested wall design will work fine. If I were building the wall, I would probably choose dense-packed cellulose rather than Icynene, but Icynene will certainly work.

Once you install enough rigid foam on the exterior of your wall, you can install almost anything between the studs. Exterior rigid foam is the wall component that makes the wall less risky: it keeps the sheathing warm, and therefore keeps the stud bays warm, greatly reducing the chance of moisture accumulation.

Your worries about choosing a foam that is vapor-permeable are groundless. Here's the way the wall works: it is designed to dry in two directions. On the interior side of the rigid foam, it dries to the interior. (Therefore, no polyethylene.) On the exterior side of the rigid foam, it dries to the exterior. (A ventilated rainscreen gap helps speed drying.) There is no need for there to be any drying through the foam layer.

While the vapor permeability of the rigid foam is irrelevant, air tightness is not irrelevant. Regardless of the materials you choose to insulate your wall, it is essential that you pay close attention to air sealing.

I would never choose to install stucco on a wood-framed wall. In my mind, stucco works fine over concrete block or stone walls, but is a problematic siding over wood framing. I've heard too many tales about wet-wall disasters under stucco to take that risk.

Sep 29, 2010 10:42 AM ET

Martin's response re Andrew's optimum wall
by David Jenny

0 Helpful?

If dense-packed cellulose is preferred for the wall cavity, and poly is out to seal the wall (due to its impermiability), how do you insure air sealing from the interior? Can Typar be used on the interior face of the stud wall before drywall is installed? Or, is there some other product?

Sep 29, 2010 10:58 AM ET

Response to David Jenny
by Martin Holladay, GBA Advisor

0 Helpful?

The easiest way to provide an air barrier on the interior of a house is to follow the Airtight Drywall Approach.

I wrote an article on the Airtight Drywall Approach that appears in this month's (November 2010) issue of Fine Homebuilding:

Sep 30, 2010 2:33 AM ET

Formula Discrepancy
by Francois

0 Helpful?

While researching the performance of a wall structure for my home, i came across a discrepancy in the formula provided in your article and that of Ted Cushman. Lstiburek's formula ( T(interface) = R(exterior) / R(total) x (Tin -Tout) + Tout ) uses the percentage R-value of the exterior insulation. Your fomula uses the percentage R-value of the cavity insulation. The difference can be significant.

[Note to GBA readers: Click "page 2" to see a continuation of this discussion.]

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