Image Credit: ASHRAE Fundamentals Dry bulb temperatures are indicated by vertical lines. Straight lines that slope down from the upper left to the lower right indicate wet bulb temperatures. 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. Temperature profiles like this one are often misinterpreted. Although some builders assume that moisture will accumulate in cold insulation, it rarely does. If condensation or moisture accumulation occurs, it happens at the sheathing layer or at an interior polyethylene vapor barrier, not in the insulation.
Image Credit: Energy Design Update Table N1102.5.1 specifies the minimum R-value of exterior foam sheathing for different climate zones.
Image Credit: International Residential Code This formula is reproduced from the book Moisture Control Handbook: Principles and Practices for Residential and Small Commercial Buildings by Joseph W. Lstiburek and John Carmody. This is the climate zone map for the 2004 Supplement to the IECC, the 2006 IECC, and ASHRAE 90.1-2004.
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 ASHRAE 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 engineers refer to ordinary air temperature as “dry bulb temperature.” 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.
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 temperature 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, 2×6 studs filled with cellulose, OSB sheathing, 1 in. of XPS foam, a rainscreen 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-T (Î”T) — 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-value 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 diffusion. 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 hygrothermal 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 2×4 walls and 2×6 walls. For example, if you’re building in climate zone 7, Table N1102.5.1 tells you that a foam-sheathed 2×4 wall needs foam with a minimum R-value of R-10 (for example, 2 in. of XPS), while a foam-sheathed 2×6 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.”