# 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

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…

1. ryan evanczyk | | #1

More foam on 2x6 walls.
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?

2. Expert Member
| | #2

Psychrometrics & Hygrothermals
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.

3. Expert Member
| | #3

More foam on 2x6 walls...
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.

4. GBA Editor
| | #4

Response to Ryan Evanczyk
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

5. J Chesnut | | #5

air leakage
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?

6. GBA Editor
| | #6

Response to J Chesnut
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.

7. Bill Rose | | #7

The ******* method
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.

8. Jesse Thompson | | #8

WUFI
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.

9. GBA Editor
| | #9

Response to Jesse
Jesse,
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.”

10. | | #10

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).

11. Expert Member
| | #11

My calculations...
Hunter,
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.

12. Expert Member
| | #12

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.

13. Expert Member
| | #13

Sorry...
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!!!

14. | | #14

ASHRAE 160
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

15. Doug McEvers | | #15

1968 Twin Cities 2 Story
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.

16. Larry | | #16

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?

17. GBA Editor
| | #17

Response to Larry
Larry,
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.

18. | | #18

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?

19. Doug | | #19

Sheathing for 12" wall
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.

20. Expert Member
| | #20

Response to Hunter
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.

21. Expert Member
| | #21

Martin
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.

22. Doug | | #22

WUFI
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 science...is 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.

23. Larry | | #23

Response to Martin re: 12" wall
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.

24. Expert Member
| | #24

Martin,
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?

25. GBA Editor
| | #25

Response to Larry and Armando
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!

26. | | #26

plywood, OSB, perms, infiltration
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.

27. Doug | | #27

Climate & OSB
Martin,
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?

28. GBA Editor
| | #28

Response to Hunter Dendy
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.

29. GBA Editor
| | #29

Response to Doug
Doug,
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.

30. Daniel | | #30

Tool for newbies
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. http://fireflyeco.com/2010/05/psychrometrics-for-the-rest-of-us/ Happy to share the SWF file if anyone wants it.

31. Pam | | #31

Green Building
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?

32. GBA Editor
| | #32

Response to Pam
Pam.

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."

33. | | #33

"setting" the interior relative humidity
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.

34. GBA Editor
| | #34

Response to Peter Yost
Peter,
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.

35. John | | #35

The good thing about Table N1102.5.1 is it provides an easy...
Is "Table N1102.5.1" available on-line somewhere?

Thanks

John

36. GBA Editor
| | #36

Response to John
John,
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.

37. Hallie Bowie | | #37

finding climate zone

38. GBA Editor
| | #38

Response to Hallie Bowie
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:
http://www.energycodes.gov/implement/pdfs/color_map_climate_zones_Mar03

39. Gene | | #39

Condensation
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

40. GBA Editor
| | #40

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

41. | | #41

I'm converting a 105 year old
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)

42. GBA Editor
| | #42

Response to David Jenny
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.

43. | | #43

Little bit of knowledge
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

44. Andrew Homoly | | #44

Unforgiving Wall Assemblies
Martin,

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".

45. GBA Editor
| | #45

Response to Andrew Homoly
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.

46. Andrew Homoly | | #46

Martin,

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?

47. GBA Editor
| | #47

Second response to Andrew Homoly
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.

48. David Jenny | | #48

Martin's response re Andrew's optimum wall
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?

49. GBA Editor
| | #49

Response to David Jenny
David,
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:
http://www.finehomebuilding.com/how-to/departments/energy-smart-details/airtight-drywall.aspx?ac=ts&ra=fp

50. Francois | | #50

Formula Discrepancy
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.]

51. GBA Editor
| | #51

Response to Francois
Francois,
Thanks for pointing out the discrepancy. I need to research the discrepancy to be sure I don't compound any errors by jumping to conclusions, but it looks like you're right.

I'll post more information on this discrepancy once I learn more. In the meantime, I have added another illustration to this page. If you click the last picture at the bottom of the blog, you'll find a reproduction of Lstiburek's formula from the book Moisture Control Handbook: Principles and Practices for Residential and Small Commercial Buildings by Joseph W. Lstiburek and John Carmody.

52. | | #52

Second Response to Martin
Martin,

I chose the Icynene to act as the air barrier. On the exterior, I don't like to use more than 1" of rigid foam as it becomes difficult to anchor the cladding through the foam. In particular, Hardie recommends a max of 1" foam. When discussing drying to the "outside", aren't we talking about the OSB drying out? If this is the case, then the permeability of the foam does become an issue.

Your comment about not putting stucco on a wood structure is interesting. I would bet more the half the homes in Kansas City have stucco over wood structures. While EIFS projects had issues, I have not heard of any widespread stucco problems. Now that being said, most are also poorly insulated with batt insulation and therefore can dry out to the inside. Most stucco installations in this area also use Tyvek Stucco wrap that has the drainage channels to form the "gap" that has become so popular.

53. GBA Editor
| | #53

Stucco problems
Andrew,
I'm glad to hear that Kansis City has mostly avoided stucco problems. Pennsylvania and Minnesota have not been so lucky.

Look for an upcoming Energy Nerd blog on the topic.

54. | | #54

What Exactly is Drying Out to the Exterior?
Martin,

When we say we want the wall to dry to the exterior, we are talking about the OSB drying out, correct? If this is the case, then the permeability of the rigid foam is relevant, correct?

55. GBA Editor
| | #55

Drying to the exterior
Andrew,
I assume that your latest question arises because of my previous post, in which I wrote, "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."

Further explanation: If you choose to build a wall with rigid foam, the following components dry to the exterior: the vertical strapping and the siding. The following components dry to the interior: the OSB or plywood wall sheathing (if any), the studs, and the insulation between the studs (if any).

Foam sheathing does a great job of keeping sheathing warm and dry. That's what you want. If you install a thick enough layer to prevent "condensation" (more properly, moisture accumulation in the sheathing), then the foam will not be very vapor-permeable. So you're not going to see any drying THROUGH the foam; you're going to see drying in two directions on either side of the foam. The foam is the least vapor-permeable layer in your wall assembly, so drying happens in both directions out from the foam.

56. GBA Editor
| | #56

More on Francois's formula "discrepancy"
Francois pointed out that the formula in Ted Cushman's article differed from one provided elsewhere by Joe Lsiburek, namely:
T(interface) = T(exterior) + { R(exterior) / R(total) x [T(interior) -T(exterior)] }

This compares to the formula in the Cushman article, namely:
T(interface) = T(interior) – { [T(interior) – T(exterior)] x (R(cavity) / R(total)] }

After a little bit of head-scratching and consultation with Bill Rose, I have convinced myself that the formulas are mathematically equivalent.

57. Rock | | #57

Comment to Andrew & Martin R:Foam Thickness Vs Attachment
Andrew, I just did my new roof with 3" foam exterior to the rafters. I used 1x4 strapping on top of the foam and screwed them to the rafters with 6" SIP screws (my metal roofing was attached to the strapping). I contacted 2 screw manufacturers, neither of which would sell directly, but found the ABC Supply distributor in Rochester NY was very reasonable (less than half the cost quoted by the Buffalo location) at just under \$150 per 1000. Includes a free T-30 tip. Screwing straight was not a problem ... it's easy to stay plumb for the 1" strapping and the foam doesn't provide a lot of resistance to the self tapping screw head. Granted, I re-built with 3X10 rafters, thus had a bigger target, but the few "misses" turned out to be miss-judgments, i.e., they were off significantly and for a reason, and were easily corrected. But I suggest a corded driver. The 14.4V cordless began to overheat and emit odors hinting at burnout. My 1/2" corded driver just smiled at the screws and kept going.

58. Peter Shepherd | | #58

Updated climate zone map url
http://www.energycodes.gov/implement/pdfs/color_map_climate_zones_Mar03 gave a 404 error, a working link for zone maps on that site was:

It would be useful if there was an extension of the US zones into Canada (even historical ones), as our map is very different http://oee.nrcan.gc.ca/residential/business/energystar/procurement/windows-sliding-doors.cfm

I guess the best proxy would be the plant hardiness map, http://www.usna.usda.gov/Hardzone/ushzmap.html

59. imallthumbs | | #59

condensation
One thing that is not included in this discussion is condensation on windows. Unless there is a hole in the wall, moisture is going to condense on windows long before it has a chance to migrate into walls. If you do not have condensation on your windows you will not have a problem with moisture within walls. I build my walls with solid foam cut on a table saw to fit between the studs. I make sure that there is a tight fit and if necessary foam or caulk any gaps.

60. GBA Editor
| | #60

Response to I'm All Thumbs
I'm All Thumbs,
That's a big "unless." You wrote, "Unless there is a hole in the wall, moisture is going to condense on windows long before it has a chance to migrate into walls."

Unfortunately, virtually every home in the U.S. and Canada has "a hole in the wall."

Air leakage through walls is all over the map, but air leakage is never zero. Most new homes have enough air leakage around electrical boxes and through cracks between drywall and bottom plates to create plenty of problems.

61. imallthumbs | | #61

condensation
It is my understanding that the models and calculations do not talk about holes, instead they address mass movement of moisture through a wall. My point is still valid, if you are going to worry about calculations, don't forget the elephant in the room, windows and the resulting condensation. If you have condensation then there is the potential for moisture to move through a wall. If you do not have window condensation then you can sleep easy.

62. GBA Editor
| | #62

Second response to I'm All Thumbs
I'm All thumbs,
You wrote, :"If you do not have window condensation then you can sleep easy." UNLESS there is air leakage through your wall!

If you've conducted a blower-door test, and you know your house is Passivhaus-tight, you can lay your head on the pillow. Otherwise, it pays to delve a little deeper into the issues discussed in this blog.

63. | | #63

how do you do the simplified calculation for SIP on timberframe?
I'm a little confused about how one would do the simplified calculations for SIP over timberframe construction?

The SIPs have OSB on the inside and outside. Does this mean that the percentage of the insulation that is on the inside of the sheathing (outside sheathing) is 100%?

In that case the exterior wall temperature would always equal the exterior temperature. Does this imply that it is necessary to always keep the relative humidity inside the house exactly the same as that outside the house in order to prevent condensation on the exterior of the walls?

thanks

64. GBA Editor
| | #64

Response to Jay Hersh
Jay,
The foam in a SIP is both a vapor barrier and an air barrier, so interior moisture cannot accumulate in the exterior facing of the SIP -- as long as the seams are properly detailed.

If a seam leaks air, all bets are off, and it's possible to get OSB rot near the seam.

When it comes to SIPs, there is no need to do a dew-point calculation. All you need to do is come up with a bullet-proof air sealing detail at the seams. Many SIP installers go for a belt-and-suspenders approach and seal their seams with spray foam, and then tape the seams on the inside with SIP tape.

65. | | #65

Effect of condensation
Condensation forms on and in all wood products unless they are surface protected, condensation on/in OSB may lead to mold and rot.
However a coat or two of yacht varnish or similar product, will keep the condensation on the surface allowing it to dry later.
When baseboard heating is used along the entire length of an external wall and it is the only source of heating within a warm wet room, then the radiated, conducted and convected heat along the wall will, providing there is sufficient insulation on the frame exterior keep it above dew point, . Leaving the water vapor to condense on a window or some other below dew point surface.

66. GBA Editor
| | #66

Response to Roger Anthony
Roger,
It is simply untrue that "Condensation forms on and in all wood products unless they are surface protected." (As building scientist William Rose likes to say, “Capillary materials do not exhibit condensation at the dew point.”) If a piece of wood is below the dew point, it will absorb moisture -- but you won't see beads of water condensing on the surface of the wood. (In some circumstances, of course, frost will form on the surface of the wood.)

Moreover, a coat of varnish is insufficient to prevent a piece of lumber from absorbing moisture or drying out. If that's all it took to keep wood sealed, then the life of a sailor would be a lot simpler than it really is.

I'm not sure what point you are trying to make when you discuss hydronic baseboard heat. If you are simply saying that, in a home with insulated walls and hydronic baseboard, window components are likely to be the coldest surface of the room -- and therefore the first place where you will see condensation -- then you are, of course, correct. But the same is true for a room with forced-air heat.

For more information on window condensation, see Rating Windows for Condensation Resistance.

67. | | #67

Are there other formulas for colder climates
I have a house with drywall, 6 mil poly, R24 fiberglass, half inch osb and siding.
I tried to calculate the dew point as explained in this thread. The variables were as follows;
Average outdoor temp 2 F
RH - 45%
Total wall R value is 25

70-(68*.96)
70-65.28
4.72

This is well below the dew point which I`m guessing would fall around the 44 mark.

Even when done with R12 of additional foam on the exterior of the sheathing I still come in at 25.8 which is also well below the dew point. I`m guessing there must be a different formula for cold climates (question mark, I triggered french keyboard and now can`t use appropriate punctuation).

68. GBA Editor
| | #68

Response to Shawn Currie
Shawn,
When you have no exterior rigid foam, your exterior sheathing is basically at outdoor conditions. So there should be no surprise that the wall sheathing is below the dew point during the winter with no exterior rigid foam.

In the example given in the article -- the example of Boston, Mass. -- the average outdoor temperature in Dec., Jan., and Feb. is 30.3°F. If the corresponding average for your climate is 2°F, you must live in a very cold climate.

According to the usual recommendations -- (read about them here: Calculating the Minimum Thickness of Rigid Foam Sheathing) -- builders in climate zones 7 and 8 with 2x6 walls need to install R-15 rigid foam to keep the sheathing above the dew point. If your average outdoor temperature in winter is 2°F, you are likely to need more than R-15 to attain your goal, so your calculations appear to be correct. In other words, it's no surprise that R-12 exterior foam isn't enough.

69. | | #69

North of 49
Yes Martin, I live in a very cold climate. We measure temperature in Celsius, that means it's cold!

After reading the articles on this site, people in colder climates appear to have a conundrum; You need foam to keep the osb above the dew point, however, building code says we need to have a vapour retarder on the interior of the wall assembly. Not the best combination so I read.

However, I do gather that the osb without foam will hopefully dry completely during the warmer months in order to prevent rot. At least with this system, we can dry to the exterior, causing less of a problem with the polyethylene.

70. | | #70

How to...
Ok, so I was curious; in my climate, how much foam is needed to keep that osb above the dew point. According to the formula provided, R60 gets us really close. Unfortunately, that would come at quite a cost. I'll wait for your response on my last post, but it almost seems to me that no foam might be the best option here. Given you have protected your shell against penetrating water from the exterior and properly sealed the interior.

71. | | #71

Need to get my numbers straight
I think I have found where I erred. I took the average "low" temperature instead of the mean average.
That being said, void the last post. R15 would be sufficient.

72. GBA Editor
| | #72

Response to Shawn Currie
Shawn,
Lots of people build walls without any exterior rigid foam. If you decide to build your walls without any insulation on the exterior side of the sheathing, and you want to encourage your exterior sheathing to dry quickly in the spring, be sure to include a ventilated rainscreen gap between the WRB and the siding.

73. | | #73

Hi, I know this is an old thread but here goes:
I was wondering if the Interior Insulation % uses the interior R-value divided by the overall effective R-value of the wall or the nominal value of the wall? This can be a big difference. According to this formula for my wall section:
Sheathing temp = 72F - [(72F-33.6F) * 48% nominal] = 54F
vs
Sheathing temp = 72F - [(72F-33.6F) * 55% effective] = 51F
In climate zone 5 Northern NJ, the average mean temp for Dec, Jan, Feb is 33.6.
My wall will consist of:
5/8" GWB
5.5" Rockwool comfortbatt R-23
2x6 stud
1/2" zip osb
6" Rockwool Comfortboard 80 R-24
1 1/2" Furring strips
Metal siding

If I use 72F interior temperature and 40% RH (both personal preference), I come up with 46F Dew Point from the psychrometric chart. While this is below my sheathing temperature, it is closer to the 51F effective R-value than the 54F nominal R-value. I would like to have a wider margin than 5 degrees from the effective interior percentage.
Maybe the is splitting hairs but any clarity from anyone would be much appreciated.

1. Expert Member
| | #74

Bump

2. GBA Editor
| | #75

Russell,
Use nominal R-values when making these calculations. (Note, for example, this sentence from a related GBA article: “The tables are based on the nominal value of the insulation layers, without taking into account the effects of thermal bridging through the studs.”)

Here is a link to the article where the sentence appeared:

74. | | #76

Hey Martin, I am a building designer, hoping to hear from you on this question of mine. I am currently wrapping up my passive house consultant course and the phpp is quite an amazing tool, anyways getting into the calculation end of building science now a days. In this article under the sub section titled "Are dew-point calculations useful?" I feel like there is considerable information missing and I'm not able to pull off a calculation correctly. Here are some questions:

- Nowhere in the article does it seem to clearly talk about how when you do this calculation in that sub section whether or not it includes the exterior insulation to the equation or not since nowhere in the equation did it ask for the exterior insulation r value.

- Please clarify as well you talk in that sub sectoin about the temperature of the interior side of the ext sheathing; at the beginning you say we need to know the interior side temperature at the end of the equation though you say the sheathing temperature not just the interior side but the actual sheathaing temperature is x.

I am trying to learn how much temperature change happens for every r value added. I know that adding more inboard insulation with no outboard insulation makes ext wall sheathing colder, I understand how that works. I'm trying to figure out exactly what you are taking into account with your calculation and it seems loosely described and lacking some details. I need to know what the temperature of the interior side of the sheathing is, however the interior temperature vs the exterior temperature should be the same true? Or is this not the case due to conduction. But the point is that I need more detail, still quite clueless as I don't know the exterior insulation effects whatsoever in your equation and it seems like when I take the same example and I add another r20 to the equation the exterior sheathing should get a lot colder, but it doesn't in the equation, not sure how that makes sense...

Very confused and would love to hear back from you on this.

• ## 5 Facts About Dew Point Temperature

A brief lesson on dew point temperature and its impact on human comfort

• ## Three Facts About Dew Point Temperature

Understanding the causes of condensation is the first step in controlling it

• ## Understanding Dew Point

If you’re a designer or builder who lacks an intuitive understanding of dew point, this article may help

• ## Is It Possible to Get Condensation on a Sponge?

A little building science puzzler on moisture and the properties of materials

• |
• |
• |
• |