GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Outside to inside perm ratio for mixed-humid climate?

HDendy | Posted in Energy Efficiency and Durability on

I’ve read several posts lately mentioning a “5:1 outside to inside perm ratio with the goal of creating walls that can dry to the outside” for cold climates. Does anyone know what the ratio (along this line of thinking) would be for mixed-humid climates? It seems the goal would be to dry to both sides…but it also seems that opens the chance for moisture problems at both extremes of the year- we get 90 degree summer days with a good deal of humidity and single digit winter nights.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. Riversong | | #1

    The only "rule of thumb" for mixed climates is no vapor barrier anywhere in the envelope.

    Summer inward moisture drives are often stronger than winter outward moisture drives, and AC makes condensation at the inside wall surface a distinct probability. So air-tight but vapor-open layers on both sides of the envelope are essential.

  2. GBA Editor
    Martin Holladay | | #2

    Hunter,
    I strongly disagree with Robert's statement. To address the problem of inward solar vapor drive, many home in mixed climates choose to install foil-faced polyisocyanurate as exterior sheathing. These houses can perform very well, as long as there in no interior polyethylene or vinyl wallpaper.

  3. Riversong | | #3

    These houses can perform very well, as long as there in no interior polyethylene or vinyl wallpaper.

    Even the EnergySavers.gov website (http://www.energysavers.gov/images/vapor_barrier_replace.gif) states that in certain mixed climate zones, there should be no vapor barrier. An exterior vapor barrier can be helpful in hot-humid climates in which there is little to no outward vapor drive, but even there it is problematic in the event of an envelope leak (think hurricanes).

    Studies have demonstrated that, once an envelope assembly gets wet (they all start out very wet and get wet at some time in their lifespan), a foam-sheathed vapor-closed assembly stays wet and warm long enough for mold and rot to occur.

    No amount of "clever" engineering can control natural forces. The wise designer/builder works with nature and creates shelters that breathe.

  4. Riversong | | #4

    REDUX with legible formatting:

    These houses can perform very well, as long as there in no interior polyethylene or vinyl wallpaper.

    Even the EnergySavers.gov website (http://www.energysavers.gov/images/vapor_barrier_replace.gif) states that in certain mixed climate zones, there should be no vapor barrier. An exterior vapor barrier can be helpful in hot-humid climates in which there is little to no outward vapor drive, but even there it is problematic in the event of an envelope leak (think hurricanes).

    Studies have demonstrated that, once an envelope assembly gets wet (they all start out very wet and get wet at some time in their lifespan), a foam-sheathed vapor-closed assembly stays wet and warm long enough for mold and rot to occur.

    No amount of "clever" engineering can control natural forces. The wise designer/builder works with nature and creates shelters that breathe.

  5. HDendy | | #5

    To make matters even more difficult I live in an area (Asheville, NC) that has very large temperature differences over the year- summers with 90's and high humidity and winters with frequent temps well below freezing and low humidity (obviously). We do have more HDD than CDD so heating efficiency is important, but as I noted we have significant cooling load days. Luckily the weather is great here for a large part of the year, it's just those few months of the extremes that can be trouble.
    I like the concept of exterior insulated sheathing to prevent a condensing surface at the outer edge of the thermal boundary, but it's just the vapor drive that I'm still not comfortable with. I think I picked the most difficult climate to live and design in.

  6. Riversong | | #6

    Even though I don't normally recommend articles written by product manufacturers, there was an article in the Dec 2006 issue of Architectural Record provided by Dupont Tyvek that is offered for continuing education on the McGraw Hill website (I omitted the link as it keeps triggering the SPAM filter) which is one of the best on the subject:

    Moisture Management in Wall Assemblies: Air, Water, and Vapor Barriers
    Selecting the appropriate protective barrier based on climate, codes, and design criteria

    Excerpts:

    The choice of materials is critical for providing a diffusion open pathway in order to promote diffusion drying. As a general rule, condensation control requires that the building envelope materials increase in permeance in the direction of vapor diffusion. This means that in predominantly heating climates, where diffusion is typically from the inside to the outside, the wall assembly must have vapor permeable materials towards the outside. In cooling climates, where diffusion is typically from the outside to the inside, the wall assembly must have vapor permeable materials towards the inside. In mixed climates, proper moisture management requires diffusion open pathways in both directions, to the inside during summer and to the outside during winter.

    ...it is recommended to protect against moisture transported by diffusion (with a vapor barrier), as long as the use of a vapor barrier does not interfere with the wall drying ability.

    Vapor Barriers should only be used in those climates where diffusion into the wall cavity occurs predominantly in one direction.

    If an air and water barrier is also a vapor barrier, its location in the building envelope is determined by the vapor barrier function and must follow the vapor barrier requirements for condensation control. In many U.S. climates (except for predominantly heating or cooling climates) such membranes should be avoided. Air and water barriers should be vapor permeable.

  7. HDendy | | #7

    Thanks, I'll read the article and go ahead and take care of a contin. ed. credit in the process. I usually shy away from info from a company trying to sell me something, but hopefully if it's AIA approved it won't be too biased. I'm good with the dry-to-both-sides concept, but I still question why there isn't a condensation concern if the vapor is allowed to pass-through. It seems like it will hit a condensing surface at/below dew point at some point. Is it the 'storage-capacity' that comes into effect here? Not passing, but stored?

  8. Riversong | | #8

    summers with 90's and high humidity and winters with frequent temps well below freezing and low humidity (obviously)

    Most Americans don't realize that average winter relative humidity is higher than average summer RH in most locations (as much as double here in northern VT). The US mean RH in summer is 62% and mean winter RH is 71%. In Asheville NC, the averages are very close: summer avg RH of 72% and winter avg RH of 70%.

    But, since vapor drive is determined by vapor pressure, which is proportionate to absolute humidity, and since warm air can contain much more water than cold air, the vapor drive increases disproportionately with temperature at the same RH.

  9. Riversong | | #9

    Hunter,

    It's not just the storage capacity (though that's an important factor), but the combined hygro-thermal properties, mechanisms and rates. It's all a matter of balance.

    From the same article:

    Moisture problems occur if buildings get wet and stay wet, because they are unable to dry. According to building scientist Joe Lstiburek, Ph.D., P.E., “moisture problems are fundamentally rate issues: moisture accumulation occurs if the wetting rate exceeds the drying rate.” Consequently, moisture management must include strategies to manage the balance between wetting and drying, that is, to prevent wetting and promote drying.

  10. HDendy | | #10

    So, the vapor drive is much higher in the summer because at 72% RH there is much more absolute humidity compared to the conditioned inside air; as opposed to winter 70% RH and the inside heated air.

  11. HDendy | | #11

    Forgot to add to above post- this means that the summer vapor drive is just as critical as winter more even though we have more HDD than CDD?

  12. HDendy | | #12

    Maybe I should research this before asking, but what would be the best way to determine what perm rating I should look for in a material for the WRB in my climate, to allow the proper breathing we've been discussing? Is there a way to get more specific to my local climate, as opposed to general mixed-humid (seems to be a fair amount of variation within that)?
    Obviously there will be no interior vapor barrier- no poly, no vinyl coverings, no VB paint. Just drywall or wood (with latex paint or stain).

  13. Riversong | | #13

    Hunter,

    The vapor drive across an assembly is dependent on the vapor pressure differential between the two sides of the assembly, which is determined by the local indoor and outdoor temperature and RH conditions. The vapor flow, of course, is dependent on the vapor drive and the permeability of assembly materials.

    In winter, if the exterior air is 0° and 70% RH (0.03" HG vapor pressure) and the interior air is 68° and 40% RH (0.28" HG, dew point 43°), then there will be a strong outward vapor drive. But liquid diffusion is determined by the relative humidity differential, so there may be a moderate diffusion potential from outside to in.

    In summer, if the exterior air is 90° and 72% RH (1.02" HG, dew point of 80°) and the interior is 75° and 60% (0.52" HG), then there will be a modest inward vapor drive, and a slight inward liquid diffusion drive, and both diffusion mechanisms are enhanced by temperature differential - particularly the strong solar radiant flux.

    There is no simple formula or rule of thumb for exactly what permeance is required for each building layer - and the relative permeance of reach layer is just as important. So decisions need to be made based on good judgement, which is dependent upon a good understanding of hygro-thermal principles and material properties.

    Code standards are the closest thing to generalized rules, and the IRC allows either the elimination of the vapor barrier or the reduction of vapor retarder from class I or II to class II under certain conditions (Section N1102.5 Moisture Control and Table N1102.5.1). In climate zones 1, 2, 3, 4A, and 4B, a vapor retarder is not required - and where it is required, a class II (1 perm) retarder is sufficient.

  14. HDendy | | #14

    Robert,
    I'm fascinated and would like to better my judgement since that's what I'll ultimately be relying on. Do you recommend and texts so I can dive a little deeper. I've tried to piece it all together over the years with various seminars and continuing education courses but they always seem to be a little too "intro" or "basics". Unfortunately I can't afford the time or costs to go back to college. I regret not taking more advantage of the building science department when I was there. I'll have to keep reading and asking.

    Thanks.

  15. Riversong | | #15

    Hunter,

    I don't know of any good text on this subject, other than the myriad articles on the web. BuildingScience.com has a lot of good material, though I differ with them on a number of recommendations.

    I have taught a 2-day workshop in Hygro-Thermal Engineering at Yestermorrow Design/Build School in Warren Vt, but am no longer teaching there because they refuse to become a smoke-free campus (while wanting to be the nation's leader in sustainable design!), and I have offered a half-day presentation at other venues.

    Jay Walsh, Energy Analyst, Energy Star Homes and LEED-H Rater, Center for Ecological Technology, said of one such presentation:

    "Robert's presentation on moisture mechanics was the best presentation I have seen on the subject. I would highly recommend this workshop to all builders, architects and building trades people. I believe the issue of moisture as it relates to residential (and commercial) construction is one of the most important pieces of building science a builder today should have a strong working knowledge of."

    I was scheduled to do such a presentation at the Natural Building Colloquium in Bath NY that ended yesterday, but had to cancel at the last minute due to vehicle problems and the unpleasant prospect of driving 8 hours each way.

    I would be glad to offer either the long (2-day) or short (4 hour min.) version again within a few hours drive of north-central VT if anyone wishes to organize a venue.

  16. Daniel Ernst | | #16

    I find both sides of the argument appealing and substantial. I have no axe to grind; I'm simply trying to better understand the various techniques and their associated risks.

    Joe Lstibureck is often quoted in these kinds of discussions. From the Building Science Corporation website, here are some recommendations for areas with potential flooding / hurricanes:

    http://www.buildingscience.com/documents/digests/bsd-111-flood-and-hurricane-resistant-buildings/?searchterm=katrina

    http://www.buildingscience.com/documents/reports/rr-0704-building-a-durable-and-energy-efficient-home-in-post-katrina-new-orleans/view?searchterm=katrina

    Note that all of the examples eschew the use of loose-fill insulation in wall cavities exactly because they do not dry out after a severe wetting event. BSC recommends only exterior insulation using rigid fiberglass, foam, or rock wool.

    Robert - Do you see dense-packed cellulose as a viable option when walls are literally filled with water (i.e. mother nature's torture test)? Cellulose can certainly act as a moisture distributor / buffer, but this concept is usually discussed relative to small amounts of moisture (i.e. condensation).

    Hunter - Here is another BSC document that discusses more general Mixed-Humid climate recommendations for Haymount, VA. This is a climate very similar to Ashville, NC: 4,455 HDD and 1182 CDD @ 65F base.

    http://www.buildingscience.com/documents/houseplans/hp-mixed-humid-recommendations/view?topic=doctypes/designs-that-work/dtw-house-plans

    To echo Robert's points, here are some quotes from this PDF on Mixed-Humid Recommendations:

    "Ideally, wall and roof assemblies are designed to promote drying to both the interior and exterior in this climate."

    "Ideally, building assemblies should be designed to dry to both the interior and exterior. In heating climates, the primary drying potential is to the exterior (but not necessarily exclusively so); in cooling climates, the primary drying potential is to the interior (but not necessarily exclusively so); and in climates with both heating and cooling, some drying potential in both directions is typically a good idea (but not necessarily exclusively so)."

    "Vapor transport through diffusion can be a benefit or a detriment. In some circumstances, vapor diffusing into a wall assembly can condense and accumulate resulting in problems with material deterioration. On the other hand, vapor diffusion can also be used as a drying mechanism that will allow assemblies to dry to either the exterior or the interior or both. In general, the vapor control strategy used should maximize the drying potential of the assembly while minimizing the potential for wetting. With vapor diffusion being affected by both permeability of building components and temperature gradients across assemblies, the vapor control strategy is often related to, and integrated in, the insulation system design as well. For mixed-humid climates, walls are generally designed to dry to both the interior and the exterior (flow through assemblies) or, they are designed with insulating sheathing in order to control the temperature of the condensing surfaces. The thickness of the insulating sheathing is determined by calculation based on the severity of the climate."

    "In moisture control, the priority is liquid water first, particularly when it comes in the forms of rain and groundwater. In these forms it is referred to as “bulk” water. Following are air-transported vapor and then diffusive vapor, all other things being equal. It’s always a question of quantities and rates, of wetting and drying, and the tolerance of materials (individually and in combination) for each and all of the above."

    Now, having said all that, BSC shows the example house completely wrapped in both polyiso and XPS insulation. The aim is to mitigate moisture issues in BOTH directions: wintertime condensation AND summertime solar vapor drive. BSC makes these recommendations with a very experienced and well-studied understanding of hygro-thermal principles. Yes, it limits the exterior drying potential. But is also limits wintertime condensation and summertime vapor drive.

    Robert - This is obviously a place where your recommendations differ from BSC. Is your problem with this technique the materials (inherent problems with petroleum based foams), or the performance / durability of the wall assembly (limited exterior drying)? Are you saying that BSC is making a poor recommendation?

    Finally, in the Haymount, VA example, if you were to replace the exterior foam with a vapor-open insulation, say Roxul, how does that change your perspective? It doesn't address summertime vapor drive, but it does limit wintertime condensation on exterior sheathing while maintaining a fairly permeable wall assembly.

    Thanks for the lively and informative discussion.

  17. GBA Editor
    Martin Holladay | | #17

    Just to provide another example of a wall that includes a vapor barrier, yet performs well in a warm humid climate: consider a urethane-insulated SIP wall. It performs fine.

    Saying, "there should never be a vapor barrier in a wall" is just simplistic. When designing a wall, you have to think. Simple ten-word rules like this often have exceptions.

  18. Riversong | | #18

    Daniel,

    Cellulose insulation can safely absorb and release up to 30% of its weight in water. Beyond that, and it's paper maché. Nothing but a solid concrete building can withstand a flood - all walls must be torn open to dry regardless of the insulation type. So that should not be a consideration in choosing insulation, but should be a consideration in appropriate siting of the house.

    BSC is not an academic or government-funded research facility, though it does conduct some publicly funded research. It's a consulting business, and it would go bankrupt if it didn't offer solutions that mainstream builders would accept. Its mission is not to save the world from humanity, but rather "better living through engineering". [We've seen where "better living through chemistry, Dupont's slogan for 50 years, has gotten us.]

    My disagreement with their hermetically-sealed approach to living in a picnic cooler is with both the materials and the performance. Most of us are aware of the environmental and public health damage that petrochemical plastics have wrought throughout all phases of their life cycle, and "alternative" bio-based plastics are little better.

    Issues:

    - all living things exist within semi-permeable membranes, conditionally separating their internal functions from the (only apparently) external environment, and a shelter for living things must be a "third skin" - after our biological skins and our clothing - in order to maintain a healthy homeostasis with our external environment. We know now, for instance, that DNA expression is completely dependent on triggers from the environment. Isolating ourselves inside hermetically-sealed chambers alters the evolutionary progress of our species, and not for the better.

    - all plastics "outgas" and release plasticizers over time, foreign chemicals which we shouldn't want in or near our environments, and they remove the negative ions that are a necessity for a healthy environment.

    - sealing a house from external elements is a futile (and arrogant) attempt at controlling nature (the goal of all science-based technology), not unlike the Army Corps of Engineers decades-long attempt to control flooding by building dams (they are now removing them). Once water (inevitably) gets in to a building assembly, it cannot dry quickly enough to prevent mold and decay, the very problems we hope to avoid by super-sealing (MEWS studies have documented this). Unintended consequence is the epilogue to all technology-based "solutions".

    Roxul (mineral wool) is a fine option for exterior insulation. As BSC states, the primary moisture problem in residential structures is environmental bulk water, and the secondary problem is air-borne water vapor. The two primary drying mechanisms are convection (in a rainscreen, for instance, not air leakage) and liquid and vapor diffusion. We all know (don't we?) that vapor diffusion is not a significant source of moisture problems as long as indoor humidity is reasonably controlled and we're not living in a rain forest.

    So, any method to limit external water intrusion at the cladding through deflection and drainage, and to limit envelope air movement, coupled with durable materials that can buffer moisture and permeable layers that allow diffusive drying are elements of a good system. To also be a healthy system as well as an ecologically-responsible system, requires the use of mostly natural materials and the minimal use of plastics of any kind.

  19. HDendy | | #19

    Excellent discussion, thanks guys. The last 5 or so posts are really bringing some sense to it all for me. Daniel, good find, I've read many documents on BSC but not that one yet. I think a big part of it is the order of magnitude of water sources (vapor diffusion being third). Controlling the first 2 most important. An interesting concept that I had not thought of before is vapor drive working in your favor, since it is one of the drying mechanism. And it needs the path to do that. Robert, I think you're right that we need to design and build with the assumption that the assembly will start wet and will get wet again at some point during it's life. You're probably right that BSC's work is targeted to mainstream builders, but unfortunately that's the audience that most of us have to deal with too (incl. mainstream clients). As a designer, I think one of my biggest challenges is to find an environmentally conscious and sustainable construction method that addresses our energy problems and is still attractive to my clients. I also see merit in the case for exterior rigid insulation, but don't want to trap moisture- Daniel you may be on to something with the Roxul.

  20. HDendy | | #20

    Daniel, have you used Roxul before? I looked into the product and wonder how well it would work as the WRB or if one would need to somehow install bldg felt (or other) over it.

  21. Daniel Ernst | | #21

    Hunter,

    I have not used Roxul in a residential / exterior application. My experience with mineral wool is limited to use on industrial utilities and equipment (not exactly apples to apples!). But I've been exploring the idea lately . . .

    Roxul, IMHO, is not suitable as a WRB. Although Roxul shows a picture of water spilling off the surface of a mineral wool "board," the product itself is extremely porous. They seem to be crafting their advertisement very carefully. No, mineral wool does not absorb water (the fiber itself), but the woven batt / board will hold a large quantity of water. It may take a little while to break the surface tension of the material, but it wouldn't act as a drainage plane for very long.

    Mineral wool board products do drain water well (they are sometimes installed on exterior basement walls to facilitate drainage). But, they drain water through the woven fiber matrix, not just on the surface. So Roxul needs to be installed on top of a suitable WRB - or behind one.

    Mineral wool is not a very dense material, not even the board products. I don't imagine you could place furring strips on top of the mineral wool. The compressive strength is low. I think Roxul specs use a lbs. / square foot unit instead of the more common psi unit - probably because nobody would think well of a product that has a compressive strength of 2.5 psi. ;-)

    I guess what I'm saying is that you can't substitute Roxul for foam board in light frame construction. The two products are more interchangeable in a masonry cavity wall (CMU / Liquid Applied WRB / Roxul or Foamboard Insulation / Air Space / Brick).

    From what I've seen online, the Europeans use this stuff on the exterior quite frequently. But they are using mineral wool batts to fill in the space BETWEEN their furring material . . . and installing it on top of their exterior sheathing. The siding is then supported by the furring strips - which are not 3/8" or 1X material. The furring strips appear to be 2" - 3" thick, creating a sizable space for the mineral wool batts. I'm not sure how they solve the problem of thermal bridging through their furring. Maybe they don't. I've even seen some European Passivhaus walls that have this same system on the interior walls, or applied to both interior and exterior walls (three insulation cavities).

    IDK. It is appealing in many ways. You get the benefits of insulating your exterior sheathing. You have a fairly vapor open wall. It creates a modified rain-screen. No petrochemical foams. It's durable (it's rock and steel slag after all). You'd just have to solve the summertime vapor drive some other way - and that's probably not a big deal with sealed wood or fiber cement siding.

    How about this wall assembly: Drywall / Double-Stud Cavity filled with Cellulose / Plywood Sheathing / Sto Gold System / Tyvek Drainwrap / Mineral Wool & Furring Strips / Siding

    I bet there are builders up in Canada that use Roxul all of the time. It would be nice to get some of their input on how well it performs in exterior applications.

    Somewhere, somehow, somebody out there could speak to this with experience!

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |