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Community and Q&A

Understanding vapor transmission and perms

Nick Welch | Posted in General Questions on

I understand the different levels of vapor retarders and how a lower perm rating means less vapor comes through the material. What I don’t understand is how this affects the amount of water vapor coming through in real life.

For example, say I have a totally unfinished basement with a damp wall that is causing high humidity in the basement. If I cover the walls with a 0.5 perm vapor retarder, how will that impact the indoor humidity? If I instead used a vapor barrier, how much further difference would that make? Is the relationship between perms and resulting indoor humidity linear or something else?

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  1. GBA Editor
    Martin Holladay | | #1

    No, it's not a linear relationship, for a variety of reasons:

    1. There is more than one source of water vapor entering a house.

    2. Occupants and plumbing usually introduce water vapor.

    3. Indoor relative humidity is affected by the outdoor temperature and the outdoor relative humidity.

    4. Indoor relative humidity is affected by the operation of HVAC equipment like furnaces, ventilation fans, dehumidifiers, and air conditioners.

    In other words, there are many, many factors affecting indoor RH. If your wall assembly is vapor-permeable, and the wall assembly starts out dry, it's even possible for a thick, dry wall that is filled with hygroscopic insulation materials to lower the indoor RH (for example, when occupants take showers) by absorbing moisture.

  2. GBA Editor
    Martin Holladay | | #2

    One more thing: I don't recommend that you try to install a layer of polyethylene on the interior side of your basement wall. It would be much better to install a layer of rigid foam instead. For more information, see How to Insulate a Basement Wall.

  3. Nick Welch | | #3

    I wouldn't install poly but there are many rigid foams that have vapor barrier facers. So, for example, what would the real world difference be between unfaced EPS and foil-faced foam? Assume that the basement doesn't have any other significant sources of humidity and that the ground is always moist. This does relate to my real-world basement but I'm also wanting to understand this in an academic sense.

  4. GBA Editor
    Martin Holladay | | #4

    Thin EPS is vapor-permeable, so when installed on a basement wall, the thin EPS would allow moisture to diffuse from the damp basement wall to the interior.

    On the other hand, foil-faced EPS or foil-faced polyiso is a vapor barrier, so when these products are installed on a basement wall, they prevent vapor diffusion from the wall to the interior.

  5. Nick Welch | | #5

    Martin, I understand the zero-versus-some difference between the two. I'm looking to quantify the "some" in a way that I can understand. I don't know how to translate "grains per inch of mercury" to something tangible.

  6. GBA Editor
    Martin Holladay | | #6

    To get a handle on permeance, Joe Lstiburek's three classes of materials is as good a method as any:

    Class I vapor retarders: 0.1 perm or less (sheet polyethylene, non-perforated aluminum foil)

    Class II vapor retarders: more than 0.1 perm but less than or equal to 1.0 perm (kraft-faced fiberglass batts)

    Class III vapor retarders: more than 1.0 perm but less than or equal to 10 perms (latex or enamel paint)

    Class I vapor retarders are what we used to call vapor barriers.
    Class II vapor retarders allow a little bit of diffusion.
    Class III vapor retarders allow some drying (or diffusion) -- significantly more than Class II retarders -- but they aren't wide open.

  7. Expert Member
    Dana Dorsett | | #7

    If you have a wall with especially high dampness it's best to deal with the drainage issues first. If you stem drying toward the interior and the concrete saturates, it will have to dry toward exterior at the above-ground section of the foundation, or potentially into the foundation sill if there isn't enough exposure for drying above grade. (About 18" would be plenty no matter how wet the foundation is, but less than 12" could have some risk.) Is there any efflorescent growing on the interior side of that wall?

    For water vapor to move across an assembly there has to be a vapor pressure difference. Increasing the temperature increases the vapor pressure, lowering the temperature lowers it.

    On the dirt side of the foundation it's a nearly saturated 100% RH, but it's at whatever temperature that is. The basement will be running a higher temperature, but a lower RH. At some RH less than 100% the vapor pressures will equalize, and no more moisture diffuses from the dirt into the basement, but that may be at a higher-than healthy RH. When a cool damp foundation wall is exposed to a drier room air, it will keep evaporating into the room air until the vapor pressures are equal. What you are trying to do is slow that down to something tolerable, something that is easily handled by the HVAC or dehumidifier systems.

    When you install insulation (any type) against inside of the found foundation in a cool climate, the temperature (and vapor pressure) at the interior surface of the concrete drops, since the temp at the surface of the concrete becomes roughly that of the soil rather than that of the basement room. That alone reduces the vapor pressure difference some, but maybe not enough(?). It depends on what your room RH limit constraints are.

    A 0.5 perm vapor retarder is actually a fairly powerful vapor retarder, and the ventilation rates & HVAC system will have a much bigger affect on the room's humidity level than whether or not you take it down to 0.05 perms with a sheet of poly. If you are finishing the room even 5 perms vs. 0.05 perms doesn't make a heluva lot of difference on the room RH., but it may become an issue if you are building a finished studwall. In the studwall paradigm you ideally you would want the vapor permeance of the layers between the foundation wall and the susceptible wood & gypsum facers in the studwall to be equal to or lower than the finish paint, so that the humidity in the susceptible layers tracks with the humidity of the room and not the the humidity of the soil.

    This isn't super-simple stuff to model in multi-material assemblies, which is why the Fraunhofer Institute came up the WUFI model for simulating heat & moisture transfer across assemblies. But as some folks at the Building Science Corp like to point out, if you have to run a WUFI simulation to know if the assembly makes it or not it's probably not a very resilient assembly. A really crude first order rule of thumb is usually good enough:

    If the cold side of the assembly has a vapor permeance less than the conditioned side there won't be much moisture moving from that side into the conditioned space. But if it's a LOT cooler there can be significant moisture migration from the warm side to the cool side, depending just how much cooler that side is relative to the dew point of the conditioned space air, and the vapor permeance of the interior side. In the basement situation it's never a problem for moisture to move from the room into the cold concrete of the foundation, but it can become an issue if susceptible materials in the wall stackup drop below the dew point of the room temp. For that reason it's usually a good idea to use air-impermeable insulation between the studwall & concrete to keep the cavity-insulation (if any) above the dew point of the room air, avoiding mold conditions at the cold side of any fiber insulation in the cavity.

    Using the IRC prescriptive values for insulating sheathing over wooden structural sheathing when using class-III interior side vapor retarders works just fine for foundation walls- it even has more margin. ( _ But in the basement case it's best to use air-impermeable insulation between the concrete particularly on the above-grade section of the foundation. Since even an inch of Type-II EPS has a vapor retardency comparable to or less than latex paint- it's enough to effectively limit the sub-grade migration of ground moisture through the studwall to rates the studwall can tolerate. At 3" (R12.6) it's a class-II vapor retarder.

  8. Charlie Sullivan | | #8

    To answer the question:

    If you had a vapor barrier with the same temperature on both sides, but different, fixed humidity, the amount of moisture going through is linearly proportional to the perms. 0.5 perm would have half the moisture coming through the 1 perm would have (though both would be small).

    As the discussion above shows, there are lots of reasons that your example is unlikely to match the simple scenario I described. Adding a vapor barrier might result in the wall being more damp, so the humidity isn't independent of the vapor retarder/barrier you choose. And the temperature of the wall is quite likely cooler than the some cases, the cool wall is damp because the room is humid, rather than the room being humid because the wall is damp.

    Now replying to Dana,

    Thanks for the good explanations of many of the complex issues involved.

    Regarding the mantra, "if you have to run a WUFI simulation to know if the assembly makes it or not it's probably not a very resilient assembly," I agree, but on the other hand, WUFI can be a great tool for learning what assemblies are more or less resilient, and when they are likely to get into trouble. The free educational version of WUFI has some limits that make it hard to use for verifying performance of a particular assembly in a particular climate, but for teaching yourself how various factors influence performance, it can be very helpful.

    Regarding "Using the IRC prescriptive values for insulating sheathing over wooden structural sheathing when using class-III interior side vapor retarders works just fine for foundation walls- it even has more margin," that makes sense if your worry is winter condensation. However, in a basement, the worst time can be spring or summer--spring because the foundation wall is still very cold while the indoor humidity is getting higher than it was in winter, or summer because that can be when there is the biggest temperature difference between the hot, humid indoor air and the cool foundation particularly near the floor.

  9. Expert Member
    Dana Dorsett | | #9

    The foam-R/thickness issue really IS a wintertime thing. Yes, the foundation is colder than the dew point of the room air in spring, but it's still quite a bit warmer than the above grade section of the wall in the dead of winter.

    Assuming you limit the basement's RH to 60% there is no scenario where the foam/fiber interface lingers at or below the dew point of the interior air long enough to matter in spring if you go with the IRC prescriptive levels for wood sheathed above grade walls. At 70F/60% RH the dew point is about 55-56F. As a dumb first-order approximation, ahead and calculate how cold the foundation has to be for the temp at the foam/fiber interface to be in the 55F condensing zone, and how long that would be likely to persist with the warming trends of spring/summer. It doesn't take WUFI to figure this one out either- there's plenty of built-in margin in the IRC prescriptives when applied to foundation walls.

    eg: In zone 5 the prescriptive is R5 for 2x4 walls. Assuming a 2x4 wall with R15 fiber and R5 foam, in a 70F room that's a 15F delta-T over the R15, or R1 per degree F. That means the foundation would have to persist at 5F colder than 55F or 50F over a couple of months. Since the even the deep subsoil temps in any zone 5 location would not be substantially below 50F, while there may be an early spring period where the foundation was below 50F when the interior air dew point hit 55F that would not persist into summer. In early spring it's unlikely that basement air would reach a dew point that high even at modest ventilation rates.

    Compare the deep subsoil map against the climate zone map, and run the dumb-arithmetic linear dew point model using the IRC prescriptive numbers. For the summertime dew point to become an issue the subsoil would have to be a lot cooler to create a dew point problem inside the studwall cavity with those R-ratios.

  10. Charlie Sullivan | | #10

    Thanks Dana, for addressing the question of when condensation is worst in so much detail. I think the reason we came to different conclusions is that you assumed the basement temperature and humidity are limited, whereas I was thinking of a scenario in which the windows are open and the outside is, for example, 80 F and 75% humidity. The basement probably won't get up to 80F, but with enough air flow it could have the same absolute humidity as the outside air, which means a dew point of 71 F. If the concrete is 60 F and the room is 75 F, for your example of R5 foam and R15 stud walls, there's condensation throughout more than half of the stud wall.

    So if you build that way, you are building in a requirement for dehumidifying that space. You might argue that it should be dehumidified no matter what, but the amount of dehumidification necessary to avoid condensation in moisture sensitive materials is more with the R5 foam and R15 fiberglass than it would be with R20 foam, and it seems to be to be better to build a building that is tolerant of various occupant behavior patterns.

  11. Expert Member
    Dana Dorsett | | #11

    You don't want to over-ventilate any part of your house with latent loads that high.

    The outdoor dew points at my house in central MA are often several degrees warmer than my basement (even when there is no sensible air conditioning load to speak of) so even though my foundation insulation is an all-foam solution there would be severe mold issues.

    The solution is to not ventilate more than the minimum required for reasonable air quality, and purge the moisture mechanically. As it happens my shading factors leave me with very low sensible loads, but the latent loads are very real.

    Basically, any insulated house in a humid climate will need dehumidification, including the basement. An RH of 60% is something of an upper bound for comfort & human health (50% is an upper bound for those allergic to dust mites.) And that goes for the basement as well.

  12. Charlie Sullivan | | #12

    Dana, we agree about the need for dehumidification, and we agree about the physics of when problems might happen. Our difference is only that I am more conservative about wanting to design a basement to be more tolerant of a wider variety of occupant behavior. Some people like having the windows open in the summer even if it is hotter and more humid than would be ideal for comfort and dusty mite control. I'm not arguing that that is a good idea--I'm only arguing that it happens.

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