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Community and Q&A

Mechanical Ventilation in a Tight, Permeable House

Pascalli2 | Posted in Mechanicals on

Ok, I am going to thoroughly expose my lack of knowledge with this question (no claims to any expertise on my part), but I’m hoping that some of the experts here can comment on something I am thinking about.

I’m contemplating theoretical house designs, and am currently thinking about a home that is fairly air-tight – say 1.5 or 2 ACH50, with thick (14″) in which all materials have a perm rating of 8 or higher (though airtight and weather sealed), the only non direct-vented appliance would be a gas range, which is vented by a range hood when in use, and all materials used inside are low or zero-VOC, interior walls are covered in lime or earth plaster, and the floor plan is quite open. In case it’s relevant, let’s say the wall is approximately R-30.

My question is whether ventilation beyond the code minimum would be necessary in such a configuration? The thing that gets me thinking about this is the purpose behind ventilation:

1) Removal of Indoor Pollutants: Ventilation is advocated for the removal of VOCs, Carbon Dioxide, and other harmful gasses. With relatively permeable walls, would some of these vapors be able to work their way out of the house through the walls? I realize that perms are a rating specific to water vapor, but is it not likely that limited amounts of other gasses, such as carbon dioxide and VOCs would follow the same principles? This would relieve some of the pressure on the ventilation system to remove those elements.

2) Control of humidity: Between the ability of the walls to allow water vapor to escape, and the natural ability of the plaster on the interior to absorb and release moisture to maintain a consistent level, would humidity still require additional ventilation to keep under control?

3) Control and filtration of incoming air: This is an important feature of a whole house ventilation system, HRV, or ERV. Being able to pre-condition the air coming into the house to avoid heat loss and filter out any contaminants from outside is a big reason to mechanically ventilate. I wonder, though, even in a house that has a full ventilation system, if some amount of untreated air still makes it in through cracks, etc, and how that volume compares to a house that is ventilated only to meet the code minimums?

4) “Fresh Air” supply: A key point of ventilation systems is to bring in “Fresh Air”, though I’m unclear what makes air fresh – is there some additional characteristic of fresh air other than having had the above points applied to it? Are we talking about bringing in additional oxygen molecules? If so, I’m not certain whether the setup I am describing would provide adequate fresh air – though I’m not adverse to opening a window occasionally.

Anyway – I would appreciate some thoughts or considerations that I might not have brought up. Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    I don't think your plan is a good one.

    If your house has a slab on grade, a crawl space, or a basement, you'll be introducing quite a bit of moisture into your house from the soil. That will occur because of your insistence to use only vapor-permeable materials.

    Almost all experts recommend installing a vapor barrier (usually 6 mil poly) under concrete slabs or on crawl space floors. In other words, vapor barriers are sometimes a very good thing.

    Lots of moisture can also enter a house through basement walls -- which is why a layer of rigid foam to insulate basement walls often makes sense.

    Among the reasons that building experts recommend the installation of a mechanical ventilation system is to manage odors (cooking odors, odors from dirty laundry, and human body odors). Moreover, it's hard to anticipate all of the airborne contaminants that might enter your home.

    In general, airborne contaminants don't magically diffuse through vapor-permeable walls.

    I recommend:

    1. That you abandon your plan to build a house without any vapor retarders.

    2. That you install a mechanical ventilation system. Once you move in to your new house, you can operate your mechanical ventilation system as you see fit -- occasionally, or 24 hours a day, or never.

  2. KSJeffery | | #2

    Stephen - I'm no expert either, but a habitual lurker and researcher as I'm building currently. What strikes my most is while you have a good list of reasons to ventilate, I'm not sure I understand your reasons behind high permability assemblies?

  3. Pascalli2 | | #3

    Thanks for the reply, Martin! You point out a big oversight in my description. I would, indeed, never consider a non-vapor sealed basement, especially as the area I am looking at is known to have radon. I should have specified "above-grade walls" would be vapor-permeable. Which also makes me think that regardless of the characteristics of the above grade walls, ventilation would be required to make sure that stale air and contaminants are pulled out of the basement, no?

    Ultimately, I think you are correct in that it is best to install adequate mechanical ventilation, and then possibly experiment with reduced levels afterwards, measuring humidity and VOCs, if curious.

    I do end up wondering about it, though. Especially since code requires a certain level of ventilation, combined with a properly sized range hood, it would be interesting to see if that level is enough to handle the removal of odors. If it were to be enough, the key would probably lie in strictly controlling all materials used in the home - cleaning products, furniture, finish materials, to avoid contaminants. Again, you're right that it's hard to anticipate what might unintentionally find it's way into your home, though.

  4. Pascalli2 | | #4

    Kent - that is a discussion for a different thread that has been had in several places throughout the internet and has the potential to hijack this discussion, so I would rather not go into detail. If you are interested, do a quick Google of Building Biology, and I'll leave it at that (especially since I am in the minority on this one).

  5. Airithol | | #5


    I realize you're interested in diffusion open assemblies, however, you also appear to be interested in IAQ. Here a few links you might find of interest related to your proposed gas range.

    As for your proposed assembly, I would suggest you set a higher air sealing target then 1.5-2 ach50. If nothing else make sure your air barrier is durable, and/or protected from misadventure or future renovations.


  6. Pascalli2 | | #6

    You're absolutely right about gas ranges - the study in those links really drives it home. I just can't give up my gas range, though. Instead I am just going to get the best range hood I can find - there are some pointers in the second article you linked to that are handy.

    My ACH target is actually much higher - I am trying for something below 1, but I am not going to be on site during the build to reinforce the importance of air sealing. Although I am hoping to find a good contractor who is on the same page as I am, I am mentally preparing to not get it as tight as I would like.

  7. Expert Member
    Dana Dorsett | | #7

    If your house was built with 8-perm walls in a cold climate you'd have ice crystals growing in the stackup during some seasons unless you kept the interior air at less-comfortable-less healthy very low moisture levels. If it is in a hot humid climate you'd potentially have mold growing on the cool interior walls, excessive latent cooling loads to dry out in summer.

    That said, it's almost impossible to build walls that vapor open (maybe you can with adobe & haybale, but even that might be tighter than 8 perms.). Concrete runs about 3 perms at 1", so a 6" wall would be about 0.5 perms. Fiber-faced gypsum sheathing, no paint runs about 4perms, OSB & plywood sheathing about 1-2 perms dry, a bit more when wet, 1x plank sheathing runs about 1 perm (wet or dry). To build a wall stackup with a total vapor permeance higher than 2 would take some VERY careful materials selection- you might get to as high 4-5 perms with low density autoclaved aerated concrete (AAC) block construction, but I'm not sure you can get all the way to 8 with any type of interior or exterior finish (even clay plasters). AAC manufacturers recommend adjusting the permeance of interior & exterior finishes on AAC to avoid frost damage from interior moisture drives in cold climates, and to limit the latent cooling loads and mold hazards in humid climates, but the material itself is fairly high-perm.

  8. STEPHEN SHEEHY | | #8

    Stephen-If you are set on a gas cooktop, because electric ones stink, I suggest you invest in a cheap single burner induction unit and give induction a try. My wife and I are avid cooks and thought we'd never give up gas, but, like you, we're in the design stage of a new house and keeping gas out of the space seems like a good idea. We bought a single burner induction from Amazon (about $70)
    to see whether we like it. We were very happily surprised to find it has the quick response of gas (in fact, much quicker) and is much faster to heat up and cool down. We're sold on induction for a cooktop and an electric oven. (Pretty much everyone hates gas ovens.) Every cook we know who used a gas cooktop but who changed to induction is glad they changed. Of course, you'll still probably want a hood to expel smells and smoke.

  9. Pascalli2 | | #9

    Mr. Sheehy - Induction does keep coming up, and I have been very hesitant to give it a shot. It does seem, though, that if I am serious about air quality, I should do as you advise and pick up an affordable countertop unit to experiment with. That's a great idea, and I hadn't thought about it. I think the main reasons I am hesitant with induction are that my mother-in-law had a bad experience with induction - she said they weren't able to achieve a simmer, everything just boiled, and that I already have copper pots and pans that I love. Attachment to my existing pans is probably not a good reason to stick with gas, though it would be a real shame to have to give them up.

    I do wonder a bit about the EMFs generated by induction cooktops, though, as there is some controversy regarding health risks there. Given the known issues with gas, it is probably a wash on that, though.

    Thanks for the suggestion!

  10. Pascalli2 | | #10

    Dana - you make some interesting observations. I'm afraid I don't have as strong of a grasp of all of the dynamics at play within a wall as I should. In order to really reach any verdict or conclusion, of course, you would need more details. I think you are right that the overall wall assembly would not reach 8 perms. I am thinking in hypothetical for that.

    I do have a question, though - If the wall has thick insulation, wouldn't warm moisture from the inside that is moving outwards condense before it reaches the freezing outside air, rather than make it all the way out and turn to frost?


  11. GBA Editor
    Martin Holladay | | #11

    Q. "Wouldn't warm moisture from the inside that is moving outwards condense before it reaches the freezing outside air, rather than make it all the way out and turn to frost?"

    A. To answer your question, there are two moisture transport mechanisms that need to be considered: air leakage and diffusion.

    The best way to address problems arising from air leakage is to pay attention to air sealing when building your wall assembly, and to verify that you've done a good job with a blower door.

    Diffusion rarely causes problems in walls, except in rare cases like indoor swimming pools and greenhouses, where indoor humidity levels are very high.

    In any case, each wall assembly has to be considered on its merits. In general, you don't want your wall assembly to have any cold surfaces where moisture can condense. One common way to make sure that your wall assembly is robust is to install an adequate layer of rigid foam on the exterior side of your wall sheathing; this foam keeps your wall sheathing warm enough during the winter to avoid any problems with condensation or moisture accumulation.

  12. Expert Member
    Dana Dorsett | | #12

    Except in the case of very severe air leakage, with typical construction using fiber insulation the entrained air in the fiber insulation stays at or below the dew point of the coldest surface of the cavity, which prevents condensation from occurring in the fiber insulation itself.

    This is because fiber insulation is EXTREMELY vapor permeable- as moisture accumulates on (or is adsorbed into) the cold surface side of the cavity, it the total humidity of the entrained air to the level where dew point of the mass of air in the cavity has a dew point of the coldest surface. Introducing a small amount of air leakage into the cavity doesn't change that much, but if it's very leaky it can.

    But in the case I was discussing (AAC, not fiber) it's high vapor diffusion, not air leakage that can sometimes cause damage. The issue isn't that it condenses (it's liquid moisture tolerant), the issue is that once it condenses it needs to re-evaporate in order to leave. AAC is for all intents & purposes air-impermeable, but it's highly vapor permeable- sufficiently vapor permeable that in a high-humidity house in a cold/very-cold climate you can end up with ice crystals forming inside the block unless you apply finishes with a modest amount of vapor retardency. AAC has much higher porosity than standard concrete- it can accumulate quite a bit more moisture, but it doesn't redistribute and wick that moisture as well as standard concrete. That's just fine until it gets loaded up with moisture and you get a freeze event at a colder exterior layer. If there is enough moisture to create ice crystals inside the material, the high mechanical stress of the ice crystals starts breaking it down, reducing it's overall strength.

    The only way for sufficient moisture to accumulate in AAC to cause that type of damage is when the exterior finish is more vapor tight than the interior finish, or if the interior finish is so vapor open and interior humidity is so high that the drying toward the exterior is at a rate too slow to keep up. It's easy to control with standard paint finishes, but it may not be possible if using non-cementicious clay plasters for an interior finish.

    This has been studied to death in Sweden & Germany, where AAC is a fairly popular building material, and though these types problems are fairly rare, it's an expensive proposition when they occur. It's enough of an issue that the manufacturers give it a couple lines of lip service in their assembly manuals, and low-permeance or vapor impermeable exterior finishes are expressly disallowed (some EIFS type stucco would potentially be a problem, others types not.)

  13. GBA Editor
    Martin Holladay | | #13

    The interior of British Columbia is probably in Climate Zone 6. (Click here to see a Canadian climate zone map.)

    According to the 2009 International Residential Code, the minimum prescriptive R-value for walls in Climate Zone 6 is R-20. For a "mass wall," which (arguably) applies to a Durisol wall, you need at least R-15 (or in some cases R-19).

    Durisol walls are notorious for their low R-value. It's hard to pin down what the true R-value of these walls is. But if your estimate of R-7 is correct, you are choosing a low-R-value wall for your climate.

  14. Pascalli2 | | #14

    Hi Guys,

    Sorry for the late response - thanks again for the feedback. So, I'm thinking about my wall assembly, and moisture movement through it. Looking at materials, I am thinking about Durisol ICF for the wall structure, with direct applied plaster on the interior and a permeable WRB and 3/4" gap on the exterior before attaching fiber cement siding.

    It seems to me that this would provide adequate ability for the wall to dry to whichever side it needs to, with the concrete fill in the middle being the least permeable component of the wall (though there are paths through the concrete via Durisol connector webs). A couple of questions based on your comments above:

    - The entire exterior of the concrete structure would be covered by 2" of Durisol material, resulting in about R3.5 continuous envelope. Not as much as a lot of rigid foam products one might put on the outside, but is it enough to significantly reduce the risk of condensation within the wall assembly? I'm not sure what zone we are in (BC Interior), but winters can get as low as -20 celsius, though not for extended periods of time.

    - Given that none of the materials involved (concrete, Roxul, Durisol) are prone to moisture damage, and all have natural resistance to molds, and the assembly should allow for fairly quick drying, is moisture in the wall assembly really that big of a concern in this case?

    Thanks for the info - sorry if I seem a bit dense as I try to interpret it, but I think I am slowly understanding more and more (hopefully).

  15. Pascalli2 | | #15

    Hi Martin - my mistake - I meant to say R3.5 (just edited my comment), and I am referring to the 2" of Durisol material that is on the exterior and interior surface before any Roxul or concrete comes into consideration. The forms I am actually looking at are R-28, with (exterior) 2" Durisol material, about 6.9" Roxul, 5.5" concrete, then another 2" of Durisol (interior). In my comment above I was thinking that there would be only 2" of Durisol between the cold exterior and the concrete, however, now that I am actually breaking it down in my head, I realize that the Roxul comes first, so we're really looking at closer to R-25/26 before it hits the concrete. That should be enough insulation to avoid condensation on the wall, right?

  16. GBA Editor
    Martin Holladay | | #16

    Calculating the whole-wall R-value of a Durisol wall is complicated. For more information, see two previous Q&A threads on the topic:

    In the first thread, I wrote:

    "As far as I know, the Oak Ridge National Lab has not tested any Durisol walls for whole-wall R-value. The manufacturer has used a computer modeling program to estimate the clear-wall R-value of a Durisol Wall. See: .

    "The R-value of a Durisol wall will depend on whether insulation inserts are used before the concrete is poured. The Durisol WF30 wall form has a clear-wall R-value of R-9.1. If mineral wool inserts are included, the R-value increases to R-13.8 (with a 1.5-inch insulation insert) up to a maximum of R-20.6 (with a 3 1/2-in. insulation insert). As far as I can determine, however, these estimated R-values have not been verified by an independent third-party laboratory."

    In the second thread, I wrote:

    "I don't know whether Durisol expects the insulation insert to face the interior or the exterior; but for the purposes of thermal analysis, it doesn't matter much. Let's assume the insulation inserts face the interior.

    "There is 1.75 inch of material (wood chips plus additives) between the concrete and the exterior. (I'm ignoring the stucco). That material has an R-value of about R-3. That's not much. That's all there is separating the concrete from the outside world. When the outdoor temperature is cold, the concrete will be fairly cold too -- definitely colder than the indoor temperature (although not, of course, as cold as the exterior air).

    "The interior heat will leak through the Durisol block and be absorbed by the cold concrete. The maximum heat flow will occur at the inside corners of the cold concrete. At those points, there is only 4.75 of Durisol material between the cold concrete and the interior. That's something -- and it may be enough to avoid cold stripes on your wall. But analyzing these two-dimensional heat flows is tricky. You need a good software program like THERM to see the isotherms created by this type of heat flow.

    "Once you introduce three dimensions into the equation, with details at the bottom of the wall and the top of the wall, the thermal analysis becomes still more complicated.

    "Most building scientists find the the best way to determine the thermal performance of this type of wall is to measure heat flow through a completed wall assembly measuring 8 feet by 8 feet or 8 feet by 12 feet. Oak Ridge National Laboratory has a calibrated hot box that is large enough for this type of measurement, but as far as I know, no one has ever performed this testing on a Durisol wall."

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