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Community and Q&A

Best way to ventilate a garage?

switters | Posted in General Questions on

I am converting my 2-car garage (432 sq. feet) into a combo exercise space / 1-car garage. I recently discovered that it had a mold issue, and had it remediated. The Indoor Environmental Professional (IEP) that I worked with suggested installing an ERV in the garage to ventilate it and also put it under supply pressure, which he said would make the mold issue less likely to recur. (If the ERV doesn’t keep the relative humidity low enough, he also suggested a portable dehumidifier.)

After doing some research I found the Panasonic WhisperComfort Spot ERV, and it looks great except that it’s a balanced ventilator—I don’t see any way to change the pressure relationship between inside and outside.

Are there any units that are similar to the Panasonic but allow for control of the pressure gradient?


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  1. GBA Editor
    Martin Holladay | | #1

    I disagree with your consultant.

    First of all, no matter what type of ventilation equipment you choose, air in = air out. If you ventilate at 30 cfm, that means that 30 cfm of outdoor air is entering your garage, and 30 cfm of indoor air is leaving your garage. Slightly pressurizing or depressurizing your garage will not change this fact.

    If you want to lower the humidity in the garage, ventilating during cold weather is good. When the outdoor air is cold, ventilating tends to lower the indoor relative humidity (RH).

    In most of the country, however, ventilating during the summer tends to raise the indoor relative humidity. So during the summer, you want to minimize ventilation if lowering the indoor RH is your goal.

  2. switters | | #2

    Thanks for your reply. Maybe an ERV is unnecessary then, and I'd be better off with a dehumidifier that I run selectively?

    My goals are to 1) reduce likelihood of mold growth, and 2) maximize air quality. I am considering an IQAir HealthProPlus (highly rated air filter) to filter the air in the garage. Along with a dehumidifier, would that be enough? There would not be any fresh air exchange happening in this case, because the window in the garage doesn't open (it was sealed by previous owners to prevent leakage).

    I should have mentioned this originally, but I am in Berkeley, CA.

  3. GBA Editor
    Martin Holladay | | #3

    Some type of ventilation is probably a good idea (because you store a car with an internal combustion engine in the same space where you will be exercising). The usual way of ventilating a garage is with an exhaust fan (like a bathroom exhaust fan). You could hook the fan up to a toggle switch or a hand-crank timer switch, and just turn the fan on when you are excercising.

    If I were you, I would install an exhaust fan and experiment with it. If it solves your mold problem, you're done. If your garage remains damp, you could always install a dehumidifier later.

  4. charlie_sullivan | | #4

    I'll add to Martin's recommendation a recommendation to put in a humidity meter, so you can find out how high the humidity is before you have a problem. Perhaps also outdoors if you want to be able to decide to run the fan when you have low outdoor humidity.

    I would also consider putting an airtight wall between the 1-car garage and the exercise room, in order to keep fumes from the car out of the exercise room. Cars release various fumes even when they are off.

    If you add a dehumidifer, it will also warm the space, which might or might not be a good thing for an exercise room.

  5. switters | | #5

    Thanks Martin and Charlie.

    Unfortunately it won't be possible to install a wall between the garage and exercise space. The garage is detached and the rear entry door is in the middle of the back wall. A partition wall would run right into that door.

    I was hoping that the ERV or ceiling fan, plus the IQAir unit (high quality HEPA filter that also effectively removes VOCs and particles down to 0.03 microns) would be enough.

    I will definitely look into a humidity meter.

  6. GBA Editor
    Martin Holladay | | #6

    A ceiling fan won't change the indoor air quality.

    The only reason that an ERV would be preferable to an HRV would be if the garage were air conditioned. Without air conditioning, an HRV makes more sense than an ERV -- and the HRV will do a better job of lowering the indoor RH during the winter than an ERV.

    That said, exhaust fans are cheap, so I would still start with an exhaust fan.

  7. switters | | #7


    I'm planning on installing a Friedrich 12,000 BTU ductless AC/heat unit in the garage. However, because the climate in the SF Bay Area is relatively mild, I would only likely be using the heat 3-4 months a year and the AC even less frequently.

    Sounds like you're suggesting starting with the exhaust fan and air filter, and then if the RH is problematic according to the humidity sensor I can add an HRV or dehumidifier? Any advantage to one over the other?

  8. Irishjake | | #8


    Check out the thru the wall ERV - TwinFresh Comfo RA1-50-2. They are relatively inexpensive, and can operate in singles or pairs. (When installed as a single unit, it brings air in and out, when installed in pairs one brings fresh air in, and the other exhausts).

    I'll be using one of these in my garage and two in the apartment above, that I'm currently building. Maybe others have input on these too?

    Good luck.

  9. switters | | #9


    Thanks a lot—that looks like it might be a great option for me. I noticed it says that one unit serves up to 350 sq. ft. Our garage is 432 sq. ft. Would that suggest that I need a pair?

    I like that these don't require duct work.

  10. GBA Editor
    Martin Holladay | | #10

    An HRV won't do anything that an exhaust fan won't do (other than save you some money on days when you are operating your space heating or cooling equipment). Either method of ventilation will lower the indoor humidity when it's cold and dry outside, or make things worse when it's warm and humid outside.

    If you have a stubborn humidity problem in your garage (due, for example, to a high water table and a garage slab without a vapor barrier under it), a dehumidifier will be more effective than an HRV. But it will cost a lot more to operate.

  11. nvman | | #11

    If you are going to get an IQ Air, get the Multigas as it will filter VOCs and odour.

  12. charlie_sullivan | | #12

    I just took a look at some weather history for Berkeley. The dew point outdoors is often in the 45 to 55 F range, with occasional days or weeks when it's lower (down to ~20 F), or higher (up to ~60 F). If the space is unheated, and the temperature hovers in the low 50's, that means that outdoor air ventilation, whether it's with an HRV or an exhaust fan, would result in humidity in the 80s or higher, on the typical days.

    Three possible solutions would be:
    1) Heat the space to at least 60 F or so, even when you aren't using it. Then with the same dew point, the humidity would be lower. This could be combined with HRV, but just the natural air leakage might suffice.
    2) Run a dehumidifier inside. That would provide some heating and some moisture removal. Perhaps combine this with an ERV, like the panasonic, which would provide fresh air while preserving the heat and humidity removal you paid for. This would allow lowering the humidity without raising the temperature as much, and I think it would thus use less energy than heating the space.
    3) Monitor the outdoor dew point, and run and exhaust fan or HRV on the days when the dew point is low.

    I would be inclined to start with 3), using an exhaust fan; then implement 2) if necessary; and only add a heating system after trying the other two--put the money you would otherwise put in a heating system into a really efficient dehumidifier, which could provide a modest level of heating with better efficiency than an electric heater, while also providing dehumidification.

    But some of this depends on what kind of exercise you plan--a cool dry room would be ideal for high-intensity vigorous exercise, whereas for low-intensity yoga, stretching, etc., you might prefer a warm space.

  13. switters | | #13

    Thanks Charlie, that makes a lot of sense.

    After more thought I've decided to abandon the idea of exercising in the garage. With one car in there and the cabinets we're planning to install, space will be very limited.

    My sole priority now is controlling RH and preventing mold, since my daughter is very sensitive to it. I'm assuming your recommendations above would still apply?

    The garage is in a completely unfinished state now (down to the studs), so I have the opportunity to build it out with these priorities in mind.

    P.S. Any recommendations for a dehumidifier or HRV?

  14. charlie_sullivan | | #14

    If you look at the energy star web site list of dehumidifiers, there are lots and lots that are designed to just meet the minimum efficiency to get the energy star rating. And then there are very few that exceed that. But some of the ones that exceed that level exceed it by a lot. The ones that exceed it are almost all made by Thermastor; their free-standing models are branded "Santa Fe". Sadly, they are probably all bigger than you need, and the most efficient ones are bigger still. So I don't quite know how to choose--it's hard to calculate payback for a more expensive more efficient model without knowing how much it is going to need to run. If you value efficiency regardless of payback, you could simply go ahead with one of the most efficient models (impact or advance), or you could find the cheapest energy-star rated one you can, and keep track of energy use and/or the amount of water collected to estimate energy use, in order to decide whether it's worth upgrading.

    If you get a cheap energy star model, make sure that it has a humidistat, and that it doesn't run the fan when the humidistat is satisfied. The fans are pretty inefficient and it is a shame to run the fan when it's not really needed anyway.

    If you are running a dehumidifier, I'd get the Panasonic ERV; if you are not running a dehumidifer, and not heating, I'd recommend a Panasonic exhaust fan, not ERV or HRV, specifically their "Whispergreen" models that have awesomely low electric consumption.

  15. switters | | #15

    Thanks again. We are getting a Santa Fe for our crawl space (after having it sealed with a vapor barrier), so I'm familiar with those units. The problem is where to put it? Can you just set it on the ground? It doesn't seem like it, since it says "Sized to fit between floor joists for convenient hanging installation."

  16. Expert Member
    Dana Dorsett | | #16

    Something like a Santa Fe dehumidifier would be extreme overkill for a sealed crawlspace w/vapor barriers in a climate like the San Francisco Bay area. What would the moisture source be in your garage?

    Ventilation alone would be sufficient to mitigate mold growth in most CA garages.

    Using an HRV for that ventilation function would only be important when you're heating or cooling it.

    Using an ERV rather than an HRV would only be useful if...

    A: The outdoor dew point is north of 60F (which almost never happens) ...

    ... and ...

    B: You are actively dehumidifying the garage to some dew point well below the outdoor dew point (which is never necessary in your climate.)

    So, skip the dehumidifier, skip the ERV. You could operate an exhaust-only or supply-only ventilation fan under dehumidistat control, or duty-cycle it and it will probably do just fine. The expense of an HRV would be a bit silly for the miniscule amount of heat recovered. You would be better off spending the HRV money on upgrading to a more efficient mini-split than the Friedrich.

    It's also important to size the mini-split appropriately for the loads and let it modulate so that it can achieve some efficiency. The heating and cooling loads of an insulated garage (or even an uninsulated garage) are well below capacity of any 1-ton minisplit, unless it has some big west facing picture window driving the cooling load through the stratosphere. Without running the heating & cooling load calculations I suspect you'd be better off with a Mitsubishi FH09NA 3/4 tonner, since it's output can modulate down to ~1600-1700 BTU/hr under light load rather than cycling on/off.

    And, if the garage is not insulated, spending the would-be HRV money on insulation would be even more valuable than spending it on a higher efficiency ductless.

  17. switters | | #17

    Thanks again to everyone. I'm completely new to this stuff (first time homeowner), so doing my best to keep up and sort through the varying opinions.

    Although I mentioned that I was planning to use the garage as an exercise space in the first message, I've mostly decided against that at this point.

    I'm not aware of any moisture source in the garage, I just want to control RH and prevent mold issues. I'm (admittedly) paranoid about this because the whole reason we bought this new house is that we're living in a moldy rental home now and my daughter is very sick from it. We are going overboard to make sure that our home—and even garage—are as protected from mold growth as possible.

    With this in mind, and given that I (probably) won't be using it as an exercise space, I'm leaning toward the following:
    - insulate walls and ceiling and use fiberglass-backed drywall to finish
    - do not install heating or cooling
    - get a humidity meter and monitor RH. If it is higher than desired, figure out what to do then.

    What do you think?

  18. charlie_sullivan | | #18

    That makes sense to me. I would be somewhat inclined to go ahead and install a panasonic whispergreen exhaust fan while you are doing the drywall, because it's a little easier to install at this point than it would be later, and if you are looking at a humidity reading, you might as well compare it to an outdoor humidity reading, and turn on the fan when outdoor humidity is much lower.

    I suspect that Dana missed the fact that you are planning to drop the heating from the plan, based on his comment that 60 F dew point is fine.

  19. switters | | #19

    In our case we can easily install a ceiling-mounted exhaust fan or ERV anytime, since there won't be drywall on the ceiling. There are horizontal joists going straight across and then there is a sloped roof. The horizontal joists can't support drywall, so we are going to put insulation between the studs in the sloped roof and put some kind of covering over that. We can install a fan/ERV on one of the horizontal joists anytime.

    Here's a picture that shows what I mean:

    Ultimately I'd love a solution that requires less direct intervention. I don't mind doing measurements and manually turning on the exhaust fan for a while, but is there some way to set something up that is more automated for the long term?

  20. charlie_sullivan | | #20

    Unfortunately, I don't know of a commercial product that would run a fan only when the differential humidity or dew point means it would be beneficial for drying out the space. There are plenty of commercial products that activate an exhaust fan when the interior humidity gets too high. But in your scenario, some of those activations would happen when it's even more humid outside, and they would also miss the opportunities for drying the space out beyond the activation threshold during dry periods. So I would suggest two things there:

    1) Just use it as a learning experience. Maybe you only need to run the fan on a few times a month, and that level of manual intervention is OK. Or maybe you learn that it's harder to keep it dry than expected, and you decide it's worth buying a dehumidifier.
    2) Take up Arduino programming as a hobby and program one to turn on the fan when it's dryer outside than in, or visit one of the many hackerspaces in your area and find a youngster who would be thrilled to get paid to do it for you. Maybe you can even get a few million in venture capital funding for the project if you come up with a cute name for it.

    And another unfortunate fact: insulating under a roof deck can be problematic from a moisture point of view, because the roof deck can't easily dry to the outside through the roofing layers. See In your mild climate, it's not nearly as big a deal as it is in severe climates, especially without heating the space, so you could probably get away with just mineral wool insulation and drywall underneath, if the drywall is air sealed well. To be safer, you could install a "smart" membrane between the insulation and the drywall, such as MemBrain or Intello. See the discussion here, for example:

  21. switters | | #21

    Man, what I thought would be relatively straightforward is not turning out that way! I appreciate the heads up on the ceiling; I think you've saved me some potential trouble down the line there.

    The contractor who gave me an estimate for finishing the garage suggested using fiberglass insulation under the roof deck, but putting something else over it other than drywall. I forgot the name of it (will find out today), but he said it was lighter/thinner than drywall. I think he was concerned about whether the roof deck would adequately support the weight of the drywall.

    I will contact him and see if he can install the MemBrain between the insulation and whatever material is being used in lieu of drywall.

    Thanks again, this is very helpful.

  22. Expert Member
    Dana Dorsett | | #22

    If unheated, duty cycle the ventilation, don't sweat the dew point issues.

    The indoor temperature will always be warmer than the outdoor dew point in that climate (with the possible exception of the three highest dew point hours every century :-) ), or at least never long enough to create a mold risk by ventilating with outdoor air.

    Seriously- the CA coast is close to being a desert climate, despite the morning fog issues right at the waterfront, and you don't have to get very sophisticated with the ventilation controls to keep moisture problems under control. This is one of the EASIEST climates to deal with from that point of view.

  23. switters | | #23

    What does "duty cycle the ventilation" mean?

  24. GBA Editor
    Martin Holladay | | #24

    I'm not sure, but here is my best guess. It means: "Turn the fan on and off periodically, so that it is sometimes on duty, and sometimes off duty."

    You could do this manually (with a toggle switch), or automatically (with a timer that allows you to program the operation of the fan -- for example, with 20 minutes of operation per hour).


  25. charlie_sullivan | | #25

    I'm not sure you need smart ventilation control or a dehumidifier, but I can't agree with Dana's assessment that there's no risk. In particular, I think he might be mistaken about the climate. Berkeley can be quite damp. Right now the dew point at the Oakland Airport shows 55 F, with the forecast predicting a drop in outdoor temperature to 42 degrees overnight and through the day tomorrow. That's the kind of scenario that can lead to very high indoor humidity, and even condensation. That kind of humidity in the winter is not rare, although perhaps it's rarer than it used to be, with the recent drought.

    Insulation will help--if you have 55 dew point air inside, and the temperature stays above 50, you'll have less condensation than you would have if it dropped to 45 inside. And mold-resistant materials will help. So you might be OK without actively trying to manage the humidity.

    Berkeley does have some microclimates that might matter. For example, a friend lived in a house in the hills across from the Golden Gate, directly aligned with the plume of fog that regularly comes in the Golden Gate and across the bay, such that that house would be immersed in fog while the rest of the city was sunny.

    The fog plume:

  26. switters | | #26

    Yes, I wondered about that too. Our current home is 10 doors down from the home we just bought, where the garage we're discussing is. I've measured the RH in this current rental and at times it was in the high 60s. We're up in the N. Berkeley Hills so perhaps it's more damp than usual. I definitely do think we need to be aware of RH and take steps to control it.

  27. Expert Member
    Dana Dorsett | | #27

    A duty-cycle controller is a timer that allows you to adjust the fraction of the "on" time to "off" time, usually called a "duty cycle control" or "duty cycle timer".

    This is a common method of adjusting ventilation rates with fixed-speed blowers, and is even spelled out as an acceptable type of ventilation control for whole house ventilation under CA CEC Title 24:

    "If intermittent fans are used, they must be controlled by a timer, and they must have an increased airflow rate to compensate for the off time. Time-of-day timers or duty cycle timers can be used to provide intermittent whole building ventilation."

    With a cheap humidity monitor and cheap duty cycle control (no dehumidistat), if the RH starts creeping north of 65% you can tweak the duty cycle up for more ventilation as needed.

  28. Expert Member
    Dana Dorsett | | #28

    Charlie, when it's 42F outside the dew point is less than 42F. It's literally never going to be below 55F inside when the outdoor dew point is 55F.

    To have condensation indoors requires that the indoor temp be below the dew point of the ventilation air. It's unlikely that an insulated garage would drop below 60F unless there is VERY high ventilation rates (like leaving the door open), but the very high ventilation rates alone would prevent condensation. The only time the garage would drop below that temp is during the seasonal low temps, but that's when the ventilation air would have an even lower dew point.

    A high relative humidity when it's 50F inside is not a mold problem, but a high relative humidity when it's north of 65F indoors is. But ventilating with 55F dew point air into a 65F garage results in an interior RH of 70%, which is the very beginning of mold hazard threshold. That's your worst-case scenario, and it never persists long enough to be a problem. Most days when the outdoor dew point is 55F the outdoor air temp is not 55F (fog), and at 55F outdoor temps most unheated garages would be coasting along north of 65F, which would lower the interior RH.

    Measuring the indoor humidity inside an occupied apartment of unknown ventilation rates doesn't tell you much. Most people aren't cooking or bathing in the garage nearly as much as in a rental apartment. But even in the latter, in the Bay Area simply increasing the ventilation rate will manage the humidity quite well. At 70F a dew point of 55F corresponds to an RH of 59%, which is below the mold threshold.

    About the only temperate west coast region where ventilation alone won't reliably manage mold hazards in unheated buildings is in the foggy-dew western slopes of the Olympic Mountains in WA, a true temperate zone rain forest. (There may be pockets on the Oregon coast where it's iffy too, but they would be VERY local microclimates.) In these areas the outdoor dew points track the outdoor temperature very closely, and it's always hazy/misty saturated air in the shadows, often raining lightly under the forest canopy even when there's sun breaking out above. That's not a Berkeley type climate, or even a San Fransisco dock front climate. It's not humid enough in Berkeley to even grow good moss! Pull up both a dew point graph and a temperature graph and look at the seasonal avearges:!dashboard;q=berkeley%2C%20CA%2C%20USA

    The dew point averages are about 8F below the outdoor temperature averages, year round, and the summertime peak dew points are below 70F. that's a semi-arid climate.

  29. charlie_sullivan | | #29

    Dana, I must not have explained my scenario for how those weather conditions can cause high humidity well enough. I agree that what you say won't happen won't happen, but it's not what I was describing. I was imagining bringing in 55 F dew point air while it's 58 F outside and then, after the fan cycles off and the outdoor temperature drops to 40 to 45 F, the inside of the building cools off to 50 F inside (if it's uninsulated). If that happens, a bunch of the moisture in that air has to drop out.

    I agree that insulation reduces that hazard, as I said before.

    I also agree that low temperatures reduce mold growth, but it doesn't need to be above 65 for mold to grow. Many basements offer proof to the contrary, as do studies which show mold grows about as fast at 5 C, 85% humidity as it does at 20 C 80% humidity. So my scenario of trapping higher-dew-point air inside the building as it cools isn't necessary to have mold-growth conditions.

    Moss can certainly grow in Berkeley, although it's not as thick or as common as pacific northwest moss.

    A low summer dewpoint doesn't equate to semi-arid, if the summer temperature is also low, which it is. The average humidity in berkeley is 79% in the summer and winter, and 73% in the spring and fall. The low humidity seasons are higher average humidity than the highest humidity season in Boston (72% in August) or Atlanta (Also 72% mid-summer). It's true that a heated space doesn't need dehumidification in Berkeley, because the outdoor air dew point is low enough. But a dew point around 50 with temperatures barely above that doesn't eliminate mold growth potential.

    I do agree that it's not a mold disaster--it might be fine with the insulation, ventilation, and materials that don't support mold growth.

  30. charlie_sullivan | | #30

    Cycling fans is kind of obsolete--if you get the panasonic whispergreen with selectable speed, and you select 50 CFM, it only draws 3.5 W. A lot of timers would draw that much power just to run the timer.

  31. Expert Member
    Dana Dorsett | | #31

    When the temps drop to 42F, so will the dew point of the ventilation air along with it. There's no way an insulated garage that started at 58F+ temperatures will drop to 50F on a night that drops to 42F, unless it has high infiltration rates or the ventilation is (over) active, which of-itself fixes the condensation risk problem. Deep subsoil temps in that area are well north of 50F, there would even be ground heat supporting the temperature to above 50F, (beyond just the thermal mass of the slab and the rest.)

    It's not about the 3.5 watts- 50cfm of active ventilation is more than you need or want as a background ventilation, since it would also affect the temperature of the garage. You'd probably want that much when actively using the place, but not as background. Without an active humidity source 50 cfm would be an order of magnitude overkill as background ventilation for humidity control in an unoccupied 432 square foot garage. Duty-cycling based on the humidity levels gives you more control over the wintertime-cooling & summertime heating effects of the ventilation, while still dealing with the humidity.

    The RH% of the summertime outdoor air is a useless number, since it's relative to the outdoor temperature, not the indoor temperature. Web bulb temp and dew point temp are the measures of absolute humidity, and very relevant. Boston's 72% summertime RH is a lot sticker than Berkeley's 79% due to the higher dew point temperatures, and the algae & mold growth shows it. The average summertime temps in Berkeley are much lower than Boston. The average mid-summer LOW temp is about 67F in Boston, which is approximately the mid-summer AVERAGE daily temp in Berkeley. The average mid-summer outdoor dew point in Boston is ~63F, to Berkeley's ~55F. According to ACCA datasets Boston has a real latent cooling load (26 grains @ 50% RH), whereas Berkeley has a negative latent cooling load (-6 grains @ 50%RH). The notion that Boston is somehow drier than Berkeley in summer based on the outdoor relative humidity numbers is simply not the case, which is why relative humidity on it's own isn't a good indicator.

  32. charlie_sullivan | | #32

    Dana, your first paragraph explains why something can't happen, implying that said it would happen. But the scenario you describe is different from what I described. In your scenario there is no condensation; in mine there is. Bottom line is that insulation helps. And timing of ventilation matters.

    You are right that 50 cfm is overkill. I am not sure that there is value in avoiding heat loss/ gain in a space that is not heated or cooled. A smaller fan would be a good choice, but that would likely require re-puposing a computer fan or the like, which would work fine and could be ecnoomical to buy and run, but is outside the scope of a typical contractor.

    If you want to know what equilibrium moisture content to expect and in wood in a ventilated apce that is not heated or cooled, relative humidity is a much more useful number than dew point. The cooler temperature will slow mold growth, but counting on the lower temperature to prevent mold growth isn't reliable.

    Chris, sorry we are clogging your inbox with this tangential debate. Putting in insulation is something we both like, and then if you monitor humidity, you can see what it really is and whether it is worth worrying about further.

  33. Expert Member
    Dana Dorsett | | #33

    50 cfm is a complete air exchange every 90-100 minutes in a cathedralized ceiling garage that size. At that rate of exchange there's almost no weather transition that could happen fast enough even at the extremes that would cause a (very temporary) condensation condition as a front moved through. (I've very occasionally seen crazy shifts between cool continental flows and humid southerly flows fast enough to create condensation on the exterior windows in New England, but I've literally never experienced that any where on the left coast, where I've spent a substantial fraction of my life.)

    A 5 cfm background rate is enough of a background ventilation rate to prevent mold, but you may want more than that when the place is occupied. Duty-cycling a higher rate fan when you're not there, but increasing the duty-cycle to 20 cfm or more when you're going to be busy out there for a few hours. Even at 5cfm it's still a complete air exchange over night, keeping up with any potential for the temp of the room dropping below the dew point of the previous afternoon's ventilation air, even if it somehow managed to dropped below the 99% outside design temp overnight after a day when it was in 58-60F with a dew point of 55F. (Does that happen even once per decade?) Even if that were to happen, to become a mold problem it has to be persistent & chronic type occurrence, not some random 1% event.

  34. switters | | #34

    No problem. I'm in medicine and in my field there is plenty of disagreement on just about every topic!

    I know the original topic was ventilation in the garage, but Charlie you mentioned Membrain between the insulation and drywall. Not sure if I mentioned this, but the original moisture barrier between the outside siding and studs had to be ripped out because it was moldy. That's what started this whole thing.

    I'm now working with the restoration arm of the remediation company I used. I want to make sure they are following best practice in terms of mold/moisture prevention with the rebuild. They said the ideal thing to do would be to remove the siding, put the moisture barrier there, and then put the siding back on, but that's cost-prohibitive for us right now.

    Would installing Membrain between the insulation and drywall be enough, or is there something else that could be done as well? Thanks again.

  35. charlie_sullivan | | #35

    I usually agree heartily with everthing Dana says here, and I really appreciate how much he contributes. And I agree 100% with his latest comment.

    Chris, what kind of siding and barrier? If it was a simple polyethylene sheet, that might have helped cause the problem. Housewrap materials like tyvec and typar are water resistive barriers, but can pass water vapor to facilitate drying. I also wonder if there might have been some kind of flashing problem that got water in there.

    Best practice is to have a gap between the siding and the housewrap to facilitate drying of the siding. That configuration is called a rainscreen. Adding that from the inside is not standard, but could be done, perhaps with a product like the below.

  36. switters | | #36


    The siding is wood and I'm not sure what the barrier was; it was pretty old and the contractor referred to it as a "paper barrier". Not sure about the flashing.

    How would the rainscreen be added from the inside? Just cut and applied between the studs?

  37. GBA Editor
    Martin Holladay | | #37

    If the WRB was removed from the interior, you need to think about how to proceed. You certainly need an air gap between the back of the siding and the insulation between the studs.

    There are two methods I've heard of:

    1. You install vertical 1" x 1" sticks in the corners of the stud bays, tight to the siding, followed by rectangles of rigid foam, caulk in place. The rigid foam is an air barrier and (sort of) WRB. The sticks maintain the necessary air space. The rest of the stud bay is filled with fluffy insulation.

    2. You install rectangles of asphalt felt in each stud bay. The asphalt felt is installed in such a way as to create the necessary air space. The felt is cut wider than the stud bay, so that you can create and fold tabs on the sides of the felt; these tabs are stapled to the sides of the studs. Here is a link to an article that describes this method -- the article also has a useful photo: Sticking With Spray Foam for My Renovation.

  38. switters | | #38

    Thank you, Martin. I'm glad I asked as I don't think the contractor was planning to do this.

    Would you recommend the same approach for the roof deck? This is what it looks like:

    We'll be putting insulation up there, and then some kind of paneling over it. The contractor who gave me an estimate said that drywall would be too heavy.

  39. GBA Editor
    Martin Holladay | | #39

    Here are links to two articles that explain different ways to insulate your sloped roof assembly:

    How to Build an Insulated Cathedral Ceiling

    Site-Built Ventilation Baffles for Roofs

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