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Venting attached garage to create negative pressure, good idea?

Steve Mackay | Posted in General Questions on

Climate zone 6B new construction with an attached garage.  I have an attached 3 car garage that will serve as a hobby, car tinkering/building workshop in summer and winter.

I have a sealed combustion natural gas heater planned to heat the garage in the winter.  The plan is to keep the garage between 35 and 45F all winter and bump it up on the weekends when I am working in it.

I’m conscious of the types of stuff I work on in my garage and toxic fumes and car exhausts leaking into the attached house.  I’ll have the garage house partition sealed as well as possible. 

I don’t think I want a full blown extraction system in my garage but I think it warrants a slight negative pressure in the garage compared to the house therefore I was thinking of putting one of those Panasonic whisper wall mounted units:

Do you think the whisper 70cfm will serve me well?  The garage is 1300 sqft  with 11ft ceilings on 2 of the bays and a 13ft ceiling on the workshop and 3rd bay.  

Does it make sense to have the fan run 24/365 or set it up as an interval such as 10 minutes run time every 30 minutes? 

I there anything I’m overlooking with respect to this plan?


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  1. Steve Mackay | | #1

    Hmm I just calculated that at about 10 cents per Kw and it being an 18 watt unit that's 157kW per year so about $16 per year for continuous operation.

    I guess my question becomes does it need to be on continuously?

  2. Jon R | | #2

    Plus the cost of the heat you are exhausting.

    Think about humidity. For example, when you pull in with a wet snowy car you will need a rather high fan rate to remove the moisture. But at other times, you probably don't need any ventilation. So no, don't use a fixed rate.

    1. Steve Mackay | | #5

      Thought about the heat loss but did not think of the wet snowy car. Would the humidity be much of an issue for mold growth or rot? It's likely the garage air will be low humidity in the winter (as it's a leaky room). Lets say I keep the garage at 45F and the RH is about 30% the dew point is 15F. IIRC my outside 99% design temperature is about 4F and the average of the 3 coldest months is about 20F.

      A 50 CFM fan will change the air in the garage in about 5 hours (assuming 16000 cubic feet of air) but the air change might be quicker assuming a somewhat leaky garage door.

      I could look for something with a timed boost mode so when I drive in with a wet snowy car I can hit the boost switch as I go into the house.

      Any suggestions on a solution?


  3. Expert Member
    Malcolm Taylor | | #3


    There are a lot of other posters who understand the implications of pressure differences better the I do, but I wonder, in a space that is very difficult to air-seal due to the overhead doors, whether the fan will be powerful en0ugh to induce useful negative pressure?

    1. Steve Mackay | | #4

      Yup I thought about that but don't know the answer.

    2. Expert Member
      Zephyr7 | | #7

      It will, just not very much. It takes only a very small amount of air movement to depressurize a space. Whether or not the amount of depressurization you get is “useful” depends on your application. At work, I have one ~ 8,000 square foot datacenter facility that is intentionally kept at a slight positive pressure by what is essentially a 1/4HP blower smaller than a typical 1/2 ton AC. The slight positive pressure keeps out dust and other airborne contaminates, and keeps humid air from entering the space (the pressurizing blower also has an AC unit).

      For your garage, you could get by with a tiny amount of airflow to get some useful negative pressure. I’d wonder if you’d really accomplish much though — at night, the outdoor air is pretty humid, often near 100% in many areas, so bringing in outside air to exhaust humid inside air isn’t really going to do anything.

      If your concern is to remove toxic fumes ORIGINATING IN the garage, then you need to exhaust at least as much air as the volume of contaminants being introduced. It can be difficult to know those values though. You might get some ideas by looking at laboratory fund hoods and checking their CFM ratings and the kinds of solvent fumes they are rated to handle. It may be easier to look up the volume of car exhaust you car puts out, and use a fan rated slightly higher than that.

      Personally I think if you just seal up any dividing walls between your garage and house you’ll be ok. Garages themselves are rarely well sealed from the outdoors and the normal leakiness is usually enough to prevent any problems from getting into the house through a reasonably well sealed wall.


  4. Expert Member
    Akos | | #6

    Garage doors are notoriously leaky. The best I could find is a ONRL study which puts them between 100 to 700 cfm/Pascal.

    So your 70cfm will at best create 0.7Pa of negative pressure. With a two story house and a colder garage, your stack pressure would probably in the order of 2-3Pa, never mind any wind driven pressure.

    With an attached garage instead of trying to depressurize the garage, the best is to treat the partitions wall as an exterior wall and detail your air barrier the same way. This means continuous air barrier from the foundation up to the roof/ceiling. A door with good seals is a must.

    If your tinkering is with older cars, either install a larger extractor fan or an operable skylight. I find a bit of cross ventilation to be the most effective way to clear a larger space.

    1. Expert Member
      Zephyr7 | | #8

      If the OP wants to run cars inside a closed garage, a simple exhaust hose connected to the tailpipe and going outdoors would be a good idea. Commercial garages do it this way.


  5. Steve Mackay | | #9

    Thanks guys.

    The tinkering is old cars, a bit of woodworking, welding etc.

    I already planned to treat the partition wall as an exterior wall. I did have a planned operable window in my garage for cross flow and ventilating but after the foundation/ framing was done i realized I'd never be able to reach the winder mechanism without a step ladder. As a result I'd never open the window so I made it a fixed window.

    It sounds like a 50cfm fan is not going to do much for me and I'll be better off with a bigger extraction rate to account for car exhaust and use it on demand. Or attach a pipe from the exhaust and run it out a cracked garage door.


    1. Expert Member
      Zephyr7 | | #10

      You can get operable windows made for out of reach locations that you operate with a pole that has a hook on the end. They are similar to operable skylights. This type of window might be a solution for you.

      If you want to run the car inside a closed garage, DEFINITELY run the exhaust through a hose to the outdoors. You can use a dryer vent port as a wall passthrough. Using the hose is the best option, running it inside otherwise always carries some risk, and you'll always end up breathing some amount of fumes.


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