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Community and Q&A

Unfinished Garage With Mold

couloir007 | Posted in General Questions on

We are Zone 3/4 Northeaster VT. We finished our attached garage attic into an apartment 3-4 years ago and this summer I took notice of mold on the north and north-west facing walls. The garage attaches on the south face and the mold problem is less or non existent, it gets sun baked. There is no mold on the ceiling, but the plywood walls are getting moldy. We get to -30 and that wall gets NO sun for most of the year, and only in the early AM in the summer. We treated it and plan to insulate and finish the garage. The issue is snow, we get a lot of snow and the floor of the garage is wet from November to April. I shovel and sweep the water out frequently, but the floor is never dry. I leave the window open in the summer and moisture is not an issue, the area the house is built on is a hill of well drained glacial debris. A builder friend told me to be sure the walls can still breath or I’ll get condensation in the walls even though I plan to use green board dry wall and seal it up like it is a bathroom with anti mold primer. There should not be condensation on the walls since the garage temps will be much higher, they should remain above freezing and I was going to run a de-humidifier and blast a propane heater once in a while to dry out the floor. I was also going to seal the floor to make it easier the squeegee the water out. The roof has 2-3 eaves overhang and I don’t mind drilling soffit vents into each bay at the top of the wall and using vent baffles, can’t find any info on anyone doing this but it seems reasonable given I don’t mind the vents, brown ones blend in well and it’s not an aesthetic issue on that side of the garage. If this makes sense should I put a 2nd vent lower? Snow doesn’t pile up against the wall, a 45 degree metal roof sends the snow a good distance away.

Perhaps I leave the window cracked in the winter too. If I finish the garage it will still be a lot warmer even if I crack the the window. Or this or this I want to keep the garage as warm as possible since there is some exposed plumbing for the apartment. The pipes freeze on the coldest evenings and I’m hoping insulating will reduce this headache for me.

My question is am I going about this wrong? Is it overkill? Is there something I’m missing?

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  1. mr_reference_Hugh | | #1

    You mentioned
    There should not be condensation on the walls since the garage temps will be much higher

    The high heat inside and cold outside would cause condensation. If the garage was the same temperature as outside, risk of condensation would significantly decrease.

    1. couloir007 | | #2

      Currently the garage is not insulated, but not the same temp. There is a finished apartment above and the temperature inside can be 20-30 degrees warmer on the coldest nights. It can hit -30 outside but the garage will only drop to 13, but these nights are not the issue, it's that it's always warmer inside than out, which means during the winter when the floor is always wet there is always condensation on the walls. I plan to insulate with Rockwool 6" R23 Batts, so the garage will be even warmer, and finish with bathroom type drywall and seal everything, but this will raise the relative humidity in the garage more than it already is. My real question is is venting the walls with vent baffles and soffit vents overkill? Should I install something like

  2. Expert Member
    DCcontrarian | | #3

    I've lived half my life in New England, and I associate mold like that with summer, not winter. Specifically humid summer weather where the dew point of the air is higher than the temperature of a concrete slab in contact with the ground. Mold tends to grow along the bottom of walls that are in contact with the floor. I don't know how much your garage resembles a basement, but generally in New England ventilating a basement in the summer introduces more moisture than it removes.

    Mold doesn't grow much below 70F and it sounds like the space isn't seeing anything near that in the winter.

    If the water that comes in during the winter doesn't have much of a chance to dry out then that could contribute to humidity for a long time. The solution there is to seal everything, floor and walls. Paint is all you need. If the slab doesn't have a vapor barrier you could also be getting moisture rising through the slab. Vapor barrier paint would take care of that.

    In terms of the strategy for improvements, you have to make a decision about whether the garage is indoor space or outdoor space. A lot of attached garages are built by people who are confused on this point. If you treat the garage as unconditioned space then you can't have plumbing running in that space. The problem with garages as indoor space is the door, it's really hard to get a garage door that seals tight.

    My recommendation would be to treat the garage as unconditioned space. That means the walls between it and the rest of the house should be sealed and insulated like they are outside walls, the door between them should be an exterior door. For fire safety there should be an airtight barrier anyway and the door should be fire-rated. Move your plumbing to the inside of the insulation. Then if you want the garage to stay warmer you can also insulate the exterior walls of the garage, the garage will pick up some heat from the house and the slab and stay warmer than the outside. Paint the whole interior with vapor barrier paint and provide a drainage path for the snow melt.

  3. mr_reference_Hugh | | #4

    I basically agree with DCContrarian.

    The only point I will raise is that if you have a floor that is contantly wet, you have water that is constantly evaporating. Green drywall is not meant for that type of application. The USG website states it is for: APPLICATIONS: Exterior Soffits and Indoor Parking Garage, Kitchens and Food Prep Areas, Multi-Family Residential and Condos Restrooms, Utility Rooms and Loading Docks, Single-Family Residential.

    You can easily call USG to ask if they think that their product will perform well in the contantly damp garage. They are easy to reach at USG Technical Support 1-800-USG-4YOU

    I would compare your humid garage to a space housing an interior swimming pool that is not heated in a cold winter environment. That might sound wild, but you indicated that have a floor that is almost constantly wet. The only difference is that the water is too shallow to dive into and the indoor temperature is much lower.

    Good reading from some experts at Building Science

    I suggest that unless you are able to manage the water, everything else is going to be a waste of time, effort and money. I would hate to see that happen to anyone.

    I would consider how water is managed in basements that are built near or below the water table. Sump pumps are used. Water is channeled to the sump pump using channels or using pipes.

    Certainly, the Lunos product is not designed to manage humidity in a garage with a wet floor. In addition, the Lunos product "recovers" or retains 20% of the humidity that is in the air being exhausted. You need to buy these in sets of 2 so you are talking about $1600. One pair is designed to exchange air for one small 450 sq foot apartment with regular humidity levels.

    If ever you seriously thought of buying the Lunos product, I would say instead buy a very large capacity dehumidifier. The are about the same price if not less expensive. They would also be giving off heat and so the energy they consume would not be exclusively for dehumidification.

  4. mr_reference_Hugh | | #5

    On the question of keeping pipes from freezing, this could be done relatively cheap with Pipe Heating Cables if this problem was looked on its own.

  5. couloir007 | | #6

    The garage is about 400+- sq/ft and the plumbing is a done deal. The issue with the heating cable is it is in the rafters and I need to cover it with fire rated dry wall because of the living space above, and those cable are not to be put into walls or floors due to fire danger. I have them but got nervous about using them. The moisture isn't a water table issue but mainly snow melting and some rain wet cars other times. It makes sense the garage is cool and summer RH being high will condense on the walls, and it's the walls all the way to the ceiling on the north facing side of the garage, not just along the bottom. I did install insulated garage doors last fall and they are much better sealed and have been wondering if this is when it started. I was thinking of using vent baffles and soffit vents in the walls, insulated, dry wall sealed and primed. My eaves are big and no threat of wind blown rain getting in if I put them high enough. I was planning to seal the floor and all the cement to make squeegeeing easier, I will also get two containment mats. I do need to do salt damage repair first though. I was also planning on a dehumidifier, but it seems like that is a summer time thing when the garage is significantly cooler than the outside. A wet floor is not much of an issue outside of the winter months but I'm sure moisture is evaporating through the floor. Should I at least put a bathroom exhaust in for colder and dryer months bringing in much drier air? With the Lunos I did see that it captures 20%, but that to me means it's venting 80%. If I can keep the garage significantly warmer and with the floor mats remove the majority of the moisture routinely it seems like this should keep the RH much lower, but then they would be useless in the summer. At this point it's a small cost compared to the potential for mold remediation in the future with a living space above the garage.

    1. Expert Member
      DCcontrarian | | #8

      I think you're on to something that adding the insulated doors made the mold worse by making the garage cooler in the summer.

      In my experience, in the summer the only thing that works is sealing the space and adding dehumidification or air conditioning. In the winter ventilation can get rid of moisture.

  6. mr_reference_Hugh | | #7

    I understand now that the plumbing is hidden and you need to cover with a fire rated material.

    I understand now what you mean by venting through the wall. If air goes out, it needs to be replaced by colder air. You would need to consider where that air comes from.

    If there is an appartement above, there should be a fan to remove exhaust fumes from the car. I saw one guy with a bathroom fan wire to a switch that activated based on co2 levels.

    If you already doing what you can to manage the water, then that is off the list. Unless you decide to park the car in a $1500 semi-temporary car shelter instead of parking in the garage for the winter. Depends on how much space you have on your lot and zoning bylaws. Some are designed to last for 10 years without being taken down.

    Myself I still think that I would put a really big dehumidifier. A whole house dehumidifier is easy to install in this type of setup (making lots of assumptions) Unlike Lunos, you would have a unit dedicated to solving the humidity issue plus it would add heat to the garage.

    There are dehumidifiers designed to work in cool places. Even if a large capacity (maybe even commercial Grade) dehumidifiers have an initial cost, I think that they would dry up 400sf in no time at all.

    I feel like I am being pushy so I will let you make your own decision. Wishing you success.

  7. mr_reference_Hugh | | #9


    I heard about this article on BS&Beer and looked it up in case it could help you.

    It is about conditioned attached garages and there are a few issues that are addressed in the article. I thought that the issue of moisture management apppeared to mirror you situation almost exactly. They speak about solutions and these are people who deal with this over and over.

    This is a link to the BS and Beer show in question. You can start listening at around 1:00:00 (one hour mark).

    Hope this helps you decide how to remedy the situation.


    1. couloir007 | | #10

      Thank you, I watched it and I think it confirms what my plan is. The delivery guy for the rockwool insulation and greenboard drywall said he'd spray foam the walls, but I don't have the budget or the time to find a contractor. It's not a living space, I just want it to be warm enough and vented well enough with no issues in the walls after I seal it up. I'm going to put baffle vents in the walls with 2" soffit vents low and high like Eventually clean and repair the salt damage and seal the floor. For this year get two containment mats. Drywall and seal as if it were a bathroom and get a HRV, I think this one I calculated I needed 27 cfm, and this is 50ish cfm on the low side. Not sure what too much ventilation would do, on the low flow side I think it is ~70% efficient. I'll use a dehumidifier in the summer since I won't want to bring warm moist in from the outside into a much cooler environment.

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