Ventilation for musty/humid concrete bunker?
Our house’s basement has an original cold war era bunker with concrete floor, ceiling, and walls, and no insulation or heating. The bunker has a door connecting it to other areas of the basement that range from “semi” finished ytility room (hydronic floor heating but no drywall or insulation) to fully finished living area (hydronic floor heating, insulation, drywall).
I’d like to use the bunker as my workshop – workbench, drill press, grinders, saws, etc.
However the bunker always has a bit of a musty smell, and relatively high humidity. My humidity monitors show that when humidity is 50% in the “finished” areas, it is about 60% in the semi-finished area and about 67% in the bunker.
As you can see in the photo, the bunker has two 3″ cast iron pipes that go to the outside. One is just a stub (to the left of the light), the other has a non-functioning 1950’s “fallout shelter ventilator” attached to it (just to the left of the column). Both pipes were capped on the outside when we bought the house.
As an experiment, I installed a Panasonic WhisperValue bathroom fan in the bunker and have it exhausting through one of the 3″ pipes to the outside (which I uncapped and finished off with a roof hood). I’m running the fan around the clock at 50 cfm, so it’s pulling the air from the bunker and presumably from the semi-finished room next to it, and exhausting it outside.
This brought the humidity down to about 62%, much closer to the humidity level of the semi-finished room 59%. The mustiness level has also decreased.
I’m wondering if there’s an additional benefit to opening up the second pipe, so as to allow outside air to be pulled directly into the room? I’m not sure how much air would get sucked in through a 3″ pipe when there’s already a fully open door in the room. But the outside air would obviously be “fresher” than anything coming in through the door to the other room.
Any thoughts/suggestions would be appreciated.
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Adding foam insulation to the exterior walls and the floor would likely eliminate the odor, which is a result of the high humidity condensing on the cool walls.
Relative humidity doesn't mean much without also knowing the temperature. Is the temperature consistent between the spaces or does it vary?
Michael, the temperature in the bunker is hovering between 70 and 72F. In the semi-finished room next to it the temperature is between 72 and 74F (RH around 59-61%). In the finished area the temperature is between 71 and 73F (RH around 55-63%). All of these areas are not conditioned in the summer. The outdoor temperatures have been in the 80's and 90's for the past few weeks.
Those are nearly identical moisture levels; just a couple of degrees makes a difference when it comes to RH.
As I think I've heard Joe Lstiburek say, dilution is one solution to pollution--i.e., venting the air continuously is one way to address your problem, but it's better to find and address the cause. I still believe that the most likely cause is moisture in the walls--from moisture in the soil and also from indoor air condensing on the cool concrete. Adding foam insulation, either rigid or closed-cell spray foam to the walls, and adding something like Dricore or Delta-FL dimple membrane, with at least 1/2" of foam and plywood or Advantech floor sheathing, should greatly reduce odors and improve indoor air quality. Having some ventilation is always a good idea as well, but if a single bath fan isn't doing the trick, I'd address the cause.
Yes, the bathroom fan has helped bring the temperatures and humidity levels in the bunker closer to the rest of the basement. That makes sense since the makeup air is coming from the other parts of the basement.
I hear you about adding insulation etc, but I like the raw/authentic look of the bunker, it has a certain charm to it that I don't want to change. So for now I think I'll keep running the fan and watching the conditions over a few seasons, including winter.
Thanks for your thoughts!
Seems to me you can’t vent this problem away by venting to the outdoors. The more semi conditioned air you vent to the outdoors the more totally unconditioned air you draw into the house elsewhere and the add load to the system.
If the current system had enough capacity for this space them circulate enough air in and out of the space to keep it more or less the same temp and humidity as the rest of the house. If the wall temp is lower than the dew point of the air you will need to insulate the walls to keep them dry.
This article should be helpful.
https://www.greenbuildingadvisor.com/article/three-ways-to-insulate-a-basement-wall
Walta
I'll look into dumping the exhaust air into the semi-conditioned part of the basement instead of to the outside. It'll require me to drill a 4" hole through 8" thick concrete, but that's an excuse to buy better tools right? Alternatively I can run a long duct through the existing door and to the semi conditioned room, but that may look awful.
I guess I need to revisit my understanding of the "exhaust only" ventilation strategy. I thought that was considered a reasonable and in some cases preferred approach, especially on a budget? Our house is not very tight, so I'm expecting that most of the fresh would get pulled in through windows and doors, not some bizarre and unpleasant paths (though obviously I cannot be sure of that). And is that something that would be alleviated if I opened up the other 3" pipe to the outside, thus providing an easier path for air to come in?
https://www.greenbuildingadvisor.com/article/revisiting-ventilation
It is one strategy, but you are literally removing air that you just paid to heat, cool and/or dehumidify, which will be made up with outdoor air as Walta stated. Continuous venting is usually something like 80 cfm for the entire house.
The odors won't go away unless you insulate; if you exhaust into other parts of the basement, you are just spreading the odors around until they aren't detectable.
Thanks Michael. Can you educate me on how odor relates to insulation? Is the idea that the uninsulated walls cause condensation and thus the odor? (my measurement say that I'm currently at almost 10F above the dew point, but that may not always be the case).
The odor is almost certainly from moisture in the walls and floor sustaining microbial activity. Some of is probably condensation from indoor air, and most of it is probably from soil moisture. Either way, adding closed-cell foam insulation or open-cell foam with a vapor retarder on the interior, with all seams taped, would block the moisture from condensing or getting through from the exterior. Check out this recent blog post for related information: https://www.greenbuildingadvisor.com/article/vapor-retarders-for-foundation-walls
I don't like old houses, but glad that you do. Every time I visit my mother in laws 1940 home I cringe especially when in the basement.
Consider putting the fan on a timer, you can reduce run time by a bit and operating cost and prevent over ventilating like others said previously and the penalties associated with it.
5 on 10 off perhaps?
I have a Panasonic dual speed bath fan with an Ecobee timer in a mechanical closet in my basement, its where the cats litter pan is kept. Wall mounted about 16 inches off the floor, directly over the litter and vents out through the rim joist. Works like a charm for odor and brings fresher air from the upstairs across the basement and out.
I had it running continuously to test out the effectiveness, but I agree with putting it on a timer for the longterm, if I continue this route.
Sounds to me like your "litter box" fan is doing the same thing as my bunker fan, right? Out with the bad air, in with the good air? (who else is old enough to remember this line?)
I would combine your exhaust only venting with a dehumidifier in the bunker. This will drop the humidity and add a bit of heat which might reduce the smell.
If that doesn't work, insulation is the only way.
There was an Aprilaire dehumidifier in the bunker when we bought the house (you can see it in the left corner of the photo), but even though it ran, it did not dehumidify (apparently all the charge escaped). I suppose that's another thing to try in the future, though I'd prefer to minimize mechanical devices if possible.
I don't know if condensation in the bunker is a problem, at least not at the moment - the dew point is around 56F, and I measured the walls, floor, and ceiling at around 65-69F. Does that make sense, or am I looking at it wrong?
My gut (admittedly not a scientific measure) is telling me that the biggest problem with the bunker is that it's just not getting any fresh air - it is a box with 8-12" of concrete on all six sides, and a door connecting it to the rest of the basement, but no existing way to move air in and out. So I think it's more "stale" than "musty". But perhaps if it were better insulated, the air would not be stale?
You don't need water condensing for mold to start growing. Those temperatures put the air near the concrete around 70% RH. Perfect humidity for mold growth.
If you want to stop that, you have two options. Reduce the dewpoint, ie dehumidifier, or increase the surface temperature by insulating.
Dehumidifier is the simpler start. Try to limit the amount of exhaust venting, depending on where the moisture source is this might bring in more humid air from the rest of the basement which could make things worse. The dehumidifier doesn't have to be anything fancy, cheap box store one is a good to start.
The way I see it if it smells musty it is because something is damp. If something is damp most likely it was below the dewpoint and the water condensed out of the air.
Consider finding someone to fix the dehumidifier. My guess is it was a $1200 machine and if you could take it to a shop and get it fixed for 3 or 4 hundred that is better than spending the same money to buy new plastic junk.
Walta
What climate zone are you in?
What is above the concrete ceiling?
If the dehumidifier is over 25 years old, it may use refrigerants that are no longer used and thus not be not worth fixing.
In my climate zone I would insulate in a heartbeat, but there are a bunch of things you can do wrong there. People end up building stud walls, which might sorta make sense for a finished space,. but a part time use shop? Now you have stud wall you didn't need and the space is 8 inches narrower and....
Thanks for the responses. Unfortunately the Aprilaire dehumidifer is long gone, the customer service person said it wasn't not worth fixing. Maybe I should have tried a second opinion, but too late now.
What size/capacity dehumidifer should I aim for? The space is about 300 sq ft; 2000 cu ft. Any particular "starter" model to try?
We're in CZ 4C, marine/mild climate. As to the question about what's above the bunker, the ceiling/roof functions as a patio above, so it's essentially exposed. Part of the patio is covered with wood decking on sleepers, and there's a roof overhang over that area. A very small portion (maybe 20-30 sq ft) has no decking and no roof, so it's exposed to the sky.
I'll be asking more questions later about HVAC in this house, as I'm struggling to keep the main floor properly conditioned. The house was recently remodeled top to bottom (just before we bought it), and they tried to make good choices (insulation, mini splits, hydronic floor heat), but the execution was lacking in a number of areas. As to the previous comment, I also am usually not a fan of old houses, but this mid-century modern has tons of character and was very nicely remodeled, so we took the plunge (after trying and failing twice to get a new home built). Here are a couple of pics.
The portion under roof is probably ok but the exposed portion is not. You had said it was part of your basement so I assumed it was inside your basement, not outside your house. I have been in one similar space, which was saturated with water. You absolutely need to protect the exposed portion with something waterproof.
i'm trying to piece together the disparate pics to think about how much is not under a roof, might be they thought of that? If water isn't pouring in during a rainstorm after 50 years the probably did an ok job. I would get a name brand dehumidifier. I bought one with a pump for my garage because I need dehumidifying in the winter.
The use of this space is more like an attached garage than living space, so if mildew can be kept at bay with a dehumidifier, and you don't feel the need for a lot of heating, maybe not go crazy until you have spent enough time to determine how it lives.
I'll try to describe it better with the attachments. First one shows the main level, with the roofline of the covered patio outlined in orange. Second one shows the basement, including the bunker shaded in magenta. It's a walkout basement in two areas, but the bunker portion is fully underground. Third attachment is an overlay of the two floors, where I try to show that most of the bunker is covered by the roofline, except for a small bumpout. I'm not sure what happens in the bumpout, it's the area where the vent pipes exit, but it's not above the bunker directly, it's right next to it. Without digging everything up I can't tell why it's there, it looks like it's where the 3" cast iron pipes are routed to the outside. Finally there's a photo to hopefully illustrate this a bit better. Yes, it's a bit complicated.
The bump out is where the fallout detector goes............I would make sure the gutter stays functional
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I put in an 80 pint portable dehumidifier yesterday, and within a few hours the humidity dropped from 60% to the 45% I set it at, and generated over a gallon of water. It also dropped the humidity in the adjacent room from 60% to about 42%.
So hopefully this will work as a solution without resorting to insulation - time will tell.
Be aware a dehumidifier is a super-efficient heater with a COP well over 1 and will noticeably change your electric bills.
Let say you have expensive electricity at say .20 a kWh the unit runs 75% of the time and draws 7 Amps.
120v X 7 A = 840 Watts
840 W X 18 Hours a day = 15,120 Watt hours a day
15120 x 30 days = 453600 Watts a month = 4536 kWh
4536 x.2 = $900 per month.
$900 a month and my guess is it will make the houses AC use about $200 to cool the house so $1100 a month all summer long would make me cry.
All the numbers are made up worst case but still plausible.
Walta
I think $900 / month is a bit implausible. 453,600 Wh is 453.6 kWh, so 10x lower.
FWIW, my portable dehumidifier in my leaky basement, zone 4a, used 1327 kWh in 2022 and 986 kWh in 2023. Roughly $200 / yr at my rates. It's my 3rd biggest user after heat pump and water heater.
I agree I did misplace the dismal point.
I thought the number was impossibly high even for the worst case.
My point remains if you run a dehumidifier, you will feel it in your electric bills.
Walta
That's a valid point about the electrical use, I'll plug in my Kill-a-watt and see what it shows over time.
So it sounds like if I wanted to save on the electric bill, I would put up insulation on the walls, either in addition to or instead of the dehumidifier. I'm still learning about moisture, humidity, and condensation, so can you help me understand what the insulation would fix? I understand that it would help with condensation by removing the cold wall surface, but would it also help with the moisture/humidity level? i.e. would it reduce the amount of moisture coming through the concrete from the outside soil? If not, would the humidity level stay the same, even if the condensation were reduced?
My guess is most of the moisture is from moist air finding a cool surface and condensing.
If you insulate the wall with an imperviable insulation like foam board the moist air does not have a cools surface to condense on.
https://www.greenbuildingadvisor.com/article/three-ways-to-insulate-a-basement-wall
Walta
Opening up the second pipe could help by providing a direct airflow path, which might improve ventilation and reduce humidity even more. If the outside air is drier than the air in your basement, it should help balance things out and tackle that musty smell.
I'm pretty sure if a lot of moisture was moving through the wall/ceiling that paint will blow off in a hurry
Where is the moist air coming from if not through the walls/ceiling/floor?
There is a normal amount of moisture and then there is problematic amounts of moisture.
IF the roof was dumping bulk water onto the patio surface or ground water level was high I cannot see how the paint would stay on
Since the paint seems to be staying on it seems like it can be easily dealt with, where if it was peeling off, more extensive work might need to be done.
Moisture can come from moist air infiltrating. It can also come from moisture wicking from the ground, or in this case the ceiling, through the concrete via capillary action. There could also be liquid water leaking in when it rains.
I don't think we have a bulk water infiltration problem, because the humidity is about the same during the rainy season as it is now, when we've had no rain for several months. There is one tiny crack in the middle of the wall that leads to a small leak during very heavy rains, but we're talking about a cup of water total. And I don't know how to go about fixing that type of a crack.
The paint is staying on very well, I don't see any signs of peeling or loosening.
DCcontrarian, thank you for replying here, I read with interest your comments on that "poly vs no-poly" discussion, and appreciate the simplicity of your statements that "The concrete must not be allowed to dry to the interior" and "moisture is better left in the concrete than making its way to the conditioned part of your house." That's is why I'm asking if/how rigid insulation would help in my case - I can see that it would eliminate the cold wall and thus reduce condensation risk, but would it also reduce the amount of moisture coming in, and the overall humidity? And if not, then how do I keep out moisture, without resorting to a dehumidifier?