GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Prioritizing Humidity Control in Basement

kenmarcou | Posted in General Questions on

Hi! I’m Ken. My 180 year old house in central MA has some sill/beam rot/WDI damage in basement caused by moisture issues. Since I started monitoring the temp & RH in Jan it’s never gone below like 85%. I was thinking of making a negative pressure environment down there with a 2300 cfm box fan in the window to exhaust the humid air – but how do I ensure the make up air is coming from the much drier upstairs air (where there are a couple window air conditioners) and not from the incessantly crazy humid outdoor air?
It’s a walk out basement. The south facing front of the house is the walk out part and has windows to put the fan in. It’s an exposed structural 2 layer of brick wall with many many air leaks. Should I be focusing more on trying to strongly blow air into the basement from upstairs to *positively* pressurize the basement with the upstairs lower humidity a/c air (not the outdoor air leaking in) rather than trying to go the exhaust/negative pressure route and not knowing where my make up air is coming from?? I set the fan up in the window a week ago and while I can feel quite a breeze outside coming from the window and smell the musty basement outside in that air stream, the RH hasn’t budged down. I also put 6mm poly over the ~50% or so of the cellar that’s still dirt w/stone and not yet concrete slabbed. No RH improvement

How important is it to start trying to dry it out before structural work is done (which I must do before fixing the envelope)? I was thinking I’d try to lower the humidity, do a borax clean of everything to try to clear out some mold, then do a BoraCare treatment on everything to kill the termites power post beetles etc. inside the wood. Then get to the structural work. Then the envelope work. But how much do I need to worry about getting it drier now?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Jon_R | | #1

    I'd install a dehumidifier and focus on air sealing the basement to reduce the operating cost. Look at water management and vapor sealing next.

  2. walta100 | | #2

    The way I see it this house survived 180 year because enough heat and air was moving thru the walls to dry all the water that gets into the walls before it does much damage. Everything is balanced until someone air seals and insulates.

    If you want to add modern air tightness and insulation you need to add modern water resistive barrier.


    1. kenmarcou | | #3

      Walta, oh yes absolutely. I’m planning on doing a modern WRB and will have lots to discuss here about the best way to do that! I think below the 40? yo siding (it was not new in 1988) which will be removed & replaced w/another cladding I think is just boards. I don’t think I need structural sheathing so prob won’t do taped zip for the WRB, so was thinking maybe doing a fluid applied (Prosecco etc) water/air barrier around the entire damn outside of the house (roof deck will also be replaced. Maybe that should be zip) Not sure if front south facing walk out basement structural brick wall is going to require something different though.

      The WRB I don’t think I can get into before some structural stuff is done (replace sill rot, pour a slab on half the basement that’s still dirt/stone fill, replace a couple rotted and termite/carpenter ant/beetle eaten beams). But not sure how much I should be trying to dry out the basement with things like temporary gutter to mitigate roof water intrusion trickling down the back stone basement wall, and exhausting air to control the 90% humidity. The humid air outside comes right in because no air barrier. So, I don’t know if playing with negative pressure/exhaust and supply from the air conditioners on floors above will do anything for now. Or if it matters that it’s still wet when I put brand new materials in to replace moisture/WDI damaged structural materials.

      1. user-2412144 | | #4

        Especially in a house with central AC the basement will be too cool to control humidity by ventilation. Look at your outside RH and convert that RH from outside to basement temps.

  3. Expert Member
    BILL WICHERS | | #5

    With 2,300 CFM of exhaust, your home's humidity levels are going to increase since you'll be drawing in outside makeup air. Your home's A/C will never be able to dehumidfy that much air quickly enough so be ready for that. You could potentially also have backdraft issues with any atmospherically vented combustion appliances (water heaters, etc.), so be careful with that too.

    I would go with Jon's suggestion and run a dehumidifier instead of an exhaust fan. Get a large portable unit for now, then seal things up to control how much outdoor air can leak inside. You may also find that you're getting moisture through foundation walls, in which case you'll need to do some insulating work with rigid foam to get that part under control.


    1. kenmarcou | | #9

      Thanks for your reply Bill. Interesting to hear to use a dehumidifier when there’s so much air infiltration. But ok! (Also did you mean to temporarily seal things up with a temporary sort of cheap air barrier to minimize the amount of air that leaks in?) Just how much should I even stress the humidity/moisture down there before/while fixing the moisture caused damage and putting in new materials to replace? I can’t fix the moisture problem that caused the structural damage (fixed by creating real control layers) until I do this structural stuff so I feel like I’m in catch 22 sort of place.

  4. ranson | | #6

    Does the concrete slab have a well installed vapor barrier?

    1. kenmarcou | | #7

      Hi John, no, only half the basement, (350 sq ft or so) has a concrete floor at all. The other half was dirt with a wood floor over it, the rotted remains of which were taken out maybe 15 years ago and back filled with crushed stone but no slab poured yet. I have to do that along with the other structural basement work I mentioned. I have covered that a few weeks ago with 6mm poly through and weighed it down to keep it relatively in place to mitigate soil vapor diffusion. But I think it’ll only have an affect on basement RH in the winter when the air leakage is leaking dry air instead of summer humid air like now. The walk out basement above grade south facing structural front wall is 2 layers of brick and that’s it, inside to out. Gotta replace a bunch of bricks and mortar. Lots of air leaking through there. So until the air infiltration is dry air, the soil vapor diffusion I don’t think will have much of an affect on the RH.

      1. maine_tyler | | #8

        If there is wet, saturated soil, it certainly can be a moisture source. Evaporation happens until saturation.

        Plus, if you are conditioning the air to remove moisture, either with a dehumidifier or the AC, your RH should go down, which would in turn increase evaporative rates from wet soil or concrete. In other words it'd be fighting you. But I agree that your outdoor air leakage needs to be taken care of. Exhausting to the outside won't help this, as has been said.

  5. Expert Member
    NICK KEENAN | | #10

    I don't think you're going to get where you need to be just by removing moisture, you have to keep it from getting in.

    Moisture gets into basements a lot of different ways. In Massachusetts in the summer the outdoor air contains a ton of humidity, you want to keep that air out of your basement by air sealing. The ground contains moisture, and that moisture can wick through soil and masonry into your basement, you want to have a vapor barrier on the walls and floor to block that moisture. You also need to keep rainwater out, as well as groundwater.

    It sounds like your big sources are air infiltration and wicking. You've made a good start by putting plastic sheeting on the floors. Foam insulation board does a good job of sealing walls both against air infiltration and wicking if the joints are taped and the edges sealed. The rim joists really need to be sealed against air infiltration.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |