How does a foundation breathe in a finished basement?

I live in Long Island New York, basements in my area tend to be moist.

My house is about 80 years on a block foundation. I've done everything I can to reduce moisture coming into the basement, graded the ground outside, repaired and extended gutters, installed an internal french drain system for any excessive water that might make its way in (more of an emergency type thing) - exterior membrane waterproofing is out of my budget and would require tearing up some driveway. Lastly I run a dehumidifier that runs a fair amount.

Basement finished: batt insulation, greenboard sheetrock, on metal studs. No other types of moisture/vapor seal. Studs are set off from the walls about 6 inches (for the most part).

I have vinyl flooring, drywall, paint, trim, caulk. I had been battling the occasional spider-cricket that make their way into the basement - so I had been really sealing everything up real good and tight outside and in the finished interior.

Here's my question: I'm wondering if the foundation is breathing alright? Its not as if the foundation walls are wet - I'm sure they've gotten that way at points though - but with the amount the dehumidifier runs - there definitely had always been a fair amount of humidity getting into the room. But once everything is sealed up and the finished space has no air gaps (aka bug gaps) - can/how-does humidity come out of the foundation, into the interior and eventually into the dehumidifier?

I've read quite a bit on proper basement water control and insulation techniques. But I'm not just not understanding this issue of airflow.

I dont want to suggest anything dumb, but im almost imagining that you'd need some sort of vent (similar to a dryer vent) on the drywall spaced out to allow flow... but I'm inventing that, and it might not make real sense.

Your thoughts?

 

If your foundation wall is not sealed on the outside then a tremendous amount of water can soak or wick through the block where it evaporates on the inside. Your grading work outside helps but if the ground is wet that water can come inside. Once it evaporates the water is a vapor/gas and just like oxygen so you can breathe the water vapor can pass through most porous surfaces and get into your finished space.

If the humidity problem is quite bad then maybe forced ventilation of the dead space between your foundation and finished walls would help. I'm sure the dehumidifier helps but it's more expensive to run than a simple ventilation fan. The question is is running the dehumidifier a big enough issue to go to the trouble of other measures?

 

Thanks for the feedback, Dane.

Actually I have no issue running the dehumidifier, and expect that I'll have no choice. I think on Long Isle we have a higher water table and more moist earth so without exterior sealing - the water will wick.

My question was more so - once the water gets thru the foundation - and eventually evaporates to vapor - you said the vapor can pass through most porous surfaces - but I wouldnt think drywall with latex paint is porous. So how does the vapor get into the main area of the room (allowing foundation to dry) and allowing the vapor to be dehumidified? Am I risking "sealing it in" (between drywall and foundation) because my finishing area is 'tight'?

The idea of forced ventalation in the dead space is sorta what I was thinking. Actually I was thinking of a passive "vent" and allowing the rooom dehumidifier to create the circulation. I googled this and can't find any examples of either such system, suggestions on any more specifics?

 

Drywall and most water based latex paint are not a moisture barrier. Water vapor can still get through. That is why separate vapor barriers are required when building or insulating a home. There are moisture barrier paints that can slow down or almost stop the vapor though I have never tried them. In your case it makes you wonder if you seal off the finished room where's all the moisture in the dead space going. Will it condense and rot the wood or cause a mildew problem?

Nature also hates things to be out of balance. If the humidity is higher on one side of something like your wall than on the other there is a natural pressure for the two to equalize. So, without a very good moisture barrier the water vapor will work it's way into the room.

Water vapor does not pass too easily through sheet rock & paint and there is not a lot of pressure driving the vapor from one side of the wall to the other. Consider the amount of surface on the walls, floor & ceiling and you can see how a very little bit working 24/7 can add up. Turn off the dehumidifier and it may take days for the humidity in the finished area to balance out with the dead space between your walls but it will eventually come into balance. If the dehumidifier can take the moisture out than comes in then you get a dry feeling room.

 

I wont doubt you, but I had read that drywall+paint is considered at least to be a vapor retardant. Though as you said, likely not vapor proof.

I just want to make sure that I have accommodations to support that natural balance. The basement is comfortable and dry (with the dehumidifier on). But I dont know what is going on behind the walls. For all I know is its just dehumidifying airleaks from windows and whatever might make its way thru flooring gaps and what not. I dont know if its not actually drawing the humidity back from behind the walls where it is born. I'm sure outlet holes, loose construction, gaps in trim, and that sorta thing - as well as drywall's vapor transferance - all help to open up a natural convective flow.

I just want to make sure - simply because I am sealing everything up relatively tight - that I'm ok. I googled alot and can't find anything about passive or active vents on drywall between finished space and foundation (though to me it seems sensible.

 

Latex paint/drywall are in the lowest Class of vapor barriers; "A Class III vapor retarder is located on both the interior and exterior of the assembly (the latex paint on the stucco and on the interior gypsum board)." From; BSD-106: Understanding Vapor Barriers — Building Science Information

And; Info-500: Building Materials Property Table — Building Science Information

You may be getting air/moisture infiltration through the rim joist with seasonal changes, pp.51; Troubleshooting Guide to Residential Construction - Google Books

Basement walls are required to be fire-stopped at top and every 10' horizontally per code (including chases or air spaces);Chapter 6 - Wall Construction

The moisture may also be from the block walls through the framed walls as said already; http://www.buildingscience.com/docum...lation-systems

BSD-103: Understanding Basements — Building Science Information

Gary

 

Thanks for the references and info, I actually read every page of BSD 103 And 106. I'm still not clear on my specific question. Should I do anything different or is vapor transmission thru drywall and rim joists adequate enough to dry out a foundation ?

 

Adequate really depends on how much moisture is coming through the foundation walls. Relying on vapor to pass through finished walls is not an option I would consider if drying the foundation walls. Normally I would not finish a basement until the water/moisture issue is solved. I think the real question is since your's basement is already finished what options are willing to do?

 

I guess my question is - what are my options? My understanding is that not every basement humidity issue can be prevented. For example, I have no doubt that the floor is a source of vapor entering. And there'd be no option to put a membrane beneath the concrete. And due to my property line and driveway (and budget) I just don't have an option to membrane seal the entire outside of the foundation walls. I should reiterate - as far as I know, I dont have an active leak, or even water seepage, just high humidty from vapor penetration.

How would you handle this? What would you have done different prior to finishing (knowing that nothing else could be done on the exterior) - and what would you do now knowing that its mostly finished?

 

I would keep the dehumidifier running and go on with life. Any other option that would work is a lot of work and money so just I would run the dehumidifier and enjoy the finished room.

 

What if I put a register (not connected to anything) in a few walls to allow some sort of airflow from behind the walls that the dehumidifier can draw from.

 

Walls are drywall to act as an air barrier (against fire travel) and/or cover your thermal barrier. No openings are allowed. You would be growing some nice mold cultures...

Gary