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Humid walkout basement; solutions?

My wife and I just recently moved into a renovated 1950's mid-century modern home. The master bedroom/bathroom are in a large walkout basement; the walkout portion has walls of (new) windows, as well as some ground-level windows on the other sides. The problem we're experiencing is a pretty large temperature and humidity differential between upstairs and downstairs. Upstairs is usually 3-5 degrees hotter and 40-50% RH. Downstairs it's consistently 50-60% RH and on hot/humid days can get beyond 70% (we live in Northern Virginia). We're wondering what our options are for bringing down the differential--in particular the humidity. We've had success with a standalone dehumidifier (got the basement down to 41%), but really don't want a standalone unit and would rather address the root of the problem rather than treat the symptoms.

As far as we can tell, there are no foundation leaks or signs of mold anywhere. Behind the master bathroom is a utility room with the HVAC and I can actually touch the foundation wall--feels completely dry. There are also some areas in that room where the vinyl plank flooring is not covering the foundation floor--and it feels dry there, too, despite the room's humidity being high. Gutters, with the exception of one, drain at least 5 feet from the house. All of this leads me to believe it's just a matter of humid outside air getting in, though I am surprised at that given all the windows are new.

We've talked to several HVAC guys who made some recommendations (move AC condenser out from underneath deck, re-do the entire ductwork in the house), and we're in the process of contacting some basement waterproofing companies to see what they think, but my gut is telling me that basement waterproofing is not necessary for us (we've had some massive rains recently and no leaks). The HVAC guys don't seem to think the humidity is a problem and that a whole-zone dehumidifier is not necessary. They also didn't seem to think moving from a single zone to a dual-zone would be necessary.

Wondering if anyone has had similar experiences and/or could help me diagnose the source of the problem and potential solutions. We'd really like to do our due diligence first to understand what's going on rather than throwing money at something that won't fix it!

Thanks for your help.

Re: Humid walkout basement; solutions?

Welcome to the forum.

A dehumidifier is really an air conditioning unit which blows hot air back into the same room. If you want dehumidification, run the house AC to achieve the same effect. You can run the fan on the HVAC all the time (instead of just when the AC turns itself on) to help balance the air between the upper and lower floors.

An air intake in the lower floor will help remove more humidity.

While you have no leaks, you're still getting moisture inside the house from under / next to the house. You can dump a pile of money into waterproofing, or just run the HVAC a little more.

Do you have the same problem in the winter or does the heat cycle dry out the air ?

Re: Humid walkout basement; solutions?

There will always be a temp and humidity differential between house levels because perfect sealing and isolation of the two areas is almost impossible in a livable home. I'd say yours is barely within "normal" limits. Humid air is heavier, so it will settle to the basement, thus by putting your HVAC return there it can treat the most humid air in it's normal operation using a single zone system. You might have to do other work to allow this such as ensuring airflow between floors, and that may require the use of a fire damper, but it shouldn't be a problem for a good HVAC contractor to resolve. Beware those who want to re-do everything unless all of them recommend it. And get written guarantees that the work they do will produce the specific results you want- that will "separate the wheat from the chaff" and get you someone who knows what they're doing.

Basements tend to be humid, especially masonry ones, since that material tends to retain it and release it slowly. As long as there are no actual leaks from the outside you shouldn't have to do any waterproofing with it being as dry as you describe. If someone suggests that anyway have them show you the actual leak, then show them the door if they can't do that- they're just out to take your money.


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