Originally Posted by
Ultrarunner
Lots of good advice above, but I have a few questions to hopefully sort things out a bit and correctly identify the concern. First, is the condensation inside two window panes (indicating a damaged thermal seal) or is it on the inside pane of glass on the interior face, assuming that the windows are double-paned? If they're single-paned, the former condition obviously won't apply. Second, assuming you've been at the same address for at least several years, has this occurred previously? Lastly, not prying, is the home a single family residence or multi-family structure? I count townhomes in this category as there are shared walls.
Moving forward, I am assuming double- or triple-paned windows.
If the moisture is occurring between panes, that's a pretty straightforward age-related deterioration. The advice above about getting the glass swapped out is a solid, fiscally feasible fix.
If the second condition applies, it's time to look at indoor humidity. The ideal range for relative humidity is 30-50 percent. Get too much above that and you run the risk of fungal growth. This is the reason for the second question. It sounds as though this is a new adverse event rather than a constant condition that replicates over multiple years so we're looking for something that has failed that can increase indoor humidity. This can be as simple as a change in cooking styles (some cuisines have a lot more moisture released) to a bathroom fan that no longer moves sufficient air to a roof leak entering the wall assemblies.
Off the top of my head, I would test: the bathroom fans by using double-ply toilet paper (2 sheets) to see if there is sufficient draw from the fan. Verify on the exterior of the home that the vent flaps are opening as confirmation that the vent pipe lines are unobstructed. Ditto for the kitchen vent fan. On the human behavior side, makes sure that the bathroom fans are used every time someone bathes. Kids are pretty lackadaisical about using the fans.
Check the dryer vent line to ensure that it is not disconnected. If you haven't had the dryer vent line cleaned in a while, consider doing so. This is good advice for fire prevention, too.
If the house is new, it may be very tight on air transfer. If so, there should be a ventilation fan of some sort to provide air changes. Many times, this will be on the furnace system. Check to make sure the timer is working if your home is equipped with a fresh air damper.
Check for plumbing leaks - even a small leak can substantially raise indoor humidity. To test sink drainage for leaks, stopper the sink, fill it, and check on draining for drips. Physically touch the supple and waste drainage piping as you may not see a leak but can feel it. If you have vinyl flooring in the bathrooms, look around the commode. A purplish staining color in the vinyl would indicate a past or current leak at the wax ring. Don't forget the dishwasher plumbing and drainage and the water line to the refrigerator. Both are prone to leakage in hidden locations. Check the water heater, especially the temperature pressure relief valve. It might pop off and create a slow drip.
On the exterior, check your gutter conditions - do they drain freely and does the flow of water terminate at least ten feet from the foundation? Is the roof in good condition? I would check in the attic for signs of water leakage and fungal growth as high indoor humidity can lead to issues in the attic spaces. If you have a crawlspace or basement, check for moisture there. If on a crawlspace, ensure that the vapor barrier (6 mil polyethylene plastic sheeting is most common) is intact. Check caulking around window and door penetrations as failure there can introduce moisture. If on slab-on-grade, make sure that you have at least six inches of clearance from soil to the base of your siding.
So, that's the starter list of possible sources for excess water/humidity.
On the last question, if you live in a multi-family structure, you could be doing everything right but your neighbor doing everything wrong which will, unfortunately, impact you. In this case, a dehumidifier may be your only solution.
As for professionals to deal with this, I would start with a company that specializes in indoor air quality or, my preference, a competent Heating/Ventilation/Air Conditioning (HVAC) contractor but after I verified the bathrooms and kitchen fans are properly operating and getting the dryer vent cleaned.
Take care and I hope this gets resolved quickly for you!