ceiling moisture

I inspected a modular on slab home two years ago and got a call back over tehhe weekend.

My clients have been experienceing water droplets above the living room window which was not very well installed and certainly not the best quality. Small room, temp kept at 68 degrees. Attic and insullation dry with proper ventilation.

I am leaning towards condensation (a fish tank is also in the room) and advised to seal wondow leakage and circulate the air in the room more.

Any opinion? Would a ceiling fan help to keep the moisture down?

Thanks, todd

Sealing the window will only keep out dry cold air .
I think you are looking in the correct direction .
Fish tanks can add moisture especially if the are running a bubbler .
I bet the room is kept closed and not much air movement or air exchange .
How do they keep the rest of the humidity down in the home.
I would look at the humidity in this room it more then likely is elevated.
The fan might help but only if the humidity is kept under about 50% in cold weather.

Circulating the indoor air will likely increase condensation (unless the condensation is occurring directly above the fish tank or other source of moisture). The more air that comes in contact with the object which is below the dew point temperature, the more condensation that will occur.

We are not worried about air infiltration providing the source of moisture. We are concerned about air infiltration cooling objects to below the dew point temperature of the indoor ambient air.

This is all about the laws of psychrometrics. Excessive humidification in the house should be avoided. We cannot reduce the humidity in the house below required levels so cold objects must be insulated or heated for remediation.

By above the window, is it on the ceiling or on the small section of wall above the window head trim?

If it’s on the ceiling, does this eave face from northeast to northwest? If so, there may be some air from the soffits causing “windwashing” on the fiberous insulation installed in the wall top plate area. The air moves through the fibers or under the fibers to cool the drywall to below the air’s dewpoint and “Voila” droplets on the room side even with regular rh’s sometimes. Have seen this phenomenon quite a few times.

Guys with IR gear should go out to some homes when a good stiff very cold wind is blowing. Scan along the windwardinterior wall/ceiling area to see if there is much cooling not from infiltrating air but windwashing.

When people cannot afford new super windows (double or triple glazed, single or double Low E with argon/krypton gas and “warm edge” spacers that resist higher levels of moisture that are better for humans (40-55%), they must run their houses in the dry zone of 25-40% to keep excessive condensation from causing mold/rot.

do they have a ventless gas fireplace or a gas stove ???..both can cause condensation near windows…

The consendation consists of water droplets above the windows in three of the front rooms, all small, but not in the kitchen where I would expect most of the moisture. The droplet are on the ceiling and not on the walls. Moisture meter readings on the walls were normal. As stated earlier, the attic space above these areas are dry and there is a vented soffit of about 1-1/2 feet

Thanks for the help.

Don’t forget that the blowing air will have a fairly good velocity which will tend to evaporate the water.

Negative, you must understand the psycrometric property of air. You can not add just one property of air and not have it change the other properties of that air. Regardless of air velocity, the psycrometric property of the air must be changed. Air movement (if from the same source of the indoor ambient air) will not evaporate moisture when the air is 100% saturated (at the dew point temperature) and as a result is condensing on objects cooler than the sensible heat temperature. Stagnant air will precipitate out moisture onto objects at/below the dew point temperature to a point where the psycrometric property of the air surrounding the cool object loses moisture and the dew point temperature falls. Air is a good insulator and less condensation will occur until you start moving the air. The more air you move, the more moisture comes in contact with the cold surface.

Also, the large volume air blowing onto the cool surface contains heat also, not just moisture, and will warm the surface to above the air’s dewpoint, hence moisture will disappear as long as the fan works and the air is at room temp. and not extreme humities.

Moisture is heat (latent). Unless you raise the sensible heat level , the surface temperature will not rise. If you raise the sensible heat in the room, you lower the relative humidity which allows air to absorb even more moisture (grains per pound of air). This will then raise the dew point temperature of the ambient air and cause the condensation to occur at even higher room/wall temperatures.

You posted that the room set point temperature was 68°. If you raise the temperature to 70° the situation will get worse if the source of moisture is still present and is allowed to saturate the heated air again. I mentioned controlling the source of humidity. If you can identify and control the humidity source and then raise the temperature of the ambient air in the room, the moisture on the wall will evaporate back into the air.

This is when the IR camera answers your questions.

This picture was taken in summer, but you can imagine what it would look like today.

99% of all homes in this area has some about of thermal bridging or reduced insulation at the corners of walls and ceilings. When the temperature reaches the dew point temp, water happens. It is dependent on the extent of the conditions as to the severity of the issue.

Turn on bathroom & kitchen exhaust fans. A dehumidifier would also help. What was the attic temp compared to indoor & outside temp?



On cooler mornings, do you ever have a condensation film on the inside lower glass/sash intersection of windows? If so, direct a fan at this area and go back an hour or so later and see if the moisture has disappeared? This condensation is another localized condition that will occur in even moderate interior rh’s of 40-50%. When you blow the house air at the window area of 100% rh, there is opportunity for evaporation since the house air is not saturated!!

Due to the window sash geometry of the right angle where the glass enters/sits in the sash, a small triangular pocket of still air develops- naturally convecting air moves in gentle curves and does not move into the right angle pocket of air. As outdoor temps drop, the glass gets colder and the pocket of air gets colder (especially in sealed glass units with metal tubular spacers), until it hits its dewpoint and is deposited on the glass. By blowing warm room air (68 deg) at this location, the glass temp is raised above the dewpoint temperature and moisture disappears.

How is this example different than the condensation above the window where there may be cool/cold infiltrating air from the vented soffit* getting under/through the batts to cool the ceiling to what…35-40 deg…anyway below the dewpoint of the room air? Blow room air of 40-50% rh and 68 deg at it and the moisture will disappear!!

  • Note: It does not even have to be a vented soffit. There is a lot of air movement around hidden gaps at the edges of the soffit pieces and F/J trim & fascia trim. Over time, it may become more than actually is moving through the small orifices in the vented style. The edges of any opening produce a drag/resistance on air moving through the opening. The smaller the opening, the more the edge effect of drag/resistance predominates and restricts air movement. As time goes on, dirt/dust begins to collect on the edges of the small orifice further restricting air flow. So the bigger hidden gaps in the trim may actually be doing a good part of the ventilating.

Back in the early 1980’s, used to see 2-4’ cold streaks on insulated ceiling drywall caused by exterior air moving in the cavities formed under the insulation batts laid over 1"X3" strapping for the drywall attachment. For the older thermographers, the IR unit was a Swedish Thermovision 720 or 700 series…cost about 40 grand and you’d have to carry liquid nitrogen around to supercool the sensing device so it could pick up the thermal differences with better resolution. Things are much better now!


I am not disputing your observations about blowing air on moisture. However, your solution has no scientific basis as a method of remediation without fully evaluating the psychometric properties of the air in question.

This is my point .Moisture does not disappear.
Just because you can’t see it, does not mean it’s not there.
There are several solutions to the problem once the actual cause is evaluated and determined.
Blowing air as well as opening a window or door may make the condensation go away but neither is desirable or effective as a long-term solution. Blowing air on a bathroom mirror after taking a shower works and works best when you add heat. Adding heat changes the psychrometric properties of the air. This is what I am referring to. Simply moving air will not guarantee moisture removal. Changing one of the psychometric properties of air will.

We have not determined this! It may very well be! I can give you a laundry list of possible causes.

The point I’m trying to make is that we must determine the source of the moisture, determine the quantity and evaluate the best means to prevent the occurrence of condensation under all potential conditions. Recommending to a client that they can mediate a moisture problem by simply turning on the ceiling fan may result in an opposite effect. Without further evaluation increasing air movement may increase moisture condensation (which I’m trying to point out). Additionally, the client may not be comfortable with the ceiling fan running all the time in the heating season. It may be a child’s room and may cause health issues.

I am not saying that moving the air in the house will not change the condensation condition. There is a high probability that there will be air in other parts of the house/room with different psychrometric properties. What I am saying is that it is not a definitive conclusion to the issue. By not determining the source and quantity of the moisture in the house and simply recommending turning on the fan is not going to hold up under the scrutiny of litigation.

The majority of times that I have found cold spots at the ceiling/wall juncture it is simply a matter of putting the insulation back where it belongs or installing batt insulation properly so air convection does not occur. As in this photograph where a ceiling repair was made and the insulation was never put back.

In this case would you recommend turning on the ceiling fan forever or replaced the insulation?