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Sky light condinsation ruining drywall

Big Will
Big Will Member Posts: 396
Although some customers do want A/C the need is minimal. This house is heat only. However the need for heat is a constant.
On Friday I had two guys working in Napa valley north, complaining about the 110deg heat. The same day I am with a guy in San Fransisco looking for a coat at 4:30 with the wind blowing.
The San Fransisco fog comes in all year round so the dew point is low in the evenings and mornings and sometimes all day.
"The coldest winter I ever spent was a summer in San Francisco." Mark Twain.


  • Big Will
    Big Will Member Posts: 396
    Its huge

    I have a contractor that is doing a light remodel that I work with. The home in question has a sky light in the stair well that runs the length of the house. The house is three stories and forced air heat so the top of the stair well gets very warm and humid. The sky light is a peaked type that is about 50 feet long and six feet wide. Because it is peaked the condensed moisture runs down the glass and hits the frame and then down the wall. I suggested heat recovery ventilators removing the humid air from the area and bringing in outside air with the registers pointed towards the sky light. Any thoughts?
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Is this an old mansion with a replaced skylight or a new mansion made to look old?

    I've seen similar in old houses. Skylights in general are well known for condensation problems and with that sort of placement and design I'd say that condensation is a guaranteed to occur at times even with modern multi-layer, insulating glass panels.

    In the days of the dead men with single pane construction only, they assumed that significant interior condensation would occur and entire unit was designed to collect and drain the condensate. Of course this was difficult and expensive and some tried to shortcut, but the plaster and skylight didn't last very long...

    Unless provision to collect and drain the condensate can be added, you'll need to directly heat the glass. Heating the glass itself is best, but you'd have to replace the panels. Practical heated coatings are here but not yet easily available. You could always blow quite a lot of hot air across it constantly (or under reasonably failsafe control), but the delivery mechanism would probably be unsightly and definitely wasteful.

    Just how big are the stair hall and skylight? What location? Climate greatly affects potential of condensation.
  • Big Will
    Big Will Member Posts: 396
    old yes

    mansion no. Its in San Fransisco. The climate here is real mild. almost never freezes and 85deg for ten days is a hot summer. The glass is single pane. About sixty feet long and the vault is about 48" wide. Since the glass is peaked the panes are about 36" on each side. The GC'c first reaction was use a fan to pull air out of the space and exhaust it outside. Kinda rough on the enegry saving idea. Spent all that mony on a condensing furnace to blow air outside. I sugested pulling the air out and blowing it back into the house. After reading your post it seems like it would make more sense to pull the air from main house and blow up into the vault. The heat recovery ventilators came to mind on the drive home after looking at the job. The vault is accesible from the roof along the long side of the skylight. I was thinking of four registors blowing up into the glass.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Is this an original part of the design or was the skylight added? Why single pane glass? I know your climate is mild, but aren't fog, dew and frost common in many locations?
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Can at least the bottom edge of the panels be accessed without staging? If so, and especially with single-pane construction, I'd resign myself to accept some condensation and collect to be drained or evaporated. Could small metal trays be fabricated with low voltage electric resistance strip heaters on a simple timer?
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,230
    Not that many years ago...

    ... I used to make my own double glazing for sky lights. Possibly it could work here. Leave enough room for air to move between panes so condensation doesn't build up, yet tight enough for the inside glass to warm up. It's not real double glazing, but it works. Square 3/4" neoprene (sticky on one side) works nicely for spacing the glass and gives flexibility for shifting building parts. Why do I know that ;~)?

    Yours, Larry
  • Big Will
    Big Will Member Posts: 396
    I dont know why single pane was used.

    The house was built after the 1906 eathquake like most of San Fransisco. The windo is from the last remodel about twelve years ago.
    I want to sugguest reglazing with dual pane. With the heat collection/hummidity that collects in the sky light vault I don't know if this would do any more than help.
  • don_200
    don_200 Member Posts: 4
    Dewpoint and surface temps

    Thats what you want to measure.Is it the glass itself sweating? Or is it the wall itself that is sweating?

    I find it hard for your glass to be sweating in the summer time.Only seen it in the winter.

    Keep this in mind when your surface temps is lower then your dewpoint that when you will get water.

    Maybe you should look at the ac system itself.

    I've seen poor airflow and a stat set at 65 and below create some condensate issue.

    Have you look into all the combustion burning equipment in the home?

  • Tombig_4
    Tombig_4 Member Posts: 45

    I think the pertinent issue here is WHEN the condensation occurs. Here in the midwest it's a huge issue in the winter when outside temps dip, and even low indoor humidity levels are overcome by the temperature of the single pane glass. If the skylight assembly is steel mulled with single pane you are condensing through the steel as well.

    One solution I've seen is to build a plexi or glass panel at ceiling level to keep indoor humidity/temps from hitting the cold single pane exposed to outside temps. If it's sealed well this works. At 48" X 60' this could be a daunting project especially retaining the architectural attractiveness.

    If the condensation occurs during normal non heating/cooling periods, ie. cool nights after a hot day, an exhaust fan is the answer.

    I can see no scenario that they would condense during the cooling season.

    Will, your idea of recirculating the hot, stratified room air back to the bottom of the stairwell or elsewhere below is a good one. I'm actually doing that in a new construction home now for the architect/owner. His idea and I agreed. This is in a 22' tall great room with a fireplace chase. I'm not sure it will solve your condensation issue. BTW..I've seen plenty of double and triple glazed glass panels condense in similar installations.

    Hope this helps.

    Tom G

  • Larry and Tombig are right

    I had problems with my double pane skylights in my bedroom. I heat only the mattress, so the bedroom gets coooold. I would get a lot of moisture and mold at the skylights. I have a customer who made plexiglass panels and affixed them with velcro so he could remove them in the summer to open the vent in the good weather. My glass guy talked me into tempered glass because it won't get cloudy. I installed the glass permanently and sealed it with silicon. My skylights are fixed so I can't open them anyway. My room is a lot warmer with the triple pane, I have about a 1" space and my customer with the velcro has about a 4" space. Both systems worked out well with no condensation getting between the glass.

    Thanks, Bob Gagnon

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