Condensation on skylights

[Mike Hartigan]

Well, here we are in the midst of another winter of sub-zero temperatures and I'm having the same problem I have every year at this time. It didn't go away by itself again this year, so I thought I'd ask for suggestions.

I built an office addition on my house about 12 years ago. There are two skylights in this room (the preferred lighting alternative to windows overlooking the side of the neighbor's house). I also have a central humidifier which does a superb job (Craftsman 3000, 14 years old, foam wheel type). As the outside temperature goes down, so goes the humidistat (that's normal). This seems to keep the condensation on the skylights to a minimum. Indeed, I use the condensation on the skylights as a guide to setting the humidistat (simple, yet effective). In case you haven't already guessed, these skylights are the most condensation-prone sites in the house.

When the temperature gets into the single-digits or below, I have the conflicting problems of dripping skylights and static discharges whenever the kids touch other (which seems to happen a lot more often under these conditions, but I digress). I dont want to reduce the humidity further, since the static discharges suggest that it's already too low. I don't want to increase it, since the condensation suggests that it's already too high.

I have a ceiling fan in that room, and that seems to reduce the problem somewhat (it doesn't eliminate it), but I'd like a 'better' solution. Perhaps something to apply heat to those skylights, or maybe small fans directed at them? Is this a common problem with a common solution that maybe I'm just not aware of? I've searched and can't really find anything that addresses this.

Thanks

-Mike

[Edwin Pawlowski]

Any way you can insulate or make a barrier with plastic sheeting? Since you can't change the laws of physics, the only thing you can do is add a thermal barrier. If it is the glass causing the condensation, double pane would help. If it is the frame, you have to make a barrier of sorts.

[Joseph Meehan]

You are on the right track. I would start by seeing if there is any way to insulate the skylights themselves. Adding thermo glass and replacing thing like aluminum frames with a less conductive material like plastic. would be a good start.

Adding an additional window below that one and allowing some air past the existing window should also help. The temp difference will be less for the new window and the air in-between will be less cold that outside and dryer than inside. Note: cold air hold less moisture than warm. Even if it is snowing outside with 90% humidity, if you bring that air inside it becomes very dry as it warms up. The condensation is coming from that 40% humidity 70º comes in contact with the 10º glass and frame and it cools to

10º and it can no longer hold the same total amount of moisture as it is now over 100º humidity.

Your idea of fans (including the ceiling fan) warms the window a little and that is why you have less condensation. If you can warm the Frame and glass enough it would work. Also heat lamps or any heat source would help.

However the real answer are modern skylights designed to handle your conditions without condensation.

[Hello Friend]

'I also have a central humidifier which does a superb job (Craftsman

3000, 14 years old, foam wheel type).'

Being in the Trade, i can tell you that this type of humidifer is the worst one. WHenever the humidifer wheel is not turning, you have standing water which is a breeding ground for bacteria , algae, and mold to form. Then it gets delivered thru your duct system by the furnace blower to all areas of your house. Just thought id give you a headsup so you dont get an airborne disease(s). Aprilaire Brand is the best to have to prevent this entirely.

[nicksanspam]

You might try an anti-static spray on the rug, eg salt water.

Nick

[Uncle]

It would be interesting to know if your skylights are plastic/acrylic or glass. Going out on a limb here, I would venture to guess your lights are of the acrylic/plastic dome stile.

Pella, Andersen and Velux which are glass, manufacture their skylights with a small tray at the bottom of the light. This tray collects condensation to help prevent those drips, the condensation evaporates out of the tray. I've seen the trays overflow when there is too much humidity in the home.

In twelve years since you've gotten yours, there has been major improvements to thermal breaks, R factors and general overall insulating factors in skylights with glass. If the problem you have can't be tolerated, I would look at the better skylights. In my book Velux is the best.

[Mike Hartigan]

Thank you for your concern, but I'm aware of these problems and I tend to be aggressive with regard to minimizing them. I clean and disinfect the reservoir monthly, more often during less-used periods. Aprilaire does, indeed, eliminate this problem and when this unit finally dies, I'll probably go that route.

[Mike Hartigan]

These are fairly recent technology with respect to construction and materials (about 12 years old). Veluxe (I don't recall the model#) double pane, aluminum/wood construction. Although perhaps a plexiglass indoor 'storm window' type solution might help - along the lines of your plastic sheeting suggestion. That would keep some of the warm air away from the glass.

Thanks

[Mike Hartigan]

As I said in a response to another post, it's a double pane, aluminum/wood construction. Technology-wise, it's already designed to minimize such problems but, physics being what it is, it doesn't eliminate the problem.

I understand the physice behind the problem (somehow, it sounds so much more complicated when you try and explain it ;)

The ceiling fan works fairly well and I've also closed the register vents in this room, which will reduce the temperature by, maybe 2-3 degrees. More significantly, however, there will be less humidified air directed into this room. We'll see what happens.

Again, these are 'modern' skylights.

[Mike Hartigan]

These are Velux double pane glass, aluminum/wood construction. No drip trays, though.

[HaHaHa]

Are these skylights at the top of a sheetrock "tunnel" from a normal flat ceiling to the roofline?

If so, I'd access the attic space surrounding these tunnels and insulate them.

[e-mail]

[Mike Hartigan]

The ceiling is sloped parallel to the roofline, about 25 degrees. The space between the joists is insulated with 11.5" fiberglass with a suitable vapor barrier. There's really no room for improvement here. I suspect that insulation is not the problem problem, since the condensation is occurring only on the glass.

[bill a]

I think you actually had the best answer in one of your own posts, namely that the temperature will need to be a bit lower in that particular room. That's about the only way to drop the dew point near the skylight without decreasing the humidity housewide. Any air mixing you can get from ceiling fans or more focused air drivers will help as well. By the way, in winter, you would fans moving air upward to displace the moist strata at the ceiling.

Bill

[nicksanspam]

Perhaps you mean "higher."

Why would that change the dew point?

Nick

[default]

You don't need a thermal barrier, you need a VAPOR barrier. Make a wood frame that will fit over, or inside, the window well, and cover it with plastic. Make a small cloth bag, or stuffed toy or something, and fill it with rice, or dessicant. Toss that on top of the plastic, fit the panel in place, and caulk with that nifty removable caulk. The plastic keeps household air from circulating against the window, but lets enough heat through so that you don't get condensation on the plastic. The bag of dessicant eats whatever moisture does make it into the (now closed) space. Don't let the bag rest against the edge, it shouldn't touch anything but plastic.

--Goeedjn

[bill a]

warmer air holds more moisture, greater temperature differential at the glazing

bill

[nicksanspam]

Warmer is better because that raises the glass temp, but that won't change the dew point temperature of the air, ie the temp when dew forms, which only depends on the actual moisture content, vs how much the air can hold.

[bill a]

whatever you want to believe is OK with me :)

Bill