Everything you need to know about fixing up your skylight
Answered by Member Services: Hello Terczek,
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Answered by LCD: Tough question - and also depends partly on whether you have double-pane (or triple-pane) skylight with moisture between the panes, or if the condensation is on the underside of the glass.
If BETWEEN the panes, then you have a failed seal (commonly occurs about 15-20 years life in sun-exposed double and triple-pane windows) and either the entire skylight can be replaced or only the glazing unit, assuming it is the type where glass can be replaced - some brands are not made for that so it is sometimes more expensive to replace the glass than the entire unit in that case.
If fog/moisture is forming on the UNDERSIDE of the glass, then that is household humidity trapped in the skylight well, or the glass is not insulated enough for the inside environment in the house when it is cold outside and you are getting condensation of household moisture on the cold glass. In very cold areas unless you specifically ventilate that area with a fan that is pretty much unavoidable, but if occurring at above freezing or barely below freezing outside conditions then you need to consider whether the skylights are worth it, or putting in much better insulated (probably triple pane) units - which can be done on some single-pane units, in other cases requires a totally new skylight. Ditto if moisture is condensing on the metal skylight housing - either a better insulated unit or better ventilation at the skylight (to remove the condendation as it forms) may be necessary.
Replacing the glazing unit (the glass and the surrounding aluminum frame which holds the glass elements) typically costs about half or sometimes more like 3/4 of the cost of total skylight replacement. Where your situation gets tough is that with a reroof planned in a couple of years, especially if not a high-efficiency unit and you are in a cold area, that would be the logical time to replace the entire skylight with a new better insulated one at that time. Ditto if it has leaked around the edge seal (which you say it has not - does that statement include having checked out the the underside of the seal in the attic for leakage around the base of the unit ?), in which case it would likely have to be removed to properly flash/seal it into the new roof, which of course would be a prime time for replacement.
You could certainly get a quote from a Window contractor or two for total replacement to give you an idea of what it will cost - and also from a Glass and Mirror (your Search the List cagegories) contractor for replacing just the glazing unit (the glass element) if it is replaceable - that should give you some idea of your options.
Many contractors would immediately advise replacing the whole skylight at roof replacement time - the reason I am reluctant to jump right to that is that a very large percentage of the new ones (especially those made for residential use) are made very cheaply so do not last like most of the ones made 20 years ago - so many times if the simple "inset and seal" as opposed to crimped-in glazing units glass replacement is pretty quick and easy, leaving you with new multi-layer glazing in a heavier duty, stronger frame than you would easily find today.
Another consideration is that while a roofer should (in theory) be able to properly install and flash the unit, a window replacement company may or may not get the water barrier, ice and water shield, flashing, and caulk correctly done to properly seal the unit AND not damage the roofing waterproof layers - done improperly (and I would say that is probably 25-50% of the installers) and you risk a new leak because of the new unit installation.
One other recommendation if looking at total replacement, especially if in snow country where you might get snowmelt or icing on/around the unit would be to look at commercial skylight options too. Cost typically 25-50% more or so for same size but typically have much more rugged construction (heavier gauge metal rather than tin-can metal or plastic) and commonly have a double seal on the glazing unit - the seal (compression or urethane sealant) around the glazing unit itself, plus a screw-on or pop-riveted overflashing with gasket which seals a gasket or urethane sealant tightly against the edge of the glass on top, holding it tight to the skylight frame to provide a doubled waterproof seal.
The exterior condition of the skylight unit of course factors in too - if metal and in good shape you might repair it now and hope it survives the reroof OK when that happens. If plastic (or partly plastic) housing, at that age, I would replace it at roof replacement time and live with the fogged glass for another couple of years if you can.
One other factor - do you know if the skylight is original to the house, or if perhaps it was added after the initial roofing job - in which case that might mean it is installed OVER the first roofing layer and would have to come out as part of the reroof anyway - which of course would push the decision toward holding off on repair or total replacement at that time.
One thing I specifically recommend AGAINST - the companies that claim to remove the fogging without changing out the glazing unit. Those "solutions" rarely last a year - because while they can evacuate the moisture between the panes by flushing with dry gas, there is no way they can properly reseal the mounting frame so the inert gas between the panes does not leak out again as it expands when it heats, or pull in outside moist air when it cools, and they charge so much that even for a possible 1-2 year period that is not a viable alternative to glazing unit replacement, even if it stayed dry that long. Just a waste of money in my opinion.
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