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Question DetailsAsked on 11/21/2017

Is it okay to have a skylight window bigger than ceiling lightbox?

We just had our roof replaced and the contractor installed/replaced 2 skylights that are about 4" larger in length than the "lightbox" opening. When we noticed it, he said that skylight companies have now changed the standard size of skylights and so it's better to use a larger standard window because the custom size windows are more prone to have problems with the custom flashing. He would instead install a custom seal around the inside of the lightbox so you wouldn't see the size discrepancy. Is this true that custom windows have more problems with the flashing kits and is it okay to have this size discrepancy (it's only about 4in)? One thing I read said it could cause problems but this was only one source.

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The problem with the lightshaft/lightbox being smaller than the skylight, other than reducing the light coming in, is you do not get as good an air circulation in the portions offset from the lightbox edge, promoting condensation or even freezing at those locations, which can then cause rot in the sheathing and wetting/staining/drywall deterioration in the light box - even in extreme cases to the point of free water dripping down the lightbox on the (inside or outside face) and then dripping from the ceiling. Of course, the greater the percentage of the skylight which is obscured from direct airflow, and of course the frequency and severity of skylight freezing or at least getting cool enough to cause condensation on the underside) - which is a function of household humidity, inside and outside air temps, and ventilation in and insulation around the lightbox - can make all the difference between having a problem and no problem.

Also - light boxes, especially "deep" ones reaching well up into the attic or poorly insulated ones, have these problems commonly anyway in climates with winters, so it may be hard to tell what the cause is - but if your skylights develop condensation only at the obscured ends and not along the edges, that would be a pretty good indication.

The percentage of area obscured from direct airflow matters a lot too - 2 inch extension past the edge of the liguth box would be pretty significant and be expected to be a problem in a small (say 6-12 inch lightbox) - not so much if we are talking a 2-3 foot opening in each direction.

Probably the biggest factor is the insulating value and height of the skylight (the higher it stands up the better in terms of avoiding icing/snow buildup induced leakage problems, but the more it cools off being exposed like that too), and the cross-sectional size and height of the box - the more restricted the opening and the higher or "deeper" the less circulation of household air it gets so the more the moisture from the warm household air rising in the lightbox causes condensation.

Unfortunately, a lightbox which has cold air cascading down from it in the winter is also the least likely to have moisture problems - because that cold air (assuming the box is airtight and insulated around the outside) is being replaced by household air, which is warmer so acts as a dehumidifier in the course of the airflow. That is why fans underneath, blowing air into the lightbox, are commonly used to solve condensation issues - not energy efficient but that is the tradeoff between light and energy efficiency - if you want a really energy efficient house build with very thick insulated walls underground and use monitors displaying outdoor scenes as your windows.

As to the custom windows having more sealing problems - really should not be the case, BUT because they are custom-cut and installed rather than using mass-manufacturing pressing dies, it is likely true - machine-assembled windows and skylights tend to have fewer problems than hand-done ones.

I would say, assuming these are roughly 1.5 to 2 foot x 3 foot or larger units, that the installer knows what he is talking about - I would be sure any trim at the top of the lightbox is installed so it does not trap any moisture coming down from above, and does not inhibit airflow into the skylight - keep trim outside of the "box" itself, as exterior trim, not reducing the lightbox dimensions.

It is also important that the interior "floor" under the skylight is properly handled so any condnesation coming off the inside of the skylight "walls" comes down into the lightbox where it si visible and can be handled, rather than accumulating on top of the sheathing in the "overlap" area - some ice and water shield or similar product is likely the best for that (will be out of view).

Bottom line - not "ideal" situation, which would have been flaring or expanding the lightbox to fit the skylight, but probably, if flashed and sealed correctly, not likely to be a major issue either. I would consider, if the lightbox does not have a good water-shedding coat of paint now (say semi-gloss or satin) you might consider having it repainted so any water getting on it is shed rather than absorbed, and will be detected warly on rather than occurring for years and then showing up as sheathing or attic framing rot. Make sure the paint is treated with mold-preventative powder mixed into the paint, or use Kilz or similar mold-prevention paint, to reduce the risk of staining and/or mold/mildew from any water running down in the lightbox.

[Note - condensate water in the lightbox would not necessarily be the fault of the overhang 2 inches - might happen anyway, especially in areas with winters and most especially in houses with a lot of boiling or frying cooking, not using bathroom/kitchen fans to remove humid air from cooking/showers, or with a lot of house plants.]

Answered 3 years ago by LCD

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