I have a below-grade basement window in a lightwell. In heavy rain, the lightwell fills to the bottom of the window and then the water leaks into the basement. I've been through a lot of options and my current plan (plan D) is to screw a clear plexiglass sheet to the window frame to physically block the water but still let light in.

Question: is it OK run a bead of polyurethane sealant (e.g., Sikaflex) along the edge of the sill before I screw the plastic to it, as a waterproof barrier between the wood and the acrylic plastic? Another question in this forum said you should not use silicone because that contains xylene, which is bad for the plexiglass. In net research, I've found any number of answers to my question, all different. Mostly that it won't stick, or I need some special-order primer before using the sealant. Considering that the screws will hold the plastic in place and against the wood, will just the polyurethane provide a reliable waterproof seal, especially between where the screws are?

In reply to comments questioning why I am attempting this: I've tried other things. 1) I thought the lightwell was filling from direct rain. I built an awning for it that also made sure water wasn't wind-blown in from the sides. Didn't work. 2) Turns out the light well consists of concrete walls around a mud floor. When it rains hard, the ground saturates, the water level outside is higher than inside, and the light well starts to fill. The bottom of the windows is still several inches below ground level. I looked into adding a concrete floor. That ran into several problems including not being able to secure the concrete walls firmly to the existing foundation, and creating a "boat" that will want to rise up with the water level. 3) French drains won't work--remember we are below ground level, there is no place for the water to drain to. The contractor who suggested one wanted $12k to put it in, and remained vague where the other end would be. 4) I could install a sump pump outside and pump the light well to some undisclosed location. This is getting into a bigger project that I was hoping for. I could pump it a long way across some landscaping and a driveway onto the lawn. Or I could try to pump up into the kitchen line 15-20' away that flows into the sewer.

It only rains hard enough to spill into the basement half a dozen times a year. As far as I can tell, the previous owner cleaned and painted the concrete in the basement, but this must have been going on for decades and the window frame hasn't rotted (yet). Its only the bottom few inches of the frame we are talking about. Solving the problem seems difficult and expensive--I was hoping the brute force hack would do the job, with annual vigilance.

  • I'd try a small test piece first. Depending on what your plexiglass/Lucite/acrylic actually is, organic solvents can produce self propagating cracks which eventually destroy the material. Acetone is particular death to acrylic sheet, but i'd be surprised if there were not other very bad combinations. Sep 17, 2019 at 22:48

4 Answers 4


Look into getting window well covers. They are purpose built for this type of situation.


Seems you are trying to put a band-aid on the problem instead of fixing the underlying cause (i.e. the lightwell filling with water).

First, I would attempt to fix that issue. Perhaps by putting in a drain at the bottom of the window well and channeling the water away from the basement with drainage tile/pipe/gravel into your yard. This will allow the window window well to (continue to) serve as an egress --especially if there is no other exit from this basement area/room.

The water problem is eventually going to rot or damage your window if it hasn't already. I do not believe the plexiglass solution will work and in-the-end, may cause more problems than it solves. beswald's solution may work as well, but I don't know how expensive that will be. My solution is probably about $15 work of material and $15 more elbow grease than the plexiglass solution.


Is the window the only exit in a room such that it would be considered an Egress window?

If so then it would be illegal and dangerous to block the window by screwing plexiglass to the window frame.
Just something to consider.



It seems others commenting didnt read you commet on ground saturation.we installed deep drainfields to get rid of some mountain run off this has helped. We did the yard then put well covers. My side basement i screwed and polyurethanes a piece of plexiglass over it. Id go with your idea to start. Think ships window. Id use rubber faced stainless washers on the screws to lessen cracking and distribute pressure. In fire school they showed us if you strike a plexiglass wall with a very sharp point and it will shatter.


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