# How to waterproof window sill from condensation?



## saw (Jan 7, 2014)

I have some aluminum framed windows that tend to get condensation in the winter and the water then drip down and stain the window sills.
A few years back I sanded the sills and restained them, then applied polyurethane. I thought polyurethane would protect the sills but it doesn't. Now the sills have water stains again.

I thought polyurethane is supposed to protect wood from water; why didn't it work?

If I coat the window sills with 2-part liquid glass epoxy resin (for coating table top), would it do the trick?


In the photo the window has been opened so it's not showing.


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## Steve Neul (Sep 2, 2011)

I don't think you will stop the condensation. From a finishing perspective there is so much sun coming in that window the sill is expanding and contracting so much the polyurethane which is an interior finish can't keep up. You need to take the polyurethane off and finish the sill as though it was on the outside of the house. This would mean using 100% tung oil or a marine grade spar varnish. The best spar is Epifanes.


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## Toolman50 (Mar 22, 2015)

The condensation is standing all night and maybe all day on the sill. No finish can take standing water very well. Combine that with the exposure from the window and it's just too much for a natural wood finish. I recommend either covering the sills with laminate or replacing the sills with stone (either natural or man-made). Of course the moisture can still run off the sills and stain your walls. 

I have the same problem but I now have painted sills. A quality paint holds up slightly better than a stained finish.


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## Rick Mosher (Feb 26, 2009)

Many times when we installed windows the carpenters would trim the sill to fit and then not finish the cut edge right next to the window. This is where the moisture gets in under the finish. I would always either coat that edge with West Systems Epoxy or a quality exterior paint before installing the window. May not be an option for you now unless you have to replace the window.


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## Bron (Jan 14, 2019)

I have exactly the same problem with my wooden stained window sills. Did you come up with a good solution? If so, can you pas it on? Thank you


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## Steve Neul (Sep 2, 2011)

Bron said:


> I have exactly the same problem with my wooden stained window sills. Did you come up with a good solution? If so, can you pas it on? Thank you


Adding to what I already wrote if you look at the picture the window sill was probably installed and then finished. The surface finished with polyurethane if not in the direct sun could have had water run between the window sill and the window. The raw unfinished edge of the board could easily soak water where it could bleed under the finish and fail. To truly make it waterproof the edge and underside of the sill should have been sealed with a waterproof finish prior to installation. Then use a rubberized caulk such as a gutter and flashing caulk between the sill and the window. 

Polyurethane is a very hard interior finish. It is waterproof however if the sill sits in the sun a lot the wood expands and contracts more than the finish can tolerate and cracks. This can allow water to seep in. A spar varnish is made for exterior use. It's formulated to be more elastic to deal with wood movement.


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## 35015 (Nov 24, 2012)

saw said:


> ...I have some aluminum framed windows that tend to get condensation in the winter and the water then drip down and stain the window sills...


Hello Saw,

As you have read by now, many have this challenge, and it is more common in modern buildings than we had in the past. A challenge, to be sure, yet it can be dealt with...



saw said:


> ...A few years back I sanded the sills and restained them, then applied polyurethane. I thought polyurethane would protect the sills but it doesn't. Now the sills have water stains again. ...


Polys and Urethanes are just modern plastic film finishes...

They are going to always fail given enough time, whether of interior or exterior grade. 



saw said:


> ...I thought polyurethane is supposed to protect wood from water; why didn't it work?...


Many (most I would suggest) of the modern finishes are mostly strait up "plastics" for all practical purposes. As such, they are a "film finish" and even the one you do get to "soak in" are not a natural material. If we take a synthetic material and try to get it to work with a natural material it very seldom "plays nicely" and in a short time either delaminates and/or breaks down of its own accord. 

Modern finishes are incapable of "aging" and/or "weathering" in the same way as traditional finishes do as they develop a patina over time. Something that can only be "faked" and never really achieve without the same materials being used on a project...



saw said:


> ...If I coat the window sills with 2-part liquid glass epoxy resin (for coating table top), would it do the trick? ...


Fine Woodworking (someplace in the first 20 additions?) did an article profiling the "water proofing nature of finishes." I'm sorry I can't recall which issue. This was one of the first "non-academic" good articles I had ever read on the subject. 

In it, the reader quickly learns that most finishes are anything but "water proof," even most epoxies and not until they are highly specified in application (special epoxies) and applied in layers number over 5 or more does any great "proofing" take place. Even then, unless a solid build (aka artificial coral, rock or other artifacts as found in zoos) they will delaminate and fail in a relatively short time.

Oddly enough, one of the least durable finishes was the only thing to fall into the 98% or higher for stopping moisture in or out and that was either a beeswax or related family of waxes...

The next was traditional finishes (as already mentioned by others)...that blend these...

*What I use and have used for 30 years plus:*

I use on most wood working be it a floor, timber fame or furniture is a blend. This is made of flax, tung, citrus oils, blended with beeswax and pine rosin. Its a millenia old blend found in different forms from a number of cultures. Any like it or similar to it will work for you. 

Recommended application is to get the wood sill stripped of the old modern finishes down to bare wood.

Make sure the wood is dry (at least to the touch) and warmed up in some way (sun, heat gun, heater blowing directly on the wood, etc) then start brushing or ragging in the oil finish...

To maintain, this doesn't usually need being done but every 3 years with just a touch up with a rag or brush. Some folks just use a rag dampened in the same finish to clean the wood once a year in the spring. The patina and characteristic of such finishes are hard to really describe well and not until you start using them do you really learn how pleasant and easy they are to use and maintain...

Good Luck,

j


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## johnep (Apr 12, 2007)

We suffered from condensation for years. It vanished when I installed double glazed UPVC windows.
johnep


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## woodnthings (Jan 24, 2009)

*I have the same issue....*

I've refinished my sills twice so far after 30 years, I was thinking maybe a clear Lexan sill cover or glass that would keep the water off the wood. You know it will get on there, so something other than varnish may be better? :vs_cool:


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## Tennessee Tim (Dec 15, 2010)

Stopping it, I don't know of a way without replacement. The way some handled this issue over the years was a marble/cultured marble/stone/ceramic/dark or colored glass glued on the window sill. I mostly seen cultured marble due to sizing and costs at that time, granite can be used also. Most were installed instead of wood BUT it can be glued on provided properly sealed at aluminum contact on back edge.

History and cause. This is a condensation issue due to thermal temp transfer. It has to do with temperature changes and moisture in the air, don't ask me to explain how it works as I'm not that educated on the subject...it just happens!!! Aluminum frames are the worst about the issue due to it's quick ability to move/change tempature. One of the options years ago was a "thermal break" which was basically a rubber/plastic/vinyl/epoxy layer between the inside metal and the exterior metal. This prevented the frame from transfering the 2 sides temp and causing the frame sweat BUT that doesn't always stop the glass sweating BUT it is greatly less than the aluminum. 



Fixes.....a good dehumidification system for your interior air helps (this is moisture in the warm air reacting with the cold framing. I would do a large DH as most houses need it anyway for the air, then for the moisture running down I'd do a marble/granite overlay (glued on top of existing wood) with the back edge sealed with a silicone/urethane based caulk to prevent under leakage. This is your simpliest long term that will also look good for years without maintaining.

edit...LOL...I didn't catch till after posting this was a old thread....maybe it'll help someone in the future!!!


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