A Charleston home inspector discusses windows with integral j channel

Submitted by Ray Thornburg on Mon, 11/19/2012 - 14:24


Blue Palmetto Home Inspection serves Charleston logo

     About Vinyl Windows with Integral J Channel




This article is about houses with windows which have built in j-channel. This style of window is popular among builders because it negates the need for purchasing j channel in those areas. However, in the opinion of this inspector this type of window has a design flaw that necessitates installing flashing in a way which is rarely done. Moisture could penetrate the siding at the corners of the windows and enter the structure if not done properly. Flashing should actually overlap the next lower piece of siding for it to be effective as seen in the following photo.

correct flashing under vinyl window

    Click image to enlarge. This is the correct way to handle this problem. 





The next photo shows a window with integral j channel.

Water can enter here

     This picture shows how moisture can enter at the corner. This water will flow over the house wrap. However since house wrap is not waterproof moisture can penetrate into the structure. House wrap is actually designed to allow a certain amount of water vapor to pass through so that wall cavities can dry out.



Windows which are exposed to direct rain such as first floor gable end walls are more susceptible to moisture intrusion than windows located under roof overhangs. Because this style of window (and vinyl mounting boxes) are not often properly flashed;  we suggest investigative measures to be certain. Windows which do not have an integral J channel but are wrapped with regular j channel are also vulnerable to this however to a much lesser degree. Since the amount of leakage is usually small it may take years before anyone notices. I've actually had to repair homes that were improperly flashed. Below are some pictures of homes under construction which show the typical way these windows are installed now days.


Typical construction method

    This picture shows how the flashing is not there. This is how it typically is done. However it is not correct to allow moisture to flow behind the siding like that. 




Typical window

    This the same window. They did a real good job of installing the house wrap. Notice the house wrap over the top window fin and tape down the side with the bottom window fin over the house wrap. They just forgot the most important part - the flashing at the bottom over the vinyl siding nailing fin.



house wrap at to of window

     They actually leave the house wrap off at the top until the window is set.... then it is folded down so moisture will flow over the window flange instead of behind it. Good Work Here. 




tape at top

    This is how they tape the house wrap at the top of this door. Again.... more good work; however you can see the duplicity here. Good work all the way until the vinyl siding is installed. This is where they forget to install the flashing at the place which is most likely to protect the home from moisture intrusion. So at the top they are very concerned with controlling moisture vapor only but at the bottom where actually leakage could occur there is no flashing.



All these photos were taken on local area homes in the Charleston lowcountry area under the most modern building codes. If your home has vinyl windows or even windows which have regular j channel in my experience it is unlikely to have been flashed correctly. Will this be a problem for you?..... maybe.... if your windows are exposed to direct rain like gable end walls or the bottom windows of two story homes. The only way to be certain is unsnap the bottom piece of vinyl and take a peek. One thing to note however is that the removal of the siding for inspection is beyond the scope of a home inspection. You're on your own there. However; getting a home inspection is one way to protect your investment by alerting your builder to the proper procedures. 


Remember Blue Palmetto Home Inspection serves the entire Summerville, Charleston lowcountry area! 


Submitted by Ray Thornburg on Mon, 11/19/2012 - 14:24


Anonymous | Wed, 08/11/2021 - 16:11

What would be your advice on how to fix the problem of improper flashing on windows.

Ray Thornburg | Thu, 08/26/2021 - 08:53

Thank you for your question. You're probably have to investigate a bit to see what is going on. It could be numerous things....even a crack in the plastic window jamb. If it's just water running down the outside of the window frame then the flashing like what is shown at the top picture of this blog will probably help.