J-channel around windows

Should this window(s) have J-channel installed around them or is the flange on the window(s) considered to be an “integral” channel whereas J-channel isn’t needed? New construction.

As always, thanks in advance!

Integral “J” is fine, its actually better. no pieces to separate.

Thanks. Just wanted to double check. How much snow you have up your way?

December was a heavy snow month, January for me has been a slow snow month, Lots of lake effect snow, not even worth snow blowing. Havent started it in a month. We are just above the current storm, lucked out again.

That’s fine. Question is, where is the water going as it enters that big hole at the bottom of the channel? Was it flashed at all?

The intergral "J’ is a real bad idea!!

It does not drain water off the top of the window but to both sides where some it will end up behind the siding when leaving the bottom of the side “J’s”…It’s a built in fault with regular and integral J’s and the way 99% of people install vinyl and similar sidings.

When a window with the integral J is installed , how are you going to install a premoulded drip cap or bent “Z” flashing at the top.

The fix is to add a 9-12" wide hidden flashing in behind the bottom of the side J’s to drain water back outward to the back of vinyl “hook” seam where it will run horizontally to corner trim or a sidng joint.

The belief is that as long as the window is properly flashed / taped around same and a moisture resistant barrier has been installed, any water that would get behind the siding is negligible.

Some folks believe in ghosts, seances, re-incarnation, etc.

When water gets behind siding/trim and is held in place by surface forces such as being sandwiched between materials, a potential rot probelm begins. The phenomenon of “reverse vapor drive” from sunlight heating the outer materials is now well accepted*. If you check the permeance of all weather resistive barriers, you will find the highest (let’s vapour through the easiest) is TYVEK…and the vapour can flow both ways!!!

  • I worked on the first major example of “reverse vapor drive” in Nova Scotia in 1994/5- a house that won an award for the architect in 1989. 4-5 years later the house has severe rot; the pine clapboard siding and some sheathing were replaced. It now has vinyl siding. The award should be taken off the architects wall!!!

The first document I have about “the rainscreen principle” (to stop/reduce the reverse vapour drive) was published in 1985 and cost a whole $4. There are examples from Quebec from the early 1970’s. It took about 30 years for any reference to rainscreens to appear in our codes (2004).

The above issues have been written about for years in various periodicals such as Energy Design Update, Journal of Light Construction, Solplan Review. There is a book written by architects from Mill Valley, “Installation of Windows and Doors”, in the early 1990’s…it’s about 225 pages!! we have a lot to learn yet!!

:mrgreen: Yep! Sure keeps me busy with repair work!

Good to see at least 2 of us are on the same page, Frank!


It is like mining GOLD right in plain sight!

I actually just returned from fixing several locations (residential and commercial) that had vinyl siding issues…in the majority of the cases the issues has to do more with fastening problem…too many installers are simply nailing the stuff to OSB, Plywood or Celotex board…they rarely hit studs.

As to the integral moulded j-channels; I didnt say I think they these type of windows are ideal, yet the windows meet industry standards as well a being code compliant.

Most manufacturer advice install a moisture barrier around the exterior sheathing of the home with specific instruction on now to cut out around windows. These windows often come with a foam tack strip for corners which are to be installed after windows have been properly set using sealant around the outside fins and correctly using the prescribed fasteners. Flashing tape is then to applied starting with the lower sash, vertical sides and then the upper sash fin. When applied correctly I have yet to find a home where moisture intruded same to such an extent that the home was adversely affected… if any of you have then please submit pictures along with details whereby the product failed because of manufacturers instructions… I am sure the ICC would be glad to follow up on same.

And speaking of the ICC, it is from them where many of the installation instructions orginate; often it is their engineers who make up the ICR reports from which manufacturers derive their installation instructions.

Where most problem crop up is where installers screw up and building code officials misinterpret the IC reports.

As far as the question is concerned the answer is simple; an additional J-channel is not required.

As to a person relying on J-channel…please, we are taking vinyl siding here.
How many inspectors actually inspect the stuff according to manufacturer specs…I venture to say hardly any. How many inspectors and knowledgeable builders actually think its a great product… very few; yet the public wants it because its an inexpensive product (as a builder, vinyl siding cost me $2.00 per s.f. turnkey; fiber cement $5-6 per s.f.; brick ~ $6-8 per s.f., one can see why builders use it)

Does it have limitations…of course yet when installed according to manufactures specs then its the manufacturer who is taking responsibility.

Finally let me say that most veneers are not installed correctly, I dont care if we are talking brick, vinyl siding, EIFS, Fiber Cement, etc… unless the inspector is doing phased inspections, of which few inspector are or even have the knowledge to do such inspections, they often are left with looking for a handful of clues which would indicate a particular product is deficient to such an extent that it needs to be addressed lest it creates an adverse affect to the property in question.

Many inspectors would do well to actually learn how to install various products for which they are inspecting so that they can actually understand what they are looking at.

warm regards


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I address all these issues and show how to keep the water out, even 99% when somebody is pressure washing the siding and windows. I also detailed some other types of siding installations.

Pressure washing vinyl siding and keeping 99% of all water out…I take it you mean the moisture from entering the building envelope…however it will get on the moisture barrier…it is at that point where most installers screw up.

applying fortified tape over felt paper is useless…the only way it will stick is using some sort of fasteners such as stables which I suspect was used. Eventually the tap will curl up. There are better wraps that can be used giving better results.

Much of preventing moisture intrusion relies on the hope that multiple tradesmen have done their work correctly…with much of the emphasis on getting homes up as quickly as possible, it is unlikely that under the right (or wrong) conditions, the building envelope will be compromised… its just a matter to what degree.



I should have been clearer. After installing all the flashing ans counter-flaashing, we used a pressure washer around a window. No moisture got to the water/vapor barrier. It stayed dry too.
I just didn’t want to go on the hook for nthat last 1%.

The test proved to me that proper flashing prevented the troubles.

I have an e-mail request to rework homes in a development in Chesapeake this year. Hope to get nice pix. Those 20 year houses are rotting away already. Ought to be interesting.

Not sure how you determined that no moisture got behind the vinyl siding without taking same back down to verify…that or you simply kept the pressure setting very low.

Anyway, why are you pressure washing vinyl siding or any exterior veneer of a home… I can take a garden sprayer with a mix of Jo-Max, water and out door bleach and accomplish the same thing without using a pressure sprayer. Pressure washers have their place but on exterior veneers…they often cause more problem then they are worth.

For those that haven’t looked, Frank has a lot of good info on his site and knows what he is talking about.



We were doing tests a few years ago to determine how to keep the water out after replacing some sills. We were finding lots of rotted walls, sills, framing, etc. on aluminum and vinyl sided homes.
Tried a simple piece of flashing which worked well. Tried a water hose on the j-channel. Some water got to the barrier. Went wider on the flashing. BIG help and almost bullet proof.
After watching a house get power washed, we loosened some siding to see how it looked underneath. Terribly wet. ( Not one I had flashed).

Later, we got another to flash and hit it with a power washer later. You could still get water in there by spraying upwards, but no water trails led back to any windows I had flashed.
And yes, I do see homeowners with power washers doing their siding on occassion. I just hate to do anything twice. You’d be hard put to get water into a window cavity I flash. But, it’s not impossible. It seems that some welded windows leak at the corners, and elsewhere in the framing. But, I’ll keep trying to keep that pesky ole water out.