# Should I caulk the gap between exterior trim and window?



## woodsmith.ryan (Nov 2, 2015)

Im referring to the gap where my 5/4 miratec trim meets my vinyl windows. It's about 3/16" all the way around. They have a nail flange which prevents water from entering the window opening, but as is, any water that flows over the drip cap on my head trim, or down the window itself, would get behind the trim and flow down the house wrap. It seems like it would be better to keep it on the outside of the cladding in the first place. 

If I do caulk it, should I not cap the gap on the bottom trim? Not caulking that gap could potentially leave a space for water to escape, but it could also leave a space for water that hits the windows to just flow behind the trim and down the house wrap.

My siding is Hardie board, btw.


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## XSleeper (Sep 23, 2007)

Caulk it to keep water out. Hope you taped the nail flanges to the wrb.


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## kwikfishron (Mar 11, 2010)

woodsmith.ryan said:


> Im referring to the gap where my 5/4 miratec trim meets my vinyl windows. It's about 3/16" all the way around. They have a nail flange which prevents water from entering the window opening, but as is, any water that flows over the drip cap on my head trim, or down the window itself, would get behind the trim and flow down the house wrap. It seems like it would be better to keep it on the outside of the cladding in the first place.
> 
> If I do caulk it, should I not cap the gap on the bottom trim? Not caulking that gap could potentially leave a space for water to escape, but it could also leave a space for water that hits the windows to just flow behind the trim and down the house wrap.
> 
> My siding is Hardie board, btw.


It needs to be caulked. I always blind caulk 5/4 trim since the trim ends up practically flush with the face of a vinyl new construction (fin) window and a clean aesthetically pleasing caulking job after the trim's installed can be challenging (for many).

Just be sure not to caulk over the weep holes at the bottom of the windows.

PS...The nailing flange is not considered flashing and all of the vinyl window manufacturers that I've dealt with go out of their way to let you know that (paragraph 14 in subsection 6 of the install manual).

3/16" is a huge gap. You need to trim off the excess extrusion created when the bot welds the frame before you trim out most vinyl windows.


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## woodsmith.ryan (Nov 2, 2015)

I definitely flashed the nail flanges. 

And yes, I guess 3/16 is a big gap. I Planned for the 1/8" gap, but I didn't know if it was ok to trim the welds. Im not looking forward to trying to do a neat caulk job. Thats a good idea to blind caulk them.


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## kwikfishron (Mar 11, 2010)

woodsmith.ryan said:


> Im not looking forward to trying to do a neat caulk job.


Use blue tape. Tape off the vinyl and the trim. Fill the void, smooth off with your finger and then pull the tape (slowly) right away. 

Try not to overfill the gap. 

A 'delicate' tooling with your finger may be needed after you pull the tape. 

Again, try not to apply more caulking into the void than is actually needed. 

Flush caulking takes finesse.


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## mark sr (Jun 13, 2017)

It helps to have a rag/sponge handy wet with the appropriate thinner [water for latex caulk] to wipe off any errant caulk - also helps to keep your fingers cleaner.


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## carpdad (Oct 11, 2010)

Your caulk will be more than 3/16. Caulk's edge is feathered to almost nothing, so you must have some caulk that stays on the hardiboards. That means you have to tape off at least 1/4" away from the siding edge and that makes the caulk about 1/2" wide. I would think about some kind of extra trim to cover the gap.


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## XSleeper (Sep 23, 2007)

Carpdad, I believe his Miratec is 1" thick, his integral fin is 1" thick. Like kwikfishron said, the two are practically flush with each other just gapped apart. You don't smear caulk so it overlaps on the face of each side, you neatly mask it so the face stays clean and you apply the caulk down the middle. As you do so it adheres to either side. 

And a 3/16 gap is not bad. If you read the mfg installation instructions (who does), a lot of window mfg's recommend an even bigger gap- like 3/8" which IMO is insane. They are coving their butts in case the surrounding brick, siding or trim expands and puts pressure on the window.


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## carpdad (Oct 11, 2010)

Sorry, thought the gap was between the trim and siding. Between the window and trim, caulk. If trim was painted, another coat of paint over the caulk. I only use osi quad, partly because urethanes may be curing in the tube already.


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