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Sealing gap under basement window sill?


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08-25-16, 10:33 AM   #1  
Sealing gap under basement window sill?

There is a large gap underneath the bottom sill of the window, that is currently caulked when the house was built.

I would like to strengthen the waterproofing here. I'm thinking backer rod with Vulkem 116 caulking.

Would this be the best way?

Here is a picture of the window, it's the channel underneath the sill:
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08-25-16, 01:19 PM   #2  
Welcome to the forums! Is that a concrete sill support under it, or is that plastic. I can't tell from the picture. How is drainage provided for this window? Can you see better under the window on the inside? Is there an obvious gap? They method you suggest is viable, but I would really be concerned with drainage if it is not up to par.

 
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08-25-16, 01:31 PM   #3  
Thank you!

The ledge is the concrete foundation under the window. I can't be certain, but there is drain tile by the footing, not sure if the window well itself has a drain. It was built in 2000, which i believe, is before it was code.

Here is a picture of the left edge under the sill showing a gap where the original caulk shrunk and exposed the wood frame:

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The rest of the caulk appears intact, do I need to remove all of the old stuff first? I was thinking since it's such a deep gap, I could simply put back rod in front of the old caulk.

Another method I've read is low expanding foam, then caulking.

 
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08-25-16, 01:35 PM   #4  
I'd remove what old caulk comes out easily, then go with the backer rod and caulk. The biggest issue with expanding foam is it can get messy especially if you spray in too much ..... and it's hard to clean up the excess.


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08-25-16, 02:32 PM   #5  
I dont see where you told us the width of the gap... but I would be inclined to cut a piece of pvc (like Azek) and stick it into the gap to create an additional "step". Then caulk the smaller gap on each side of the pvc.

 
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08-25-16, 07:06 PM   #6  
Thanks guys.

I hadn't thought of pvc...

The gap is 1". The largest backer rod I've seen is 5/8", can I simply squeeze 2 pieces on top of each other?

 
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08-25-16, 07:11 PM   #7  
5/4 pvc is 1" thick. You could also use 3/4 pvc and squeeze some smaller backer rod in the gap.

You can squeeze pc of backer rod together but I don't know that a 1" bead of caulk is preferable to two smaller ones.

 
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08-25-16, 07:25 PM   #8  
I'll check into the pvc, thanks.

I came across this today with seemingly good reviews across the web for exterior use. Any experience with it?

https://www.amazon.com/Winder-Advanc...dap+sidewinder

I've used Vulkem many times and it's not the easiest thing to work with... Plus it has to be painted so it doesn't crack from sunlight. But if it's the best tool for the job, I'll definitely use it.

 
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08-26-16, 03:28 AM   #9  
I like X's suggestion for using PVC, caulking always works best when it's not applied too wide.


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08-26-16, 04:40 AM   #10  
Another vote for the PVC. Is the concrete slopped? What kind of shape is the wood under the sill in? Is it water stained? Can't see it all but you may want to consider weep holes.

 
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08-26-16, 11:04 AM   #11  
The concrete has a very slight slope. The wood appears to be ok and is not soft, I can't tell very well if it's water stained.

With the pvc, would I only be caulking the edges at the top at bottom, so the middle of the pvc would be exposed? I haven't ever used that method before, want to make sure I'd be doing it correctly?

I am thinking i'd use a round pvc tube.

 
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08-26-16, 11:45 AM   #12  
We aren't talking about PVC pipe but rather dimensional boards like these - AZEKŪ PVC Trimboards | Buy Online | Vintage Woodworks


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08-26-16, 12:27 PM   #13  
PVC boards can be cut with regular wood working tools....same as wood. Don't use fine tooth blades....too much heat build up which will melt the PVC. This stuff will not be affected by water and can be left unpainted. If possible I would cut the part that will be in contact with the concrete to the same slope angle as the concrete. This can be done on a table saw, if you have access to one. Caulk the top and side edges. If you caulk the bottom edge you may trap any water which gets back there.....that's why I asked about water stains on the wood.
If you want to paint the PVC it takes latex paint very well....be careful if you want to use a dark paint that will be in the sun...there are special dark color paints that can be used.

 
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08-26-16, 12:57 PM   #14  
This is fascinating to me, thanks for the insight!

In my case, I don't have access to tools that I can rip the pvc to right depth in the channel. I will see if I can't find a table saw.

In the event that I can't get a table saw, I'm thinking a foam, like this, might be the best option.

I'll have to cut away any foam that expands beyond the edge, and/or caulk around the entire face of it.

Foam won't be uniform like pvc or backer rod, that's my only concern, having to caulk the entire face of the foam, top to bottom. Not sure if that makes a difference in the integrity of the caulk. But at least with foam I know there would be a good seal from air and water.

 
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08-26-16, 01:18 PM   #15  
Im pretty sure a pvc 1x2 will fit, and around here you can buy it that size. I suggested caulking both sides because the wood looks weathered and I thought the cement looked flat, not sloped.

Contrary to what it might say on the can of foam, foam does not provide a good barrier to water. Any backer you use to fill a void needs to be recessed to provide depth to the bead of caulking or it will quickly fail.

 
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08-26-16, 02:15 PM   #16  
No tools? A lot of lumber yards will mill parts to size for you. I know HD will do cutting....mostly large plywood sheets or crosscutting with a miter saw. I've never seen HD do any ripping. PVC can be cut with hand tools or small power tools.

 
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08-27-16, 09:38 AM   #17  
Ok, I want to make sure I have this right...

The caulk needs to be recessed under the sill, not flush, to be effective, and use pvc.

Here's a diagram I made to make sure I'm on the right track, feel free to double check my work.

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08-27-16, 04:35 PM   #18  
Hi, I would clean it up then use stucco or mortar and fill it in.
Good Luck
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08-28-16, 03:43 AM   #19  
Picture looks ok....but I would leave the bottom between the PVC and concrete open to allow water to drain out. If water is trapped it will rot the wood.

 
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08-28-16, 06:37 AM   #20  
I was suggesting the 1x2 protrude beyond the sill nose an out 1/2" or so, so that you could caulk both sides of 2 small gaps.

If you use backer rod, it would need to be 1/2" back for a bead of sealant that is 1" wide.

 
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