Caulking a 3/4" gap?

Old 01-01-06, 09:55 AM
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Caulking a 3/4" gap?

I've read that the ideal size for caulk is between 3/16" and 1/2". I have a space that is about 3/4" (betweem a window and a rough sill). The window did not use a nailing flange. I want to seal the gap at the exterior. Then from the interior, I will use expanding foam for "windows and doors" to insulate.

What's the best way to seal the exterior? Can I use silicone caulking and a foam backer rod to fill that large a space?

Thanks in advance.
Old 01-01-06, 11:43 AM
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Caulking a 3/4" gap?

You are right on the backer.

Use a foam backer rod set below the surface so you can force the caulk into the joint and guarantee that it adheres to both sides. Make sure everything is very clean and dry.

Make sure you use the foam designed for use around windows and doors. The other type expands too much and binds the doors and windows.

Old 10-23-07, 10:02 AM
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please expand

I am doing the exact same thing around my 2 new windows, with no nailing flange... What is a foam backer rod? I am trying to figure out how to caulk a 1/4 inch gap, with nothing behind the gap to stick the caulk to.

Then should I use window insulating foam, or will this expand and push the caulk out of the seam?

My biggest concern is the caulkiing.

Thanks for your help.
Old 10-23-07, 10:18 AM
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Backer rod is a spongy, round, flexible foam rod that comes in rolls. You'll find it in hardware stores with the weatherstripping. The bag will say the size of gap that the backer rod is made for. There are various sizes. You push it into the gap until the gap has been filled, then you're ready to caulk. If you push it in too far, it can sometimes push through and get loose, so pushing it in just the right amount is the key.

Do NOT use expanding foam as a backer for caulk, unless you'd like to have a big mess.
Old 11-01-07, 09:17 AM
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Thanks, the backer rod worked great.

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