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I noticed that the caulk has failed where the bottom plate meets our concrete slab foundation along an exterior wall. I’ve removed the old caulk and vacuumed the area. The bottom plate is sitting on a foam gasket, but I’d like to fix it to help with air sealing.

This is a wood to concrete joint. I need something that will adhere to both and stay that way through temperature changes.

Should I re-caulk it with a flexible caulk or use a small bead of great stuff from my foam gun?

Josh Bush
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  • Is this on the inside or the outside of the sill plate? And how wide is the gap? – SteveSh Dec 07 '22 at 15:25
  • Inside, just below drywall. Gaps look the be about 1/4”. – Josh Bush Dec 07 '22 at 15:34
  • "Best" is asking for opinions. Give us some objective criteria that you're looking to measure by (most waterproof, longest lasting, lowest cost, there are lots of possible "bests") and we'll be able to answer the question. – FreeMan Dec 07 '22 at 16:00
  • When building a house back in the 90's the power company had best energy practices. Their guidance was to caulk the sill plate and use expanding foam on all the wire penetrations. Maybe things have changed though. – RetiredATC Dec 07 '22 at 16:08
  • Amended the question. The main concern I have is the concrete to wood joint. It needs to stick to both and flex enough for temp changes. – Josh Bush Dec 07 '22 at 16:13
  • Some type of Silicone, like GE Silicone II, will adhere to both those surfaces, so long as they are clean (this applies to any sealant). You'll have to read the label to determine the maximum crack size the material should be used for. – SteveSh Dec 07 '22 at 16:27
  • If there's a foam gasket ("sill seal"), what do you hope to accomplish? Is the gasket not adequate at some point? – isherwood Dec 07 '22 at 17:36
  • There was caulking there that had failed and I can see some evidence of air movement. Just trying to seal everything back up before the baseboards go back on. – Josh Bush Dec 07 '22 at 18:13
  • I've never seen that joint caulked in decades of building and remodeling. If the gap is that large I'd be using spray foam. – isherwood Dec 07 '22 at 23:17
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    @isherwood I see a high flex sealant used pretty commonly on YouTube on high efficiency houses at the sill plate, but I think tape is more common on the exterior. Example: https://www.youtube.com/watch?v=rQj1wPyvsio – UnhandledExcepSean Dec 08 '22 at 04:00
  • @JoshBush Can you provide a picture of the area? – UnhandledExcepSean Dec 08 '22 at 04:04
  • I went with a small bead of spray foam. One of the “good” images from this guide showed spray foam along that joint. https://basc.pnnl.gov/resource-guides/air-sealing-sill-plates – Josh Bush Dec 08 '22 at 14:27
  • Please move your answer comment to the "Your Answer" box below. That's where answers go in this forum. – FreeMan Dec 08 '22 at 16:47
  • [What is the best way to caulk this?](https://diy.stackexchange.com/questions/260347/what-is-the-best-way-to-caulk-this). Not how; what : **Vulkem**. – Mazura Dec 10 '22 at 00:36
  • [Best method for sealing underneath basement window sill?](https://diy.stackexchange.com/questions/98051/best-method-for-sealing-underneath-basement-window-sill) - same thing. – Mazura Dec 10 '22 at 00:39

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I went with a small bead of spray foam. One of the “good” images from PNNL's guide (reproduced below) showed spray foam along that joint.

image of spray foamed sill plate

ThreePhaseEel
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Josh Bush
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