Sealing sill plate
WHEN SHOULD THE SILL PLATE NOT BE SELED BY THIN FOAM BEAD TO THE FOUNDATION WALL? ONE BUILDERSAID THAT THERE ARE TIMES WHEN THAT SHOULD NOT BE DONE. THERE IS NO BARRIER BETWEEEN THE SILL AND THE WALL. MY PLAN IS TO STOP OUTSIDE AIR FROM COMING IN THRU THAT AREA WHICH I HAVE SEEN DAYLIGHT COMING THRU BEFORE.
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Let me guess, your builder also says things like "you shouldn't build too tight" and "a house has to breathe"? The mudsill (or greenplate) is one of the largest and most common sources of air leakage in homes and should definitely be sealed. If you don't have a capillary break--an impermeable layer between the concrete and the sill plate--you should try to add one. Tapes may work better than foam; I've found that Siga Fentrim is absolutely tenacious about sticking to clean concrete. (https://sigatapes.com/product/fentrim-f/) Foam won't stick well to dirty concrete either.
ONE BUILDER SAID THAT THERE ARE TIMES WHEN THAT SHOULD NOT BE DONE ....
... and these situations are?
On an uninsulated rim joist with no capillary break between the concrete and sill plate.