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Our basement dryer vent was cut too big. There's a gap of approximately 2" all the way around. We want to fill it (crickets, and cold air in winter come in). I bought a can of spray insulation but am worried about using it. Is there a flat cover/collar that would cover the excess hole? Or should we make one?

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Was the hole cut through the basement wall (concrete) or through the rim joist? – Niall C. Sep 7 '12 at 17:10
Got a photo or 5 you can post? – The Evil Greebo Sep 7 '12 at 17:18
Dryer vent is usually 4", so that would make your hole twice as wide as the vent pipe? Is that the case? That would be really strange. Can you measure the hole and the pipe and post the numbers? – dbracey Sep 7 '12 at 19:39
What worries you about the spray insulation? You could doubtless make a collar (cut materail to shape, screw to wall, clamp or screw to vent, gasket or caulk around the contact points), but the spray insulation seems easiest. – Jeremy W. Sherman Sep 7 '12 at 19:51
Spray foam will not provide a reliable water seal. Also, if the spray foam is exposed to sunlight, make sure you get one designed for that -- most spray foams will break down in a couple of years if exposed to UV rays. – Shimon Rura Sep 7 '12 at 21:45

A good solution would be to make a collar/cover of a material that would fit visually with the outside wall, and would provide a reliable seal to prevent water, air, and pest infiltration. Then to support a good seal, use spray foam to insulate around the duct.

The appropriate material for the collar depends on the wall you're going through. If it's a masonry wall, you can use mortar or a concrete patching compound. If the outside is vinyl siding, a small sheet of vinyl could work, with the appropriate trim pieces to terminate the cut vinyl boards (you may want to cut them square so this looks better). If the outside is wooden shingles or clapboard, a painted piece of pressure-treated plywood or cellular PVC trim board would look OK.

Depending on what exactly was cut, and how much, you may want to replace some of your sheathing to support the siding or collar pieces.

Make sure to use the appropriate trim and caulking to seal the collar to the exterior wall. This is really important to seal out water. Consider how water would flow onto and off of the surface - you want gravity on your side. So if you're doing this with wood shingles, make sure the shingles overlap your collar piece on top, and your collar overlaps the shingles below. You don't want a situation where if your caulk wears out you are funneling water behind your siding.

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Try strip caulking or Dum Dum sealant. I live in a -40°C winter climate and just installed a new dryer duct in January.

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Dum Dum sealant is no longer produced. – Tester101 Jan 22 '14 at 17:16

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