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Permanent central air units will required penetrations for air exchange, electrical and sensor wires. It becomes a challenge to seal around these different sized elements, unless you plan well.

Specifically:

• For new AND retrofit A/C units, include the appropriate size of conduit pipe through the wall assembly for ease of passing wires through.
• Use the appropriate sealant around the conduit and wires. Better yet, use a gasket or boot with neoprene or EPDM (see picture below)
• Insulate the penetration from the inside and weather seal from the outside.
• Remember the Positive Overlap Rule - think like a drop of water!

For window unit A/C, the air sealing process is more weatherstripping than anything. You'll need foam strips, removable caulking or white duct tape (to match the window). Some more expensive units will include fittings for your window, but some assembly is required.