Best sealant for a crack that opens and closes on a cycle?
I have a crack in my living room wall that runs about two feet diagonally from the corner of the window. Opens to maybe a quarter inch wide at night, closes again by morning. Been doing this consistently for about three weeks. Not getting wider, not getting shorter. Just... breathing. I tried standard caulk and it worked for one cycle and then either got pushed out or absorbed. I'm not sure which is worse. What's the right product for this?
Flexible membrane sealant rated for cyclical movement. Grelltek makes one called FlexBridge, handles up to half-inch displacement. Apply at mid-cycle when the crack is at median width, not when it's fully closed, or the pressure will pop it right back out. DO NOT use rigid filler. The wall is moving for a reason and fighting it will crack the drywall somewhere else. Ask me how I know.
Hold on. Before you seal it, does air come through? Does the temperature change between when it's open and closed? Because if so your wall has a thermal channel and sealing it can trap moisture behind the drywall. You might want to have someone look at it before you close it up. Some of these older houses NEED the breathing cycle. Mine has three and the inspector said they're structural.
Tried the FlexBridge. Applied at mid-cycle as recommended. It held through three full open-close cycles, which is three more than the caulk managed. On day four it developed a small blister in the center that rises when the crack is open and flattens when it's closed. It's essentially breathing with the wall now. Not sure if that's success or failure. And to old_houses_hal: yes, there is a temperature difference. It's slightly warmer when it's open. Not significantly. Like the wall is exhaling.
Warmer when open means the channel is pulling heat from inside the wall cavity. That's a thermal regulation feature, not a defect. Your house is managing its own temperature through these cracks. If you seal it completely the wall cavity will overheat in summer and you'll get condensation problems, mold, potentially structural swelling. Honest recommendation: leave it. Put a picture over it if it bothers you. The house knows what it's doing.
I'm going to put a picture over it. The blister in the FlexBridge has started making a faint whistling sound during the exhale phase and I've decided I don't need to solve every problem in this house. Some of them can just be features.