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My house was built in the 1950's and have plaster walls. I am in the process of repainting and have noticed that the plaster above the baseboard air vents has cracked above almost every vent in the house. What could cause this? Moisture in the air coming out the vents?

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  • Pictures would improve this question. Any idea how long it's been cracked? i.e. you notice now because you are painting, but perhaps it's been cracked a long time (in which case I suspect plastering in the winter with the heat on as a cause going back to the start...)
    – Ecnerwal
    Commented Feb 13, 2020 at 14:12
  • It has been like this for a while but I am just now thinking about it as I am having to repair the bad spots before I paint. The base layer of plaster is fine, it is just the finishing layer is separating from the base layer. I will try and get a picture of a spot I havent touched yet.
    – lakehouse_dreaming
    Commented Feb 13, 2020 at 15:17
  • There's some fiberglass tape behind that mud bridging the crack, right? Otherwise you'll be doing this again shortly.
    – Mazura
    Commented Feb 15, 2020 at 22:29
  • Get yourself a proper taping knife with a handle, so that you can flex the blade and work the mud. Your application of joint compound should be nearly as flat as the original wall. Sanding should take about 5 seconds.
    – isherwood
    Commented Feb 16, 2020 at 15:37

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That probably happened in the 1950s, not long after the house was built, especially if you have wood lath and not gypsum panels. The corners of the cutout for the vent create weakness, and normal seasonal house movement results in cracks.

Rub some painter's caulk in there before you paint. Doing more than than will probably just make it look worse unless you completely redo the drywall. It's one of the traits of older homes.

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  • The cutout for the vent is in the hardwoods. The vents sit on the floor and are in the baseboards.
    – lakehouse_dreaming
    Commented Feb 15, 2020 at 14:43
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    But, @lakehouse_dreaming, behind the baseboard is more plaster and lathe. Just like with drywalling, the entire wall is covered down to ~1/4" from the floor then the baseboards are put on.
    – FreeMan
    Commented Dec 8, 2021 at 13:19
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" What could cause this? "

My guess would be the furnace was turned off and the house got cold....real cold.

Then someone turned the heat back on and the flow of warm air caused materials to expand around the vents.

Too bad they didn't reheat the house slowly and incrementally (assuming my theory is correct) and avoided the problem.

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I guess the crack was caused by the cyclic movements of the baseboard due to change in moisture/humidity around the house. The baseboard was attached to the studs through nailing over the drywall panel, however, when the baseboard underwent change in length, the drywall would not (it is less sensitive to moisture/humidity change), thus cyclic tension and compression forces had pull/tear the drywall apart, that resulting in cracks on drywall through its weak point (top of the cutout). Humidifier can help to maintain constant indoor humidity, and reduce the chance for reoccurrence.

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