Gap filler

Use standard filler for most of it, then a large helping of decorators caulk.

Its not the plaster shrinking BTW, its the wood.

Thanks, hadn't thought of that. Never used decorator's caulk before. Do the various brands differ much, or can I just get the Screwfix cheapo stuff?

All the wood? From 400-year-old-beams to 7-year-old window frames?

|We live in an old barn, converted in 2000. Moved in a year ago. | |The plaster between the beams has shrunk, leaving gaps that the wind |just loves to blow through. The narrow straight gaps are no problem, and |SWMBO is a dab hand with the sealant gun and does a neat job. | |Where we are having trouble is the larger gaps, mostly irregular, around |and almost behind the older misshapen/damaged beams. They're too wide |for sealant, and I don't want to use rigid stuff like Polyfilla because |there still has to be a degree of flexibility because of the building |moving in the wind. | |Is there a suitable foam for this job, one that doesn't expand like mad |and fill the whole room in a nanosecond? Or something else?

Shove paper into the hole, to about 1/4 inch below eventual surface, then set it as a challenge to SWMBO.

Especially the 400 year old beams. They've probably never experienced central heating before. This kind of shrinkage is usually a one off event as the wood adjusts to the new ambient conditions, but in larger sections that can take a long time. I'd fill a couple of sample areas and see when the wood finally stops shrinking before you do the whole lot.