Use Lime Skim. Not Gypsum. you can get breathable emulsion which covers as well if not better than modern emulsions.Using Lime will maintain your house for years to come as well as being better for your health. If you have the whole house to do the cost of using Lime instead of gypsum is very little. All the guys using Gypsum in old buildings are just ensuring work for me and a few others in the future.