The problem with gypsum plaster

Why to use lime plaster when comparatively cheap gypsum plaster is available.

If you could go back in time you’d find small villages and hamlets scattered with houses of cob, wood, thatch and stone construction. There is little to no evidence of these today but if we could take a walk down the main track, in say the year 1200, you’d probably find you got lots of attention from the domestic dogs as well as the locals who'd look at you askance. 

In a self-policing society where strangers were reguarded with suspicion, dogs were the safeguard of many a poor subsistence farmer living in the most basic of accommodation. However, if you wanted more security and potentially a better class of living then a dwelling within the city gates of places such as York, Worcester, Newcastle - upon - Tyne, or Berwick on Tweed would be your target providing you didn’t mind the incumbent issues of cost, smell and overcrowding. 

As you can imagine, lifestyle then was very different to its modern day equivalent. Houses were predominantly shelters from weather and places to sleep. And unless you were of means, the idea of being particular concerning plaster-work would be pretty low down on your remit.

Traditionally, both internal and external walls were covered in just about anything anyone could lay their hands on including mud, clay, dung and straw. Lime plaster for the masses was yet to come and would act in the same way as the above in that it helped to windproof homes as well as allowing rain to soak into the plaster rather than the stone. As lime garnered popularity its application often consisted of one coat of lime mortar pasted so thinly over the wall that all it resembled was a bad case of repointing. Nevertheless, the moisture inherent in the mortar evaporated as the weather improved. The lime and sand mix also aided the displacement of water which may have soaked up from the ground. 

If one wanted a more even surface then the first application of mortar was scratched to form a key on which to apply a more even second coat. Finally a third coat was applied for internal work and this consisted of a higher ratio of lime in tandem with finer grains of sand which is now commonly known as plaster.

In sum, a coarse mix of lime and sand simply thrown onto the surface of the wall would be an adequate covering but when a smoother more high quality finish was required then the size of the grains went down and the amount of lime used increased. Obviously those who owned smarter and more prestigious buildings were the ones who enjoyed such luxuries.

Lime served its purpose for thousands of years until after the Second World War when comparatively cheaper cement render and gypsum plaster was employed. This was predominantly out of economic need as houses had to be built quickly because many had been destroyed in the Blitz. 

By now, damp courses were being employed in every build.

Membranes were being used to restrict moisture from entering a property at its base and this now meant there was less need for lime mortar. Solid walls became a thing of the past meaning cement render and gypsum plaster could be used. This, in tandem with the manufacture of tougher bricks, cavity wall and the use of air bricks to encourage airflow, meant that lime had had its day. The upshot was that skill with lime became absent; and very quickly. 

 

This was fine where new build property was concerned but the need for lime remained where the maintenance of older properties was required as these still needed porous materials which would facilitate the expiration of moisture. That’s when the cowboy contractors and snake oil salesmen moved in. 

With the greater use of cement came all of the problems of which older building now suffer. As cement was applied this meant that there was no means of escape for moisture therefore many owners of older properties resorted to damp proof treatments. Companies which offered these happily charged unsuspecting and ignorant members of the general public hundreds – sometimes even thousands – of pounds to eradicate their problem. However, all they were left with were empty promises based on “scientific proof" that certain products would totally and with absolute certainty eradicate moisture. Yet nothing can stop this from traveling higher up a wall if it’s been coated with cement as this will not allow it to escape. The term rising damp was coined and a whole industry evolved around it. Contractors used their products on new buildings and assumed incorrectly that they would do the same job on older properties. What owners were left with were products which would totally waterproof their properties or which would hold water within walls. 

 

If you bear in mind that gypsum plaster is a product which allows for the passage of moisture and has some porosity then you'll see my point. All that happens is that it absorbs moisture from the base of a property and holds on to it. Thus, when paint is applied it will only be a short while before it peels off. When you have a damp wall then often the best solution is to totally remove the gypsum and cement and apply lime. Adding components to lime plaster such as pumice, perlite and glass can also improve insulation – especially with hemp lime plasters. Combined with the use of porous paints the issue of damp becomes a thing of the past and so does condensation.

 

In sum, owners of period buildings who employ the use of gypsum plaster must prepare for certainties. One of which is cracks. Older properties are set on less than substantial foundations and are therefore prone to settlement. Lime plaster will accommodate this but inflexible gypsum will not. So mold, condensation, cracks, poor insulation quality, rising damp and salts bubbling up from the surface of your wall are the mainstay of gypsum plaster.