I’m writing this from the point of view of a householder and what I have learned from discussing our problems with experienced builders, architects and surveyors. The property was built in 1924, and one common theme seems to be that old houses “need to breathe.” This can obviously include air circulation but can also refer to the way moisture comes and goes. The two things are related though. We had a small problem with some damp in a corner of one room and when asking a very experienced builder (who had done some plaster repair work ten years previously) about it, he replied “I shouldn’t have skimmed that wall with gypsum, we know better these days. Should have used lime, these old ‘ouses need to breathe.” (TLDR: More on that later)
It seems that back in the 20s, before central heating, air and moisture movement was very different to the more modern concept of an efficient airtight and watertight building.
First of all let’s think about some basic building concepts that have changed over the years:
Older properties with clay bricks and lime mortar (which are quite porous) tend to absorb moisture when the weather is wet, and then dry out when it is dry and sunny. Part of this process is aided by the airflow in the building, something we now describe as draftiness. In many cases air bricks have been blocked up or replaced in a misguided attempt to make the house warmer, without paying attention to the fact they were there for a good reason. Insulation wasn’t really something people thought too much about when they mostly had one or two coal fires and a kitchen range.
And of course energy was not so expensive. One thing to bear in mind if you live in an old house is that you can expect there to be some odd bits of damp here and there. It only becomes a real problem if it is widespread and causes smells, or cosmetic issues or threatens the structural integrity of the building.
Houses (post 60s) tend to built with the idea of keeping the weather out and the heat in. Impermeable bricks, cement based mortar, sealed window units etc. are all great at keeping the weather out altogether. Attitudes about this concept of sealing the house are changing, and new regulations are now coming into place in regard to ventilation.
- The walls had air bricks that let air into the space under the floorboards.
- The bricks were soft and porous clay bricks
- Mortar, render and plaster was lime which is softer and more flexible than sand and cement
- No roofing felt so the tiles allowed air (and often wind blown rain ) into the loft space
- Open fireplaces with open chimney.
- Windows and doors were often drafty.
Coal was relatively cheap, so in the winter a roaring fire was lit, everyone gathered round it before going up to bed with a hot water bottle and loads of blankets.
Basically what happened was that sometimes damp would get in through porous bricks, lathe and plaster gables, leaky roof tiles, eaves etc. or else would form due to condensation within the house from washing etc. But then thanks to the ventilation (or drafts) this damp would dry out again. The heat from the roaring fire up the chimney would pull air in and ensure good airflow so damp often didn’t get a chance to linger.
On the other hand, a decade or so after WW2 a different concept of building meant the that instead of water and air getting in, the house was more sealed off from the cold and wet. Bricks fired at a higher temperature were used and are less porous. Portland cement mortar is less porous. Gypsum plaster is less porous. Bitumen roofing felt was used. Floors were more often solid concrete or floorboards were tongue and grooved to minimise drafts and fitted carpets were installed. All of these things were designed to help keep the house warm and more efficient.
Both of these concepts can work although it has to be said the idea of breathability seems to be coming back into fashion. We now see breathable roofing felt, breathable paint, interest in lime plaster and breathable insulation.
Looking more closely at air and moisture:
We know that a house needs to have proper ventilation to allow for the circulation of fresh air. This can be achieved through a variety of means, such as opening windows and doors, using extractor fans, and installing ventilation systems. More on ventilation here.
The ability for moisture to dry out is also an important aspect of a breathable house. Excess moisture in a home can lead to mold growth, which can cause health problems for the occupants and damage to the structure of the house.
A breathable house should be able to allow for moisture to escape, either through ventilation as above or through breathable (porous) building materials This can help to prevent the buildup of moisture and ensure a healthy indoor environment.
Moisture-resistant building materials including gypsum plaster, modern bricks as opposed to older clay bricks, and some forms of insulation.
The ability of the house to dry out also depends on factors such as the climate, the level of humidity, and the type of insulation and building materials used in the construction.
Mixing breathable with non-breathable materials
There can be big problems when the two concepts are mixed.
After 100 years of decorating and redecorating which often includes stripping wallpaper, some of the old plaster may be in a sorry state, as if it had only been held together by the wallpaper. So the solution is to call in a plasterer, who will, more often than not just do a skim of gypsum plaster ( the modern brownish pink stuff) because it is cheaper, easier to store and much nicer to use than traditional lime.
However lime and gypsum plasters have different properties and react differently to moisture, which can lead to compatibility issues and potential damage to the plaster.
Lime plaster is more breathable and flexible than gypsum, allowing it to better handle moisture and movement in the underlying structure. Gypsum plaster, on the other hand, is more rigid and can be more susceptible to cracking and damage from moisture. Also, lime plaster is alkaline with a high pH value. As such it is a fungicide and keeps away mould.
But the biggest issue may be that any dampness in the wall cannot dry out so well through the gypsum as it would have done previously.
If you need to repair or resurface a lime plaster wall, it is best to use a lime-based product such as Solo. It may not be easy, but it is well worth finding a plasterer who is happy to work with heritage materials.
Mortar and pointing
Porous clay bricks held together with lime mortar is all part of breathability. However many such houses were repointed with sand and cement once it became a more popular material. Again, we can get big problems because moisture in the walls that would otherwise escape through the lime is held in by the cement. If this freezes it can crack or worse, cause damage to the brick facing, known as spalling.
The Damp issue
Getting back to the damp issue. This was not an obvious serious issue showing as black mould, but we had noticed a vaguely damp smell in that room. The first surveyor decided there was some rising damp due to a failed damp course. He said the remedy was to strip the all the plaster off that wall, line it with a waterproof membrane and replaster, effectively making a watertight seal. This is known as tanking. However other advice was that although this would work in the short term, it wouldn’t stop the damp that existed on the other side of the membrane. They discovered this issue was actually not due to a failed damp course, but the wall cavity had become blocked over the years with debris and this was actually forming a bridge across the damp course allow moisture to permeate from the outside skin of the wall to the inside. The problem was made worse by being a wall exposed to a lot of wind blown rain.
The solution was to remove the debris from the cavity and paint the outside of the wall with Masonry Creme which is a product certified by the Energy Savings Trust. This stops damp penetrating, while at the same time allowing the brickwork to dry out. The damp skim of gypsum plaster on top of the old lime just peeled off and reskimmed with lime “fine stuff”.
All of this cost less than half the price of tanking the room, and allowed the old house to continue “breathing.”
Things to avoid (or at least think twice about)
This list doesn’t mean you must go out and change previous work. In many cases they won’t cause problems. However if you have any issues with damp, mould, condensation, rot, blown plaster or brickwork it’s worth looking these potential causes before treating the symptoms rather than the cause.
- Non-breathable roofing felt
- PUR spray insulation Note that in some cases companies will not provide a mortgage to properties with spay foam insulation.
- Gypsum – either a full plaster or a skim over lime plaster especially on external walls
- Waterproof Sealant over mortar (unless it is a breathable type), or bitumen painted over mortar.
- Sand and cement mortar pointing over lime mortar