Breathability took me ages to understand, partly because the term is misleading. For once the clue is absolutely not in the title.
Breathability is all about the movement of vapour. I know – it didn’t make sense to me either. Lately I’ve noticed the phrases Vapour Open or Vapour Permeable being used – makes much more sense.
Difference between vapour and moisture
Getting a bit technical here, so bear with. Both vapour and moisture are about water held in air.
- Vapour is water held in the air as a gas
- Moisture is water held in the air as tiny liquid droplets
Obviously both vapour and moisture must be dealt with in every building. (I’ll talk about moisture and how to deal with that in another blog)
The good news is that as long as materials are breathable, then vapour can look after itself. It will flow easily through the walls from inside to out or vice versa.
‘As long as’ is the key phrase here. The whole system must be vapour open or there will be problems. If the vapour flows through breathable brick, then hits non-breathable insulation or paint, it has nowhere else to go. And we all know what happens when vapour gathers, you have condensation, mould and damage to the structure of the building.
How to know if your house is breathable
You can tell if your house is breathable by checking the build. If it has single brick wall (rather than a cavity wall) and you have suspended floors (a gap under the floor and no concrete) then you have a breathable house.
If this is you, you’re part of a long tradition that includes the Great Pyramids and the Houses of Parliament. Originally building materials were based primarily around lime, sometimes clay or earth. It was readily available, cheap, versatile and easy to work with. It could bond with stone to make a strong structure, and it could flex and expand with changes in temperature and humidity, preventing cracking and water damage.
Bit daunting, but you’ll soon get the hang of it.
Get the Insulation right – your builder will probably want to use PIR (polyisocyanurate) – brand names Celotex and Kingspan. You’ll have seen it leaning up against skips in driveways. Like a thick foam with a tinfoil covering on it. It is a highly efficient insulation material but with certain drawbacks:
- It is made from petrochemicals so definitely not good for the environment
- It is not breathable, so will disrupt any breathable system
- It gives off VOC’s – Volatile Organic Compound – which are toxic
Sustainable insulation is easy to access – just needs to be ordered. You can see the options here. They are breathable, sustainable and climate friendly. You ‘ll need to use a slightly thicker layer to get the same thermal performance (U Value) but it’s worth it to know you’re not damaging the earth. You can get everything from wood fibre to old denim and velvet!
Airtightness membrane – to make your home airtight you’ll add an airtight membrane across all the walls. Just need to make sure it is also breathable. Which sounds like a contradiction in terms – just remember that it’s air and vapour we’re talking about so it is possible.
Plaster – this is the big challenge. Not because you have to use lime plaster – that is getting easier every day with products like Lime Green Solo. The big challenge is getting a lime plasterer. If you fancy a new career I can guarantee that this is one that’s in short supply. I would suggest sorting this out before you start anything!
Paint – finally the paintwork – one that’s easy to miss. By this time you just want to get on and make the house comfortable again. But if you put on standard paint that is non-breathable (and the vast majority are) then you are creating a barrier to vapour flow. There are at least a couple of truly breathable makes – Earthborn and Graphenstone
Look here for more detail about breathable paints
All this will take a bit more time exploring, ordering and delivery, but once you get into it, I’m sure you’ll find it really interesting. And you’ll understand your house better.
Most of all you’ll have a fully breathable system which is long lasting and robust. There’s a very good reason so many of us live in Victorian and pre-Victorian houses!
Next blog – ventilation to deal with the moisture.