In its essence, a building serves the same function it has for the past thousands of years — as shelter.
At first glance, it is pretty simple, a wall is a wall. It protects us from the outside environment, while we carry on with our everyday lives inside.
What if I tell you that there is more to a wall than meets the eye. More precisely several layers that are meant to keep the outside out and the inside in.
Of course, it is without a doubt that the current building technology allows us to design and create amazing buildings with crazy performance. But this article is meant to keep things simple.
There is a really simple way of looking at a high-performance wall that is widely used in the construction industry. It’s called “The perfect wall” and it was first described by Joseph Lstiburek, B.A.Sc., M.Eng., Ph.D., P.Eng., and a principal of Building Science Corporation.
“The perfect wall” principle refers to four essential layers that protect the building structure. Because of all the bad effects (expansion, contraction, corrosion, decay, UV radiations, etc) are functions of temperature, it’s very important to keep the structure safe and away from outside exposure. So they are typically placed on the outside of the structure.
It is important to mention here that these protective layers are presented in order of their importance.
Rain protection
This is the very first layer that is typically represented by the cladding and drainage plane.
As its name suggests, it is meant to protect the building structure from water in its various forms (rain, snow, etc.) as well as the first layer of defence against mould.
Even though the cladding material is repelling the majority of water, some still find its way behind it. This is why a layer of water repellent material (building paper, house wrap, sheet membranes, etc.) is usually installed right behind the cladding. So it allows the water to drain away from the wall.
Air protection
Air control is the second most important aspect we need to consider when designing our wall.
This is because when our wall has a leak, it allows the air to carry a lot of moisture to the structure. And if it’s trapped there, it can lead to a lot of problems such as mould and rot due to condensation. This not only affects the structure of the building, but also the indoor air quality.
Air carried through the building envelope can also carry a lot of pollutants. This is especially concerning when it comes to the air we breathe.
Fred C.Malven, Associate Professor from Iowa State University, tackles the issue about how air leaks can affect the building’s behaviour in the case of a fire as they can help spread smoke or toxic gases, or feed the fire itself.
Another problem with air leakage is how much energy is lost through the building fabric. And according to John Straube, Principal and Senior Building Science Specialist, RDH Building Science Inc., up to 50% of your energy consumption can end up wasted through air leaks.
So overall, it’s a good idea to keep your building airtight and one of the good things about this is that you can control the air that comes in and out of the building. But more on this in another article.
The most commonly used air barrier materials are rigid materials (gypsum boards, plywood, OSB) or spray-on materials (heavy coatings or asphalt-based material - that are used over uneven substrates or cavity systems).
Vapour protection
Third on our list is the vapour control layer, and yes, it has to do with moisture. But there is a difference between moisture transported by air and vapour diffusion in that there is significantly more moisture in the first case.
And although they’re both on the list, air barriers should be used at all times, where vapour barriers are best used in cold climates.
Generally, vapour moves from the warm side to the cold side of the wall. In theory, this seems easy, but where do you draw the line? This is very circumstantial, depending on building use, climate, season and even materials used.
There are also different terms that can easily be confused:
- A vapour retarder is, as per Wikipedia, “a layer of material or a laminate used to appreciably reduce the flow of water vapor”.
- A vapour barrier is “is any material […] that resists diffusion of moisture through the wall, floor, ceiling, or roof assemblies”.
Note the fact that it’s a matter of how much vapour is allowed to diffuse through the material. So based on this you may want to either restrict the movement of vapour or allow it to pass in order to not accumulate moisture.
Vapor diffusion is generally thought of as a negative process but in reality it’s an important drying mechanism that can be used as part of the solution. — RockWool
Although all of this seems confusing, there is a general consensus of how not to use a vapour barrier. There are four principles that you can follow:
- If you can choose between drying and wetting prevention mechanisms, always go for the drying mechanisms. Meaning that if a vapour-permeable material has a decent performance, choose that over a vapour retarder. The same goes if you have to choose between a vapour retarder and a vapour barrier.
- Avoid installing vapour barriers on both sides of the wall construction.
- If the interior is air-conditioned, avoid instaling materials such as polyethene vapour barriers, foil-faced batt insulation and reflective radiant barrier foil insulation on the inside of the wall as you risk to get mouldy buildings.
- Continuing from the above point, avoid installing vinyl wall coverings and finishes on the inside of the wall to avoid mould growth.
- Bonus point: make sure your building is properly ventilated.
Heat protection
The fourth and last layer is the heat protection layer. This is an important aspect of building design because let’s face it, no one likes a too cold/warm house.
And the best way to achieve heat protection is through insulation. You can also regulate the temperature through heaters or air conditioning, but the topic of the article is about walls. Plus, the better job you do at insulating your building, the less need you have to regulate its temperature.
Insulation comes in all kinds of forms, from mineral wool, vacuum panels, cellular glass, aerogel to spray foam insulation and even natural fibre insulation like wool or wood fibre.
The idea is, that whatever type of insulation you use, you want to make sure that there is the least amount of heat transfer through the wall (and other building parts) as possible.
So we got this far and you put about 150–200 mm of insulation on your wall, but you still feel a bit cold when you’re next to the wall. That is because even though you put in the ideal amount of insulation when the wall meets the floor there is a break in the insulation layer.
This is called a thermal bridge and it means that heat will escape even through the smallest gap if given the opportunity. And putting all of that expensive insulation on your building will be a waste, ending up with a chance of mould and a high energy bill to keep the interior at a comfortable temperature.
There can also be different types of thermal bridges:
- structural thermal bridges — between building components with different thermal conductivity;
- geometric thermal bridges — at projections and corners;
- material thermal bridges — by using different materials,
So the point is to have an uninterrupted layer of insulation all around the building, with special consideration to building part joints. And if you simply can’t avoid a thermal bridge, you should aim to make the external surface area in contact with the cold outside temperature as small as possible.
The good thing with all of this information is that you can apply it to both roof and slab construction. Joseph Lstiburek himself says that “the physics of walls, roofs and slabs are pretty much the same”.
Of course, there is a lot more we can go into detail with, but these are some basic principles to consider when designing or renovating your building.
Finally, thank you for reading and if you liked this article, don’t be shy and give it a couple of claps 😁
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