What are Vapour Barriers?

“So, what are vapour barriers?” That’s a question I get asked that a lot when I talk to people about installing/retrofitting insulation to their homes.

Basically a vapour barrier is a sheet of plastic which is impermeable to water vapour. It’s installed on the warm side of the insulation and prevents moisture getting into the insulation and wall.

If water vapour were to get into the wall and insulation it would reduce its insulation properties and potentially allow defects to occur such as damp or rot etc.

A vapour barrier can be as simple as a heavy gauge plastic sheet or an aluminium foil bonded to your insulation sheets. But no matter what type or style of vapour barrier you choose you’ll need to understand why you need it, how it works and how to install it correctly.

Why you need a vapour barrier?

A vapour barrier, as I’ve already mentioned, is there to stop water vapour, which floats around inside your home, from getting into the walls of your house where it could cause damage.

Where does the water vapour come from?

Water vapour comes from a lot of different sources in your home but the most common are cooking, washing and drying clothes and bathing. If you think about it most of us cook using open saucepans which allows a lot of water vapour to escape.

We also love to have showers and baths with lots of hot water and in the winter we tend to dry our clothes inside on radiators etc.
In addition to the above we all breathe out water vapour in our breath all day every day, so it might be no shock to realise that we each can breathe out 2-3 litres of water vapour per night while we sleep.

All this water vapour floats around your home until it finds a cold surface such as an uninsulated wall or a single glazed window etc. where it condenses into water droplets. If these water droplets are left for around three weeks black mould can and will start to grow and that’s not something that you or your family want to be breathing in.

What are vapour barriers (in a little more detail)?

Now you know the answer to the question "what are vapour barriers?" and you know what you want it to do.

But why is it so important to use one to stop the vapour getting into the walls of your house?

Most old buildings built prior to 1920 will have solid walls (i.e. no cavities). These walls are actually permeable and allow moisture to pass through them (albeit slowly). It is very important that the outer surface of your main walls is more permeable than the inside surfaces.

Without any insulation the inner surface of the walls are at a similar temperature to the outer surface meaning any condensation is more likely to occur on the inside wall surface (which is cold).

Once you insulate the inner surface you make the masonry section of the wall colder making it more likely that the condensation will occur within the structure of the wall (interstitial condensation).

Solid Breathable Wall

To avoid this interstitial condensation it is extremely important not to let the water vapour reach this cold section of the wall which is why the vapour barrier is placed on the warm side of the insulation. (Good ventilation is also important when trying to reduce the risk of condensation in order to keep the relative humidity low)

A basic rule of thumb is that the outer walls of your house need to allow moisture (water vapour) to escape to the outside more easily that they absorb water vapour from the inside. That way they stay dry and don’t allow condensation to build up within the structure of the wall.

What are Vapour Barriers - Example 1:

Let’s say you have a solid wall house with no insulation and you plan to insulate the walls internally by building a timber stud wall against the inside surface and then insulate the gaps in the stud wall before fitting plasterboard and decorating.

By doing the above your insulation makes the original masonry wall (on the cold side of the insulation) colder and therefore increases the chances of water vapour condensing to form water droplets within the wall (interstitial condensation) which would allow damp to occur and potentially rot any timbers in contact with it!

Insulated solid wall wi no vapour barrier

So how do you stop this happening?

What is a vapour barrier?

You install a vapour barrier on the warm side of the insulation i.e. after you’ve built the stud work and insulated you then install a vapour barrier which stops the water vapour getting through to the areas of the wall that are cold enough to allow the water vapour to condense. End result – no interstitial condensation.

N.B. You’ll probably never manage to stop 100% of the water vapour getting through the vapour barrier. But by installing the vapour barrier correctly you’ll only let a very small percentage of water vapour through and that shouldn’t present any major issues.

Do You Need a Vapour Barrier?

If you’re adding insulation to the inside and/or the outside surface of your main external walls you will likely require a vapour barrier. A good rule of thumb to remember is that the vapour barrier always goes on the warm side of the insulation.

So depending on where you live that vapour barrier will either go to the inner surface of the insulation (if you live in a cold climate) or the outer surface of the insulation (if you live in a hot climate).

Installing a vapour barrier:

It's not enough knowing what vapour barriers are, you also need to know how they should be installed and thankfully the principle is very simple.

  • When installing a vapour barrier it’s important to maintain the integrity of the barrier i.e. make sure there are no holes in it. 

  • If you’re using a polystyrene sheet to insulate the inside of a solid wall you can't assume the insulation sheet acts as a vapour barrier in its own right (like foil backed insulation) make sure you’ve checked with the supplier how permeable it is. For example, expanded polystyrene sheets are a lot more permeable that extruded polystyrene sheets. Your best bet is to go to a builder’s merchant and tell them what you need or search online for vapour barriers and you shouldn’t have any major issues. Or assume you will need to fit a proper vapour barrier before installing the plaster board.

  • Make sure all joins in the vapour barrier (or foil backed insulation) are taped and the puncture holes from where you nail it, or staple it, onto the timbers are also taped. Pay particular attention to gaps at the bottom and edges where the timber battens finish. You could use expanding foam to seal these areas (preferably a closed cell foam rather than an open cell foam as it’ll be less porous).

Some rigid foam insulation sheets have an aluminium foil layer bonded to them which is a vapour barrier. However you’ll still need to make sure joints and edges are well sealed. Foil tape which is readily available on-line or at your local suppliers is great for sealing joints in the insulation, but it doesn’t stick to timber very well.

Vapour barriers on timber frame houses:

What are vapour barriers on timber frames houses and are they any different?

The principle is actually the same with timber frame houses however they will also have a second form of barrier installed.

The vapour barrier is fitted on the warm side of the insulation (the inside in the UK) then on the outside of the timber frame there will be a moisture barrier which stops any moisture that may penetrate from the outside of the wall getting through to the timber frame (where it could cause rot) and the insulation.

This moisture barrier allows any water vapour that may get past the inner vapour barrier to pass through it on its way to the outside environment but will stop any moisture trying to go the opposite way (i.e from the outside in).

Timber frame with vapour barrier

In conclusion,

Now that you know the answer to the question "what are vapour barriers?" you’ll know that adding insulation to the inner or outer surface of you main external walls will mean that you'll likely need to install a vapour barrier of some sort.

Make sure it’s fitted to the warm side of the insulation and make sure all gaps are well sealed to make sure as much water vapour as possible is prevented from getting past your vapour barrier. That way your walls, timber battens and insulation will remain dry and healthy.

A note of caution though, there are a lot of builders out there who may not fully understand the importance of vapour barriers or the need to make sure they are well sealed etc. so it’s up to you as the client to make sure you tell them that you need one and to ensure that it is fitted correctly.

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