When old houses were built there was no such thing as a vapor barrier. Sure there may have been felt paper or rosin paper installed under the hardwood floors or behind the siding but these pieces weren’t as much about vapor protection as they were about stopping rainwater.
Vapor barriers are designed to prevent wall assemblies from getting wet. But when they’re not designed and installed properly, they can actually prevent assemblies from drying, which leads to big problems.
What Do Vapor Barriers Do?
Vapor barriers do exactly as their name suggests. They keep water vapor (and bulk water) from passing through them. This works well to keep moisture from getting into the wall assembly, but what happens when the water inevitably gets behind the vapor barrier?
Well, the best designed vapor barriers are supposed to be better at letting water vapor pass through them from the inside to the outside. Without this permeability the house wouldn’t be able to breathe and moisture would be trapped in the walls causing issues.
Things like felt paper and house wraps essentially work best at keeping bulk water from getting into the wall and at the same time allowing water vapor to pass through them to some extent. The extent that vapor passes through these materials is called permeability and is a big area of debate in construction.
Permeability: How much is enough?
15 lbs felt paper has for decades been the standard housewrap. It originally got it’s name from the fact that 100 SF of it used to weigh 15 lbs. Sadly, it’s a little more like 7-10 lbs. today because like everything, manufacturers have gotten cheaper.
Felt paper has a decent permeability of about 8 perms when dry, but when it gets wet its permeability jumps up to around 50 perms (Source: GreenBuilding Advisor). This makes felt a great performer for walls where there is a good chance of water intrusion. When it gets wet it can easily dry out and let vapor pass through it. When dry its permeability leaves something to be desired.
Housewrap products like Tyvek are made with one of the highest ratings of 58 perms so they can easily allow water vapor to pass through them. Where these house wraps fall short is if bulk water gets behind them. When this happens the housewrap will still allow vapor to pass through, but not the bulk water.
Unlike felt the housewrap will not absorb the water and disperse it to the outside which can lead to water being trapped and the resulting rot or mold.
Plastic sheeting, often called Visqueen, is the least permeable of all vapor barriers having a rating of less than 0.1. This plastic sheeting is meant for serious vapor stopping ability and wherever it is applied it will provide almost complete blocking.
So what is right for your house? Well, the big question is where do you live? Requirements in cold northern climates are completely different then in the hot and humid south. The NAIMA website has some very good guidelines for the different climate zones and what type of vapor barriers are recommended.
Where to Install a Vapor Barrier
Going into the specifics of what is appropriate for your house in your climate is something that only a local building science or insulation specialist can prescribe, but I can give you a few rough guidelines for retrofitting old houses.
Firstly, the vapor barrier should always be installed on the warm side of the wall. That means that if you live in a cold northern climate where you have more heating days than you have AC days your vapor barrier should be on the inside of the wall assembly (just beneath the drywall).
Conversely, if you live in a hot climate like I do here in Florida the vapor barrier should be on the outside of the wall assembly (just beneath the siding).
One interesting note is that in cold climates vapor barriers are usually required by building codes whereas in warm climates the vapor barrier is often an optional portion of the wall assembly as prescribed by the local building codes.
Retrofitting an Old House
So what if you are doing work on your old house and have some walls opened up? Should you install housewrap or felt paper of something else? I hate to cop out on this but the best advice I can give you is to ask a local specialty contractor.
In most of the projects I have worked on here in Florida we have followed the lead of the original builders and installed 15 or 30 lbs felt. We’ve also stayed away from any interior visqueen because we are primarily a cooling climate.
Another thing I keep in mind for our warm, wet, sunny climate is that the sun shining on rain soaked siding has a tendency to drive vapor inward into the wall so even if there wasn’t any felt paper originally adding it now creates a necessary barrier to keep as much water out of the wall as possible.
I’m a simple builder and these are simple buildings so a simple solution tends to work well. Were I to be working on a more complex building then my techniques would have to be brought into line with the way the building was designed originally.
Keep that in mind always when working with historic buildings. Make sure anything you add to the structure can work in harmony with the original elements of the building. Putting a Corvette engine in a Model T Ford may seem like an upgrade but it’s likely a recipe for disaster.