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Vapor Barriers 101

Vapor Barriers 101When 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.

Improper use of vapor barriers is one of the leading causes of moisture-related issues like rot and mold in buildings today.

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?

Felt Paper

15 lbs felt paper has, for decades, been the standard housewrap. It originally got its 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.

Tyvek

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

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; 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. If I were 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.

 

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34 thoughts on “Vapor Barriers 101

  1. Hi,

    I’m in Jacksonville, FL. Older bungalow, wood exterior.
    I’d like to insulate the exterior walls (I’ve read all the prior comments). Would rigid insulation panels be an option?

    If so, what is the order recommended from inner surface of exterior siding to inside finish?
    (Ex. Tyvek, rigid/other/no insulation, visquine, Sheetrock, breathable paint.)

    Thanks in advance.

    1. Hi Grant. A few questions before we get started with proper procedure. What are you doing with the interior and exterior wall coverings? Removing both or one or the other? Is this your full time home or a partial year residence? If part time residence, what months do you use the home? Are you on a budget for this project? There are so many different ways this project can be done so it will help me answer your question if you answer mine first.

      JC

  2. Beth,
    Please read rule #2 very carefully. Sounds like you are asking for one big problem WITHOUT applying any interior vapor barrier. I guarantee that with no interior vapor barrier, and you applying the felt, within a very short period of time you will start to trap moisture in the wall cavity and create major, major problems.

  3. I made a few comments earlier in the year and would like to clarify and simplify a bit. There seems to be quite a few different ideas on how/what to use for vapor barriers. Here is the “bible” on vapor barriers. First off, Tyvek type products are NOT vapor barriers. They are used mainly on the outside of homes to “help” any major water intrusions such as wind driven rain and siding leaks. Using Vek type products serves very little purpose on the interior walls. Here goes the vapor barrier rules.
    1. Vapor barrier on the interior walls ONLY is OK.
    2. Vapor barriers on the interior and exterior walls is desired no matter if you insulate or not. If you don’t insulate your walls, vapor barrier on the inside walls is OK. If you do insulate you MUST have both vapor barriers applied in order to insure little or no moisture infiltration into your wall cavities.
    3. Vapor barrier on the exterior walls only is NOT OK under any circumstances. This will guarantee moisture/mold buildup in your wall cavities. And believe me it does not take very long.
    4. Applying an exterior vapor barrier from the inside of your home will work if done properly. Properly means VERY CAREFULLY cutting the barrier to be applied so that there is an exact fit. A proper caulking seal along the entire perimeter of the wall cavity, after the barrier is installed, is a must so that minimal moisture intrudes the wall cavity.
    These are the accepted rules of thumb for all parts of the country and all of the different conditions that may be thrown your way. Of course special circumstances pop up, available monies for the project, etc. do arise. If/when they do, get the advice of an expert for your certain circumstance.
    Hope this helps some.

    1. I am doing a addition to my house and would like to put 1×6 on the interior walls. IWe have already put tyvek on the outside. Should I put 15lb felt on the interior wall after the insulation is installed?

  4. So… I’m a purist. Love love my old 1920s craftsman & everything about it. Dont want to change or improve a thing. Everything is original in my lovely home of 24 years EXCEPT the kitchen which is a bad bad 1969 reno 🙁 They nailed firring strips & ugly thin paneling RIGHT OVER perfect plaster, molding and beadboard. 🙁 Saved the molding. Saved the beadboard. Plaster was all busted up. I didnt think to save the lath. 🙁 The kitchen is now down to the studs. My plan at this point is to apply 30lb felt to the back of the lapboard siding, install rockwool insulation batting & drywall. I want to lay mosaic tile over the walls. Cabinets will also be on these particular walls so not to much will be visible.
    I live in humid south Georgia. Does this seem like a plan that wont cause too much trouble trapping moisture?

  5. Hi Scott
    I have a 85 year old home with a stone exterior. We had a leak in the roof of a bay window which lead to a mold problem. We had a company come and open the entire wall up to get rid of mold. I want to insulate before I put wall back together. There is felt or tarpaper under on the outside sheathing under the stone. Do I need a vapor barrier inside? I live in upstate NY just outside Albany area.

    1. Hello Tony,

      Thank you so much for sharing your experience and taking the time to write in. Honestly, while we can give our opinion, our best recommendation is to get a licensed preservationist in your area to take a look in person and provide much better guidance upon seeing it. Finding a licensed preservationist is so important because they will actually approach your home with a preservationist mindset instead of a “dump it and replace it quickly and cheaply” mindset. If you haven’t found a good preservationist in your area, feel free to use our directory to aid in finding one in your area. thecraftsmanblog.com/directory

      Best of luck to you, Tony and we look forward to hearing what you end up doing to solve this issue!

      Happy Holidays!

      -Alyssa at The Craftsman Blog

  6. I think that I can help you a little bit. IF you plan to replace your siding some day but not now….this would mean you can properly place the tyvek wrap later.
    These old houses in the Tamp area are made with lathe and plaster and were built before insulation and air conditioning were readily available.
    The plaster itself allows moisture to move through it well and there is no condensation because of this. The paints they used back then did not create a moisture barrier! Sometimes the plaster used mineral dyes to color it (pastel pinks. blues and greens were common). The paints if used breathed as well as they did not create a water tight membrane…they were also mineral based…..almost like whitewash!
    I found all this out the hard way with my old house. We had black mold beginning to appear behind the refrigerator after using modern paints!!!! This meant we had to start using the AC system to reduce moisture inside the house. This was not necessary when the house was built.
    I suggest replacing the walls with sheetrock and using breathable paint until you can remove and replace the exterior walls. At that time you can add thermal reflective barrier tyvek etc before replacing the exterior.

    I hope that this helps.

    1. Thanks – yes that helps. We’ve actually moved ahead with a similar solution. We have not removed the siding (but, like you say, we may in the future). And if we do, we’ll re-visit more Tyvek at that point…
      Instead, we removed all the plaster (well – most of it fell off during structural work), but we left all the lath in place. Then we added a layer of Tyvek right onto the lath. Then have dry-walled right on to of that. We’ve not added any insulation in the walls, and are going to paint with breathable plaints. Essentially we’re trying to keep the house as breathable as possible. Whilst including a vapor barrier also… At this stage, I have no idea if this’ll work, but we were pretty limited with budget, time and options. And had lots of opinions… We’ll see how this works out…

      1. Tyvek is NOT a vapor barrier. It is designed specifically to be a poor vapor barrier.
        Tyvek is like gortex: sheds water, but lets vapor pass.
        Visqueen (plastic sheeting) is a vapor barrier .
        Visqueen is like a rubber raincoat, waterproof, but sweaty inside.
        I respectfully suggest that the Tyvek on the interior walls will not make the house less damp/humid.
        I must also admit that i am a builder in Seattle: a cool but damp environment with long stretches of damp weather in the winter. So, not the same conditions as FL. Here in the NW, shedding water well is essential and a vapor barrier is always placed on the interior side of exterior walls. (Siding, Tyvek, sheathing, framing, craft-faced insulation, sheetfock). This system permits low permeability paint to be used on interior walls without concern.

  7. We’re renovating a 1918 bungalow in Tampa, Florida. Unfortunately, we had to take down all the plaster, when major structural work caused it all to crack beyond repair. We’re now left with lath walls. We’re planning on drywalling over the lath throughout the house.
    But our GC and us cannot agree on the best way to get some kind of vapor barrier on the external walls. We know (think) that the way other people do it is to remove the siding and wrap the house with tyvek? That’s way beyond our budget (and we’d rather not disturb the original siding).
    Another option we’ve looked at is stripping out the lath from the inside of the external walls. Then inserting panels of tyvek between each of the studs (against the siding), but this seems pretty half-assed.
    Or we could have no vapor barrier and use water-resistant sheet rock?
    Any opinions anyone? Or do we just hope for the best and have no vapor barrier at all?
    (Our GC also wants to add insulation in the wall cavities – which we think is a bad idea).

    1. Hi Lynda,
      Goodness, so sorry to hear about all of the craziness during your renovation! A good preservationist friend of The Craftsman Blog is located in Tampa, FL and it’s definitely worth giving him a call or email to see what he says about it in person, potentially. https://www.woodwindowmakeover.com/
      Best of luck to you!
      -Alyssa at The Craftsman Blog

    2. Hi Lynda. Unfortunately you have a real problem here. Adding insulation without the proper vapor barriers is a bad idea and will just cause more problems, the main one being moisture and then mold in the wall cavities. In your area of the country with all of the humidity and temperature changes, vapor barriers are very important. Without removing the exterior siding I would suggest the following. Remove the plaster and lath. Do not insulate the wall cavities unless necessary. Carefully apply a 4 or 6 mil visquine vapor barrier over the interior of the exterior facing walls making sure that there are no “breaks or holes” in the visquine. When you cut around wall switch, thermostat, recpt., etc. boxes, make sure you seal around them with some Great Stuff expanding foam after you have applied the interior wall covering of your choice. If you can’t afford to remove the siding and apply an exterior vapor barrier, this is your best choice. Also, while you have the wall cavities exposed, it would not hurt to add some sort of a vapor barrier to the backside of your wall sheathing/siding. The important thing is to try and keep any moisture from being contained in the wall cavities. A home needs to “breath” in order to keep moisture/mold from forming. Hope this helps.

      JC

      1. I should clarify. Add a Tyvek type product to the backside of your wall sheathing/siding, not a vapor barrier. These type of “vek” products allow your cavity to breath while still protecting the wall cavity from moisture.

        1. Thanks again!
          I’m still a little confused. So, we remove all plaster and lath. Then, If we don’t add insulation, but do add Tyvek panels against the inside of the siding – would that work as a vapor barrier? Then leave the cavity open, and drywall on the studs? Or do you think that’s just not enough of a vapor barrier? Do we definitely need something like visquine also?

          1. Add the Tyvek to the back of your siding if you choose to. Tyvek is NOT rated as a vapor barrier This does not make a vapor barrier but forms a wind and major water infiltration barrier into your wall cavity areas. You can apply visquine to the back of your siding which will then form a partial vapor barrier, which I recommend. The proper way to do a vapor barrier on your wall cavities is to apply a vapor barrier both on the outside and on the inside of your wall cavities. Whether you insulate or not, apply a visquine vapor barrier to the INTERIOR wall studs. This prevents moisture from entering into the wall cavity from the inside. If you would care to talk to me concerning your problem, my number is 906-632-8682. We can talk and I can explain vapor barriers to you a lot easier over the phone.

      2. Thanks! This sounds like an option that might work well. We’re desperately trying to avoid insulating (due to the moisture problems we’ve read about), but we’ve just had a City inspection and they’re saying it’s a requirement. We’re in a historic area, so hoping that status may override the local code. I’ll look into visquine. The plaster is already off the external walls – so we just (!) have to remove all the lath.
        If we do have to insulate, do you have any particular product you would recommend?

  8. I have a roll of lumber wrap , similar to tyvek just heavier. Can I use it to put directly on the dirt under the house ? I mean will it hurt anything ?

  9. I am reviving an old Dutch lap cedar sided garage. Siding very old and weather permeated what I’m thinking is since the siding will breath so very well. I have put faced insulation between the studs and have covered that with synthetic roofing underlament .before covering wall’s in luan sheeting . will this work for a very breaths le garage wall. Thanks No.

  10. I live in New England and am remodeling a cottage. I put a vapor barrier on the inside wall and then sheet rocked over it. The exterior has painted shingles which I am covering with siding, should I be putting tyvek over this before I put on my vinal siding? Also note, my home is under a tree canopy in a lake community.

    1. Hi John. After 40+ years in the Upper Peninsula of Michigan as a Building Contractor, and having applied vinyl siding to approx-125-150 homes, I give you the following advice. If you are going to use vinyl siding you must put some sort of a vapor barrier on the outside because of the fact that vinyl siding “sweats” a lot when the sun shines on it and also when the temperature goes up and down. What we did on any home that we were going over old siding is this: We applied 4’x8′ sheets of any type of insulating board over the entire home. This “smoothed” out the old siding and made a much nicer looking job and also added a bit of insulating value, and also gave your exterior a vapor barrier. If the home had no interior vapor barrier we used bead board which is NOT a vapor barrier but gave you the other two “helpers”. If you need any other questions asked, just email me at [email protected] since I don’t usually post on sites and not very often go back to them. Any others need some questions answered use my email address. Hope this helped. JC

  11. We have major wicking on the under side of our home which is up off the ground on pad and pier. This is creating mild and lots of moisture inside the home. What is the best way to stop this. We are also getting some condensation on the ceiling in one bedroom. Any ideas??

    1. Hard to say exactly, but putting 6 mil plastic on the ground in the crawl space goes a long way to keeping things drier. Do a little Google on how to best install the plastic for moisture blocking.

  12. We are remodeling our 1890 home in Buena Vista, VA one room at a time and the plaster/lapboard walls and ceiling are in a bad state (crumbling and falling out in chunks) so we ripped them out down to the studs. The electrical is being redone and then I want to insulate before drywalling. What would be the best options for creating a vapor barrier and insulation? The only thing that is between us and the outdoors is wood siding and you can see a lot of light shining between the overlapping gaps.

    1. David, find out what is recommended by your building code first as to whether the vapor barrier is required on the interior side of the wall or the exterior. Also, be very careful about insulating if you can’t be sure it will stay dry. If you fill the bay’s with fiberglass or cellulose and it is continuously getting wet you will have a very moldy and rotted house that can cause major health concerns.
      Likely your best bet would be to remove the siding and insulate. Then install house wrap like Tyvek or 30# Felt paper before reinstalling the old siding. Probably not what you wanted to hear, but that’s just my opinion.

  13. I’m confused,
    some time ago you wrote about a product from Benjamin Obdyke that you liked, along with reasonable vapor permeability it had small ridges that made an exterior water shedding property.
    I’ve used felt or rosin paper at times under my cedar shingles as I work my way around the house, but I have a lot more house to do and was considering using the Obdyke product (which I assume must be more expensive, but I’m willing to pay more for better and longer performance).

    1. Mike, the product is Benjamin Obdyke HydroGap and I am definitely a fan. If I was building a new building I would probably be using this. I also like their product called home slicker for a very effective rain screen.

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