You're Using Vapor Barriers Wrong | What You NEED To Know

vapor barriers keep our buildings dry or do they in this video we're talking about how using a vapor barrier incorrectly can actually lead to more moisture problems mold and rot when you need to use a vapor barrier and some alternative strategies that you can use to keep your home dry and durable let's get into it so why do we install vapor barriers typically we install a vapor barrier to slow or stop the movement of vapor from defusing in or out of a building assembly usually because we're concerned that the vapor passing through the assembly could condense on a cold surface or raise relative humidity within the space which could impact durability and support things like mold growth and these are all valid concerns we might also want to use a vapor barrier as a capillary brake to prevent moisture from wicking into our building materials as in the case of a slab on grade moisture moves from higher concentrations to lower concentrations and from warm to cold that's the second law of Thermodynamics if let's say we poke a bunch of holes in a vapor barrier it's still going to function effectively as a vapor barrier how however it's not going to function as an air barrier and air is what concentrates Vapor since air leaks can transport moisture at a rate that is orders of magnitude higher than diffusion so if moisture ends up in the assembly it can't get out and we run into problems vapor barriers or vapor retarders fall under three primary categories we have class one Vapor retarders which are less than 0.1 perms thanks standard polyethylene SPS rubberized asphalt PVC foil faced insulation glass and then we have our class 2 Vapor retarders which between .1 perm to one perm in vapor permant and that's going to be our unfaced rigid insulation like XPS or EPS or thick polyiso and then we have our class three Vapor retarders which are between one perm and 10 perms these are materials like OSB concrete latex paint and a lot of waterproofing products anything over 10 perms that's going to be considered as Vapor open or vapor permeable however there still are varying degrees of vapor permeance that impact how a building dries and then we we have Vapor variable materials which tend to have a lower dry cup permance but have a higher wet cup Vapor permance these are materials like smart Vapor retarders craft paper traditional Roofing Felts and plywood and these allow for a lot more flexibility in the design of the assembly because we can slow the flow of moisture into the assembly or out of the assembly and that can be beneficial to prevent things like condensation so here we have a detail of an above grade framed wall assembly with a vapor barrier installed on the interior very common in cold climates especially places like Canada so why do we get mold in the wall cavity well we have seasons remember moisture moves from warm to cold and from higher concentrations to lower concentrations and often times they're moving in the same direction during warmer months Vapor Drive is predominantly from the outside inwards while Vapor Drive is from the inside outwards during the cold months of the year when vapor diffuses through the walls to the interior in those warmer months it gets trapped increases relative humidity within the wall cavity and then we get condensation on the back side of the vapor barrier especially if the interior is air conditioned that water that condensed is now absorbed by the framing materials and it can even end up collecting at the seill plates and we get mold and rot in our walls since it can't dry out this is only worsened if the cavity is well insulated because insulation slows the flow of heat and we have a reduced drying potential so we want to avoid plastic vapor barriers in our above grade walls we don't want to install any vinyl wallpaper or foil faced finish materials so how can we prevent wintertime condensation if we can't use vapor barriers well we can warm the condensing surface of the sheathing with rigid insulation to keep it closer to Interior temperatures any vapor diffusing into the walls that comes into contact with the sheathing won't condense because the temperature of the sheathing is warmer than the dupoint temperature granted that we do need to use the right ratio of exterior rigid insulation installed outboard I just posted a video about the benefits of exterior rigid insulation and integrating rigid installation we can go and watch right here so what if you're being forced to install a vapor barrier as in the case of many jurist jurisdictions in Canada specify a smart Vapor retarder instead of the standard polyethylene smart Vapor retarders are vapor variable they are low class 2 vapor retarders when relative humidity is lower but increase in perance substantially as relative humidity in the framed cavity increases which means that if we get moisture in the cavity it can actually dry out instead of being trapped what about below grade basement walls we frequently see vapor barriers being installed in the wrong location in basements this is what I call the diaper where the vapor barrier has been installed on the interior side of the studs trapping moisture that gets into the wall cavity whether it's from water leaking inside from hydrostatic pressure from interior air leakage or from moisture wicking up through the concrete footings from The Damp soils and we get mold in the cavity we can't dry through the basement walls to the exterior since the soils are pretty much always wet we have to stop doing this instead we want our moisture sensitive framing components to be able to dry out the interior so instead of a plastic Vapor Barrier we want to install either taped rigid foam insulation or close cell spray foam directly over the concrete foundation walls which will not only serve as our Vapor retarder but will prevent air leakage and condensation from moisture Laden interior air if we're building in an especially wet climate it's a good idea to install a capillary breake at the footing to prevent moisture from wicking up into the concrete foundation walls being driven inside and of course all this should be combined with good drainage practices to keep water away from the basement walls if we want to avoid foam products in our basement walls we can also use a Smart vapor retarder in combination with fibrous insulation materials to prevent condensation while allowing the walls to dry to the interior next we have crawl spaces we tend to locate the vapor barrier in a crawl space right over the soils or ground which is actually a good thing since we get a lot of moisture evaporating out of the soils and that moisture can end up on the inside of our home however if we're building a vented crawl space we still get moisture that's being transported via air flow into the crawl space from the exterior environment and that can condense on the underside of our floor framing especially in more humid climates so we actually need an additional airtight Vapor Barrier separating the floor framing from the crawl space I found that taped foil-faced rigid insulation works best here since it not only provides a vapor barrier and air control layer but it provides a nice thermal brake as well and it's lightweight if you're building a conditioned crawl space you don't actually need this additional Vapor Barrier since you're essentially building a mini basement now let's talk about slabs we need a vapor barrier underneath our concrete slab since concrete is porous and it's in contact with the ground we can get a lot of moisture flowing through the slab in both the liquid and the vapor form if we don't have a vapor barrier here as the vapor barrier also acts as a capillary breake we can't dry through the slab into the soils since the soils are always wetter than the interior conditions so we really do need a strong Vapor Barrier underneath the slab this is the perfect location for your standard polyethylene Vapor Barrier all right finally we have roofs roofs are a little bit different compared to our other assemblies and depending on how the roof is designed we may need a vapor barrier or even a double vapor barrier to prevent moisture issues as in the case of flat roof assemblies or no vapor retarder at all roofs with vented addicts especially well insulated addicts and cold re climats need an airtight Vapor retarder installed at the ceiling plane to slow Vapor movement and prevent air leakage into the attic space warm air can hold more moisture than cold air the incoming cold air in the attic space doesn't have the ability to pick up much moisture and if the attic is well insulated there's minimal heat loss from the building to warm up that cold air so vapor pressure is lower and we can actually get condensation in our vented addicts that support mole growth and rot this is where I like to use a taped smart Vapor retarder membrane at the ceiling plane as it will provide a much needed air barrier at this location and slow the flow of vapor into the attic space while allowing moisture to dry back into the interior this membrane also allows for Superior air tightness around ceiling penetrations if we're designing an unvented conditioned roof assembly we have a lot more options at our disposal and can either design the assembly with a vapor retarder or without a vapor retarder depending on how well we warm the condensing surface of the sheathing if we wanted a vapor open conditioned roof assembly with no vapor retarder at all in the assembly we would want to install an airtight self-adhered Vapor permeable membrane a vapor permeable insulation above the roof deck like mineral wool or wood fiber to warm the condensing surface of the sheathing a vapor open insulation within the rafter cavities and a back ventilated roofing material installed over a vapor permeable underlayment so the roof assembly would look sort of like our wall assembly but tilted to an angle this assembly also works with imp perable Roofing underlayments as long as the right ratio of rigid insulation is installed above the roof deck we can also manage vapor in warmer climates with a vapor diffusion Port which allows us to vent moisture out the ridge without introducing outside air if we can't install rigid insulation outboard of the sheathing we need to control condensation from the interior either with a tape smart Vapor retarder membrane installed over the rafters sealed rigid foam insulation or with closed cell spray foam while spray foam can work in some circumstances it's not a silver bullet and it has the potential to crack in cold climates which can violate the Integrity of the air barrier and it can offgas some nasty chemicals for an extended period of time check out this video that I made on why spray foam tends to crack in cold climates and how you can prevent it for more comprehensive information on building envelope design head over to A-D designs.com where we have over 150 free building science articles that cover a wide range of topics from flat roof design to insulating and sealing old buildings to basements and more for now good luck with your projects cheers

Channel: ASIRI Designs

Views: 200,304

Keywords: vapor barriers, do you need a vapor barrier, vapor barrier, do you need, do i need a vapor barrier, how to install a vapor barrier, how to, how to install, vapor retarder, do you need a vapor retarder, do i need, vapor barrier basement, basement, basement vapor barrier, slab vapor barrier, condensation, vapor barrier mold, mold, prevention, walls, roofs, vapor barrier roof, air barrier, air control layer, vapor control layer, explained, vapor barriers explained, building science

Id: K6HR2ftXFss

Length: 9min 38sec (578 seconds)

Published: Sat Mar 09 2024