Updated - January 2016
Vapour Barriers and Air Tight Building Envelopes
The Energy Efficient Home Series
In order to be effective a vapour barrier must be completely sealed.
Therein lies the problem and the intent of this article is to discuss the issues related to vapour barrier installation and the importance of an air tight building envelope. In this latest installment of the Energy Efficient Home series we'll address the following questions and concerns:
- What is a building envelope?
- What is a vapour barrier?
- Why is a vapour barrier important?
- Why are polyethylene vapour barriers only somewhat effective and why do we continue to use them?
- Why are we so passionate about this? An example of poor installation.
- Why is an air tight building envelope so important?
- If polyethylene vapour barriers are flawed, what do we recommend for new homes and buildings? (Hint....structural insulated panels)
What is a building envelope?
A building envelope is what separates the interior of the home or building from the exterior elements. This includes the exterior walls, windows, doors, mechanical openings (exhaust vents, hose bibs, etc.), the roof and the floor/foundation which connects the home or building to the ground. There is also a thermal envelope which is where the insulation, vapour barrier and air barrier occur around the interior of the home.
What is a vapour barrier?
A vapour barrier is most commonly a polyethylene plastic sheet that resists the passage of moisture through a wall assembly. The vapour barrier is installed on the warm side of the insulation (ie. the inside of the wall) behind the interior wall finish (ie. drywall) and is secured to the structure (ie. wood studs) with staples at the intermediate framing members and with acoustical sealant around windows and other openings. Tuck Tape (red tape) is also used to connect sheets of vapour barrier together and around junction boxes and exhaust ducts. At a minimum, Building Codes throughout Canada require a 6 mil (.152 mm thickness) polyethylene vapour barrier.
Why is a vapour barrier so important?
In cold climates we have to introduce heated air to keep occupants warm during those chilly months. Along with the heated air, activities like cooking, dishwashing, laundering, bathing and even breathing produce considerable amounts of water vapour which is then absorbed by the air in the house. This air borne water vapour is constantly trying to escape the home due to two factors, vapour pressure (diffusion) and more importantly, air movement (exfiltration).
As the air borne water vapour escapes through the cracks and holes in the building envelope (most important component in this case is the vapour barrier) it comes into contact with the cold air outside and then condenses from air borne water vapour to liquid water or frost. All to often, the liquid water ends up in the wall cavity which results in moisture damage to the wood framed structure, insulation, sheathing and cladding. Furthermore, once the insulation is wet its thermal resistance (R-value) is significantly reduced which in turn requires more heating or more energy use.
Why are polyethylene vapour barriers only somewhat effective and why do we continue to use them?
A polyethylene vapour barrier has had a significant improvement in cold climate home construction but it comes with many flaws.
The issue that we have with polyethylene vapour barriers boils down to human error. Inadequate detailing by their designers, poor installation practices by the installers and a lack of inspections by professionals and building inspectors lead to homes and buildings that leak and cost Owners year after year.
Designers and Architects – on a typical spec house purchased from a home plan book the extent of the vapour barrier detailing boils down to one note and if you are lucky, maybe one typical detailed wall section drawing. The note reads “6 mil poly vapour barrier”. That single note transfers the assumed responsibility to the installer to correctly install the barrier.
Installers - All it takes to install a vapour barrier is a staple gun (which creates holes), Tuck tape (red sticky tape) and some acoustical sealant. Quite often the result is a patchwork of taped together sheets of vapour barrier, Tuck tape wrapped around wire penetrations and electrical outlet boxes and acoustical sealant smeared and not properly adhered to the barrier.
When the vapour barrier installation is complete, the wall finish (typ. drywall) is secured to the framing members and it does help to seal the barrier to the framing but it still leaves a lot to be desired in terms of a quality well sealed vapour barrier system. In addition to that, if the drywaller misses a stud with a drywall screw the vapour barrier is punctured. A properly installed 4'x8' sheet of drywall requires a minimum of one drywall screw every 12" so that means that there are 38 screws per sheet of drywall. Times that by an entire house and the installer has to be pretty accurate to never miss a stud and puncture the vapour barrier.
Building Inspections – in building construction Architect's review (not guarantee and install) vapour barriers prior to the walls being closed up (drywall finish added) but in the end it is the responsibility of the builder to ensure that the final product is completely sealed. Some building Inspectors are really good at reviewing vapour barriers but more often than not their main concern are three elements – fire, life and safety elements. In home construction, an Architect is not required so home designers are tasked with reviewing vapour barrier installation. Unfortunately, the bulk of Canadian homes are never reviewed by their designers because most homes are built on spec typically from a set of plans purchased from a book. The inspection then lies on the builders shoulders and just ask Mike Holmes what he thinks of builders inspecting their own work. Enough said…
The other issue that we have with polyethylene vapour barriers is that even if an installer does properly install the vapour barrier, it has been inspected and a blower door test has proven its effectiveness, once the homeowner moves in it is only a matter of time until the vapour barrier is punctured. The second a picture is hung inside the house on an exterior wall, the nail that is used to hang the picture punctures the vapour barrier.
Even though is widely acknowledged throughout the industry why do we continue to build homes like this? The only thing that we can conclude is that we have become complacent. North America hasn't seen our energy costs rise like they have in Europe but as provincial and state governments become more indebted they will not be able to continue the energy subsidies that we have enjoyed for years.
By building homes and buildings like this we are guaranteeing huge energy bills each and every year. We are pretty confident that energy costs will only rise in the future so let's start building better homes!
So why are we so passionate about this? Just one example of poor installation.
We don't know how many times we have arrived on site to find openings and holes in vapour barriers that haven't been sealed, unsealed vapour barriers around mechanical penetrations (exhaust ducts) and in some instances no vapour barrier whatsoever?
In one particular case in 2010 we had to have a builder pull down two full layers of drywall because an inspection wasn't completed before the drywall went up. The result, the vapour barrier had been installed but it was poorly sealed, had numerous holes in it and what was really scary was that incorrect insulation had been installed. What is even scarier is how often stuff like this happens and it is exactly why we had to instruct the builder to pull down the layers of drywall because we couldn’t take their word for it. On a construction job the builders main concern unfortunately is not the homeowner, their main concern is the schedule because if a home or building can be finished faster that results in more profit.
Our intent here is not to point blame because not all vapour barrier installations are done poorly, but when significantly better and simpler practices exist why do we continue building to the absolute minimum requirements which only cost homeowners more and more money each and every year.
Why is an air tight building envelope so important?
Air tight building envelopes are so important for two significant reasons, to reduce overall energy loss and to ensure a durable long lasting building envelope. No building envelope can be 100% air tight due to openings like windows, doors, mechanical openings, etc. but we can take steps to ensure minimal air leakage through the building envelope.
Polyethylene vapour barriers have been used since the late 70’s and yet practically every other technology that we use day to day has changed around us so why is it that in home construction we continue to use 70’s era technology especially when we know the flaws of the system? This isn't logical? Would you go back to your original cell phone? Remember that one that came complete with the big carrying case, large antennae and cigarette lighter plug (that is if your car still has a cigarette lighter?), or would you like to stick with your new smartphone? It's time to build better, smart homes and buildings!
If polyethylene vapour barriers are flawed, what do we recommend for new homes and buildings?
Building Science and Technology has improved immensely over the years. Fortunately, in the case of building envelopes, the technology the we are recommending hasn’t become any more complicated, it has actually been simplified in terms of its components. Where they system has been significantly improved though, is that it has been designed to remove the human error component.
Rather than typical wood frame wall construction we recommend Structural Insulated Panels (SIP’s) with an installation or service wall cavity installed inside of the thermal wall panel. Not just any typical SIP will do however. Due to the consideration of embodied energy and moisture migration through building envelope assemblies, we are firm believers in foam-free construction whereever possible therefore rather than using a typical SIP which consists of an insulating layer of EPS foam sandwiched between two layers of OSB, we recommend SIP's that consist of a layer of OSB, densepack cellulose insulation and either a highly breathable wood fibreboard sheathing or a combination of a high performance weather resistant barrier with two layers of cross battens for shear strength and cladding attachment.
This leads us to our next article in the Energy Efficient Home Series – Structural Insulated Panels. There is so much to cover that we feel that we need to dedicate an entire article to the topic so stayed tuned as we have much to discuss.
Brett Sichello Design specializes in
High Performance Home + Passive House Design