Smart Vapor Retarders
If you are looking to build a new home or make an existing home more comfortable and energy efficient, you need to look further than just insulation. In fact, insulation is the last of 4 “layers” a home needs to be comfortable.
In our article Understanding Air Barriers and Vapor Retarders: Why and Where to Place Them, we walk you through these 4 layers in their order of importance, and yes, insulation (thermal control) is last. The reason is that air, the second-place layer, carries water vapor and heat with it, so if you have a well-functioning, well-placed air barrier, you actually take care of a lot of water vapor and heat issues! There are actually products now called “smart” vapor retarders that function as air barriers AND vapor retarders. The reason they’re called smart, however, is because they allow variable vapor permeance (a measure of how readily water vapor can pass through) according to humidity.
It used to be that when a homeowner decided to renovate an old, drafty home to increase comfort and save on energy, it was recommended to add polyethylene sheeting to prevent air from moving through the walls and roof. Polyethylene sheeting works great as an air barrier, but when it’s placed between warm and cold areas, it can also cause condensation and mold! This excellent video (only 5:29 min) shows how it can actually damage a wooden structure (studs and rafters) from rot and mold, and it shows how newer “smart vapor retarders” do a better job of stopping air but allowing some moisture to pass through.
The goal of smart vapor retarders is low permeance in the winter when humidity is low but it’s critically important to block moisture flow and prevent condensation, and high permeance in the summer when humidity is higher and you want drying potential to both the interior and exterior. (Smart Vapor Retarders) Actually, the old “kraft” paper facing on fiberglass insulation acted as a “poor man’s vapor retarder” because it becomes more permeable to moisture as the humidity increases in the summer, and less permeable in winter as humidity drops.
Some of these smart vapor retarders are plastic based and some are paper-based. They are mainly installed by stapling to studs, then sealing the edges together using an approved tape (to maintain air barrier integrity of the layer). Popular brands include:
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Intello and DB+ by pro clima
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Majrex by Siga
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MemBrain by CertainTeed
It’s important to check with the manufacturer on where to install their particular vapor retarder. Most are made to be installed on the “warm side” of your wall or roof, meaning inside of insulation and studs, just before drywall is installed. In the photo below you can see a blue vapor retarder, pro clima DB+, that is installed on the warm side of an attic room. Some are made for external vapor control, such as just under the siding or rain control layer, so you need to know what to install where.
Image Credit: Alex Wilson, Smart Vapor Retarders for Walls and Roofs
The other important preliminaries to installing a smart vapor barrier are to address any water management problems first (repair all leaks, because a smart vapor barrier is not going to help with those!), address and close any voids that may convey air currents, and to install the appropriate insulation (fiberglass, rockwool, blown-in cellulose, etc.). These requirements are addressed in this followup video by the same company.
Then, after the smart vapor retarder is installed, you’ll want to make the most of its air barrier functionality by limiting penetrations. This is where horizontal strapping is useful (see photo above). The strapping allows electrical and some plumbing runs behind the drywall, in order to minimize penetrations through the air barrier (smart vapor retarder). The previous video also suggests order of insulation and other products to use with masonry walls, which are notoriously tricky to insulate without condensation and mold issues.
There is even a software program that can calculate the vapor permeance needed for your particular building. WUFI® is an acronym for Wärme Und Feuchte Instationär—which, translated, means heat and moisture transiency, and can be used by your architect to make sure you get the air barrier, vapor control layer and insulation according to your climate and structure. Here’s the thing: moisture and mold just form according to the laws of nature, but we need to use extra reasoning skills and diligence in building to keep them out! Thankfully, science and product development is making it easier to build or renovate a healthy home, and smart vapor retarders are definitely a big improvement on kraft paper and polyethylene sheeting.
