This summer, we are renovating the two-story bay windows of our house. We had reclad these bays some years ago with a fabric house wrap and fiber cement panels. This time, we decided to use a true air barrier, exterior insulation, and triple-pane windows with nailing fins. For the air barrier, we went with a fluid-applied product, Tyvek WB. Fluid applied air barriers are not common in eastern Maine. In fact, it took a little digging to find a product we could get shipped to our region.
An air and moisture barrier can offer a real line of defense against the passage of air and water. They are essentially air- and water- tight. Those on the market today also allow the passage of water vapor, those molecules of water that have not condensed into droplets. Vapor-permeable barriers will let a wall breathe. High concentrations of water vapor in the wall can get out.
How do air barriers differ in terms of vapor permeability? Typar Housewrap DW, which is Typar’s premium house wrap, has a perm rating of 16. This is significantly higher than their standard housewrap, rated at 11.7 perms. Tyvek Home Wrap comes in at about 56 perms, making it significantly more vapor permeable than Typar. Tyvek Home Wrap also has about twice the air porosity of Typar, using the Gurley-Hill Porosity test. Grace Ice & Water Shield is also an air and moisture barrier, but it is also practically impermeable to vapor transmission: Its perm rating is .05.
House wraps come as rolls of non-woven fabric. They are installed by literally wrapping a building in the material, with seams overlapping each other. The fabric is held in place with staples, and seams can be taped to create a seal. Where a building has a simple geometry, creating a good seal should not be difficult. Where the geometry is challenging, creating a seal can also be challenging. Think about using a house wrap on complicated geometry with windows and doors, and then spraying the house with a garden hose. How confident would one be that water will not penetrate the house wrap? What if one tested the house with a blower door?
Fluid air barriers offer significant quality benefits over fabric house wraps. Operationally, these barriers are literally painted onto the building. Challenging geometries are not an issue. The installer can visually see that the barrier is creating a monolithic seal around corners and at openings. Tyvek WB fluid applied air barrier has a perm rating of 25, and an air porosity rating of about one quarter that of house wraps.
After we removed the existing siding, house wrap and windows, we prepared for new windows by creating plywood window frames, or bucks. These bucks gave us some structure to move the windows out of the wall’s stud plane and into the exterior insulation plane. Our house was originally sheathed with 1-inch nominal (about 7/8-inch actual) wood boards, so we added a layer of ½-inch oriented strand board (OSB) to provide a smooth substrate for the fluid applied air barrier.
Fluid-applied air barrier manufacturers offer a system of products, including Tyvek. A thick product comes in caulk tubes, and is designed to act as a flashing to fill and bridge joints. A “thinner” product (Tyvek WB) comes in 5-gallon pails, and is designed to cover everything. We first applied the flashing to every single joint on the wall: between sheets of plywood sheathing, between sheathing and buck, and between buck and window. We then rolled the WB product over everything, including the joint flashing. To say the WB is "thinner" is relative. It is thinner than the the joint flashing, but has the viscosity of ketchup. It does not drip or sag once rolled on.
The recommended thickness for Tyvek WB is 25 mils. Tyvek shipped a small material thickness gauge with the product. After rolling the material with a 3/8-inch nap roller, we realized we needed to go over areas twice (while still wet) in order to reach 25 mils.
The end result is a monolithic membrane whose integrity can be visually inspected. One can imagine taking a garden hose to this air barrier and having high confidence that the membrane will do its job.
The Tyvek WB becomes the air and water control layer behind rigid insulation. We used two-inch extruded polystyrene (XPS), commonly known as blue-board in eastern Maine. XPS has an R-value of about 5 per inch at 75 degrees F. The R-value drops off somewhat in colder temperatures. In the dead of winter, this exterior insulation may only be providing an additional R-8. Nevertheless, the insulation is continuous, meaning there are few thermal breaks. The insulation is increasing the time during which the air barrier surface is above the dew point. Therefore, vapor on the inside plane of the air barrier can stay a vapor until it passes to the outside plane of the air barrier. In other words, the exterior insulation is not only increasing the wall's R-value, it is also decreasing risks of cavity water vapor condensing within the wall.