Building Science-Building Shell Layers-Rain Control Layer

The next four blog postings are all going to discuss a few of the most important parts of a building, the four layers in a building shell.  I’m leaving out the “structural layer”, though it’s the most important part of a house, the other layers are less understood.  These four layers are:

1. The rain control layer or WRB
2. The air control layer
3. The vapor control layer
4. The thermal control layer

These layers are listed in their order of importance with the rain control layer, sometimes called the weather resistant barrier (WRB), being the most important.  If we can’t keep bulk water out of the house, the rest of the barriers just don’t mater.  Let’s get started with the rain control layer.

As I just stated, the rain control layer is the most important layer after the structural layer in a building’s shell.  This layer controls the movement of water down and away from the structure and includes the roof, walls, windows, doors, and the foundation.  The main purpose is to keep the inner layers dry.  It is also the first line of defense to keep outside air from moving through the assembly.  There are several layers of building materials that can be considered part of the rain control layer, we will start with the roof.

The exposed side of the roof will have a roofing material, such as asphalt shingles, steel shingles or panels, rubber, wood, or some other less common roofing material such as photovoltaic roof panels.  These materials are the outermost WRB, they see all weather, sun, rain, snow etc…

Directly under the roofing material is a secondary water-resistant material used to back-up the main roofing material.  Sometimes water will end up under the main roofing.  Ice dams during the winter or a mistake during installation or error in a flashing detail will allow water to get under the main roofing material.  This secondary material, often tar paper or a peal and stick membrane, is designed to keep the water from entering the roof.

The final layer in the roof is the structural sheeting, usually oriented strand board (OSB) or plywood.  Older roofs can have individual boards nailed to the roof framing or rafters.  This layer has gaps where the boards meet, not very water-tight, but because the sheeting is made of wood, the materials can absorb a small amount of moisture that ends up under the other layers.  If water makes it through all these materials, it’s in the home and causing trouble.

A good roof design will have a large overhang, one to two feet, which will reduce the amount of moisture ending up on the exterior wall and move bulk water further away from the foundation.  When I was building new homes, all my roof overhangs were at least a two-feet.

Next up are the exterior walls, and the first protecting surface is the siding (or the paint or stain on the siding).  The siding will shed almost all the water and moisture during rain events, but it’s inevitable that some will end up behind the siding.  The choice of the material behind the siding is just as important as the siding itself.  Modern buildings use some sort of house wrap, (commonly referred to as the WRB), with one of the most popular being Tyvek.  The products are designed to prevent bulk moisture from getting to the framing structure but still allow vapor to move through the house wrap.  The trick is to make it as easy as possible for bulk water to move down and remain on the outside of the house wrap and still allow for an outward vapor drying potential. Vinyl and steel siding naturally produce a gap between the siding and house wrap, other types of exterior finishes have the best drying potential with a planned gap.  Sidings that are not constructed from plastic, metal, or glass are considered “reservoir claddings” and will absorb at least some moisture.  In the case of stucco, brick and stone claddings, lots of moisture can be absorbed.  Without an air gap, this moisture can move through the house wrap and sheeting and end up in the wall cavity.  Because many homes in Minnesota use polyethylene for interior vapor control, there is no drying potential to the interior.  This means any moisture that ends up in the wall cavity is stuck until the assembly can dry outwards.  This type of wall design can be high-risk.  That air space behind the siding or cladding is key!

Poorly designed water management details can be trouble.

Windows and doors are also an important part of the water control layer, they need to be connected to not only the WRB, but also the air barrier, vapor barrier, and thermal barrier.  Usually, but not always, windows or doors are caulked to the siding or cladding as the main barrier to moisture. They also need to be connected to the house wrap, typically with caulking and tapes.  There are many ways to accomplish this connection, I recommend following the window and door manufacturers installation guidelines or the directions from the house wrap manufacturer.

Lastly, we must extend the weather water control layer to the foundation.  Any water that flows down the siding, either on top or behind the siding, should end up on the ground and should move away from the foundation.  Most new buildings in a northern climate will have insulation on the exterior of the foundation or slab.  This insulation should always be covered to protect it from damage.  The covering needs to connect to the house wrap and be lapped to shed water.  I recommend mechanically connecting this lap by either taping, caulking, or using a fluid applied product to produce a good seal.

Below grade water-proofing must also be provided.  This layer, installed mostly on homes with a basement or crawlspace, keeps the basement or crawlspace walls dry and helps control humidity levels within the space.  It’s best if the below grade water-proofing ties to a sub-slab vapor barrier, which not only will help control humidity levels, but also reduce the potential for radon gas to enter the home.  This sub-slab vapor barrier is also required in a frost protected slab on grade foundation.

There are lots of devices and equipment inside our homes that end up producing a hole in the water control layer.  Electrical service, outdoor lighting and plugs, plumbing vents, heating exhaust vents, dryer vents, ventilation dampers…lots of holes that need to be addressed through all the building layers.  The easiest way to deal with these holes is to seal them when the building is new.  This process becomes much more difficult after the building is completed.  Plan carefully!  The photo shows just one way to address penetrations through the WRB, Tyvek’s Flexwrap.

Tyvek Flexwrap

As I said earlier, the most important job of the water control layer is to move moisture down and away from the building.

Bad things happen when water enters a home. Next week we will be discussing the second most important barrier, the air control layer.

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