GreenDwellingSeattle

The structure is in place, the interior systems are coming along, and it is time to start applying the layers of material that will make up the exterior of Columbia City Green. 

Going back a few posts, after we framed the walls and roof structure, a layer of 4’ x 8’ panels was placed on the outside of the framing; forming an exterior wall surface.  This exterior wall surface (sheathing) will form the base layer for the various measures that are taken to protect the house from the elements, properly manage moisture, and maintain the integrity of the structure indefinitely. 

For Columbia City Green Case has decided to use what is known as a Rainscreen Wall for the exterior of their homes (See below). 

A Rainscreen Wall (also known as Rainscreen Siding) has four major components:

1)      Rigid Support Wall (Sheathing)

2)      Drainage Layer (Weather Resistive Barrier)

3)      An Air Gap

4)      Exterior Cladding (siding)

 A Rainscreen Wall is an extremely effective siding technique for the Northwest climate.  In almost every home, water WILL find its way behind the siding at some point.  Rainscreen Walls are designed to effectively deal with this water.  With this system the exterior surface of the home is actually designed to be porous.  Behind the siding there is an air gap that separates it from a weather resistive barrier.  The air gap serves a number of functions.  Open at the top and the bottom and fitted with protected vents, the siding system allows moisture to easily drain to the bottom or evaporate out the top.  The convective air movement behind the siding can also provide some amount of cooling effect during warmer months. 

Perhaps the most important component of a Rainscreen Wall is the type of weather resistive barrier that is applied to the rigid support wall.  Many moisture barriers that are available on the market today are designed to create an airtight, watertight seal around the exterior of the home.  While this sounds like a great idea, there is a middle-ground that provides for a much healthier home and its occupants.  Think of this moisture barrier like you would a rain jacket.  You want something that keeps the moisture out, but let’s your body breathe at the same time.  Case has decided to use a product called Vapro Shield for the weater resistive barrier on these homes.  Vapro Shield is a moisture barrier that works very much like Gore Tex.  Moisture and vapor can travel one direction: from the inside out, but not the other direction.  Vapro Shield’s permeability allows the house to “breathe”, reducing the likelihood of mold, mildew, and rot.  This can have a significant effect on the indoor air quality within a home. 

Behind the vapor barrier is the sheathing that we have already discussed.  In conjunction with the 2 x 6 advanced framing, cellulose insulation, drywall, and interior air sealing, this wall system will provide comfort, protection, health, and durability.  We will continue to talk about siding and exterior details in the next post so stay tuned.  If you have any questions or anything to add please do.