DOUGLAS E. SMITH
        Smith Custom Builders

    The building envelope (the products that surround the structure/frame of the home) keeps out rain, ground water, wind, noise, heat in summer, cold in winter and protects it from damage.
  The climate we live in determines the products used in the building envelope design.
  The home must be designed with products that stop bulk water intrusion into the home, but allow the house to breathe. This is accomplished by the use of weather barriers like housewrap, plastic, peel and stick tape, spray foam, metal flashings and sheathing.
  With the high moisture content in our air, we must use products that have high "permeability" (the ability to holdout bulk water, but allow moisture vapor to pass).
  Moisture travels from hot hu- mid space (outdoors) to cooler dry space (indoors). Your home should allow moisture vapor to move through its envelope by way of diffusion" (movement of moisture in the vapor state as a result of 81 vapor pressure difference or a temperature difference).
  Once moisture vapor has passed through the outside envelope, it must not be slowed or stopped before it enters the conditioned space -this means "no" double vapor barrier, like paper on the inside of the insulation, plastic under the Sheetrock and/or vinyl wall coverings.
  
 

   If you have ever seen windows sweat on the inside where water runs down onto the window sill, this is a perfect example of the same problem that happens to Sheetrock when insulation is compressed or has flattened out over the years.
  This allows the hot moist air from outside to come in direct contact with the cool dry Sheetrock inside, causing mold to grow.
   So when you hear that builders design homes much tighter than we used to, this really means we seal the homes with better products that breathe but hold out bulk water. This is what is meant by the term "tight homes".
  The strength of the overall structure also is a factor in the home's envelope. Plywood sheathing installed on the outside of the complete frame increases the racking strength by a factor of eight times. (Racking refers to wind-driven movement of a struc- ture.) This means windows, doors, roofs and imy component used in the structure will be less likely to have potential Ieaks or problems in the future.
  Determined by the "zone" you live in, resistance to wind loads is met by different applications of plywood, sheer walls, hurricane steel, steel members and hold- down anchors. The structural engineer uses this information in his/her calculations.
  In remodeling, when structural changes are to be made, it is man- datory to have the structural engineer review existing conditions to best meet the needs of that structure.
  Remember, different builders use different products but the end result should be the same protecting your home's structure from the elements while allowing it to breathe.
  

  Applications such as these cause the moisture vapor to become trapped inside the wall cavity. With no way to dry out. The moisture vapor must flow through the Sheetrock evenly, allowing it to dry out with the air-conditioning system.
   This brings up the problem of air leaks through the home's envelope via holes from pipes, wires, plate line (bottom of walls), around windows, dryer vents or any other penetrations through the home's envelope.
   Air leakage in any wall can allow as much as 15 times more moisture to enter through that small penetration than through a complete wall. This moisture can condense on any cold surface, Sheetrock, wood and tile, creating moisture-related problems (mold).
   Problems also are created by the improper installation of insulation. Insulation must never be compressed smaller or thinner than its factory design.