Tyvek and House Wraps

This type of material is a one-way door system. It allows humidity in the stud cavity to migrate out to the exterior air when the humidity level is lower than the relative humidity in the stud cavity. Great concept. In actuality it has some drawbacks;
First you must make sure that wall sheathing is working with you on removing the humidity.

1. Using the house wrap only works when the humidity level is below 17% this is the level the framing (wood and sheathing) has to remain at to keep its structural stability. At higher levels of humidity in the stud cavity on exterior walls with cool evening temperatures, condensation forms on the exterior wall sheathing, runs downward to the sill plate and starts the decaying process. If the wall sheathing is porous enough to allow the humidity to pass through, and with drop in outside temperature the humidity will condensate or freeze on the back of the house wrap.

2. When moisture level exceeds 18% in the lumber and the temperature is above 40° fungi begin to inhabit the wood.

3. Where water is present bacteria can grow, when the bacteria reacts with the nails rust forms. Nails holding the framing together start to rust, more the bacteria the faster the stability of the structure is affected.

In my personal opinion House wraps causes more problems than they solve.

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