So when you install wood siding over a Tyvek house wrap, you take care to flash and seal all penetrations from plumbing, HVAC and electrical connections to make sure no water can get in... then you riddle it with nail holes when you nail the siding in place!
Why isn't there any concern about water penetration into the house through the nail holes? Does Tyvek seal itself around the nail holes? Or does nobody worry about it?
While products like Tyvek do act as a drain plane to some extent, they're not truly waterproof. They're designed to allow vapor to easily pass, and therefore they're inherently an imperfect waterproofer. They should be considered a second line of defense against flawed siding installation and occasional wind-blown water intrusion.
The flashing around windows and other penetrations has as much to do with stopping airflow as it does with stopping water. Housewrap is mostly concerned with air leakage, which can dramatically affect the energy efficiency of a building's envelope.
The bottom line is that nails in the field are less likely to be subject to water on a regular basis, and they do somewhat self-seal. Window flashing is much better at self-sealing and is used in high-risk areas. Housewrap should not be considered a substitute for proper siding drain planning. Steel and vinyl siding channels, for example, must lap properly in order to direct the flow of water outside the siding.
I have experience with that issue from back in 1999. I was building new homes in Southwest Washington in an area exposed to higher than average wind. We removed siding on two new homes for different reasons and what I saw ended my use of Tyvek ever since. Both homes had horizontal lap siding so there were rows of nail penetrations on every stud. Rain water was hitting the siding and traveling upward under the 1-1/4" overlap. They say that rainwater can travel 1" vertically for every 10mph of wind blowing on it. I can confirm that it happens.
There were no problems within 6' of the roof overhang. The water ran horizontally across the top of the siding until it hit a window or corner where it then poured down over the Tyvek. Every nail hole in the field had a 4" to 6" wet spot behind it. It was probably sucked in by a pressure difference. Once it was in it stayed there. The Tyvek held it in better than keeping it out.
At the lower part of the corners and below the sides of the windows were the worst situations. So much water got through that there were large wet areas with swollen OSB. The water was trapped and not evaporating through the Tyvek or nail holes.
I never built another Tyvek house. I switched to 15lb felt over plywood and then to 30lb. 30lb felt seals the nail holes, keeps water out and lets vapor pass through. It will probably still be good in 200 years. I never had another water problem on a single house after switching.
You are correct about how Tyvek works. Canada will not allow Tyvek in its country as a building material as it does hold in the wetness and moisture that causes mold to grow. Most builders install it wrong and do not use the correct fasteners, do not properly tape the seams or seal through wall penetrations like windows, doors, plumbing and electrical systems. You’ll find most mold issues under the tyvek from contractors cutting holes in the exterior walls. The sawdust falls in between the wall and tyvek and is the perfect food for mold to grow. I used to properly seal houses after mold was found from forensic house detectives after someone from the household either died or became deathly ill from mold in their homes.
Use felt paper or tare paper
