I have moisture in my attic. I have rusted nails through my sheathing and it looks like mold is starting to form.
I have soffit and continuous ridge ventilation, and the bathroom and kitchen vents are vented to the outside. What can I do?
Moisture in the attic oftentimes can be frustrating. More so when you have provided what is considered all the right things in ventilation and exhaust installs.
To understand why this is happening you must figure out what the moisture source is. It has been my experience when issues such as yours has come up that the moisture source is coming from the inside living area of the home and is being drawn into the attic through was is called Stack Effect.
Stack Effect is the result of poor air sealing both in the basement and in the attic combined. Believe it or not, the basement has a lot to do with the performance of the attic vent system. What happens is as the air in the house is warmed up it rises. Much like a chimney. When this happens it moves up taking indoor moisture with it. I am sure you or someone you know has a home that suffers from dry air in winter months? This is all caused by air moving out of the home. When we think this air is moving out of the walls, and it is, most is going up and out through the roof. The result is the moisture condenses on the colder areas of the attic. The nails and the area closest to the ceiling which is around the soffit locations. As the air move up in the middle of the attic it tends to cool down and mix with the air in the attic and is drawn out through the vents. But when it enters near the soffit it hits the cold roof before it mixes with the air in the attic and condenses. This is why in most cases we find the attic stained near the soffit areas. But quite often then not we blame the soffit area for pulling in the damp air in that location.
In any case we need to stop the air flow entering into the attic. This will stop the moisture flow and the result will be no moisture, no mold, and no rusted black stains where the nails enter into the attic.
To do this we must first find were the air is entering. This is easy. If the attic has insulation, pull it up wherever there is a wall below. This can be the outside wall area or where any wall that makes up a room below is located. Look at the insulation. If its fiberglass you will see black stains in the fibers. This is dust. This dust is the result of air movement with the fiberglass acting like a filter. This is a sure fire method of finding exactly were the air is entering. You will find it wherever wires enter into the attic, plumbing pipes, chimney come up and out. And even around the access hatch or staircase is placed.
To fix this you need to purchase spray foam insulation. I use the window and door type as it is a little less foamy then the regular stuff. The standard foam in can expands way to much and can get a bit messy wen working with it. In anycase, regardless of what foam you purchase, you need to pull up insulation and foam both sides of the exposed board that is the top plate of the wall below. Any wire openings you need to do the same thing. Plumbing as well. When you get to the chimney you need to baffle this. Normally there is a fairly large space between the brick and the framing of the ceiling. This is done for fire protection. Purchase thin metal sheets and form and cut them to fit around the opening blocking off as much of the hole as you can. Then using fire blocking foam. Seal off the metal to the chimney and framing area. Once done this would be a great time to add insulation while your up there. But your not done yet.
Go to the basement and do the same thing. Purchase foam 2" boards and fit them into the mud sill area over the block foundation wall. Using the high expansion foam (the messy kind) spray around the edges of the framing, place the precut foam boards into this space and foam them tight to the wood. This will air seal the wall so no air moves up it towards the attic. Foam all the pipe openings, wire openings and around the chimney in the same manner as you did in the attic.
Once done you not only will prevent this moisture from collecting in the attic on the underside of the roof, you will save a lot of heat and increase your comfort level by stopping drafts that pull the moisture out of the house.
In effect this is the same thing we do as Building Performance contractors only using large foam machines instead of the little cans.