Hello everyone,
I am finishing my attic to give my wife a quilting room. The floor is 2×8″ truss cords with 3/4″ advantech tongue and groove OSB. There is 8 inches of cellulose insulation underneath this floor. The ceiling below does have a poly vapor barrier.
The cellulose, as well as the bottom the subfloor is a little moist (throughout the whole 12x40ft attic.) Also, the subfloor nails are rusty and the joists have a little black around the nails. Is this because there is not enough cellulose? We live in Vermont and It does get below 0 degrees F quite often.
Thanks, Eric
Replies
Is it possible your poly vapor barrier has some holes in it or is not properly sealed around penetrations?
Is there moisture between the poly and the drywall ceiling, or just above the poly?
Do you have a bath vent or dryer venting into the attic?
I can't find any holes in the poly and it seems to be sealed pretty well. We have 2 bath and 1 dryer vent that go thru metal pipes to the outdoors, but there doesn't seem to be any extra moisture around them. The moisture is pretty evenly distrubed throughout the whole floor area.
There is not moisture between the ceiling and the poly. It is only in the top couple inches of cellulose and on the bottlom of the subfloor.
Thanks for responding, Eric
I'll go out on a limb here. There's moisture in the attic and it's coming from inside the house. Even if there's a poly VB there's also a staircase that you're using to get up there, and moisture is migrating into the attic from the house as the warm air escapes up the stairs. The moisture is finding the dew point within the insulation, where it's warmer than in the attic per se.
If the attic is going to be finished space, will you be leaving the insulation in the floor? I would assume that you will need to insulate the rafters and kneewalls in order to make the attic into conditioned space.
Eric:
Where do you live????
He's in Vermont.
Oh right, it's all coming back now!!! DOOOHHHHH!!!
Thanks- checked his profile and not the first post again!
Could be a combination of interior air leakage to attic through loose exhaust ventilation joints (I even tape all adjustable elbow jointsas they are not airtight), other sites not fully sealed and possibly the stairwell. Another source of moisture that no one usually considers is the moist outdoor air that enters during the day in certain weather and then cools as night falls.
The phenomenon is best known to cause condensation/corrosion in outdoor oil storage tanks that aren't kept full during the summer, fall and spring. During the day as temperatures rise, the air in the tank expands and some leaves the vent. By late in the day, the outside air has picked up moisture as it is at a higher temp and evaporated the dew/frost from the previous night plus maybe more moisture from the ground and waters due to winds. As the air in the tank cools and contracts, outdoor air with more misture is drawn into the tank and, at some point, as temps drop, water condenses and falls to the bottom of the tank, leading to longterm corrosion/leakage.
A similar phenomenon has been postulated to happen in some vented attics by research engineers. They say that damp maritime climates would be more suscepable to the increase in moisture by having vents in attics. I have seen signs of this in only a few attics I have inspected in my own locale. One was very perplexing until I remembered reading this research paper. This roof was OSB sheathing and had the venting channels like Raftermate to bring air from the soffits up to the higher vents. The only place I saw mould growth was at the sheathing above the inner mouth of the vents where theoretically the outdoor air should be drying and keeping this wood clear of condensation and mould!! Obviously,there was more moisture coming in with the ventilation air as evidenced by the mould growth!!!!