Aug 2014 I had my roof reshingled. Total strip down, new vent boots, ridge vent, 6 feet of ice and water barrier, felt on remainder of roof. In the middle of the winter, which was very cold and snowy here in Massachusetts, I saw condensation on the attic sheathing and nails on the North facing side of the roof. The South facing side had no condensation. I have never noticed any condensation during previous winters. The roofing company has been in business for a long time so I assume that they know their business but what I wondering is if the condensation problem is due to an undersize or incorrectly installed ridge vent. There has always been a ridge vent. The soffits run the length of the roof and are not blocked. Something else….I replaced all the wall-to-wall carpeting with solid hardwood a little over a year ago and have used a humidifier during the heating season to keep the house in the rh range recommended by the hardwood floor manufacturer….could this be the cause of the attic condensation? Concerning the ridge vent, is there any place to get some education on sizing, installation, etc.?
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You have condensation because humid air is leaking from the house into the attic. The roof replacement could have made this worse due to a lower-flow ridge vent, or due to a failure to cut back the shingles at the ridge. Or not -- last winter was a very weird one from several standpoints, and we had attic condensation worse than in previous years, even though nothing was changed.
Verify that the shingles are not blocking the ridge vent opening, and try to seek out and seal air leaks into the attic. Be especially suspicious of any furnace supply ducts in the attic.
As to the humidifier, be careful to not set it too high. Max of 40% when the temp is above 30F, and reduce by 5% for each 10 degrees below 30F (expected nightly low).
What was the R.H. of your home and moisture content of the hardwood you installed during last winter? What is it now?
My guess is that you have a significant amount of air leakage from your conditioned living space flowing into your attic during the winter. This warm (probably moist) air condensed on your cold sheathing. Prime culprits of air leaks can be bath fans and recess lighting.
RH and MC
The hardwood installation went on for several months. All hardwood was acclimated to the annual average mc (9%) for my location no matter what time of year it was installed. The flooring manufacturer recommended that rh be kept between 45% - 55% year round. Originally I tried for 45% during heating season with the humidifier. I dropped it to 35% when I found the attic condensation. Central a/c in summer especially when rh goes up. I have a humidistat which I monitor year round so that I can avoid extremes. I have 14 inches of fiberglass insulation in the attic....6" between joists and 8" run perpendicular on top of joists. An energy audit recommended air sealing the leakage between living space and attic. Bath exhaust fans are covered with insulation and the ductwork in the attic is wrapped. The new ridge vent seems to have slightly less visible light than the previous ridge vent but I have been told that product designs change so I don't know if this is due to a newer design or if it is not installed right.
The point isn't to insulate air ducts in the attic but to seal them. Generally all joints should be sealed with foil duct tape or with duct mastic.
Aid Ducts
Bathroom exhaust air ducts have been in place for 20+ years. Condensation problem is recent.
You should verify that the ductwork has not been accidentally knocked loose, and that the connected roof vents (which presumably were replaced with the reroof) are indeed connected. It's often fairly easy to knock bath vent ducts loose.
Ductwork
I'll verify as you suggest. Thank You. My dilema is I can check all ducting, living space to attic penetrations but if it is the ridge vent that is the culprit I won't know that until next winter when it won't be a good time to do anything about it.
For the ridge vent, go up in the attic and look up at it. There should be an opening cut in the sheathing, with the vent over it. Make sure that there are not shingles that you can see blocking the opening. It's OK to see maybe an inch of shingle intruding into the space, but no more than that.
If that check is OK, and the vent was a decent name brand, then I would assume the vent is OK.
air sealing
If you believe it was the ridge vent making the difference, then why would condensation only be on the sheathing installed on the northern exposure? Your positive roof ventilation system doesn't have a bias with regard to the cardinal directions of the compass rose. However, reaching dew point at the surface of certain materials does.
I believe air leaks from conditioned space during the dead of winter is your primary culprit.