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Question DetailsAsked on 11/29/2015

venting of attic space

There is a small addition (laundry room) on my house. This additon was connected at the gutter and extends out from the house. There is no attic space above this addition. When they created this addition, they abandon the eave vents (bird blocks).
I was in the attic, and noticed that the plywood roof deck is wet (well it's cold out, so it was a thin layer of ice)....but it is only in (2) truss bays, that abut to this addition, and this moisture is only half way up to the peak.

Seems like a venting problem. If so, can I add a roof vent down low near the "old gutter line"......to act like an eave vent?

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Yes you can, and unless it lets water in because of ice damming, MIGHT mitigate your problem assuming you have ridge vents for it to vent to.
However, your problem is moisture getting in there, not ventilation as such - though good ventilation might minimize the problem. Could be leakage from the eave connection of the extension, could be failure to block warm moist air from coming up from the addition (assuming you never had this before in those bays) into that area. I would start with looking into that area of the new roof from the existing house attic - crawling to the old eaves and seeing if you can see down into the new roof, and seeing if you can see up into the after bays of the addition to the existing attic. If totally blocked off at the junction, that is likely a significant source of the issue too - by blocking the eaves they eliminated all your ventilation for those truss bays. There are under-shingle vents designed for this type of condition - I don't like that idea myself, especially for areas with driving rain or potential for ice damming (which it sounds like you have at least the latter if attic is frosting up this time of year) because they are an invitation to leakage in volume. Since frost only extends half way up to the peak, unless that is how far down the roof the sunny part of the roof was at that time, sounds like you are getting too much warm house air up in there - so condenses on the underside of the roof low down where the airflow area is small and water content in the air is still high, but higher up where the warmer air accumulates (so also sounds like no ridge vent, or at least not operating well ?) it evidently is thawing out - or else the lower part is getting besically no ventilation, as your concluded. How to solve that issue - roof vents low down maybe, or possibly (though an energy drain) capturing some air from an eave vent a bay or two away and using a fan to inject it into those two bays. Another thing I have done in a few cases - run large diameter ducting from an adjacent eave vent or two (maybe one on each side) - leaving enough open eave area to serve that bay, but capturing some with ducting leading sideways to the "blind" bays to serve as artificial eaves - I would use non-metallic (to reduce frosting issues and easier to install) uninsulated flex duct, probably 8 or 10 inch. If you can stick it a bit out the eave into open air on the inlet end so much the better - probably have to squash it out of round to get it in place over the blocking, or cut a bit of a swayback into the blocking (not over 1/2 way) to provide an opening. Also - if the ice on the underside of the sheathing was smooth and uniform, rather than very bumpy and "bubbly" like frost in a freezer, than it was likely from a leak rather than freezing condensation. Also - freezing condensation, while it can form icicles when it melts and refreezes during the day, almost always starts and is thickest (at least until the general depth exceeds the nail stickthrough) on the nails sticking through the sheathing because they are colder and conduct heat better, but tends to be pretty unjiform thickness on the underside of the sheathing otherwise. Water coming through the sheathing, unless melting and forming icicles on the nails, tends to be smooth and fairly non-uniform thickness with more buildup where it is actively running (like a stalagtite growing on its side), generally with ice buildup and glaciering as it runs down the sides of the rafters. Another thing to look at - if the rafters on the attic addition run the same direction as the roof rafters and are open into that attic space, I would guess your moisture is coming from the addition - openings around ceiling penetrations like lights and such, dryer vented where the moist air goes right back up into the eaves/rafters, room vent or such maybe venting into the attic instead of outside, etc.

Answered 3 years ago by LCD




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