Cape Cod kneewal attic ventilation

[Brian]

My older (50’s) Cape Cod developed a pretty good case of mold growing on the underside of the roof sheathing in a kneewall attic on the north side of the house. This appears to be a wintertime problem of moisture from the living area leaking into the attic, and the attic not being adequately ventilated. During the coldest weather this winter (here in Ohio), frost had actually formed on the underside of the roof sheathing! The attic currently has just two 4″x16″ soffit vents, and two roof louver vents (about 10″ diameter). And no, the bathroom exhaust fan does NOT even run through this attic.

I just got a fairly expensive mold cleaning and re-insulation job done in this attic, and obviously wish to improve the ventilation so this doesn’t recur. I plan to add several more soffit vents to at least triple the soffit venting. What I’m unsure of is which would be more effective: to add several more (like 3 additional) roof louvers, or to install louvered gable vents on both ends of the attic. I’m thinking the gable vents might be the most effective in this case, as they’ll allow any westerly breeze to pass through the attic. However, a lot of the literature I’ve read suggests that gable vents are not as good a choice as ridge venting. Of course, I can’t do a true ridge vent, since this is a kneewall space, and the “heel” (I think that’s the right term) area in the roof/ceiling above this attic is completely filled with insulation.

To complicate things further, the center 1/3 of the attic cannot be vented with soffit vents, because of an addition out the back of the house – there’s simply no soffit there. So I can only soffit-vent the approximately 1/3 of the attic at either end. Whatever ventilation is installed will have to work without air intake at the soffit in the middle portion of the attic.

Any recommendations or opinions? I have to believe that anything I do will be an improvement on the minimal venting that’s currently there. When it was in the teens outside this winter, this attic remained in the 40’s with humidity readings in the 80-90% range. But I don’t want to cut big holes in my siding or roof and then find out it doesn’t work as intended…

– Brian

[tomh]

Venting works by convectional movement of air which sweeps moisture from the soffit or edge vents to the peak of the roof. In the knee-walls, you should have a space between the sheeting and top of insulation. You can add this gap by inserting a foam channel like Owens Corning Raft-R-Mate.

http://www.owenscorning.com/around/ventilation/raftrmate_attic.asp

Where you lack soffit venting, or even soffits, you can add drip-edge venting that will blend into the roof line. This may require removing the first row of shingles and cutting back the roof deck 3/4 to 1 inch.

http://www.airvent.com/homeowner/products/intake-ventedDE.shtml

http://www.toolbase.org/tertiaryT.asp?DocumentID=2146&CategoryID=1014

There are many solutions to venting, but a balanced system is important to solve the problems you describe. Without including lower edge venting along with gable or ridge venting, you only have half the answer.

[Daniel E. Fall]

One of the reasons you had such high heat was probably due to the lack of ceiling insulation.

Another reason is heat migrating off the walls.

The first things you need to do in that space are to insulate, insulate, insulate walls and floor to R21 and R38. Once you do this and provide sufficient venting, problem solved, although vapor barriers would have helped, it is cost prohibitive to rip down walls to put in the plastic.

I don’t define the area above as the heel. The heel is the amount of space between the bottom of the roof boards and the top of the walls. This is where soffit vents go… In newer homes “energy” heels are required, something like 7″ or so, I forget the actual spec. You probably have 3″ heels or so..maybe less…hard to say from here.

If you have another roof intersection in the middle, your old house roof should have openings for that new house addition, unless space doesn’t permit. That new house addition can be equipped with ridge vent based upon your limited description, combined with gable vents and/or mushrooms and soffit venting, this would be an effective venting solution.

Additionally, if the heels (my definition) are closed up with blocking, remove it and vent all the soffit you can.

If you install a ridge vent on the new roof, don’t install a gable vent on the end of it, those are useless when combined with ridge vent. On the other hand, adding gable vents to the original house would be a good solution because you have no exhaust venting in that section except for in the middle given you cutout to a ridgevent.

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