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I have a cape cod style home, pictured below. The second story living space on the back side of the home is created by having a lower pitched roof. So, the front and back sides of the roof have different pitches. I have large eaves overhanging the exterior walls with plenty of soffit vents already installed. I recently found that these soffit vents on the lower level were rendered useless when I discovered that the space formed between the roof sheathing and the sloped interior wall was stuffed with insulation. I plan on placing baffles in those spaces to allow the colder air to be pulled into the attic space above the second floor living area.

I currently have 8 soffit vents; four in the lower eave which is the floor of the knee wall space, and four in the eave of the second floor, along with a ridge vent to be installed with a new roof coming in a month or so. My calculations call for me to add 2 more soffit vents in order to balance the system, 6 if I want to go 60/40 intake/outtake. Installing the additional soffit vents in the lower eave is obviously much more attractive to me and with that in mind this is my question:

Will the air pull from the second story eave vents more so than the lower level vents? Do I need to take this into account when making my decision? The last thing I want to do is to go through all this work only to short-circuit the system and have little air pulled through the knee wall at the front of the house.

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1 Answer 1

The lower vents will indeed be less effective than the upper ones. In addition, you are reducing the effective insulation of the wall/roof in order to install baffles. With effective cross flow ventilation and a proper ceiling vapor barrier, you only need a total of 1/300 of the attic area in open free area ventilation. IMO, getting at least 1/2 this minimum area from the soffit is the important part. Actually balancing the areas when you have more than adequate ventilation is less important.

If you can achieve this 1/2 minimum area in the upper soffits alone, then forget about the lower ones all together. Only if you have no choice would I attempt to bring in air from the lower soffit. I'm fairly dubious on just how effective these would be.

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I don't want to ignore the ventilation in the knee wall, as I get some ice dam issues as it is not. The area just gets too warm and I need to get cold air moving through it. –  Evil Elf Feb 10 at 15:17
    
Ah, OK. If you're going through the trouble anyway, I would estimate you can assume such ventilation is 25-50% effective for calculating the minimum required area. Not based on any physics, just a gut feeling, the reality could turn out much different. –  bcworkz Feb 10 at 21:13
    
My attic space is two separate divided areas. I solved really bad ice dam problems by going 60/40 intake/outtake by adding additional soffit vents. I also insulated the eaves properly. This knee wall is an animal I have not dealt with before. The huge issue that is that the knee wall is across the entire front of my home and only 1/3 is accessible through a closet. I am pretty sure it is not insulated well. The knee walls are much colder than the exterior walls. The thought of busting through the drywall to fix is not uplifting. I may see if I can work on the house when the roofers come. –  Evil Elf Feb 11 at 13:37
    
Yes, drywall work is messy, nasty work. Yet, even with shingles off, going through drywall at floor level may be preferable to going through roof sheathing at elevation. Consider your plan of attack carefully. –  bcworkz Feb 11 at 18:10

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