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Izaura 02-18-2013 11:05 AM

Attic (cross)ventilation - pre-insulation Qs
I could really use some help in determining where/how to improve my attic ventilation, before I even dare to invest in new insulation. For all my reading on ideal ventilation scenarios (balanced soffit/ridge systems), I cannot begin to visualize the current / desired airflow patterns for the shape and configuration of my attic.

Disclaimer: As will soon become evident, I am not at all a seasoned DIYer - this would be my first project of any consequence. Air-sealing and insulating an attic is probably a "weekend project" to most of you, but it seems like a herculean task to me!
  • House: 1965 rancher in zone 5A, 1400 sq. ft. attic floor area, broad eaves (2’ available surface width) on entire perimeter.
  • Existing insulation: 3-5” FB batts, might as well be considered R-0 (compression, settling, torn facing)
  • Goal: Air seal and completely replace insulation up to R-60 if possible
  • Existing ventilation: Conservatively meets 1/300 rule of thumb, but configuration probably less than ideal. Combination of 16 x 4” louvered soffit vents (NFA at most 22 sq in) and continuous perforated panels (flush with the walls). Lacks ridge or gable vents, but the rake soffits have the same louvered vents (see all 3 gables on the diagram).
  • Signs of ventilation problems: No ice dams sighted since move-in in early December (but also not much lasting snow accumulation). Some mold noted on sheathing where bathroom fan previously vented into attic -- will clean up and monitor these areas.
Please refer to my marvelous PowerPoint diagram (dimensions somewhat off) and to my questions/concerns below.
  1. Cross-ventilation, or lack thereof: In a system without balanced cross-ventilation, the applicable rule of thumb would be 1/150 NFVA, correct?
    1. I have supplied roof pitch, but unsure how to factor this into required NFVA in the absence of ridge venting.
    2. Perhaps there is some crossventilation provided by the vents at the rake soffits (gables)?
  2. Gables: To the extent that their rake vents are placed higher than eave vents, what role do they play - exhaust? How should they be factored into my NFVA calculations?
  3. Carport roof: 16’ long – continuous with house roof (4/12), soffit vents at base. Would it be acceptable to leave this portion of the roof completely hollow (unobstructed) up to an insulation dam + baffles where it meets the exterior wall? If so, what is its (potential) contribution to ventilation?
  4. Soffit ventilation options: The low-hanging fruit would be to replace all individual vents on level portions (non-gable) by continuous perforated panels as found inside the “L” but I read that vent openings too close to the walls may cause problems...
    1. Is this a concern in my climate zone, or with the rest of this setup?
    2. Where and how might I modify soffit vents to achieve required NFVA?
  5. Bathroom exhaust: Now vented through soffit. Depicted on the diagram (inside L portion of soffit). Hopefully adequate?
  6. Vapor retarder: (Never mind, Type I/II vapor retarders not needed in a vented attic in this climate zone).
My sincere gratitude to anyone who has managed to read this far and would be willing to offer suggestions or advice. I would be happy to provide pictures or additional details upon request, although the 5/12 portion of the attic is mostly blocked off from access/view by utterly useless ductwork (to be removed!)

jklingel 02-20-2013 01:32 AM

Bathroom exhaust: Now vented through soffit. Bad idea. Moisture will just float back into the attic.

Vapor retarder: If 1/150 NFVA ratio (or other requirement) could be achieved within current constraints, would there be a need for Type II/III vapor retarder (e.g., faced FB batts) beyond ceiling paint? Or would it be acceptable to just air-seal then blow in cellulose to desired R value? If you can air seal real well, the vb is not necessary. However, if you are in a cold climate a vb may not hurt. Air sealing is more important that a vapor barrier; make sure the vapor barrier, if used, is sealed tight, too.

Izaura 02-20-2013 08:40 AM

Thanks for taking a look, jklingel. You are indeed correct about not needing a vapor barrier in my climate - I knew I had seen that somewhere, and I since found the link again.

Bathroom exhaust - such was my fear. This was the PO's fix in response to problems raised during the inspection (i.e., when the bathroom exhausted directly into the attic) which, as far as I can tell, is strictly "correct" (the duct does run to the outside, after all), although far from satisfactory. Looks like we have traded one problem for another, and will have to trade it for a risk (new perforation through the roof).

Gary in WA 02-24-2013 07:27 PM

Pitch does not figure in NFVA, only floor area;

Gables are exhaust vents unless the wind is directly at one, pp.9;

Gable vents require way more area than built, scroll backward to chapter on pp. 603, read esp. 605, 616; Notice mention of center soffit vents/, gable/soffit, and placement of soffit continuous venting...

More on gables as exhaust;
Run the bath exhaust to the gable end rather than through the roof, IMO.


Izaura 02-28-2013 11:23 AM

Gary, thanks for your response and links. It was in fact links that you provided in response to other posters that had me rethinking ventilation. (If I am interpreting them incorrectly, the fault is all mine.)

First, I must apologize for an inappropriate choice of words that seemed to imply that I had gable vents. I do not - the soffits under the gables do have vents. I believe "rake" is the word I should have used for those, not "eave". Anyway, they are visible on the N, S (5' high) and E (3') sides.
I find very little information online regarding rake soffit vents - is this because:
a) they have another name unknown to me
b) they are treated like another type of vent, same considerations apply (gable or soffit vents? a bit of both depending on height? 3' seems like the magic number for gable vents...)
c) really shouldn't be there (I see recommendations for sealing them to avoid rain penetration, but I have a hard time imagining this would occur with a 2' overhang?).

The various texts you link confirm my suspicion that replacing the existing vents/panels by a continuous strip near the fascia would address most of my concerns. If the higher "rake soffit" portions qualify as exhaust vents, I would end up with more ventilation than strictly necessary; if they count as intake (or nothing much), I'd meet the 1/150 rule of thumb (already pretty close) for non-balanced systems.

I would indeed feel better about running the bathroom exhaust through the N gable than through the roof, good point! Would that require sealing off the rake soffit vents in the vicinity? (Again, same hesitation on my end - are those considered intake or exhaust?)

To be honest, at this point I am seriously considering enlisting professional help for both ventilation and insulation - that's a lot of insulation that could potentially be wasted if I make a bad judgment call. I also question my ability to crawl to and reach all the nooks and crannies of such a constrained space (not to mention whether R-50/60 is even reasonable in such a case).

... that said, I still value any input that might help me have an informed conversation with a contractor, convince my husband that one is needed (!) or minimize my chances of failure if I lose the latter argument :)

Gary in WA 02-28-2013 06:39 PM

Usually the gable overhang (on the rake) is closed off from the main roof, so venting those would not be figured in the balanced system for the house attic. If you check them from inside the attic, the truss/rafter framing or the gable end wall sheathing extends to the top, blocking any air-flow there- intake or exhaust. Builders typically soffit vent the rakes against rot/moisture accumulating in them. You are correct with no vapor barrier/retarder for your location:

Your Zone 5 requires minimum R-38 in the ceiling;

2009 IECC;

And more specific;

Air-seal first, as you said;

Weight the value vs. cost;

You may want to use some housewrap over blown-in fiberglass to stop convective loops and wind-washing near the intakes. Cellulose is fine, stand-alone. You could seal off the middle gable vents from the bath exhaust, if the aren't already (punched ones clog from air-born dust), lol. The venting may be adequate as is (esp. if no ice dams or mildew/mold on rafters), though adding insulation sometimes creates problems;

But, if you do have ice dams;


Dave Sal 02-28-2013 10:04 PM

Here's a link that may help you understand more about attic ventilation.

framer52 02-28-2013 10:24 PM

If you read the building science site you will discover that attic ventilation is not required or necessary contrary to what others here believe and preach.

Dave Sal 02-28-2013 10:54 PM


Originally Posted by framer52 (Post 1127228)
If you read the building science site you will discover that attic ventilation is not required or necessary contrary to what others here believe and preach.

I read the Building Science report about attic venting and I must have missed the part where it said venting is not necessary. :confused1:

Gary in WA 02-28-2013 10:57 PM

As there is no more ductwork in the attic (" I would be happy to provide pictures or additional details upon request, although the 5/12 portion of the attic is mostly blocked off from access/view by utterly useless ductwork (to be removed!) ")AND the roof design is fairly simple as pictured, you want to stick with a "vented" attic as BSC recommends; "Vented attic/roof designs have the advantage of a long, proven historical track-record. However, they work best with airtight ceiling/attic interfaces and where ductwork and air handlers are not located within attic spaces. The increase in the use of complex roof shapes and cathedral ceilings has resulted in problems with vented roofs." From;


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