Replies

1. GBA Editor
   Martin Holladay | Jun 26, 2022 05:06am | #1

   Mike,
   Lots of issues here. Are you talking about a roof in which the insulation follows the slope of the roof? Or are you talking about a roof over a vented attic?

   Your references to gable vents makes me think you're talking about a roof over a vented attic. But it would be helpful if you clarified the issue.

   By the way, if you are worried about moisture accumulation in your roof assembly, the way to reduce the risk is to perform air sealing work at the ceiling plane rather than to try to address moisture accumulation by ventilation. (All that said, ventilation can also lower your risk.)

1. 
   mikeolder | Jun 26, 2022 07:54am | #2

   Thanks Martin.

   Both of my homes have roofs over ventilated attics. I re-roofed and added more ventilation and insulation to the town house.. But I need to re-roof my second property in the country and I'm questioning if the "more roof ventilation the better" narrative I keep reading and blindly followed is true in a heating dominated climate?

   Or in other words, I question if homes in my 5A zone could save on heating costs by minimizing attic ventilation, because Ive seen plenty of un-ventilated local attics that didn't have any moisture damage.. But also not enough space for R60 at the eaves.

   Thanks

2. Expert Member
   PETER Engle | Jun 26, 2022 10:14am | #3

   Like Martin said, air-sealing the attic floor is more important to moisture control than ventilation. That said and in answer to the question, no, you can't really overventilate the roof, at least not with ridge and soffit ventilation. You do need to install air dams or other protection for fluffy insulation at the eaves because wind actively blowing through the soffit vents can either displace insulation or just blow through it, reducing its effectiveness. Beyond that, you should meet your local code requirements for ventilation. That's generally one sf of ventilation for every 150 sf of attic floor area, balanced between high and low vents. The low vents are required to bring makeup air into the attic. You note that humid air rises, but that only helps if the humid air can get out of the attic before its temperature drops enough to condense on the roof sheathing. In practice, the low vents are required both to move the humid air out of the ridge vents fast enough and also to provide decent cross-ventilation the rest of the time so that any accumulate moisture has a chance to dry out.

   So the short answer is: ventilate as required by your local code.

3. 
   mikeolder | Jun 26, 2022 12:50pm | #4

   No local code to adhere to for my country house.

   In a perfect new construction world where we always use energy heel's and >R60, continuous soffit & ridge vent make sense to me. Adding this to a cold climate existing older home that shows no signs of moisture damage, I'm not convinced.. But I did forget to mention my efforts (removing old plywood soffit and old blown in, sealing all holes, add baffles and 24" of new fiberglass and ventilated soffit/ridge vent) reduced my ice dams, but I didn't notice lower bills..

   One problem with the country house roof project is I have no access to the attic space. And even with new overhung 2x4 sleepers to be install over the existing trusses, I only have 7" of insulation space over the wall plate.. Naturally the attic space increases toward the center of the room, but access to the attic will only occur from above through the old roof deck, making it pretty difficult for any attic work in general. Considering I only have 7" of space after I remove the old roof deck, I didn't really want to give up 1" and allow -20 winter air to sweep over the least insulated portion of my ceiling.. How about 1/4" of space?

   I'm still not convinced there's no savings to be gained by limiting the amount of attic ventilation in older homes. Or that you may actually lose some, by installing continuous soffit and ridge venting as per the manufacturer. I'm also not convinced that you get any kind of a chimney affect with a 4:12 pitch. And if you have fresh air movement across the ridge with gable vents, how much more humid could the air be 12' further down at the wall plate?

1. GBA Editor
   Martin Holladay | Jun 26, 2022 05:24pm | #5

   Mike,
   Here is a link to a relevant article: "Insulating Tight Spaces at the Eaves."

1. 
   mikeolder | Jun 28, 2022 08:45pm | #6

   Thanks Martin.

   Is there a GBA article that describes how to insulate a roof from the outside with foam board? Including the details of converting a vented attic to non vented?