Squiner - 20 November 2009 11:52 AM

murph - 20 November 2009 11:14 AM

To follow up with what you have said, I don’t have very good ventilation. My soffit vents have a 1” hole cut into the board between the rafters. I’m not sure, but I would guess that this isn’t adequate. Do you agree? I guess I will have to take a hole saw and cut bigger openings. What is an appropriate sq-in opening to provide adequate ventilation from eave to ridge vent?

Also, just to get a better understanding; does the ice and water shield exacerbate the ventilation/moisture problem over basic felt because it has superior moisture transmission resistance?

Yes you are understanding the ice and water issue perfectly.

As far as the ventilation requirements I would need the actual dimensions of your roof / house. But that doesn’t sound adequate.

BTW - That was not meant to scare you. This might not be an issue for you at all. It really depends on your life style and how much “water vapor you cause. ie. cooking, showers, humidifiers, dishwashing etc.

murph - 20 November 2009 12:29 PM

Yes you are understanding the ice and water issue perfectly.

As far as the ventilation requirements I would need the actual dimensions of your roof / house. But that doesn’t sound adequate.

BTW - That was not meant to scare you. This might not be an issue for you at all. It really depends on your life style and how much “water vapor you cause. ie. cooking, showers, humidifiers, dishwashing etc.

I think I can figure out the calculations. The only difference I have is that my house is one story with cathedral ceilings throughout. So I think I am limited to drilling holes. Another issue with my ventilation is that I don’t have the baffles in the cathedral ceiling. Fortunately, the 1950’s insulation (thin cellulose between felt) is so poor that it doesn’t even touch the sheathing.

Another option that I have been thinking about is removing the drywall and insulation in the ceiling and replacing it with expanding foam. I have read conflicting information about not having to have ventilation at all with the foam. That way I could take advantage of the full volume of space for insulation.

Do you have any experience with this?

I really appreciate you taking the time to answer my questions, sorry for so many!

No problem at all.

I know a little about spray foam but not enough that I would want to recommend the application. I know it’s a good product and getting better.

I know there are 2 types closed cell and open cell. I believe closed cell is the type your referring to. It has a really low perm factor which makes it act as both moisture barrier and insulation.

I’m under the impression that most people use open cell due to the cost. I have a local contractor who uses closed cell, but only uses an inch or so, then adds fiberglass on top on that. (I think he said that closed cell was like $1.00 per board foot applied. ugghh)

After reading this reply it sounds like I’m trying to say I know something about the stuff. But not really, I guess I’m regurgitating stuff I’ve been told and overheard.

murph - 20 November 2009 01:14 PM

No problem at all.

I know a little about spray foam but not enough that I would want to recommend the application. I know it’s a good product and getting better.

I know there are 2 types closed cell and open cell. I believe closed cell is the type your referring to. It has a really low perm factor which makes it act as both moisture barrier and insulation.

I’m under the impression that most people use open cell due to the cost. I have a local contractor who uses closed cell, but only uses an inch or so, then adds fiberglass on top on that. (I think he said that closed cell was like $1.00 per board foot applied. ugghh)

After reading this reply it sounds like I’m trying to say I know something about the stuff. But not really, I guess I’m regurgitating stuff I’ve been told and overheard.

It’s enough for me to get started researching, didn’t know about open or closed cell.  thanks

In case any one else has this same issue with cathedral ceilings and venting, I found a website with a great article.

http://www.buildingscience.com/

Go to the information link and type in “Unvented Roof Assemblies for All Climates”.  Talks about Closed-Cell Spray Polyurethane Foam (ccSPF) and it’s correct application.

It’s amazing that with 2x6 rafters I can have an R-value of about 34 with the ccSPF.  I really need to look into this more.

Nice link. I just learned a bunch.

Makes me want to fix my attic.