Still confused about insulating roof...

[greg273]

 OK, i know this topic has been done a thousand times, but I'm still rather confused over the issue of soffit vents.   It seems counterproductive to go to great lengths to make a house weathertight, then to have massive airflow just under the roof deck. Would gable vents, without soffit vents, be an acceptable level of ventilation?
 This is the old 2x6 cathedral-type ceiling in the loft I'm talking about.  It just seems a waste to insulate, then purposefully poke more holes in the roof.
  If condensation is the issue, why is it not an issue in a stud wall, which usually has NO air gap between the sheathing and the insulation?
  Hopefully someone can provide some insight into this.... thanks...
 
  greg

[michael lopiccolo]

 if you do not vent your roof ,condensation has no where to go, roof sheathing absorbs the moisture and eventually rots,mildew,stains sheetrock wood etc.  
   If you use a insulated chase,vapor barrier,insulation and sheetrock , your R value will be just as good ,if better then no air space.
   "sealed" walls do not need to be vented per say,a wall breathes through tyveck, felt paper etc. the roof is subject to extreme heat/cold ,heat rises,and your roof becomes a magnet for moisture
   I am not an expert ,but that is my understanding of that

[greg273]

 The roof is vented, there is a gable vent at each side, in the airspace above the collar ties. Only in the lower ten feet of the rafters would insulation be close to the sheeting.
 I would think the gable vents would move enough air to prevent any moisture accumulation, as heat rises, small currents would form, even in insulation.
 Pretty much everything I've read says keep the insulation off the sheeting, but why then does it work OK in wall cavity, not a roof one?
 With a proper vapor barrier, how is moisture going to get into the rafters anyway?  How could significant quantities of water vapor  come from the outside, through the metal roof?
 I must be missing something here.... the research goes on, but I cant wait too much longer, 2 winters without proper insulation was enough!

[spinnm]

I know that there's tons of info written about this...you've probably read some of it.  Also know, that there are disagreements between experts.

Guess that venting is somewhat region dependant.  Here, the heat is the problem, not the moisture.  Gable vents are done all the time with no soffit vents.  You can do it many ways.  Think there are formulas out there to calculate.

I know this.  Roofs are likely to fail if the attic is not vented.  Extreme temp variations.  If you live in wet country, the previous post is valid.

Here's my unscientific understanding.  You have what they call a heating envelope.  That ends at the ceiling.  Outside the envelope there must be air flow.  That's all it is.

Out here we're worried more about expelling all that hot air to save our roofs and keep the houses cooler.  In cold/snow country one of the objects is keeping the heat inside the envelope so that ice dams don't form from the escaped heat.

[Ryan B]

My thoughts on the subject.
As a builder/remodeler in a climate that would be considered very dry (no rain 4+ months of the year and less than 10" for the rest of the year), distinct patterns are becoming  apparent when removing shingles or panel sheeting from roofs.

1 - If the builder of the original structure has done the correct detailing (sometimes by accident I think) no apparent damage or visible moisture is evident.
2 – Even if the builder has done the correct detailing the home owner can severely test the system – loose dryer vents, a pot of spaghetti boiling, wet clothes drying in front of a gas stove can be pretty common in any home and where to you think the moisture goes? Yep up to the ceiling and finally to the underside of the roof sheeting where it cools down and condenses, in rare occasions it will drip down to the top side of the ceiling sheetrock then follow the wall path down to the bedroom floor and ruin your vintage Rolling Stone record collection.
3 – If the builder has done such a sloppy job that the house is full of air leaks the moisture escapes anywhere the wind blows it and problems with rotten roof sheeting is not apparent. Remember your mom always complaining about a drafty house and her cold ankles?
4 – Hip roofs with large hip/ridge beams are pretty much guarantied to have some type of vapor problem. The detailing on the jack to hip connection is easy to overlook in this venting system.
5 – Shear blocking in high seismic/wind zones can also block vent path ways.
6 - Wood finished ceilings installed over a poly barrier are a problem with so many punctures in the barrier.

But don't despair it's not time to demo your house yet.
Some rules of thumb that I use.
If you pay attention to detailing – seals on penetrating wall fixtures, top plate penetrations sealed, bathroom fans with a 20 minute timers, kitchen vent fans that actually exhaust air to the outside, dry framing before the finished roof is installed etc. Roof vents at this point are a "belt and suspenders" approach – not needed but nice to have. In your case I would seal any romex holes caulk all blocking and insulate the full cavity and apply a vapor proof ceiling – presto in 10 minutes your done!

Full house air exchangers are one of the answers for moisture removal but an open window and a vent fan running for a few extra minutes is a heck of a lot cheaper.

Ok I know I did not touch upon the heat subject – perhaps in a later reply

[Shelley]

Ryan's post dovetails with our experience, tho more limited than his.

I'm pretty anal about the things he said.  Got one chance to do it, doesn't take any $$ to speak of, just time.

Last house I was obsessing over this.  Continous ridge vent, metal roof, exposed rafter tails.  Buy the Cor-A-Vent stuff?  Punch holes in the blocking between the rafter tails?  Himself told me to chill.  That no roof was tight enough to obsess over.  I did the things that Ryan mentions and a few more.  Got in the attic once it was closed in and insulated.  Still lots of light holes down near the rafter tails.  Enough to convince me that we had enough air flow.

And, Ryan's right about moisture being a problem in any climate.  We've had big problems here with contractors using Tyvek and synthethic stucco.  Mfgs say that it's improper installation.  Who knows?   Contractor's opening up a 5 yr old house and finding rot and mold.  Then they talk of tearing into a 100 yr old house with felt and and some air gaps looking like the lumber was just milled.

[n74tg]

Go to BuildingScience.com and read their (many) articles on vapor barriers, vapor retarders, air barriers, vented and unvented roofs, different insulation methods, soffitt, gable and ridge venting advantages and disadvantages and what does and does not work in each of the different geographical areas of North America.  They even discuss why tyvek and synthetic stucco sometimes don't mix.  

[John Raabe]

Ryan and others have done a good job of covering the issues. You do need to vent the roof (above the insulation) in most climates.

Soffit and ridge vents have been shown to work much better than gable end vents. So much better that in a recent house I designed we installed faux gable end vents to look right with the house but we vented the attic with ridge and soffit vents. (Doing gable end vents in addition to soffit & ridge vents actually cuts down the ventilation in the attic.)

Greg asks why this is not done in walls. It is actually!

The best framing details for walls now have the siding over a drainage plane that allows trapped moisture to escape out the back of the siding through a vented air cavity.

Also the vapor and air pressure into and through small gaps into the attic is much higher than in the walls and floors.