Came across this paper listing the advantages of unvented roof assemblies..... courtesy LaGrange Consulting

It should be noted that the document is specifically for hot humid climates.

Because heat loss is more than just R-value.  It's far easier to get the infiltration factor down to ZERO on an unvented attic, so you can punch in lighting cans, plumbing, ducts, whatever into the ceilings of the rooms below with great abandon, and you needn't worry about maintaining a tight air seal on the attic door/hatch, whatever.  The performance lost by the increased area is usually more than offset by the boost in air tightness. (Of course there will be many exceptions to prove the rule...)

Labor-wise air-sealing the attic floor to perfection can be substantial- far more than the labor for foaming the underside of a roof deck.  The increase in total (not just roof) exterior area for the structure is under 3%, and far LESS than the glazed area gain/loss factors in most homes.

Beyond heat gain/loss it means whatever gets stored/installed in the attic is within the pressure, thermal & hygric boundary of the envelope.

Also 8/12 is a much steeper pitch than most ranch homes, and of course, the higher the pitch, the greater the difference in exterior surface area.  In non-snow-zones in the US  most ranch homes have pitches under 5/12, with a correspondingly lower difference in attic-floor vs. roof deck + gable area.

as greentree stated it was for hot humid climates, if you were in a northern climate it may not be to your benefit, however I never could understand the reasoning of running warm ducts thru a cold basement or cold ducts thru a hot attic?

I dont understand ducts through an attic either, however most people condition their basements and use them for living space, so your warm ducts are in your warm basement.

How about cold climates?

I was about to start a thread, but will hi-jack this one instead...

I am planning on closing off the way-to-small vents in my attic and including it into the conditioned space.  My house is in the S.F. Bay area within sight of the Bay and thus no worry about freezing and no summer temperatures for more than a week each year (Bay Area residents melt at anything over 80 degrees F....) 

The house is a small 1960 vintage Calif. ranch with 2:12 roof pitch and with what I guess would be called a cathedral ceiling at each end.  Between the bedrooms at one end and the living room at the other is the attic area which covers about 25% of the house, over the hallway and the bathrooms.  There are a total of three vents at the eves with a total vent area of 1/3 of a square foot - just enough to let the winter winds drive out some of the heat and which appear to provide no ventilation during the one week of the year when temps are 80 degrees or higher.

The heating ducts are in this area and there is no insulation on the 2 foot sections of the upper wall shared with one bedroom and the kitchen.  The roof is an unusual construction composed of what was originally called "Hemasote" panels which are fiber panels about 3.5" thick with an R value of about 2 per inch.  I had two inches of polyiso placed on the roof when I had the hot mopped roof replaced several years ago, so the roof insulation R value would be on the order of 18 or so. 

With that background, my question: what is the proper way to include the area into the heating envelope area of the main house.  When I close off the small outside vents, do I need to vent the area into the main area of the house - perhaps one of the bedroom closets and/or the living room or kitchen.  If so, does anyone have any idea of how large the vents should be?

I was thinking of just cutting a vent into the access hatch in one of the bedrooms and having it the same size as the previous external vents.  Would vents at each end provide some advantage? 

Thanks, Casey

Whoops, not quite familiar with the mechanics of the form, this is not a response to greentree...

Casey;

plug up the exterior vents and if possible a little plug of insulation at the open end between rafter tails , if accessable?
opening a vent from the current conditioned space will serve no purpose, once you seal of exterior that void will become close to the same temperature as your living space, without cutting any holes

Casey
you already have an insulated roof; the attic vents simply short circuits the roof insulation by introducing outside air into the attic. So yes, close up the vent areas. BTW, Homosote Corp is still in business if you have questions about the product.