catsrose, that's such a cool idea! This structure isn't really a pergola, in the sense that there's two rows of posts, parallel to one another, with connecting beams above. It's more of an arcade ... but when I tell people that I'm building an arcade, they picture video games. :) As I sit here with my calculator, though, I find that it's not possible to use this formula for this structure. I changed the design so the posts are now going to be 6' apart (to add more roses, of course). If I calculated things correctly, the posts would only be 3.7' high. (?) I'm going with a different sort of historic model ... vertical emphasis, with big corner braces that make the openings between the posts resemble Gothic arches. Connie

the purpose of the air space is to allow porous insulation to dry if it gets wet As well, in cold climates it is supposed to keep the roof cold and prevent ice damming; in hot climates, a vented attic reduces the heat on the roof surface and lessens the load on cooling equipment. Fibrous insulation subject to a convective current, as in an attic, has a reduced effectiveness vis-a-vis solid insulation materials. "Convective heat loss in insulation is rare, but it can occur when large temperature differences above and below insulation create tiny air currents (called "convection loops") within the insulation. Studies have shown that convective heat loss can occur with lighter density loose-fill fiberglass at the very low attic temperatures possible in extremely cold climates. Depending on the attic temperature, the insulation's measured R-value could decrease by as much as 50%" (See link below.) I have been in vented attics in winter watching the swirling snowflakes coming in from all those mandated vents. If the whole thing were sealed in the first place, the snow wouldn't be coming in. If prices were the same, I'd go for a sealed attic next time. I don't know if I have the energy to explain to the trades how to do it. Old dogs do not like learning new tricks. Here is a link that might be useful: Energy Guide

Your insulation is below minimum recommended R Values. See charts in this thread. Ice and weather shield should be on the roof, running from the eaves to three-six feet up the roof. It has nothing to do with R Value, but protects the roof against ice damming. Since you're re-roofing you could well consider insulation both above and below the sheathing. Finally, sealing your home will do as much or more than increased insulation levels to make your home more energy efficient. (See Link Below) Here is a link that might be useful: Attic Air Sealing Guide

I tried roof vents, I do know what you're talking about. I destroyed two or three of them trying to get one put in on top of the existing insulation. I ended up ripping out the insulation in the channels and replacing it with R-13, which leaves about 1-1/2" of air between the insulation and the bottom of the roof deck. Holy COW did that make a difference already. I'm used to the house being 77 degrees after a sunny day in the summer and the bedroom being so warm you cannot have even a sheet over you without sweating to death. I got home to find the house at 75 degrees and the bedroom was cool enough to take a nap with a sheet and bedspread on top of me. Here's a picture: http://www.ankylosaur.com/images/housetopfloor.jpg A) The triangular, unheated, space in front of the bedroom. B) the "channels" The top of the 2X6 is the roof decking and the bottom of the 2X6 is where the bedroom's ceiling is nailed. These channels are about 5' long from the storage space to the attic, proper. C) the standard attic (yes, the ceiling of the bedroom has a down slope in the back of the room) D) the bedroom, heated in winter, cooled in summer.