On Wednesday, August 28, 2013 3:32:56 AM UTC-4, nestork wrote:
If by "soffit vent channels" you mean the styrofoam trays you fit

between the rafters before covering with fiberglass insulation, vapour

barrier and drywall, then...



Consider installing extruded polystyrene insulation instead. Not only

does using polystyrene foam insulation eliminate the need for those

styrofoam trays, but it gives you much better R value. Fiberglass will

typically be R=3.5 per inch, whereas extruded polystyrene will be R=5

per inch of thickness as a stabilized R value. So, if you have 3 1/2

inch rafters,

2 x 4's for roof rafters? Where is that allowed?




and you devote 1 inch to ventilation under the roof

sheathing, then 2 1/2 inches of extruded polystyrene is going to give

you R=12.5 versus R=8.75 for fiberglass. That's a 42 percent increase

in insulation.



Since he used the word "attic", I would think he isn't limited to 3.5"
of space and he also stated he was adding insulation, which implies
there is existing insulation. I don't know of anyone using rigid
board insulation for that application.




If you opt for a 1/2 inch air gap under the roof sheathing, the

difference in R value is even greater; R15 versus R10.5.



But, be aware that in order to add ventilation to your roof, you will

need to cut through the shingles at the ridge of the roof to install a

ridge vent. If you don't have a ridge vent now, it'd be best to save

this work until you need to replace the shingles on your roof, and add a

ridge vent and ventilation at the same time.



He has an attic. And even if he didn't, say it was a cathedral ceiling,
and it's not vented properly,
why should he wait 20 years until he replaces the roof? By then the
roof will fail. It's very easy
to add a ridge vent to an existing roof. Run circular saw down the
roof, nail in vent, cover with new cap shingles.





Answer this question, tho: On the coldest F***ing day of the winter, if

you go into your attic, do you see any frost or condensation forming on

the underside of the roof sheathing. If the answer is no, then you

really don't need to add ventilation to your roof. You've got as much

attic ventilation as you need to prevent damage and adding more isn't

going to do any good.



The insulation goes between the ceiling of the living space and
the attic. There isn't supposed to be insulation under the rafters
in an unfinished attic.




Contrary to popular belief, attic ventilation is used to prevent

condensation occuring in the attic in winter. While attic ventilation

can also cool the attic in the summer, a super hot attic in summer won't

do any harm to your house,

That's it's primary function when it's a cold climate.
In a hot climate, it's function is to remove hot air which
reduces the heat getting into the living space and helps
lower the temps seen by the roof decking, shingles, etc.
In most places, you have winter and summer, so it's both.



whereas condensation forming in your attic in

the winter can cause serious and extensive damage to a house. So,

regardless of what you may have been told, the reason for attic and roof

ventilation is to prevent condensation forming on the roof sheathing in

winter, not to cool the attic in summer.



Having 130F air trapped inside an attic until it cools slowly by radiating to the outside and in to the house doesn't sound like a smart idea to
me. With proper ventilation, not only is the temp lower during the
day, but once the sun goes down, it can cool off quickly via convection.