I think it vents the attic sufficiently as for the slopes it’s the same as closed cell foam, no ventilation.
How would you explain the frost on the nails in this situation?:
Home essentially vacant (owner moved to nursing home, belongings moved out, heat on about 65).
Whole house fan inside, but not used in months.
Attic hatch in garage was basically a screen door (no fire barrier).
Large gable vents, and a large turbine. (lots of ventilation)
In other words, great ventilation, and no source of inside humidity.
(And yes there were “drip” marks under the nails in the insulation where condensation had dripped in the past.)
Peter, in doing this, do you crawl in there to install proper vents or fill the whole area and loose the soffit vents?
Just want to make sure I am understanding you right.
And I too have had good luck with my own house with adequate soffit vents and two small gable vents and no ridge vents.
Peter, I think this colder climate makes a difference if one does not have drastic heat loss up in the attic space.
I’ll be checking my attic again this year.
Marcel, we eliminate the soffit vents completely to stop air infiltration.
Here is a few photo’s, kinda dark but you can get the idea.
I don’t know Peter, a few months ago, I inspected a modular home that had no soffit vents and the over sized gable vents did not seem to work properly. The underside of the plywood was black as could be from moisture and to the point where it was soft walking on the roof.
Area, construction, prevailing winds, got to make a difference I guess.
I have pictures, but would have to dig them out of a disc file. 
Ahh!!! I see said the blind man.
I agree, that is a knee wall gambrel.
Gable vents would be adequate and that is the only way, or best way to insulate that attic space.
Whole attic systems, you could use SPF and elliminate vents altogether.
Yup, you got it. It was unbelievable the amount of cold air coming into the crawl space behind the knee wall.
Thanks Peter.
Peter:
In 1983, after a couple of years of energy auditing (I was certified in the US in 1981) and blower door testing, our company starting doing outer attic slopes in this manner. Never had a call back.
On every house, the other thing we were doing was comprehensive airsealing of the attic ceilings and having clients install good bathroom fan systems. (Get the moisture out before it can get into your attic or elsewhere.)
http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/26447.pdf
See the last section on kneewalls. They should be informed that this can be done on existing dwellings and not only new construction.
Brian, that link appears to be broken. the pictures I posted are a retro fit I did for the State of NH, weatherization program.
This perticular house we did replace the bathroom fan, hard piped the dryer vent with a new hood, flashed and sealed the chimney both in the basement and in the attic ( lot of heat loss there ) installed new weatherstripping and sweeps on both front and back doors. Job included pre and post blower door testing with thermal imaging.
Pre blower door was 1980 CFM @50
Post blower door was 1770 CFM @50
Doesn’t seem like much but it made a big difference especially looking at the air leakage through the thermal imager.
Peter:
I got it to work but had to click on it twice and refresh the link.
Can lights not sealed. Humidity (even if it is low) entering the attic through the can lights or other ceiling penetrations.
Hope this helps;
Brian, got it to work, thanks good info.
Will, we always box up the can lights before insulating them.
Peter has it!
There are sources of moisture leakage from other locations in many attics, but the frost is at the soffit and that is were the infiltration is occurring.
Great article Brian!
This soffit location is always a problem area because there is little space for insulation while keeping space for ventilation. Less insulation = more air passage from the wall below.
You don’t want insulation in the soffit, but then the insulation seldom extends past the wall. This is where the moisture comes from. There are baffles you can buy to improve on this (I’ll look it up on my other computer and post).
Eliminating the soffit vents may be fine for New England and other cold zones, but you can’t get rid of them in the hot zones.
If you can’t eliminate the soffit vents, use of denser insulation at the top wall plates will reduce air-moisture passage to the roof.
One step better than the regular proper Vents.
http://www.brentwoodindustries.com/spg/building-products/index.aspx:)
Bathroom vent fans discharging into attic?
Did you read my post?
A whole house humidifier does help carry even heat throughout the home. I’m not a fan of moist air being carried throughout the ducts.
I removed mine and use stand alone units where needed.
It’s common around here to see them turned up to 60%. When that’s the case I often see condensation on walls and attic sheathing.
All this talk of proper insulation and stopping moisture infiltration into the attic… Proper insulation is important but has nothing to do with this problem. It is important that the insulation does not have a vapor barrier however, especially in ceilings and attics.
More importantly, you can not, and should not, stop moisture infiltration into the attic. Moisture is generated in a home every day… all winter. This moisture has to go somewhere, or the home will become laden with moisture. I remember the 70’s when polyethelene plastic was recommended to cover the complete exterior of homes. This was done to stop air infiltration. Instead it stopped moisture from moving thru the walls and ceilings to escape. I have drilled the sheetrock at the bottoms of walls in these homes only to have gallons of water drain out of the wall cavities. Moisture is coming into the home from somewhere (cooking, showering, crawlspace, etc.) Allow it to naturally move to the attic and then insure proper ventilation. If it is coming from the crawlspace you can help by insuring a good ground vapor barrier.