How did they insulate the 2013 FHB award house?
I was looking at the 2013 Houses issue and the award winner i sthe Passive House built in California. On page 38, they show a detail of the roof framing/insulation (image attached). There appears to be 2×12 (ish) rafters with sheathing above and spray foam between the rafters. The 2×12 rafters terminate at the exterior wall framing. Above/on top of the primary rafters, they have another layer of 2×4 rafters that extent beyond the exterior walls, providing the overhangs and go all the way to the peak. The 2×4 rafters are sitting on 3/4″ plywood spacers (presumably helping to provide a thermal break) which in turn are sitting on the sheathing of the lower primary 2×12 rafters. Phew…. I see what they mean by a picture being worth a thousand words :-).
The drawing shows the foam between the 2×4 rafters only over the soffit, but I think it was just to show that there is 2×4 framing beyond as they did in the wall framing detail. Assuming the insulation goes to the peak, I’m wondering how they applied the foam between the 2×4 rafter bays as it woudl be blocked from the underside by the sheating that is on top of the primary 2×12 rafters. The only thing I can figure is that they sprayed it before putting on the final layer of sheating above the 2×4 rafters, cut the foam and then applied the sheathing? Maybe the foam really is only applied over the soffits, but that is a pretty funky and expensive detail to end up with dead, unvented, uninsulated mini-attic space between the sheathing and lower rafter/insualtion system. Any other ideas?
Replies
Hey Huckle,
Your assumptions are correct. The foam between the 2x4 rafter is stopped short only to illustrate that the 2x4s extend up to the ridge. In reality, the entire rafter bay is full of foam. Also, they did insulate these rafter bays from above. I thought that it was a bit odd that they chose to insulate all of the way to the edge of the eave. However, I asked the builder and architect about the detail. They felt that it was faster and easier to fill the entire bay than it would have been to add additional blocking.
Hope you enjoy the issue.
Best,
Rob Y.
Design Editor
Benefit of Insulating to the Eaves
Thanks for the response. I believe insulating all the way to the eaves can be additional protection against ice dams. Under the right (wrong) conditions, when temperatures during the day are right around 30 degrees, additional heat under the eaves can warm them up enough to turn the snow above into a slushy mess. When the temperature drops at night, it can refreeze into an ice block, starting an ice dam.
While this house is particularly tight, most houses can pick up some amount of heat under the eaves from poorly insulated/air sealed top plates, windows and exhaust vents, By insulating down to the eave, it can help prevent the warmth from below heating up the roof surface enough to start the ice dam process.
If I have that right, the cost to benefit may be marginal, but it could be considered cheap compared to bad ice dam damage....
Looks to me like the foam goes all the way up but for the life of me I can't figure out why. The payback time for all the extra framing, plywood and foam is probably 100 years. This is the kind of stupidity that "Green" building has brought us.
Green building sure costs a lot of GREEN!