How should I deal with attic insulation where roof meets wall?
I’ll be blowing cellulose into a vented 1920’s attic in Oakland Ca. zone 3c. Roof has no overhang, rafters begin on top plate. There’s also a small daylight gap where top of gutter board and bottom of roof sheathing meet. Would filling that intersection and contacting roof sheathing and rafters be a concern?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
There are several issues here.
You call it a "vented attic," but you haven't described the vent inlets. It sounds like you don't have conventional soffit vents. So in what way is the attic vented?
The second question concerns the vertical distance between the top plate of your exterior wall and the underside of the roof sheathing. Ideally, you'd have about 12 inches of vertical distance... but I'm guessing you don't have that. So you probably don't have enough room to provide the code-minimum requirement for insulation (R-38), much less room for a ventilation gap between the top of the insulation and the sheathing. The usual solution is to create a ventilation gap (at least 1 inch deep) and to use closed-cell spray foam for the rest of the space -- at least until the roof provides the needed 12 inches.
Answering your questions; Yes no soffit vents, no overhang. Shingles, flashing, gutter, stucco. Eyebrow, gable end and ridge vents.. 2x4 rafters and joists. Distance from plate to sheathing starts about 1 inch.
I understand not blocking soffit vents, the minimum air gap and using baffles. Are you saying you'd recommend still treating this as if it were a typical overhanging soffit roof? Just keep insulation out of contact with sheathing?
There are three ways to proceed: the best way; the OK way; and the not-so-good way.
The best way would be to install one or more layers of rigid foam above the roof sheathing, followed by a layer of OSB or plywood and new roofing. This approach would transform your vented unconditioned attic to an unvented conditioned attic. (This approach would also require you to insulate the gable walls of your attic, if there are any.) The main advantage of this approach is that it's the only way to get adequate insulation at the perimeter of your roof assembly. The main disadvantage is that this approach is expensive. For more information on this approach, see How to Install Rigid Foam On Top of Roof Sheathing.
The OK way to proceed would be to install ventilation baffles at the eaves before blowing cellulose on your attic floor. Even though you have no air movement from soffit vents, the ventilation baffles will prevent the insulation from touching the roof sheathing (which tends to get damp in cold weather, and which will dry better with the air gap). The main disadvantage of this approach is that you will have almost no insulation at the perimeter of your roof assembly. For more information on this approach, see Site-Built Ventilation Baffles for Roofs.
The not-so-good way to proceed would be to let the cellulose touch the roof sheathing. This approach might result in damp sheathing in a cold climate. In Oakland, California, where the weather is mild, you might get away with this approach. But I wouldn't do it.
If you take the middle road among Martin's three options, you'll have to decide on how to allocate your small space between ventilation and insulation. You might consider angling the baffle to taper from 2" far from the eaves to 1" or perhaps as little as 1/2" at the very end.
How much benefit you get from the best approach depends in part on how steep the roof is. If it's steep, there's only a little space that you won't be able to insulate up to code, so it doesn't matter much.