Replies

1. GBA Editor
   Martin Holladay|Jun 01, 2016 08:02pm|#1

   Carl,
   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.

2. 
   Carl Haberberger|Jun 01, 2016 11:10pm|#2

   Martin,

   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?

   Thanks

   Carl

3. GBA Editor
   Martin Holladay|Jun 02, 2016 06:27am|#3

   Carl,
   There are three ways to proceed: the best way; the OK way; and the not-so-good way.

   最好的方法是安装一个或多个层s 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, seeHow 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, seeSite-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.

4. 
   Charlie Sullivan|Jun 02, 2016 07:09am|#4

   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.