Vented & unvented in same roof plane?
Hopefully you can all see this attached glass-house view of my proposed roof.
The open attic at the bottom right of the photo allows plenty of room to super-insulate, same too with the open attic above the 2nd floor ceiling, up in the top left/center of the photo. The sloped roof though poses a problem for super-insulation levels, just not enough depth. What I’d like to minimize is a path of least thermal resistance which would circumvent the high insulation levels in the open attics and favor “Easy” migration through the sloped roof section.
How essential is a venting channel in such roof assemblies? No venting channel frees up another inch of spray foam. Can the two attics be vented with the sloped intermediate section spray foamed through the entire depth of the 2×12 and create a system which is not compromised? If so, what potential risks are brought into play for that section of the roof?
For the two open attics, what is the best way to create air circulation in each compartment?
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Replies
Alex,
If you are planning a new house, you need to plan ahead so that your sloped roof assemblies have enough depth for proper insulation. You can create unvented cathedral ceilings if you want, as long as you get the details right. Just don't skimp on R-value. For more information, see How to Build an Insulated Cathedral Ceiling.
Martin,
The roof will be framed with 2x12.
Thanks for the link but I've already read it and was unclear on the aspects I noted, the interaction between vented and unvented sections in the same plane and the particulars of venting the two open attics, in that most venting starts at the eave and ends at the ridge. What is best practice when there is an interruption in between? If the sloped roof has air channels, then are they sufficient in scope to carry air from the open attic at the eave level to the open attic at the ridge level? If an unvented slope attic, then do I need to have vents at eave and at top of the eaved attic and for the top attic, at the ridge and then below, at the bottom of the ridged attic section, to insure cross flow?
Will 11" of closed cell in the slope perform at a level greater than it's stated R-value compared to the R-value of blown celluose or batts of Roxul on the attic floor, in that the closed cell has zero air migration?
Alex,
In general, I'm not a fan of chopped-up roofs with a complicated series of roof planes. Whenever I hear that there are several small attics separated by roof planes that will be insulated along the roof slope, I say, "Uh-oh. It sounds like we have a basic design problem here."
That said, sometimes builders have to come up with solutions to problems created by architects. It happens all the time.
If you have to create continuous ventilation channels that pass through intermediate areas (strange attics), I would continue the ventilation channels through the attic areas by using the same baffle system as is used in the insulated sloped sections. The best baffles are site-built baffles made of thin plywood or rigid foam, with a ventilation channel that is at least 2 inches deep.
The other option, as I wrote, it to create unvented roof assemblies.
In any case, it is essential that: (a) you know where your air barrier is, especially when an insulated roof assembly meets an attic, (b) the air barrier is tested, and (c) you don't skimp on R-value.
So 1.5 stories have inherent design problems baked into their design DNA? Can good construction execution completely erase the negative consequences of bad design?
The baffles certainly provide a ventilation connection between the bottom and top attic but at the expense of 2 inches of insulation.
Do you know of any research, or can you draw on industry experience, to speak to the situation of vented attics working alongside a section which is unvented? Does the nearby presence of two vented attics affect the performance of the unvented attic?
Good advice on the air barrier, I will certainly keep that in mind.
As for the R-value, the open attics allow me to not skimp, but the sloped roof has a built-in limitation on depth. Are there any studies which speak to the usefulness of super-insulating parts of the attic when the sloped roofs have less insulation? Is that practice akin to building the Maginot Line only to have the enemy bypass the fortifications?
I really would like to maximize the slope roof insulation, 11" of closed cell, maybe a bit more with furring on the ceiling to extend depth, but I'm uncertain of the interaction effect from the two vented attics adjacent to the unvented slope.
One last question. Should every small attic have an air entry vent and an air exhaust vent?
Alex,
Q. "So 1.5 stories have inherent design problems baked into their design DNA?"
A. Absolutely. No type of building is more likely to have a bad thermal envelope, thermal bridging, and air leaks. Capes are air-sealing nightmares.
There are solutions, of course. The best solution is to create an insulated roof over the entire house. The best details for this type of roof assembly include a thick layer of rigid foam above the roof sheathing, with a "vented over-roof" above the rigid foam.
It makes no sense to tell us that "As for the R-value, the open attics allow me to not skimp, but the sloped roof has a built-in limitation on depth." It's a new house! What possible reason is there for your designer to choose a type of framing that isn't thick enough for code-minimum insulation? That's nuts. A designer has to think about insulation at the design stage.