The most Underappreciated Building Assemby
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A Recent FHB Lstiburek Article
http://www.finehomebuilding.com/how-to/articles/a-crash-course-in-roof-venting.aspx
( free at BSC site)
http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting
A good article.
And a very interesting clue to affordable/buildable high performance….
Lstiburek: “A vented attic, where insulation is placed on an air-sealed attic floor, is one of the most underappreciated building assemblies that we have in the history of building science.”
I think the Vented Attic HAS been neglected by most.(but not all)
I think the trend in high performance home design has been toward unvented roofs or vented roof decks.(Almost all Building America projects, Marc Rosenbaum projects, Passivhaus & PHIUS projects…And most of what I see at FHB & JLC magazines.)
the BSC article mentions a Video by Joe Lstiburek … but I have not found a link yet
does anyone have a link to the video?
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Replies
The article links to some prefabricated baffles that seem to work with Roof trusses
What about stick frame?
Does anyone have links to prefab baffles that work well with stick framing?
John,
Accuvents work just as well with standard rafters as they do with trusses. They are just plastic baffles that fit in the rafter or truss bays.
Certainly in the northeast, vented unconditioned attics aren't underappreciated. They are the norm.
Martin, do you have a link to the video that was mentioned?
by underappreciated I am talking about what is usually published for Low Energy Enclosures including the Northeast
Riversong & Chlupp are exceptions
with stick framing .. sometimes the ceiling joists are 16 OC and the rafters are 24 OC
I think this would make the Accuvent type baffle awkward.
Even when joist/rafter spacing is the same ... the rafter & clg joist leave an odd-shape void
are there other prefab baffles that work better with stick-frame?
John,
Here's the link:
https://www.greenbuildingadvisor.com/blogs/dept/green-building-blog/lstiburek-s-rules-venting-roofs
John,
Since we need to do heeled trusses or knee walls to allow for continuous insulation in the attic floor, all you need to do is run the wall sheathing and exterior rigid insulation all the way up to create a 1”-2” air gap under the roof sheathing so you don’t have to worry about baffles and their installation.
Armando's pdf shows a great example of full insualtion over the walls and the "wind wash barrier".
The rigid insualtion or any exterior sheathing extends beyond the the top plates and to a point 1 1/2" or so under the roof decking, a very important energy detail. In MN, our code calls only for a minimum 7 1/2" energy heel so air chutes are needed to get airflow above the blown insulation.
Armando & Doug,
I am looking for a way to have Uber Attic Floor insulation AND without having to raise the overall height of the house too much. (expense of extra brick, siding or stucco)
Most of the examples I see from Cold Climates employ baffles that continue under the roof deck to allow for more insulation depth.
And if I understand Joe... we only need to meet or exceed the R-value of the exterior wall above the plate.. not necessarily match the overall attic floor R-value.
Doug, what are you using for baffles? prefab, site built .. or like Armando's dwg?
John,
This is the vent I use http://www.adoproducts.com/provent.html
Here is a source http://www.buildingforhealth.com/proddetail.php?prod=ADO-PV
John,
The new codes require continuous insulation, period. In the old ways, the issue you describe would allow to install up to a third of the ceiling area with less of the insulation required by code. How would you like to be the homeowner of that house?
I would like to know how do you proppose to install R30 or R38 insulation above the top plates without a heeled truss???
Armando: John is picking up on Joe's criterion that insulation over the top plate must be at least as good as the wall insulation. That this may be somewhat less than the general level of attic insulation does not conflict with the requirement for 'continuous' insulation as I would understand it. I accept that this may be an unacceptable standard for superinsulated homes in severe climates but for the rest of us I suspect it may be just fine. I do wonder where you get the 1/3 number for the underinsulated portion of the attic: perhaps you work a lot with very low pitch roofs. With a roof pitch of 6/12 or above there's little problem getting to R38 within a few inches of the wall line without needing much in the way of a raised heel or knee wall. A larger point may be that in the absence of serious thermal bridging, overall resistance of the entire enclosure to thermal exchange is more important than the thickness of insulation at any one point, and it's easy enough to compensate for a lower R-value in one part of an attic with a greater amount of blown-in insulation at another.
On the other hand I have found that kicking the eaves up by framing the rafters on a plate which is on top of the ceiling joists gives welcome flexibility in getting the right shading overhand with steeper roofs. And this will generally give room for 14" or so of insulation right at the plate line.
And on the other, other hand there's an interesting discussion going on in a parallel thread to this one also based on Joes' vented attic guidelines. The topic is ways to enhance and protect the ceiling air barrier by creating a service core below it. In our piedmont NC climate air sealing is at least as important as insulation thickness, perhaps more so, and I'll take a tiny bit of R20 ceiling in exchange for a whole lot more airtightness.
https://www.greenbuildingadvisor.com/blogs/dept/green-building-blog/lstiburek-s-rules-venting-roofs
Armando,
I am not arguing to not use Raised Heel trusses or to not raise the rafter by some other method.
I am talking about above code R-values for walls and far above code for the majority of the Attic.
So I see a need for a sloping baffle to contain and protect the insulation
Armando:"The new codes require continuous insulation, period."
Armando, I want to make sure I understand the new code....
Doesn't Section 402.1.4 Total UA Alternative. offer an exception?
Boy oh Boy
The Simple Vented Attic keeps looking better every day
http://www.buildingscience.com/documents/insights/bsi-063-over-roofing/files/BSI-063_Over-roofing.pdf
He is forthcoming with his learnings (mistakes). Gotta respect that.
Gotta love this guy Joe L. Hey Joe, what's your blood type AB? Can't believe it could be another. (Just read a blip on blood type personalities) Fit me to a tee.